A variety of terms, including kinesthesia, proprioception, somaesthesia, the haptic system, position sense, muscle sense, joint sense, and movement sense, have all been used in similar ways to describe aspects of the perception of bodily movements and positions.
APX. II.11 Sixth sense.
The sense of bodily movement and position of one’s own body does not fit easily into Aristotle’s classic senses; seeing, hearing, smelling, tasting, and touching. Consequently Sir Charles Bell (1833) and others more recently (Fitt, 1988, p. 266) use the notion of a “sixth sense”.
Dickinson (1974, p. 9) explains how the five senses were based on a “doctrine of ‘specific nerve energies’” whereby a particular sense is thought to emerge from a particular sensory receptor. This has been shown to generally not hold true. For example, audition and vision both utilise data from head and body movements which are used to achieve a variety of postural orientations from which to sample the visual or audio stimuli (Scharf and Houtsma 1986; Sedgwick 1986). This is especially true for the perception of bodily movements and positions which arises from receptors throughout the body, including muscles, joints, tendons, skin, labyrinth, visual, audio, and an efferent discharge loop (see below):
. . . the doctrine of ‘specific nerve energies’ [which was] held earlier this century has blinded later researchers to the fact that movement sensitivity does not depend on specialized receptors. It is not simply a sixth sense to be added to Aristotle’s five classical senses. (Dickinson, 1974, p. 9)
APX. II.12 Touch.
The sense of bodily movement and position can also be conceived to be an expanded generalised version of Aristotle’s sense of touch. Bastian (1888, p. 5), who proposed the term “kinesthesia” (see below) noted that its “cerebral seat or area corresponds with the sense of touch”. Rock and Harris (1967, p. 96) also use “touch in this broad definition” according to which “Touch includes several other components [in addition to skin sensitivity], of which the one most significant for this discussion is the position sense”.
Indeed, the sense of touch, or cutaneous sense, is itself not entirely based on data from skin receptors but also relies on sensations from muscles, tendons, and joints to perceive the shapes of objects being touched by moving the body around the object, and to perceive the texture of an object by exerting variations in pressure (Gibson, 1966, pp. 50, 53; Schwartz et al., 1975).
The sensations of bodily movements, positions, and touch, all arise from the same system of receptors and so they can be conceived to belong to the same sensory system. The specialised “touch” sub-system can be included together with the other specialised kinesthetic sub-systems including the sense of balance, force, linear or rotary self-motion, sense of limb movement and limb position (see below).
APX. II.13 Kinesthesia.
Bastian (1888) proposed the term “kinesthesis” to refer to the “sensations which result from or are directly occasioned by movements” (p. 5). Despite its etymology (Greek kinein, to move + aisthesis, feeling; American, 1982]) meaning literally “the feeling of movement”, this term was intended to replace both the terms “‘muscular sense,’ and ‘sense of force’” which were previously in use (p. 5). Thus, under the heading of kinesthesis he included the perception of “position and movements of our limbs”, and “different degrees of ‘resistance’ and ‘weight’” (p. 6). The dictionary definition of kinesthesia also groups together the various sub-senses within its class, including “bodily position, weight, muscle tension, and movement” (Collins, 1986) and also the sense of “presence” (American, 1982).
The term “muscle sense” may be synonymous with kinesthesia (Collins, 1986) in that “kinesthesia” was adopted to replace the former term (Clark and Horch, 1986). Alternatively, the muscle sense may be considered to be one of the several kinesthetic sub-senses, including “muscle sense” (receptors in muscles), “tendon sense” (receptors in tendons), “joint sense” (receptors in skeletal joints), and “static sense” (receptors in the labyrinth) (English and English, 1974).
The common accepted usage is that kinesthesia refers to the perception of ones own bodily movements and positions (Fitt, 1988, p. 266), and sometimes also the forces produced or reacted to (Rasch and Burke, 1978, p. 80). Some authors also include processes of “motor coordination” and “motor memory” as part of kinesthesia (Fitt, 1988, p. 267).
In contrast, Cross and McCloskey (1973, p. 443) distinguish between “position sense” for limb positions and “kinesthetic sensations” for limb movement. McCloskey (1973) vibrated the tendon of subjects’ biceps brachii muscle with a physiotherapy vibrator which induced the subject to perceive an illusion of movement at the elbow joint. The illusions of movement and illusions of position were able to be experimentally manipulated so that they did not correspond: A longer duration movement illusion did not result in different position illusions; A low frequency vibration did not cause a movement illusion but did create an illusion of a changed position:
. . . subjective judgements of the static positions of joints and judgements of movements of joints can use different lines of information. It is suggested that the term ‘position sense’ be reserved for the static judgements, and ‘kinesthesia’ for the dynamic ones, and that the two terms should not be regarded as synonymous. (McCloskey, 1973, p. 130)
APX. II.14 Proprioception.
Sherrington (1906) uses the term “proprioception” in his discussion of how types of sensory impulses will initiate certain physical reflexes. Sherrington draws a general distinction between internally versus externally produced sensory stimulations:
Multicellular animals . . . are cellular masses presenting to the environment a surface sheet of cells, and under that [is] a cellular bulk [which is] more or less screened from the environment by the surface sheet. (Sherrington, 1906, p. 316)
“Proprioceptors” refer to the receptor cells which are screened from the exterior environment, and so “the stimuli to the receptors are given by the organism itself” (Sherrington, 1906, p. 130). The term derives from the Latin proprius (one’s own) plus “reception” and so is defined as “the reception of stimuli arising within the organism” (American, 1982).
Sherrington (1906) distinguishes two types of sensory cells on the organism’s exterior surface; exteroceptors and interoceptors. “Exteroceptors” refer to sensory cells which are “freely open to the numberless vicissitudes and agencies of the environment” (p. 317). “Interoceptors” refer to sensory cells on “surfaces” of the body, but which have developed deep recessions, “in this recess a fraction of the environment is more or less surrounded by the organism” (p. 317).
Vestibular “labyrinth” receptors form a special case. Sherrington (1906, p. 336) describes these as being “derived from the extero-ceptive, but later recessed off from it” and which now function as proprioceptors:
The proprio-ceptors of the body generally and of the labyrinth receptors in the head appear to co-operate together and form functionally one receptive system . . . embraced within the term “proprio-ceptive”. (Sherrington, 1906, p. 341)
Sherrington distinguishes the following groups of sensory receptors:
Proprioceptors found in: muscles, joints, tendons, labyrinth.
Exteroceptors found in: eyes, ears, skin.
Interoceptors found in: mouth, stomach, nose.
Sherrington’s distinctions are often followed closely (Dickinson, 1974, p. 10; Ellison, 1993, p. 75; Rock, 1968) and also have been misrepresented. For example Wells and Luttgens (1976, p. 58) consider proprioceptors (together with visceroceptors) to be a type of interoceptor, contrary to Sherrington who explicitly distinguishes between the two.
APX. II.15 Somaesthesia.
“Somaesthesia” (or, somesthesis, somatosensory) is used very similar to proprioception in that they both refer to perceptions arising out of one own’s body (from Greek soma, the body). Somaesthesia refers to sensations of body movements and positions, and also to sensations of temperature, pressure, touch, and pain (Collins, 1986). It is generally used to refer to stimulations arising from receptors in muscles, tendons, joints and skin (not vestibular) (Bles, 1981; Lackner and DiZio, 1984; Taub et al., 1973; 1975). The muscle/tendon/joint/skin conception of somaesthesia is sometimes considered to be synonymous with kinesthesis (English and English, 1974).
APX. II.16 Haptic system.
Gibson (1966, pp. 50, 53) proposed a sub-group within proprioception which he termed the “haptic system” with its “mode of attention” as “touching” and using sensory data from receptors in skin, joints, muscles, and tendons to produce perceptions about the environment or the body. The “hands and other body members” are considered to be the “organs of perception”. The haptic system can derive information about one’s own body or about the exterior environment (eg. feeling the shape of an object) and so is both proprioceptive and exteroceptive. “Haptic” is from the Greek haptein, to touch, (Collins, 1986) and so is identical with the generalised notion of the sense of touch (see above).
This varied terminology describes perception of body movements and positions in slightly different ways. Certain criteria can be identified to asses the usefulness of each term.
APX. II.21 Invalidity of Internal / External Distinction.
The proprioceptive/exteroceptive distinction between internal stimuli from the body versus external stimuli from the environment has been found to be invalid. In many cases external stimuli such as visual-field motion or audio-field motion (see below) provide information about the body’s movement. In his landmark work, Gibson (1966) explored how audio and visual perception are extremely important for the perception of one’s own movements. It has been shown that the movement of audio or visual fields can easily induce illusions of self-motion even in the absence of joint, muscle, tendon and labyrinth stimulations (G. J. Anderson, 1986; see below). This type of external stimulation is also vital for maintenance of the body’s balance (Lee and Aronson, 1974; see below).
Thus, in addition to the traditional “muscular proprioception” (muscle receptors), “articular proprioception” (joint receptors), and “vestibular proprioception” (labyrinth), Gibson (1966, pp. 36-37) also includes “cutaneous proprioception” (skin receptors), “auditory proprioception”, and “visual proprioception”. Other researchers also use the terms “visual proprioception” (Lee and Aronson, 1974; Lee and Lishman, 1975), “visual kinesthesis” (Lishman and Lee, 1973; Rieger, 1983; Warren et al., 1988, p. 646), “visuopostural feedback” (Souder, 1972, p. 15), and “exproprioception”, literally, perceiving the inside from the outside (Fitch et al., 1982, pp. 275-276; D. N. Lee, 1978). Receptors in skin which can receive stimulation from the exterior environment will also respond to stimulations from body movement. Thus these must be classified as both proprioceptors and exteroceptors. Bastian (1888), who proposed the term “kinesthesia” stated this same fact at the outset, that “the group of sensations under the name of kinaesthesis . . . is confessedly a mixed group partly ‘intrinsic’ and partly ‘extrinsic’ in their origin” (p. 6).
APX. II.22 Inconsistent use of “Kinesthesia” versus “Proprioception”.
Another problem is that the terms “kinesthesia” and “proprioception” are not consistently defined. Typically the two terms are used synonymously (Clark and Horch, 1986; Schmidt, 1982, p. 202) and Moberg (1983, p. 1) considers “kinesthetic sensibility, position sense, muscle sense or proprioception” as synonyms. Similarly, sometimes the term “visual proprioception”(Lee and Aronson, 1974; Lee and Lishman, 1975), is used, while others refer to this as “visual kinesthesis” (Lishman and Lee, 1973; Rieger, 1983; Warren et al., 1988, p. 646).
The particular components included within kinesthesis or proprioception also vary among authors. In the narrowest view stimulations arising from receptors in muscles, tendons, and joints (not labyrinth or skin) are included as proprioceptors (Fitt, 1988, p. 266) or as kinesthetic (Laszlo and Bairstow, 1971).
In a slightly broader view, Bastian (1888, p. 5) considered receptors in muscles, tendons, joints and skin (but not vestibular) as being kinesthetic. Perhaps vestibular was not included because Bastian focused on the positions and movements “of our limbs” (p. 6) rather than linear or rotary self-motion (see below). Other writers also follow this same view of including stimulations from muscle, joint, tendon, and skin (but not vestibular) receptors as being kinesthetic (Clark and Horch, 1986) or as proprioceptive (Rothwell, 1987, p. 74), or as proprioceptive considered synonymous with somatic sensation (Taub and Berman, 1968). Similarly, Souder (1972, p. 14) considers the vestibular labyrinth to be a separate system from either kinesthetic or proprioceptive. Sherrick and Cholewaik (1986, p. 111-3) consider the cutaneous sense to be exteroceptive but also that “the senses of the skin do occasional duty as supplement to the kinesthetic senses”. Wells and Luttgens (1976, pp. 58–61) include skin receptors as being proprioceptive only when they participate in withdraw and thrust reflexes.
Other authors use Sherrington’s (1906) original distinctions of including sensations arising from muscle, tendon, joint, and labyrinth (but not skin) receptors within proprioception (Dickinson, 1974; Ellison, 1993, p. 75; Rock, 1968).
In the broadest view, visual, audio, skin and labyrinth receptors are included together with receptors in muscles, tendons and joints as all contributing to kinesthesia (Rasch and Burke, 1978, pp. 80-81), or to proprioception (Gibson, 1966, pp. 36-37), or as kinesthesia considered synonymous with proprioception (Schmidt, 1982, chapter 6):
Historically, kinesthesis . . . was a term limited to a person’s perception of his or her own motion, both of the limbs with respect to one another, and also of the body as a whole. Sherrington’s (1906) term proprioception was originally used to mean the perception of movement of the body plus its orientation in space (even though it may not be moving). Over the years these two terms have become practically synonymous, and it is probably not important to continue this distinction. (Schmidt, 1982, p. 202)
Other researchers might arrange kinesthesia and proprioception into a kind of hierarchy, however these arrangements tend to vary. The proprioceptive system is sometimes considered as a higher-order system containing the separate kinesthetic and vestibular systems (Riesser and Pick, 1976; Sherrick and Cholewaik, 1986). Strelow and Babyn (1981, p. 191) list “vestibular, kinaesthetic, and proprioceptive information” implying that the three are separate. Singleton (1972, p. 61) represents the somaesthetic system as containing the proprioceptive system and the tactile system. The proprioceptive system is then further subdivided as containing the kinesthetic system (including receptions from muscles, tendons, and joints) which is separate from the vestibular system.
APX. II.23 Kinesthesia and Proprioception as Conscious and Unconscious.
Kinesthesia is sometimes used to refer to conscious perceptions since the Greek root aesthesia means “to perceive”, while proprioception is not necessarily conscious but may occur as unconscious sensory receptions which elicit reflex reactions. This conception places kinesthesia as a higher-order derivative which calls on proprioception for its data.
Much of Sherrington’s (1906) research which distinguished the term “proprioception” focused on reflex actions produced when stimulating particular receptors. McCloskey (1978, p. 764) also describes that Sherrington used proprioception to refer to “vestibular sensations and inputs from muscles and joints that are not necessarily perceived” and other authors explicitly refer to the conscious/unconscious distinction between kinesthesia and proprioception (Ellison, 1993, p. 75; Paillard and Brouchon, 1974, p. 275). Correspondingly, in Lee and Lishman’s studies of vision and body movement, they use “visual kinaesthesia” (Lishman and Lee, 1973) when they are studying subjects’ conscious perceptions of their own self-motion, whereas they use “visual proprioception” (Lee and Lishman, 1975) when they are studying subjects’ unconscious, reflexive responses for maintaining upright posture.
Research has also focused on whether sensory discharges from muscle spindle receptors have any direct access to conscious perception (kinesthesia) or are used solely for subconscious reflexive control of movement (proprioception). This question has been referred to as the “problem of ‘conscious proprioception,’ whether there is awareness of muscle length and tension changes” (Gelfan and Carter, 1967).
Some evidence indicates that sensory reception from muscles is not consciously perceived. Anaesthestized joints produces a loss of perception of passive movement or position in the finger joint (Provins, 1958) or the toe (Browne et al., 1954) even though the muscles which act upon these joints were unaffected by the anaesthesia. Stretching a muscle by pulling on the exposed tendon does not produce any conscious perception of limb movement in the fingers, hand, or foot and so Gelfan and Carter (1967) conclude that “there is no muscle sense in man”. This effect was duplicated by Moberg (1983) who stresses the importance of skin receptors (rather than joint or muscle receptors) for conscious kinesthesia in the fingers and hand.
However other evidence indicates that muscles do play a role in conscious perception. Sensory impulses from muscle spindle receptors have been found to have direct connections to the cerebral cortex in baboons (Phillips et al., 1971) and cats (Oscarsson and Rosen, 1963). When muscles acting on the fingers are lightly tensed or voluntarily moved then motion is perceived even if the joints and skin have been paralysed (Goodwin et al., 1972a; 1972b). This sensory facilitation of actively moved versus passively manipulated muscles was also noted earlier (Browne et al., 1954). Illusions of forearm movements and false positions have also been elicited by vibrating the muscles and tendons with a physiotherapy vibrator (Goodwin et al., 1972a; 1972c).
Distinguishing receptors as to whether their stimulations become conscious or unconscious appears to be a tentative affair. This is especially true since conscious perceptions rarely arise solely from the sensations of one individual receptor, especially in kinesthesia where input from an abundance of receptors is combined into a unified perception. Conscious kinesthesia is not attributed to particular receptors per se, but as a phenomenological experience of the body’s positions, motions, forces etc. In McCloskey’s (1978) exhaustive review of “kinesthetic sensibility”, and in particular the question of “Are muscles sentient?”, it is
noted that “perceptions” are not experienced in the receptors, but in the objects perceived:
. . . we are no more likely to feel kinesthetic sensations in our muscles or joints than we are to hear sounds in our heads or see objects in our retinas - but [conscious kinesthesia] would be sensations of movement, or force, or tension, or of altered position in the parts moved by the muscles. (McCloskey, 1978, p. 777 [emphasis his])
It would not be beneficial for the raw data from receptors to be available to consciousness since the data from collections of receptors must be interpreted relative to each other and to exterior forces (gravity, momentum, external objects) and relative to any motor commands which have been executed. These will all influence the significance of any isolated receptor response:
. . . the essential point is that it would be of little value for the highest sensory centres to receive raw data from the muscle afferents, because what these mean depends entirely upon what the relevant muscle is being told to do by the motor system. (Goodwin et al., 1972a, p. 744)
Likewise, in a study of the history of proprioception Dickinson (1974, p. 10) concludes that “at a physiological level, the absence of a direct link from receptors to the cortex may not necessarily preclude some indirect participation in perception” since the perception is derived at an unconscious level anyway.
APX. II.24 Conclusions; Working Definitions.
From the overlapping concepts of somaesthesia, kinesthesia, proprioception, etc. outlined above, the following working definitions will be used in this study.
The term “proprioception” will not be used since it belongs to an interior/exterior distinction which has been shown to be invalid. In Dickinson’s (1974) historical review of proprioception it is observed that “Not only is there disagreement concerning the definition of proprioception, there is even disagreement over whether proprioception may be viewed as a sensory modality” (p. 9). Clark and Horch (1986, p. 13.2) state that “the term proprioceptive lacks a precise definition” and therefore they prefer the term kinesthesia. In light of the other terms available, the notion of proprioception is not necessary.
Kinesthesia will be used in its broadest sense to refer to perceptions arising from muscle, tendon, joint, skin, vestibular, visual, and audio receptors. In addition, an interior knowledge of motor commands or “efferent data” can be considered to be another source of kinesthetic information (see below).
Other “senses” can be classified as kinesthetic sub-systems. These include limb position sense, limb movement sense, sense of linear or rotary self-motion, sense of balance or equilibrium, and the sense of force.
“Somatic” will be used in its typical definition of referring to perceptions arising from receptors in muscles, tendons, joints, and skin. These receptors comprise a complete grouping in themselves within the larger group of kinesthetic receptors. This somatic system is synonymous with the haptic system but since “haptic” comes from “to touch” it is more related to skin receptors. Somatic is chosen here since it refers to perceptions from anywhere in the body.
Various sensory receptors and an internal knowledge of motor commands contribute data which is derived into kinesthetic perceptions. This section will briefly review the functioning of each type of receptor, the stimulation which it responds to, and the type(s) of information the receptors provide. Evidence for an internal knowledge of motor commands (referred to here as “efferent data”) is also noted.
A fundamental characteristic of receptor function is the rate of “adaptation” of a sensory receptor response to a stimulus which is steady and continual (Sherrick and Cholewaik, 1986, p. 111-6). These are classified as generally two types: 1) Quickly adapting receptors stop responding to a continual stimulus very soon are are therefore efficient in sensing rapidly changing stimulations such as quick movements; 2) Slowly adapting receptors maintain their response to a continual stimulus for a long period and are therefore efficient in sensing continuous, unchanging stimulations such as a maintained bodily position.
APX. II.31 Muscle Spindles; Primary and Secondary Endings.
Muscles are composed of hundreds of individual long slender muscle fibres which connect to tendon filaments at either end which in turn attach to bones. The large main muscle fibres are referred to as extrafusal fibres and produce the muscle’s strength from their force of contraction.
Modified muscle fibres of the sensory spindle organs are referred to as intrafusal fibers and are arranged in parallel to the longer, thicker extrafusal fibres. This arrangement allows the intrafusal fibres to shorten and lengthen together with the extrafusal fibres but without carrying any of the burden of force. A muscle spindle receptor, within the thin intrafusal fibres, has two types of sensory endings known as primary and secondary.
Secondary spindle endings increase their response linearly as the muscle length increases throughout the range of the muscle (Matthews and Stein, 1969) and so they function analogously to slowly adapting receptors. The secondary spindles provide data about the overall length of the muscle but are not sensitive to small quick changes in the muscle length (Rothwell, 1987, pp. 76-87).
Primary spindle endings are sensitive to much smaller muscle length increases but their response does not increase regularly with muscle length (Matthews and Stein, 1969). Rothwell (1987, pp. 77-79, 86-87, 97) reviews how primary endings are thought to respond to “cross-bridges” which link parallel intrafusal fibres. The cross-bridges are stiff, when the fibres slide apart (as the muscle lengthens) beyond some critical point the cross-bridges break and reform at the new muscle length. Because of this they are sensitive to very small changes in muscle length (motion) and then quickly return to a static level of response once the new length is arrived at. This pattern of response occurs irrespective of the overall muscle length. They are so sensitive to muscle length changes that they may even respond to arterial pulse or respiratory movements. However, the response of the primary endings to static positions is low, “only 10 per cent of spindles show any discharge at all at a comfortable rest position of the hand” (p. 97). This behaviour of quick responses to small stimuli changes, followed with an immediate return to a neutral response level is analogous with quickly adapting receptors.
From these patterns of responses, it is believed that the primary endings sense velocity of muscle change-of-length and muscle length, while the secondary endings sense only muscle length (Clark and Horch, 1986; Rothwell, 1987, pp. 74-104).
APX. II.32 Tendon Receptors.
Slowly adapting Golgi tendon organs are located at muscle-tendon junctions and are composed of a capsule enclosing several tendon filaments. These are attached end-to-end with muscle fibres and tendon filaments (in series) and this arrangement allows the Golgi tendon organs to respond to muscle-tendon tension regardless of muscle length. Muscle length and tension are separate, for example in an isometric contraction the muscles contract (increased muscle-tension) but limbs do not move (identical muscle-length). Golgi tendon organs have been shown to increase their response to increases in muscle tension very accurately (Crago et al., 1982).
Two other types of receptors in muscles and tendons seem to be of a lesser importance and have not been widely studied. Paciniform corpuscles are mostly found near the Golgi tendon organs are are sensitive to vibrations. Free nerve endings are found throughout the muscle and tendon structures and seem to be sensitive to mechanical pressure and pain stimulations (Clark and Horch, 1986; Rothwell, 1987, pp. 74-104).
APX. II.33 Joint receptors.
Slow adapting Golgi sensory receptors (similar to Golgi tendon organs), are found in the ligaments which connect bone to bone and form the outer layer of the joint capsule. Slow adapting Ruffini receptors and quick adapting paciniform corpuscles, similar to those found in skin, are also found in the tendon material of the joint capsule. Free nerve endings are found throughout the joint connective tissue (McCloskey, 1978, pp. 766-767).
The functioning of joint receptors is debated by physiologists (for reviews see Clark and Horch, 1986; and McCloskey, 1978). Skoglund’s (1956) findings that cat knee joint receptors are selectively activated by certain positions of joint angle led most researchers to believe that the slow adapting Golgi and Ruffini receptors within the ligaments around the joint respond to being stretched. Contrary to this other researchers (Burgess and Clark, 1969; Clark, 1975; Clark and Burgess, 1975; Grigg, 1975) found that most cat knee joint receptors respond only to extreme joint angles. Still other researchers (Carli et al., 1979) found that cat hip joint receptors responded at all angles with the same increasing rate of response with increased flexion or extension of the joint.
The cat knee has also been shown to respond sensitively to pressure into the joint capsule, leading some researchers (Clark, 1975; Clark and Burgess, 1975) to hypothesize a pressure response (rather than a stretch response) for joint receptors. The quickly adapting paciniform corpuscles probably respond to high speed vibrations as they do in the skin. (General references for joint receptors; Clark and Horch, 1986; Rothwell, 1987, pp. 74-104.)
APX. II.34 Skin Receptors.
Free nerve endings are close to hair follicles and stimulated by movements of bodily hairs (resulting from bodily moves or external forces). Slow adapting Merkel disks are close to the surface of the skin, respond only to vertical skin pressure (ie. pressure into the body, not lateral stretch of the skin), and may maintain their response to a constant pressure for up to ten minutes. Quickly adapting Meissner corpuscles are also close to the surface of the skin and sensitive to pressure but will cease responding in seconds.
Slow adapting Ruffini sensory endings are deeper in the skin and demonstrate a directional specific response to stretching of the skin. One direction of stretch will elicit a response, but a stretch at a right angle to that direction will elicit no response (Knibestol, 1975). Quick adapting Pacinian corpuscles are also deep in the skin and respond to stimuli in an area “almost as large as the whole palm in some cases” (Rothwell, 1987, p. 99). Its sensory ending is surrounded by concentric rings which eliminate low frequency vibration and so they respond only to rapid vibrations. (General references for skin receptors; Clark and Horch, 1986; and Rothwell, 1987, pp. 74-104.)
Receptors in skin provide kinesthetic data about the stretching and bending of the skin during movement and within poses. Skin kinesthesia may be especially important in areas of dense skin receptor populations such as the hands, feet, face, and mouth (Moberg, 1983; see below).
APX. II.35 Vestibular Receptors (Labyrinth).
The vestibular system is the non-auditory part of the inner ear. There is one vestibular system for each ear (bilateral). Each system is composed of two parts, the otolith organs and the semi-circular canals. Both the otolith organs and the canals consist of chambers filled with a think “endolymph” fluid. When the head moves through space the inertia of the heavy endolymph fluid causes it to lag behind the movement and thus push against a gelatinous membrane connected to tiny hairs which are connected to nerve endings. When a steady velocity is reached the endolymph fluid stabilises in its chambers and so the sensory response stops. Because of this, the vestibular system responds to accelerating or decelerating changes in speed but not to constant speed.
The otolith organs consist of two sack-shaped chambers, the utricle and the saccule, each filled with endolymph fluid. The hairs connected to nerve fibres are arranged on the floor of the utricle and around the wall of the saccule. Because of this symmetrical arrangement of hairs a rotary acceleration around a vertical axis passing through the head causes opposing forces in the otoliths which cancel each other out and therefore cause no sensation. A linear acceleration through space will cause the endolymph to push unevenly on the hairs and elicit a sensory response. The nerves of the otolith organ also have a constant discharge which continually indicates the direction of gravity.
The semi-circular canals consist of three ring-shaped chambers, each forming a compete circuit of endolymph fluid approximately 3-4mm in diameter. The three canals are oriented at approximate 90° angles from each other so that one canal is roughly parallel to the frontal, medial, and horizontal planes of the body. This mutually perpendicular arrangement allows rotation around any axis to be registered in at least one of the canals, however there is little response from purely linear motion.
In each canal there is one cupula which is the gelatinous projection into the endolymph fluid which is connected to the sensory hairs. When the head undergoes a rotary acceleration the inertia of the heavy endolymph fluid causes it to lag behind the motion and push against the cupula which elicits the sensory response. If the rotary speed is constant after a short time the fluid will stabilise in the canals and the nerves will stop responding. If the motion is then abruptly decelerated the fluid will continue moving and push against the cupula in the opposite direction. This can cause the sensation of turning in the opposite direction accompanied by post rotary nystagmus (see below). (General references for vestibular receptors; Kapit and Elson, 1977; Howard, 1986.)
APX. II.36 Visual Receptors.
The visual-motor system plays an important part in kinesthesia by sensing visual field motion and vision of the body moving (general references; Hood and Finkelstein, 1986; Hallett, 1986; Westheimer, 1986).
Each eye is roughly spherical. At the front of the eye the cornea bulges forward which serves to gather electromagnetic light rays into itself and thus expand the visual field.* The light which is collected by the cornea passes through the adjustable opening of the pupil and into the oval shaped lens.
__________
* The visual field consists of the spatial range of electromagnetic radiation reaching the retina at any one moment. This is also known as the visual array or the optic array.
__________
The retina is a layer of photo-sensitive sensory receptor cells covering the interior surface of the eye. There are two types of visual receptor cells, approximately 120 million rods and 6.5 million cones in each eye. Cones occur in high density in the central fovea region of the retina (more than 140,000 / mm2), low density in the peripheral region of the retina (less than 10,000 / mm2) and are sensitive to high intensity light (daylight brightness), colour vision, and fine detail. Rods occur in low density in the central fovea region (virtually 0), high density in the peripheral region of the retina (from 50,000-160,000 / mm2) and are sensitive to low intensity light (night-light brightness).
The fovea is a small area on the retina which contains a high density of cone photoreceptors. This is the retinal location where visual stimuli can be seen in greatest detail and so is where visual images fall when a person fixates her vision on a point in space. The size of the fovea can encompass stimuli which fills approximately 0.5° of visual angle,* or about the same visual angle occupied by a view of the moon from earth (Westheimer, 1986, p. 4.6).
__________
* Visual angle refers to the amount of space filled by an object within the visual field, measured in angular degrees with the angle’s vertex at the centre of the eye (Sedgwick, 1986).
__________
Monocular focus, also called accommodation, is accomplished by the ciliary muscle adjusting circumferential tension around the lens, thus allowing the lens to bend the light rays in variable amounts. This adjustment of the lens’ shape takes approximately 0.6 sec. to complete. The lens’ accommodation bends the diverging light rays and converges them onto the retina at the back of the eye. When diverging light rays from a single point in space are converged by the lens into a single point on the retina, than this point is in monocular focus.
Binocular focus, also called vergence, is the only type of eye movement when the eyes do not follow parallel pathways. To keep a stimulus in binocular focus (ie. its image falling on the fovea of each eye) the two eyes either rotate closer together or farther apart in response to a stimulus moving closer or farther from the observer respectively.
Two types of data can be distinguished which contribute to visual kinesthesia. These can be referred to as visual field motion and vision-of-the-body moving.
APX. II.36a Visual field motion.
Early analysis of kinesthesia from visual field motion (Gibson, 1958; 1966) discussed how the visual field will appear to move across the retina when an organism travels or turns through space. Patterns of visual field motion, or “optical flow” become associated with the self-motion which usually produces them. For example, when traveling forward the entire visual field will expand and appear to move past on the sides while the point traveled toward remains in the centre of the visual field while gradually becoming larger (G. J. Andersen, 1986; Gibson, 1966).
Other cues within the visual array will add to the details available about the visual field motion (Sedgwick, 1986). “Motion parallax” refers to how visual stimuli close to the observer move across the visual field faster than visual stimuli far from the observer. “Occlusion” refers to how visual stimuli farther from the observer will sometimes disappear behind visual stimuli closer to the observer.
Visual nystagmus will also occur when the visual field flows across the retina. This is a basic orienting reflex which helps stabilise the perception of the visual world when the head is in motion. There are two phases of nystagmus which alternate slow phase, fast phase, slow phase etc.
During the slow phase of nystagmus the eyes remain fixated on a location in the exterior environment while the head is in motion. Thus, in the slow phase the eyes are rotating in the head in the opposite direction as the rotation of the head. This stabilises the perception of the exterior environment while the body is in motion.
During the fast phase of nystagmus the eyes quickly catch-up with the head, re-centering the eye in its socket and fixating on a new location in the exterior environment. Thus, in the fast phase the eyes are rotating in the same direction as the head motion.
When the body turns around the vertical axis the visual field flows across the retina right-to-left (or vice versa) and “horizontal nystagmus” occurs with the eyes also moving right and left. When the body rotates around the lateral axis (eg. somersaults) the visual field flows across the retina top-to-bottom (or vice versa) and “vertical nystagmus” occurs with the eyes also rotating up and down (also called “doll’s eye reflex”). When the body rotates around the sagittal axis (eg. cartwheels) the visual field rotates around the retina clockwise or counterclockwise and “torsional nystagmus” occurs with the eyes also torqueing clockwise or counterclockwise. (General reference for nystagmus; Hallett, 1986.)
The motor commands for the eye movements will effect the characteristics of the nystagmus. Focusing and fixating on objects in the visual field results in less frequent, higher speed and larger distance quick phases of nystagmus than non-focused staring (Honrubia et al., 1968). When the eyes have a visual fixation goal the sequence of motion is as follows:
1) The eyes begin moving 20 msec before the head begins moving.
2) The eyes reach the target before the head reaches the target.
3) The head completes its movement to the target while the eyes remain stable and fixated at the target via a compensatory reverse motion relative to the head.
When there is not a visual fixation goal for the eye movement (eg. in a dark room) then the eyes respond to, rather than guiding the movement. The sequence of motion is as follows:
1) The head begins moving while the eyes begin slow phase of nystagmus in the opposite direction, remaining stable in the environment.
2) The head continues moving while the eyes alternate slow and fast phases of nystagmus.
3) The head completes its movement to a new position.
4) The eyes catch-up to the head with the nystagmus fast phase.
This reveals how eye motion can function to guide the body movement or the eye motion can occur as a reflex response in reaction to body movement (Bizzi, 1974; Howard, 1986). In dance practice a similar technique is used known as “‘spotting’” in which “the eyes [are] focused at a definite point” while the body is turning (Grant, 1982, p. 84):
[Spotting] is a term given to the movement of the head and focusing of the eyes in pirouettes [and other turning movements] . . . In these turns the dancer chooses a spot in front and as the turn is made away from the spot, the head is the last to leave and the first to arrive as the body completes the turn. This rapid movement . . . prevents the dancer from becoming dizzy. (Grant, 1982, p. 113)
Though the description of spotting often focuses on the use of the head, spotting is essentially an eye fixation which is maintained until the last possible moment during a turn. Then the eyes lead the movement around the turn and back to the same fixation point. This helps stabilise the perception of the exterior environment.
Nystagmus elicited by visual field motion is referred to as optokinetic nystagmus but nystagmus can also be elicited by audio, vestibular, or somatic sensations. The occurrence of nystagmus from a variety of stimuli, and the accompanying perception of self-motion, adds to evidence for “sensory convergence” which posits that afferent signals from different sensory receptors converge together within the nervous system (Bles, 1981; DiZio and Lackner, 1986).
During vestibular nystagmus (vestibulo-ocular reflex) stimulation from the vestibular canals and otolith organs elicits visual nystagmus (even in the absence of visual stimuli). Extrinsic eye muscles and vestibular canals are directly linked. Electrical stimulation to each of the six vestibular canals excites one of the corresponding six eye muscles which is responsible for moving the eye in the same plane of orientation as that canal (Hallett, 1986, p. 10.13).
The quality of vestibular nystagmus, and so kinesthetic perception, depends on types of visual fixation which are used while the body is rotating. Focusing on, or imagining, stationary objects in the environment results in a nystagmus slow phase of equal velocity as head movement speed. Non-focus in darkness results in slower slow phase speed than head movement speed. An imagined focus on an object moving along with the subject (imagined visual tracking) results in even slower phase speed, and focusing on a real object moving along with the subject (eg. focusing on your hand in front of your face while turning) virtually eliminates vestibular nystagmus completely (Barr et al., 1976).
When a subject rotates at a constant velocity after about 20 seconds the vestibular fluid stabilises and therefore stops stimulating the nervous system. If there is no visual field motion (eg. eyes are closed) then the nystagmus may stop. If bodily rotation decelerates the vestibular fluid will keep moving and therefore stimulate the nervous system just as if the body was rotating in the opposite direction. This deceleration elicits post rotary nystagmus where the body may have stopped turning yet the visual field appears to be rotating and nystagmus is occurring. The direction of this nystagmus may reverse several times while the vestibular fluid is gradually stabilising. Similar effects may be caused by alcohol consumption (Howard, 1986, p. 11.14).
A source of sound which rotates around a stationary subject will also elicit a nystagmus reflex and perceptions of self-rotation (Lackner, 1977b). This is termed audio nystagmus.
Various types of somatic stimulations will also elicit perceptions of self-motion accompanied with the nystagmus reflex and so are termed arthrokinetic nystagmus. For example nystagmus and (illusory) perceptions of self-motion have been shown to occur when a hand is placed on a circular wall which rotates around a (blindfolded and stationary) subject (resulting in shoulder articulation) (Brandt and Buchelle, 1977), or when blindfolded subjects walk in place (ie. stationary) on a circular conveyer belt (resulting in spinal, hip, knee, and ankle articulations) (Bles, 1981).
Nystagmus is a visual reflex which contributes to the perception of the orientation of the body relative to a stable exterior environment while the body is moving. It also assists rapid scanning. When the eyes scan across an environment then slow/fast nystagmus phases will reflexively occur. If the eye movement was continuous (no nystagmus) then the visual image would be blurred. Smooth eye movements (continual smooth eye motion with no nystagmus) will only occur during voluntary “tracking”, that is, maintaining a fixation on a smoothly moving stimulus. When the background is visually distinctive then nystagmus will still tend to occur. Tracking the moving stimulus against a black background will allow most success in completely inhibiting the nystagmus reflex (Hallett, 1986).
APX. II.36b Vision of the body moving.
Sensory data from the vision of one’s own body is also a dominate source of information contributing to kinesthesia. We know our body positions and movements because we can see them. This source of data is overlooked in most discussions of kinesthesia, perhaps because it would be considered visual-spatial information. Since the vision of the body provides data about body moves and poses it can properly be considered to be kinesthetic.
Indeed, information from the vision of the body appears to be given more perceptual reliability. For example, somatic sensory data can be intentionally ignored but data from the vision of the body cannot be ignored and thus will dominate the spatial memory (Klein and Posner, 1974; Reeve et al., 1986). Greater amounts of vision of the body results in more accurate recall of body poses, regardless of greater or lesser amounts of somatic sensory data (Adams et al., 1977). Visual spatial data typically tends to dominate the perception of kinesthetic spatial data (see APX. IV.12). The perception and learning of limb positions and movements is also possible even when all somatic nerves have been severed (in monkeys) and vision of the body together with efferent data (see below) are the only sources of kinesthetic information (Taub and Berman, 1968).
APX. II.37 Audio Receptors.
The structure of the outer ear truly begins with the body itself. As sound approaches the ears it may bounce off, or be “shadowed” by the shoulder and the head. The neck and entire body allow the position of the ears to be rapidly and precisely shifted. The bilateral positioning allows two separate samples of the incoming auditory stimuli.
The outer ear proper begins with the outermost, visible, cartilaginous part termed the auricle (or pinna). Its expanded shape serves to collect sound waves and channel them into the narrow auditory canal which guides the waves to the tympanic membrane (ear drum).
The middle ear consists of three tiny bones forming a linkage which transfers and amplifies the vibration from the tympanic membrane to the inner ear. The bony linkage may also be vibrated directly through the bones of the skull (eg. the sound of clicking you own teeth together).
The vibrations are transferred to the thick lymphic fluid within the spiral-shaped cochlea of the inner ear. The vibrations create waves in the lymphic fluid which move up the cochlea and stimulate tiny hairs on its interior surface which are connected to nerve endings. Stimulation of hairs at the base of the cochlea’s spiral produces a perception of high pitch and stimulation towards the tip of the spiral produces a perception of low pitch. (General references for audio receptors; Kapit and Elson, 1977; Scharf and Buus, 1986; Scharf and Houtsma, 1986.)
Similar to the processes of visual kinesthesia, two types of audio information can be distinguished. These can be termed audio field motion and audition of the body moving.
APX. II.37a Audio field motion.
Analogous to visual field motion, when an organism travels or rotates through space the surrounding sounds of the stable environment will be in motion relative to the organism’s ears. This audio field motion contributes to the subject’s perception of self-motion and a stable environment. Audio field motion is an important kinesthetic cue and can create illusions of self-motion and accompanying nystagmus by rotating a sound around a blindfolded, stationary subject (Lackner, 1977b).
APX. II.37b Audition of the body.
Our use of sensory data from the audition of the body is included in Gibson’s (1966, p. 37) concept of “auditory proprioception” and its importance can be practically experienced by attempting a kinesthetic task with plugged ears. Body movements can be heard internally through the bones and externally through the outer ear. We also hear the effects of our movements (eg. the sound of each key tapping on a typewriter). This auditory feedback provides data regarding the bodily movements.
APX. II.38 Efferent Data.
A mechanism is hypothesized whereby we have an internal knowledge of the motor commands which have been initiated. This is sometimes termed “efference” (efferent commands as opposed to afferent feedback). It might be considered that efferent data is fundamentally different than the peripheral sensory feedback of other types of kinesthetic data. However, efference can be considered to be a central or internal feedback loop which serves as a “motor memory storage system operating without the requirement of peripheral feedback” (Kelso, 1977b, p. 34). This central feedback is thought to be available to establish a stronger memory representation together with other peripheral kinesthetic feedback (Larish et al., 1979). Efferent data provides useful information about the body’s movements and positions and so is a vital contributor to kinesthesia.
One source of evidence for the existence of efferent data is that monkeys who have had their somatic nerves surgically severed can still learn and perform gross limb movement and positioning tasks (eg. walking, climbing up a wire cage, reaching and grasping for food) (Taub and Berman, 1968; Taub et al., 1973; Bossom, 1974) or be trained to point at visual targets without sight of the limb (Bossom and Ommaya, 1968; Taub et al., 1975) (though accuracy for fine movements such as grasping small objects did not develop normally). After more time these monkeys were able to execute normal gross movement while also blindfolded or with the reaching hand out of view. In addition, monkeys with surgical somatic deafferentiation of forelimbs and blinding on the day of birth still learned to use the limbs for gross tasks such as supporting weight, walking, linking forearms (though more learning time was required overall and reaching toward objects could not develop) (Taub et al., 1973). Since sources of somatic information have been eliminated it is hypothesized that efferent information, and also sometimes vision of the body, is used to perceive body movements and positions.
Efference is also indicated by research which demonstrates that subject’s actively produced movements to end-positions of their own choice can be recalled better than if the experimenter manipulates the subject’s passive arm. Presumably efferent data is produced when the movements and positions are generated actively by the subject and this data is available to derive an accurate perception (see active versus passive limb positioning; APX. II.43).
Other evidence comes from the reaction time required to correct an error in an executed movement. The time required to perceive, process, and react has been measured at 190-260 msec for visual feedback (Keele and Posner, 1968) or 108-169 msec for somatic feedback (Higgins and Angel, 1970). However, movement errors can be corrected as fast as 83 msec from the moment of initiation (Higgins and Angel, 1970). This rapid ability to correct one’s own movement errors is therefore attributed to a knowledge of efferent data rather than sensory feedback.
Two theories about the nature of the interior knowledge of motor commands are termed “efference copy” and “corollary discharge”. Corollary discharge (Teuber, 1974) posits that a copy of motor commands is sent to perceptual centers where it influences the interpretation of the raw sensory data. Efference copy (Jones, 1972; 1973; Von Holst, 1954) posits that a copy of the motor commands are saved for future executions of the same movement and to compare to other kinesthetic data (eg. from joints and muscles). Clark and Horch (1986, p. 13.57) illustrate the two theories with diagrams. Kelso (1977b) compares efferent copy with corollary discharge in a linear positioning task and found that simply forming the motor plan in one’s mind did not result in as accurate kinesthetic memory as when also actively executing the motor plan. This indicates that the most useful “efferent discharge” (p. 34) or “efferent information” (p. 42) is generated by actually executing the motor plan, rather than simply reading an efferent copy of the commands without having executed them. For the purposes of this study this difference is not critical. The knowledge of motor commands can be generally referred to as efferent data of efferent information.
Kinesthetic perceptions are rarely derived from a single sensory organ located in one part of the body. Instead, sensory data from many types of receptors is integrated into a single kinesthetic perception. The variety of receptors contributing to kinesthesia provides sensory redundancy so that if one group of receptors fails to function another group can still provide the necessary information.
This section reviews how particular stimulations are derived into particular types of perceptions. Types of kinesthetic perceptions distinguished here include the sense of balance or equilibrium; the sense of linear, rotary, or circular self-motion; limb position sense; limb movement sense; and the sense of force or exertion.
APX. II.41 Sense of Balance, Equilibrium.
The sense of balance is closely related to the perception of the gravitational vertical which is most readily sensed by the vestibular otolith organs. Also, the physical weight of the body and its gravitational alignment through the joints (translated from the upper-most to the lower-most body-parts) is sensed by pressure-sensitive joint receptors throughout the body (Clark, 1975) and the entire weight of the body is sensed by pressure-sensitive skin receptors against the ground.
The alignment of the body-weight relative to gravity may also be sensed by tension-sensitive tendon receptors. That is, the closer the body alignment to the gravitational vertical, the more equal the tension between opposing tendons. Though, prolonged periods of adaptation to an off-vertical position may create a somatic misperception of vertical since the receptors have become so “used to” the off-vertical alignment. When this subject’s body is placed into gravitational alignment the subject will (mistakenly) perceive that they are out of line with gravity because their somatic receptors have become so adapted to the off-vertical alignment.
Visual field motion (or non-motion) may be the most important for the sense of balance as anyone can testify who tries to stand on one leg with their eyes closed. Posture is most stable when the eyes are focused on a fixed point (Hellebrandt and Franseen, 1943). Visual field motion which occurs when a subject sways or falls off of balance will induce reflex motor reactions to counter the sway and maintain balance. When this type of visual field motion is artificially induced with a “swinging room”* infants will fall over (Lee and Aronson, 1974) and adults will experience body sway (Lishman and Lee, 1973), especially in unusual stances (Lee and Lishman, 1975). Similar postural sway will occur when projections of moving visual scenes are presented to subjects’ peripheral visual field (Lestienne et al., 1977).
__________
* The “swinging room” moves around the subject while the floor remains stable. Thus the subject experiences visual field motion but no accompanying stimulations from somatic or vestibular receptors (pictured by Lishman and Lee, 1973, p. 289).
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Maintenance of equilibrium through visual field stimulations is most effective with a textured visual stimulus whereby even slight lateral visual field motion can be readily detected (viz. a texture of vertical lines is more effective than horizontal lines), and this texture is most effective when presented to the peripheral visual field (Amblard and Carblanc, 1980).
APX. II.42 Sense of Self-Motion.
The term “self-motion” is used here to refer to motion which either 1) translates the entire body in a straight line through space to a new location (linear self-motion), 2) turns the entire body around an axis (rotary self-motion), or 3) a combination of translation and rotation (circular self-motion). In contrast, “limb–motion” refers to the motion of body-parts relative to other body-parts (see IIA.32).
Limb-motion often occurs together with self-motion (eg. motion of the limbs while walking) but the two can also be separated (eg. self-motion forward while riding in a train with the limbs held still). When self-motion occurs by riding on a train, boat, elevator, etc. without any active participation by limb-motion it is referred to as “passive movement” (Rock, 1968) or “passive locomotion” (Johansson, 1977). Three types of self-motion will be considered here; linear self-motion, rotary self-motion, and circular self-motion (For reviews see: G. J. Andersen, 1986).
APX. II.42a Sense of linear self-motion.
During linear self-motion the subject travels through space along a straight line to a new location. This is also referred to as “translation” (Andersen, 1986, p. 56; Warren et al., 1988) or “locomotion” (Johansson, 1977; Strelow and Babyn, 1981).
Somatic and vestibular receptors would seem to provide the basis for perception of linear self-motion since it is the muscles, tendons and joints which produce the movement and the vestibular receptors sense the acceleration and deceleration. Despite this, somatic and vestibular sensations are dominated by reliance on visual field motion. This is so robust that illusions of self-motion can be easily induced. Common examples are when the train on the next track begins to move and it is initially perceived as one’s own train moving (or if a large truck begins to move next to one’s car), or when standing close to a large river a perception of self-motion (rather than motion of the river) may occur.
Self-motion illusions, referred to as “linear vection” or “induced translation” (G. J. Andersen, 1986; Bles, 1981), have also been produced in the experimental setting. Lishman and Lee (1973) used a “swinging room” to induce illusions of self-motion and found that “the effects are practically universal” (p. 292). Subjects’ knowledge that they were actually standing on a stable floor and that the “room” was swinging around them did not change the strength of the illusion of self-motion.
When visual field motion is presented only to the periphery of the visual field the illusion of linear self-motion occurs even when accompanied by conflicting vestibular and somatic sensations (Berthoz et al., 1975; Johansson, 1977). This agrees with studies of the effect of visual field motion on the sense of balance when the greatest effect came from peripheral rather than central vision (Amblard and Carblanc, 1980; Lestienne et al., 1977). This same effect has been found in studies of rotary self-motion (see below). Illusions of linear self-motion can also be induced from a radially expanding pattern with an apparent internal depth (eg. from motion parallax and occlusion visual cues) which is presented only to the central visual field (Andersen, 1986, p. 58; Andersen and Braunstein, 1985).
Conversely, if visual field motion is absent (in a dark room) then subjects (incorrectly) perceive that an exterior object is moving rather than (correctly) perceiving their own passive self-motion. Even if somatic and vestibular stimulations are available from repeated accelerations and decelerations (Rock, 1968) or from voluntary jogging (while attempting to stay in place) (Glanzmann, 1987) subjects will usually not perceive their own self-motion if visual field motion is absent.
The direction of linear self-motion can be more accurately performed when visual field motion is available (ie. with a visible background texture rather than darkness or isolated objects in the foreground) (Strelow and Babyn, 1981). Judgements of the heading are more accurate when a high density of visual field texture is available (Warren et al., 1988).
APX. II.42b Sense of rotary self-motion.
The perception of rotary self-motion is closely associated with the vestibular semi-circular canals since these respond to rotational accelerations and decelerations. Nevertheless, even in the absence of vestibular stimulations illusions of rotary self-motion with accompanying nystagmus can be induced from isolated somatic stimulations (Brandt and Buchelle, 1977; Brandt et al., 1977; Lackner and Dizio, 1984), somatic stimulations together with efferent information (Lackner and Dizio, 1984), audio field motion (Dodge, 1923; Lackner, 1977b), or visual field motion (see below). Illusions of rotary self-motion induced by visual field motion are referred to as “induced rotation” or “circular vection” (around the vertical axis), “roll vection” (around the sagittal axis) or “pitch vection” (around the lateral axis) (G. J. Andersen, 1986).
Circular vection is typically induced by rotating a circular wall with a textured surface around a stationary observer. Reducing the luminance levels so that the visual field motion is only perceptible by peripheral vision (Leibowitz et al., 1979), masking the central visual field, or presenting stimuli rotating in the opposite direction in the central visual field, has no effect on the illusion of self-motion, whereas masking peripheral vision eliminates the illusion (Brandt et al., 1973). This type of evidence indicates that peripheral visual field motion is primarily responsible for the perception of rotary self-motion. The importance of peripheral over central vision has also been found in inducing roll vection (Brandt et al., 1975; Held et al., 1975; Reason et al., 1982).
Although illusions of rotary self-motion can be induced by visual field motion alone, when both visual field motion and vestibular stimulations are available the perception of self-motion is quickest and speed estimates are the most accurate (Melchner and Henn, 1981). This indicates how visual and vestibular data function together in the perception of self-motion. Illusions induced from visual field motion are tied to characteristics of the vestibular receptors. With very slow accelerations of the visual field, the self-motion illusion is immediately perceived (Melcher and Henn, 1981). This immediate perception presumably occurs because during slow rotary accelerations very little vestibular stimulations would be expected. However, when the visual field accelerates quickly the self-motion illusion is perceived only gradually, not reaching its full effect until after 30 seconds (Brandt et al., 1973; Wong and Frost, 1978). This latency in the onset of the self-motion illusion presumably occurs because during quick accelerations the vestibular canals would be stimulated during actual rotary accelerations. The illusion of rotary self-motion occurs after the same time that it would take the lymph fluid to stabilise in the vestibular canals during an actual rotation.
The importance of vestibular stimulations in perceptions of rotary self-motion also reveal themselves when illusions of rotary self-motion are induced around a sagittal axis (roll vection). To induce roll vection a subject observes a rotating visual field on a plane parallel to the frontal plane of their body (Held et al., 1975, p. 258). In this case, rather than perceiving a continuous bodily rotation (as in circular vection) the observer perceives that their body is tilted up to a maximum of about 15° (Dichgans et al., 1972; Held et al., 1975). The greater the visual field texture, the greater the illusion of tilt (Brandt et al., 1975; Reason et al., 1982).
Presumably an illusion of a complete 180° roll vection does not occur because during an actual rotation around the sagittal axis (eg. a cartwheel) vestibular otolith stimulations would occur indicating the body’s reorientation relative to gravity. When the same rotating visual-field in the frontal plane is presented to subjects who are lying on their backs, then full 180° roll vection illusion is perceived. In this case the sagittal body axis is oriented along the gravitational vertical and so vestibular otolith stimulations would not be expected during an actual self-motion. Furthermore, when subjects are lying on their backs and the rotating visual field is presented at an angle directly in front of their tilted head, then subjects (incorrectly) perceive that their head is horizontal and their body is tilted. This incorrect perception of head orientation allows the full 180° rotary roll vection illusion to be experienced, presumably because when the head is perceived to be horizontal then there would not be any vestibular otolith sensations expected during an actual rotation (Dizio and Lackner, 1986).
APX. II.42c Circular self-motion.
Locomotion in a curved path can be referred to as circular self-motion and consists of a combination of rotary and linear self-motion. These translation and rotation components must both be distinguished by the perceiver for accurate performance of circular self-motion (Rieger, 1983). As with rotary and linear self-motion, visual field motion is most important for the perception of circular self-motion. When visual field data is not available (eg. a dark room) then circular self-motion will not be perceived even with available vestibular stimulation. Visual field motion alone, or somatic stimulations with efferent knowledge (walking on a circular conveyor belt) are sufficient to elicit illusory perceptions of circular self-motion when the subject actually remains in the same place (ie. no vestibular stimulation). When somatic and efferent data would indicate that the subject was traveling forwards (walking forward on a circular conveyor belt) but contradictory vestibular stimulation would indicate that the subject was traveling backward (the high speed of the conveyor belt caused the subject to actually be traveling backwards) then the vestibular data is ignored and the illusory circular self–motion is perceived in accordance with the somatic and efferent data (Bles, 1981).
APX. II.43 Limb Position Sense.
Sensory data from the vision of the body dominates the perception of limb position. Recalling an arm position is more accurate when the arm can be seen than if it can not (Adams et al., 1977; Posner, 1967). Seeing one’s own limbs is the most accurate way of knowing where they are.
A great deal of research has been devoted to discerning the non-visual mechanisms of position sense (for reviews see McCloskey, 1978; Clark and Horch, 1986). Early work recorded discharges from sensory endings in cats’ joint receptors and found that each sensory ending had a maximum response which ranged over a few degrees of joint positions and that these ranges were different for different sensory endings. This was interpreted as indicating that perception of limb position arises solely from joint receptors (Adams et al., 1977, p. 13; Andrew and Dodt, 1953; Gibson, 1966; Roland, 1979; Skoglund, 1956). Even though this notion has been overwhelmingly shown to be erroneous (see below) it is sometimes still adhered to in current texts on dance and exercise (Ellison, 1993, p. 75; Fitt, 1988, p. 266).
In subsequent research joint receptor responses were found to be inadequate for the perception of limb position. Using a different technique for recording joint receptor discharges (for details see McCloskey, 1978, pp. 766-767), Burgess and Clark (1969) found that only 4 out of 278 slowly adapting afferent fibres from the posterior nerve of the cat knee joint were maximally activated at intermediate (rather than extreme) joint angles. In addition, 140 fibres were maximally activated at both full flexion and also full extension of the joint. Clark and Burgess (1975) continued this research and found that only 6 out of 672 fibres tested in the medial nerve, and 45 out of 713 fibres tested in the posterior nerve of the cat knee joint gave slowly adapting responses to intermediate joint angles. Similar results were found in other studies (Clark, 1975; Grigg, 1975). Because joint receptors respond primarily to extreme joint angles, rather than intermediary joint angles, the general conclusion of these findings was that “articular receptors in the knee are not capable of providing appreciable steady-state position information over most of the working range of the joint” (Clark and Burgess, 1975, p. 1462).
Receptors in different joints may have different response characteristics than those in the cat knee joint. For example, nerves for receptors in the cat hip joint have been found to discharge at intermediate joint positions (Carli et al., 1979). Nevertheless, other research findings have shown that anesthetized knee joint capsules in man (Clark et al., 1979) surgical replacement of finger or toe joints with silicone implants (Cross and McCloskey, 1973; Kelso et al., 1980), or deafferentiated joint and skin receptors from a pressure-cuff around the limb (Goodwin et al., 1972a; Roy and Williams, 1979) has no effect on position sense. Therefore, while joint receptors provide some data for position sense their role does not appear to be crucial.
Other research has indicated the role of muscle spindle receptors in the perception of limb position. When the joints, muscles, and skin of the hand are paralysed (by local injection of anaesthesia or by a pressure cuff which cuts off circulation causing total numbness) subjects can still perceive flexion/extension movements and the resultant positions of the fingers, presumably because this perception arises from the long muscles acting on the fingers but which are located in the (non-paralysed) forearm. This perception became even more accurate when the subject lightly tensed the muscles in the forearm (which would increase the data coming from muscular receptors) (Goodwin et al., 1972a; 1972b). The completeness of the anaesthesia within the hand was verified since subjects could not detect lateral finger movements nor could they distinguish between movements at different joints of the same finger. These are movements which are not effected by the long muscles of the forearm, rather, in order to detect or perform these movements, the muscles, skin, or joints within the hand would have to be active (Goodwin et al., 1972b, p. 327).
Further evidence for the role of muscular spindle receptors in the perception of limb positions comes from experiments in which a biceps or triceps tendon is stimulated with a physiotherapy vibrator (placed on the skin above the tendon) which causes a reflex contraction of that muscle (thought to originate from the stimulation of the muscle’s spindle receptors). When the other arm is used to duplicate the perceived motion and position of the vibrated arm (which is hidden from vision), a position illusion (sometimes more than 40°) was evident. The subject perceived the arm position as if the vibrated muscle was stretched longer than it actually was (Goodwin et al., 1972a; 1972c).
If the tonic vibration reflex is allowed to shorten the vibrated muscle then the correct direction of motion will be perceived (viz. elbow flexion if biceps are vibrated, elbow extension if triceps are vibrated) but the distance of movement will be underestimated so that the vibrated muscle is perceived to be stretched longer than it actually is (Goodwin et al. 1972c). If the tonic vibration reflex is not allowed to articulate the elbow (the forearm is restrained by the experimenter or if the subject voluntarily contracts the antagonist muscle) so that an isometric contraction occurs (no change is muscle length), then the elbow will be perceived to be articulating just as if the vibrated muscle was lengthening (Goodwin et al., 1972a; 1972c; McCloskey, 1973).
These position and movement illusions are so robust that even subjects who are informed about the procedure will still perceive the illusions (Goodwin et al., 1972c, p. 1383). This vibration technique can even lead to perceptions of impossible wrist positions, or to perceptions of simultaneous multiple forearms (Craske, 1977).
Using the same vibration technique, McCloskey (1973) separated the illusion of movement from the illusion of position. Velocities of the motion illusion were too fast to have been equated with the size of the position illusion. Longer durations of the vibration caused no change in the position illusion whereas it produced a continual illusion of movement. Loading the muscle (placing a weight on the end of the limb) slowed the velocity of the movement illusion but increased the size of the position illusion. And, with lower frequencies and greater amplitude of vibration the movement illusion was eliminated but the position illusion persisted.
Since muscle spindle primary endings are the most sensitive to vibration it was concluded that these receptors are responsible for the illusions (Goodwin et al., 1972a, p. 744; 1972c, p. 1384). But during lower frequency and greater amplitude vibrations the secondary spindle endings may also play are role in the position and movement illusions (McCloskey, 1973, p. 130).
Further support for the role of muscular receptors in position sense comes from the common finding that arm positions which are actively moved to are recalled better than positions which are imposed by the experimenter onto the subject’s passive arm (Jones, 1972; Kelso, 1977b; Marteniuk, 1973; Paillard and Brouchon, 1968; 1974). When muscles are actively contracting there will be greater sensory discharges from muscle spindle receptors (Matthews, 1933; McCloskey, 1978, p. 770), tendon receptors (Jansen and Rudjord, 1964) and joint receptors (Grigg, 1975; Skoglund, 1956) which should provide more sensory feedback.
This effect of better position recall from active movements also indicates the use of efferent data for deriving the perception of limb position. This is demonstrated when subjects are allowed to actively move their arm to an end-location of their own choosing. In this type of subject-generated movement efferent data would be available and recall of the end-location is most accurate.
Kelso (1977b) distinguishes between factors which may lead to superior memory for self-produced movements: 1) The movement may be actively executed by the subject (active) or the subject’s arm may be passively moved by the experimenter (passive). 2) The end-position of the movement may be chosen by the subject (preselected) or it may be defined by the experimenter with a physical stop on the experimental positioning apparatus (constrained). The combination of active movements to preselected end-locations produces the greatest accuracy. When subjects actively execute preselected end-locations then these can be recalled accurately even with deafferentiated joint and skin receptors in the hand during finger movements (Roy and Williams, 1979). When the active movement is abruptly stopped at an unexpected (constrained) end-location, then this cannot be recalled any better than if the movement was passive (Jones, 1972). A limb position produced by active limb movement can be matched by the opposite limb better than if the position was produced by passive movement (Paillard and Brouchon, 1974). These results indicate that efferent data and muscle spindle receptors are both contributing to the perception and recall of limb positions.
Other studies have also indicated the use of efferent data in limb position sense (eg. Lashley, 1917). Even without any somatic stimulations (when nerves have been severed) after two to six months the vision of the body and efferent data can be sufficient for successful limb movement and positioning in monkeys (eg. climbing up a wire cage or reaching and grasping for food), though this condition does not yield as much accuracy as when somatic stimulations are also available. After more time these monkeys were even able to accomplish the movement task while blindfolded or with the reaching hand out of view (efferent data being the only remaining source of information) (Bossom, 1974; Taub and Berman, 1968).
However, in these deafferentiation studies fine movements such as grasping small objects do not develop to normal accuracy without somatic stimulations (Taub and Berman, 1968; Taub et al., 1973). This indicates that somatic stimulations are necessary for fine positioning and movement.
Skin receptors have also been shown to play a vital role in position sense for certain body areas with a high density of skin receptors. Anesthetized skin around the hands has a detrimental effect on its position accuracy (Moberg, 1983) though anesthetized skin around the knee has no effect (Clark et al., 1979).
Another approach is to suggest that it is not joint angle which is sensed but limb orientation relative to gravity. Soechting (1982) demonstrated that producing an identical forearm orientation was more accurate than producing an identical elbow joint angle. In this approach the pressure sensitivity of joint receptors (Clark, 1975; Clark and Burgess, 1975) could be contributing since the pressure torque in a joint would be in a constant relation with that limb’s orientation to gravity.
Position sense degrades over time. The longer an arm is held in a static position, the less well its position can be matched by the opposite arm (Paillard and Brouchon, 1968; 1974). Presumably this occurs because sensations from quickly adapting receptors are no longer available after a brief time with no movement.
APX. II.44 Limb Movement Sense.
The perception of limb movement is tied to the perception of limb position since a position can only be reached by a movement, and every limb movement leads to a new position. This relation is exemplified in the studies of active versus passive produced movements in which the accuracy of body position recall was dependent on the quality of the movement (active vs. passive), with active movement leading to the most accurate position recall (see APX. II.43).
However, other evidence reveals that limb movement and limb position are separate. For example, if a very slow speed is used to articulate the knee joint the new position will be perceived but the movement will not (Horch et al., 1975). This leads to the conclusion that perceptions of movement arise from quickly adapting receptors while perceptions of position arise from slowly adapting receptors. The uncorrelated illusions of limb position and limb movement induced by placing a physiotherapy vibrator against the skin over a muscular tendon (see APX. 11.43) also indicate that position and movement are separate perceptions (McCloskey, 1973).
Just as with position sense, limb movements can be learned with information from the vision of the body together with efferent knowledge, and eventually with efferent knowledge alone (Bossom, 1974; Taub and Berman, 1968; see APX. II.43). Vision of the body can also dominate perceptions of limb movement. Subjects can learn a letter-writing task from vision of the body and efferent data with all somatic stimulations eliminated (from a pressure cuff causing total arm numbness) though fine control is best when both vision of the body and somatic stimulations are available (Laszlo and Baker, 1972). A sequence of horizontal linear arm movements was also learned better after one practice trial by simply watching the visual pattern of the movements than by watching the pattern and also bodily performing it (Klein and Posner, 1974). (For discussion of memory and recall characteristics of visual, somatic and audio space, see APX. IV.)
APX. II.45 Sense of Force; Sense of Exertion.
Perceptions of an object’s weight, or of the amount of force exerted against an object, can be derived from pressure-sensitive skin and joint receptors and tension-sensitive tendon receptors. In certain situations audition of the body would also contribute information about the amount of force which the body has exerted (eg. the sound of the body impacting upon an object). A typical procedure used when estimating an object’s weight is to actively move the object upwards and downwards. This may assist perception since the force required to set the object into motion and then to stop its motion, provides amplified pressure and tension sensations.
The weight of an object is perceived to be heavier when the muscles lifting it are fatigued (McCloskey et al., 1974). Therefore the perception of force appears to be related to the efferent data about the amount of exertion expended. When a reflexive contraction of the muscle is induced (by a physiotherapy vibrator) then subjects can distinguish between the force encountered by the muscle, and the exertion ordered by the motor commands (Ibid). This indicates that the sense of force may be derived by somatic receptors while the sense of exertion is derived from efferent data.
Kinesthesia is identified as arising from sensory stimulations via receptors in muscles, tendons, joints, skin, vestibular apparatus, eyes, ears, and also from an interior knowledge of motor commands (efferent data). This assortment of stimulations from throughout the body are derived into perceptions of balance and equilibrium, self-motion, limb–motion, limb position, and force or exertion.