The Non-visual Perception of External Motion
J. J. Gibson, Cornell University
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There are two ways in which the “sense of touch” can be stimulated: either by touching something or by being touched by something; that is, either by a movement of the individual relative to a stationary solid environment or by a movement of an object relative to the individual (and the environment). We are here first concerned with the latter. What external mechanical events involving contact with the stimuli are perceptible? Also, what are the stimuli for such perceptions? What illusory perceptions may occur?
The following classification, based on observation and experimentation, is proposed.
1. A solid body moves against the skin, perpendicular to its surface (e.g., a stylus or other laboratory stimulator). The stimulus is a depression of the skin; the experience is that of an object “touching” (“pressing,” “contacting”) the skin at the locus of stimulation. There are stimuli which produce equivalent experience — upward deformation of the skin, air – jet, etc. This view is inferred from the work of Nafe and Wagoner.
2a. A small solid body moves along the skin with friction (e.g., a stylus or other stimulator is drawn over the surface). The stimulus is a moving depression of the skin with the addition of lateral tension and with changes in tension due to friction. the experience is that of a moving (“scraping”) object. The “form” or pattern of the path traced may be readily perceived.
2b. A solid body moves along the skin without friction (e.g., a small wheel is rolled along the skin). The stimulus is thus a moving depression of the skin without lateral tension and vibratory changes in tension. The experience, perhaps, may be a brushing or rolling rather than a scraping object. Is the surface felt to be slippery?
2c. The stimulus of two successive separated depressions of the skin, without a moving object, yields an illusory movement (“apparent movement”).
3a. A large solid body slides over a small fixed area of the skin with friction (like a violin bow draws over the string). The stimulus is a depression of the skin, like #1, with the addition of tension and changes in tension. The experience is that of a moving object (like 2a without, of course, any experience of a path) with a rubbing quality. Thus the perception of motion does not depend on change of cutaneous locus, but only on lateral changes in tension due to friction. When the person himself moves, this is “rubbing” on the surface and texture is perceived (Katz).
3b. An elongated solid body slides over an area of skin without friction (like an under-rosined violin bow drawn over the string). If friction is indeed eliminated, the stimulus should be identical with #1, and the experience should be simply that of pressure without lateral movement. (Does this illusory perception in fact occur?)
4a. A solid object on which the mobile, say, hand rests is moved relative to the observer. The stimulus is skin depression, like #1, with the addition of rotation of the joints of the limb. The experience is of lateral movement of the object, as in #2 and #3a, but with concurrent movement of the hand.
4b. A solid object touched or grasped by the hand is caused to move by a movement of the limb. Here active movement of the individual is included in this discussion. The stimulus, then, is skin depression plus rotation of the joints, like #4a, plus contraction of the muscles and tension of the tendons. The experience is that of moving the object with the hand.
5a. A solid but non-rigid body in contact with the skin changes shape (e.g., a bulge produced on the surface of the body moves without motion of the surface itself). The stimulus is thus a fixed large-area depression of the skin combined with a small – area moving depression of the skin without fractional accompaniments (like 2b). The experience is that of “deformation”: a surface which is not displaced together with a bulge of the surface which is displaced.
5b. A non-rigid body in contact with the skin of, say, the hand is caused to change shape by movement of the hand and fingers (e.g., palpating, kneading, or squeezing clay or putty). Here again active movement of the individual (at least, his hand) is included. The stimulus complex is a combination of skin depression, joint sensations. and muscle and tendon sensations. The perception is of the changing protuberances and indentations of the substance and also of its qualities (elasticity, viscosity, softness), etc. An important variable is probably the degree of skin depression corresponding to the degree of tension in muscles and tendons (i.e., the force exerted). As this ratio decreases, the impression of softness increases.
In short, lateral movement of an object relative to the observer can be perceived on the basis of several different types of mechanical stimulation: (1) change of position of cutaneous depression, as in 2a,2b, and 5a (in part); (2) lateral tension and vibration (skin friction), as in 2a, 3a; (3) joint friction, as in 4a, 4b; (4) certain combinations of these. In addition, the non-rigid movement of elastic body can be perceived as such, as in 5a.
The Non – Visual Perception of Self – MovementNow, the difference between the perception of touching something and that of being touched by something is that the former is accompanied by classical kinesthesis (muscle-tendon-joint stimulation), while the latter is not. The same difference exists between the perception of moving something and something moving. The skin may move against a solid oject instead of the object moving against the skin (“Type I event”). Likewise, the skin may move over a small object instead of the object moving over the skin (“Type II event”). And a small area of skin may slide over a large surface instead of the reverse (“Type III event”). The physical frame of reference for these movements is the earth, and the phenomenal frame of reference for their perception is given by contact with the surface of support, by thevestibule (and other gravity receptors), and by joint-sensitivity, all acting together in what we might call postural sense (vision here is left out of account). Hence both self-movement and external motion are perceived relative to this framework.
The relation between the perception of a cutaneous locus (“local sign”) relative to the surface of the skin, and the perception of the locus of felt objects relative to the horizontal – vertical axes of the earth and gravity is a puzzle! One is subjective, the other objective; they are “poles” of experience, like the visual field and the visual world. The old theory started with the sensation of kinethesis. A better theory would be that cutaneous stimulation and postural stimulation are inseparable and that together they yield both “where on the body” and “where in the world” a touch is located.
For example. Behind a screen O’s hand is lowered on a knob in one place, then raised, shifted, and lowered on a similar knob to the right of it. the cutaneous stimuli are the same and the cutaneous locus is the same. O feels the two pressures as same but nevertheless he feels two knobs in difference objective places, not one knob in the same subjective place. If the hand is simply raised, not shifted, and lowered, he does feel the same objective knob in the same objective place. The only difference is a shift at the elbow joint, but this postural stimulus makes all the difference in the two perceptions of “touching.”
Another example. The subject is asked to judge the direction of an edge (e.g., a ruler) placed briefly on the skin on the hand with the palm up, resting on the table. When the forearm is held out from the body a pressure across the palm is judged as a right-arm direction, but when the forearm is bent and held parallel to the shoulders it is judged in a front-back position. Hence the cutaneous impression, although introspectively isolable, arouses a perception of direction only in intimate combination with the concomitant postural impression.
We may now list the proximal stimuli of cutaneous and postural sensory events.
The Modes of Stimulation of the Skin (hairless regions) Produced by External Mechanical Objects
1. Perpendicular deformation (depression, pressure, contact) Note. – according to Nafe and Wagoner, the effective stimulus is not the state of deformation of the skin but the process of deformation; when the process ceases, the excitation ceases (in normal life, tremor will prevent such adaptation). Nafe & Wagoner, “The nature of pressure adaptation,” Jour. Genl. Psych., 1941, vol. 25, pp 323-351.
2. Lateral deformation (wiggling?)
3. Lateral friction (rubbing) and perpendicular (?) vibration
4. Translocation of a deformation (motion)
5. Multiple location of deformation (pattern)
The Modes of Internal Mechanical Stimulation (superficial vs. deep sensitivity)
1. Contraction of muscle (also the tension, or load)
2. Tension (stretch) of tendon
3. Rotation of a joint, and angular position of a joint
4. Linear forces, including gravity, acting in the statocyst (show little or no adaptation)
5. Angular foces acting in the semicircular canals
As to the nature and structure of the receptors for all these stimuli, the evidence is not clear. There is rich afferent neural supply in the skin, muscles, tendons, and joints. And the nerve fibers sometimes terminate in specialized end-organs (“corpsucles”) and sometimes are “free” nerve endings. We must remember, too, that a nerve fiber can be excited by pressing or squeezing it!