The Non-visual Perception of External Motion
J. J. Gibson, Cornell University; 1963
The World Wide Web distribution of James Gibson's "Purple Perils" is for scholarly use with the understanding that Gibson did not intend them for publication. References to these essays must cite them explicitly as unpublished manuscripts. Copies may be circulated if this statement is included on each copy.
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 - Movement
Now, 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 the vestibule (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!
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