The Implications of Active Touch

J. J. Gibson, Cornell University; March 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 References to these essays must cite them explicitly as unpublished manuscripts.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.

Active touch, referred to as "touching" is an exploratory sense in which the impression on the skin is brought about by the perceiver himself. That is, variations in stimulation of the skin are caused by variations of an individual's motor activity, as when he runs his fingers over an object or surface. This is distinguished from passive touch in which stimulation is caused by movement of the external object or surface against or relative to a stationary tactual receptor surface. Tactual sense organs have frequently been conceived as passive receptors (receptor mosaic), but they also serve as active "tentacles" for sensory adjustment and sensory exploration. Such active, exploratory touching movements of the fingers have been termed tactile scanning (Gibson, 1962). These exploratory movements can be described by such terms as feeling, grasping, rubbing, groping, palpating, wielding, and hefting.

Does a sense organ merely (1) analyze the impinging energies of the environment (analyzer function), or does it also (2) explore the available energies of the environment. C. J. Herrick (1924, p. 20) has described the sense organs as "windows, each penetrable by its own kind of energy". The Pavlovian "analyzer" theory does not do justice to the exploratory adjustments of "aesthesic systems". Animals are not merely "receptive" and "responsive"; they are stimulus explorers. There is exploratory mobility as well as goal-seeking mobility. Hence several sensory systems are "proprioceptive"!

Some implications of the above discussion follow:

There are not so-and-so many senses; organs do not define senses (e.g., the proboscis).
Modes of awareness are not good indicators of the input of information.
Exterospecific and propriospecific information may be concurrent.
Sensory quality is not specific to "nerves" or receptors.
A receptor is sensitive to energy, but an organ is sensitive to information (energy changes and relations).
Sensations are not the basis of perception. There is, of course, the sensitivity of each sensory mosaic (retina, skin, cochlea, statocyst, tongue, olfactory), but its sensitivity is radically different when it is active!

Both the "accessory equipment" and adjustment" of sense organs have been too narrowly conceived. For example:

Vision includes not only the retinal and ocular adjustments, but also head-turning, going closer, holding closer, or walking around. (Examining)

Hearing includes not only the tensor tympanic reflex, but also ear cocking and head turning for localization.

Chemical system includes tasting, smacking, sniffing, but also trailing (the hands and legs are accessory to the chemical senses!)

Haptic system includes grasping, feeling, rubbing, hefting, palpating, groping, etc. (See footnote 1).

Temperature systems includes not only vasodilatation and constriction, piloerection, shivering, sweating, or panting but also behavioral adaptations to cold or heat (clothing, shelter). (See footnote 1).

Pain System: the whole section of the animal may be called into play.1

These systems extract information from the obtained stimulation and seek information from the obtainable stimulation. The invariants of available stimulation specify sources in the environment. The variants specify either external change or self-produced change. The latter both isolates the invariants and controls performance.

The exploratory activity of sensory systems implies: (1) the assumption of available stimulation and of unlimited potential stimulus information; and (2) the assumption of exploratory activity as distinct from performatory activity.

The sensory systems seek and extract information generally by selecting, maximizing, optimizing, isolating, scanning, integrating, and accumulating. (See footnote 2). Specifically, the various sensory systems operate as outlined below:

Vision:

(a) By locking-on, fixating, accommodating, pursuing, scanning (the array)

(b) By adapting to, or moderating (illumination).

(c) By vergence, so as to register disparity (of the dual arrays).

(d) By optimizing acuity (at the best looking distance).

(e) By transforming perspectives ("looking at all sides").

Hearing: By registering and nullifying binaural disparity (locating).

Chemical system: By maximizing concentration of certain odors and tastes.

Haptic system: By isolating certain invariants of mechanical interaction between object and skin (not merely by registering outaneous locus and form).

Temperature: No special adjustment to the organ (?).

Pain: No special adjustment to the organ (?)

. The preceding discussion of exploratory activity suggests an experiment to distinguish exploratory responses from performatory responses of the hands. Behind a curtain might be arranged pairs of objects which can be fitted, fastened, or attached together. Examples of such objects are nut and bolt, rod and sleeve, plug and socket, jar and stopper, button and buttonhole, etc. The groping, grasping, and feeling movements for distinguishing and matching objects will be very different from the movements of putting together. Only the latter are responses proper which, by S - R theory, are modified by the reinforcements and rewards of problem solving (See Berlyne on Exploratory Behavior.)

The experience of the tactual world may be contrasted with the experiences of the tactual (cutaneous) field (by analogy with the phenomenal visual world vs. the visual field) (Gibson, 1950, Ch. 3):

1. The world is constant (invariant), but the field is changing in time. The constancy of the tactual world is characterized by:

a. Unity of the phenomenal object. When a single object is felt with two or more fingers, only one object is perceived in spite of the separate outaneous pressures.

b. Stability of the phenomenal object. Movement of a skin surface over a corner or protuberance of an object does not usually give rise to recognition of the "tactile motion" or displacement of outaneous pressure.

c. Rigidity of the phenomenal object. A solid object pressed or squeezed by fingers or hand appears rigid and resistant. The increase of intensity of outaneous sensation is not usually noticed.

d. Shape of the phenomenal object. When one feels the corners, edges, or other protuberances of a strange object, one is able to distinguish the pattern which these make to one another, but not the pattern which the various cutaneous pressures make to one another. In other words, one perceives the object-form but not the skin-form.

2. The world is three-dimensional; the field is two-dimensional.

3. The world is located with reference to the ground and gravity; the field is located with reference to the skin.

4. The world is unbounded; the field consists of local impressions.

5. The world is integrated from successive patterns of stimulations; the field is itself a pattern of stimulation.

There are, of course, many differences between the field of outaneous impressions and the field of retinal impressions, but the analogies are nonetheless suggestive.

Note that the properties of the tactual world (as opposed to the outaneous field) prove that the sensitivity of the skin is different when it is active than when it is passive. Passive and active sensitivity are not the same. (See footnote 3). The tactual world, then, demonstrates the following properties:

Object unity, not cutaneous unity.
Stability during cutaneous motion.
Substantiality, instead of cutaneous intensity.
Tridimensional constant form instead of bidimensional changing form.
Tridimensional space localization instead of bidimensional skin localization (and the same for direction of edges, by analogy with the constancy of the visual vertical and horizontal.)
Unbounded space, instead of bounded cutaneous figures, or patterns of impressions (no figure-ground phenomenon in active touch.)

A system like vision (or touch) can bring about inputs caused by its own activity, as well as receiving inputs caused by external events. Inputs are called efferents by physiologists. But the total afference to the NS can be divided into two parts (von Holst): reafference and exafference. (See footnote 4). Von Holst has realized that these are not the same as Sherrington's proprioception and exteroception (supposedly dependent on different receptors). His question is "how the central NS can distinguish the one from the other." (An old puzzle.) What von Holst has not done is distinguish between exploratory and performatory activity. Certain components of the total input are exploration-produced not just response-produced (as e.g., Hull). That is, it is a case of exploratory exteroception not just proprioception.

Let us call the two components of a stimulus flux (during the process of exploratory observing, or exploratory touching) the exterospecific component and the propriospecific component. I believe that these components are distinct as stimuli!

What sensory exploration does, I suggest, is to isolate the invariant properties of a stimulus flux. That is: getting new visual perspectives on an object and similarly getting successive tactual perspectives of an object ("getting all sides of a thing." When there is permanence underlying the change of a stimulus-flux, this residual is exterospecific. It is what specifies the constant object. But note that a sensory system can separate the permanent variables from the varying variables in a flux of sensory impressions only because of the variation.

Finally, we need to understand the function of:

A stationary receptor-mosaic on which a pattern of energy "impinges."
A stationary receptor-mosaic on which impinges a changing pattern of energy (a "motion")
A mobile receptor-mosaic automatically excited by the changing pattern resulting from its mobility (reafference) but also sensitive to the invariants specifying external objects (exterospecific) and to any component of changing pattern due to external motion (also exterospecific.)

Footnote 1 -- Involves adjustment of the whole body.

Footnote 2 -- Instead of assuming that perceiving is based on reasoning (Helmholtz, Brunswik), I would assume that reasoning is based, phylogenetically and ontogenetically, on perceiving. It is not that attention is a rational process, but that reasoning is an attentional process.

Footnote 3 -- The demonstration that sensitivity to form under transformation is better than sensitivity to form under nontransformation should be repeated. Cf. Caviness

Footnote 4 -- For example, the shaking of a branch vs. a branch shaken by the wind.

References

Caviness, J. Unpublished Master's Thesis. Cornell University.

Gibson, J. J. Perception of the Visual World. Boston: Houghton Mifflin, 1950.

Gibson, J. J. Observations on active touch. Psychological Review. 1962, 69, 477-491.

Herrick, C. J. Neurological Foundations of Animal Behavior. New York, H. Holt, 1924.

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