June 1968
Consistency vs. Discrepancy of Stimulus Information
J. J. Gibson, Cornell University
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 References to these essays must cite them explicitly as unpublished manuscripts.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.
I. If stimulus information is equivalent across sense modalities, as I argue, then a new problem arises of the consistency and discrepancy of information, either within a perceptual system or between perceptual systems. (cross-modal, supra–intermodal, amodal, the “cooperation” of the senses, “unity” of the senses, etc.)
Examples:
(1) The information about the vertical from the statocyst, from the skin, and from the eye. Covariance and coincidence (p. 62 ff.)
(2) Information about fire by sound, odor, heat, and light.
(3) Information about concavity-convexity (shape of “feelies” from vision and haptic touch.
II. How to define consistency of information? Covariance, coincidence, concomitance. The “invariant“.
How to define discrepancy of information? Conflict of cues, contradiction of information. Mismatch of information.
III. The problem of dual stimulation with the perception of one thing; in two eyes on two areas of skin, and in two ears.
(a) Two stimulus patterns on the two retinas yield the perception of one object when the patterns differ (are “disparate”) by a lateral perspective transformation. They are then mutually consistent. But they yield the perception of two objects when the two patterns are strongly disparate (diplopia) and they yield the alternating perceptions of two surfaces when the two patterns are really discrepant(binocular rivalry). In short, there can be a certain amount of difference or disparity between the eyes without a discrepancy between them.
(b) Two stimulus patterns on the skin will yield the perception of one object when they are on opposable areas of skin (as defined by the reciprocal action of joints, or mandible action) and when the cutaneous impressions are concomitant and covariant, e.g., the pencil between two fingers. But they seem to yield the perception of two objects when they are on non-opposable areas of the skin that are forced into an opposable posture, and also on non-opposable areas of the skin that are forced into an opposable posture, and also perhaps when opposable areas of the skin with flexor action are forced into a posture that makes them come together only with extensor action (Aristotle’s Illusion and its variants).
(c) Two acoustic stimuli in the two ears will yield the perception of one event (“sound”) when the wave trains are not too disparate in time and are not wholly discrepant with respect to the transients and the sound spectra. Otherwise they will yield the perception of two events. It would seem that the binaural system, like the binocular, can use certain kinds of mismatch as information is perceiving a single thing, but that other kinds of mismatch cannot be tolerated and yield either anomalies of perception, or the experience of different events.
IV. The problem of consistency and discrepancy between the visual and the haptic systems.
When an object held in the hand is looked at, there is a perception of one object in one place although the information is obtained separately by the haptic system and the visual system. Either system alone, or in combination, will yield the same perception.
The illusion of two objects, one seen and one felt, does not occur even when the visual information for location is displaced relative to the haptic information for location by prismatic spectacles.