December 1967
Conflicting Object Information on the Two Retinas and Conflicting
Object Information on Opposable Areas of the Body
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 References to these essays must cite them explicitly as unpublished manuscripts.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.
When “sufficiently discrepant” patterns are imposed at the same time on the two central areas of the human retinas they result in binocular rivalry (Levelt, 1965). There is some sort of “conflict” between the inputs of the two eyes, with alternating “inhibition” of the corresponding conscious impressions. Note that a haploscope has to be employed in order to impose “discrepant” impressions; a stereoscope imposes impressions that are “disparate” by a special perspective transformation of the pattern, not impressions that are “discrepant.” In ordinary vision the same single external object provides two mutually consistentimpressions on the central areas of the retinas. I assume that two perspectives of the same object contain the same object-information and, therefore, yield the perception of the same single object, not of two objects.
The situation is similar when one single object makes two impressions at the same time on two opposable areas of the skin. When an object (e.g. a pencil) is pushed between two adjacent fingers (or between the fingers and the palm, or the opposable thumb and a finger) one object is perceived with two sensory impressions made by the two opposable sides of the object. The information for the haptic system specifies one object. But now suppose that a pair of fingers is unnaturally twisted so that non opposable areas of the skin are opposed. If one object (pencil) makes two impressions on those areas at the same time, two objects are perceived. This is Aristotle’s illusion.
My explanation of it is that the information for the haptic system specifies two separated pencils. The illusory separation of the pencils is just the normal haptic distance between the non opposable areas (Ch. 7 in Gibson, 1966). The crux of the problem in both situations is the normal perceiving of one environmental object where there is actually one source of stimulus information (the fact of two anatomically separated loci of sensations being irrelevant). In both situations, however, the illusory perception of two object when there is actually only one source of stimulation can be brought about. With a haploscope the experimenter can impose on the retinas the stimulus information for two quite different objects in front of the eyes at the same place at the same time. The perceptual system is then frustrated and comes up with impossible kinds of phenomenal disappearances and reappearances. In the case of persons with abnormal conjugation of eye posture (inability to converge both eyes on the same object) the system may come up with two separated but transparent identical objects in front of the eyes at the same time, diplopia. (The binocular system usually adjusts by not attending to the input of one eye and eventually by not “using” one eye). The haptic system of the two fingers when they are crossed comes up with the perception of two separated pencils being pushed between the fingers at the same time.
An experimental investigation of Aristotle’s illusion might be very profitable if it concentrated on the haptic perception of objects. This theory implies corresponding areas of the skin, by analogy with corresponding areas of the two retinas, that is, areas whose stimulation is always concomitant, covariant, and consistent, except when it is experimentally interfered with.
References
Levelt, W. J. M. On Binocular Rivalry. Soesterberg, Institute for Perception, 1965.
Gibson, J. J. The Senses Considered as Perceptual Systems, 1966.