The Relation Between Retinal Stimulation and Visual Sensation
J. J. Gibson, Cornell University
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The theory (a) that sense impressions are the basis of perception and that an inventory of these impressions is possible, the supposed law (b) that sense impressions are specific to particular afferent neurons or (nowadays) receptive units, and the assumption (c) that a visual stimulus is analogous to a prod applied to one of the receptors or receptive units of the retina, albeit by light, in the way a sharp point is applied to the skin (the retinal image being a pattern of such stimuli) ñ these assumptions have led to a set of psychophysical expectations about vision. That is, experimenters will expect that certain corresponding variables of the stimulus applied to the retina will yield certain corresponding variables of the sense impression.
These expectations can be listed. (1) The intensity of a stimulus, either a spot or a patch, is supposed to determine a sensation of brightness. (2) The wavelength of a stimulus is supposed to determine a sensation of hue. (3) The “purity” of the wavelength-composition is supposed to determine saturation. (4) The location of the stimulus on the retina is supposed to yield a sort of sensation of location, the “local sign.” (5) The form of a luminous patch is supposed to yield a sensation of “extensity.” (7) The beginning and the end of stimulation (if abrupt) are supposed to yield sensations of “on” and “off,” and a pulse of stimulation is supposed to yield a flash of brightness. (8) The duration of the stimulus is supposed to yield a sensation of duration (propensity). (9) The motion of a stimulus over the retina is supposed to yield a sensation of motion, and then velocity a sensation of velocity (the “retinal image displacement” hypothesis, Gibson, 1968).
Not one of these expectations has been borne out in simple form during a century of psychophysical research. There is no correspondence of brightness to intensity except in the night sky or under wholly artificial conditions. The same is true of hue and saturation except when a spectroscope is used. There is no impression of stimulus location on the stationary retina (as there is on the stationary skin) and not even an impression of external location when a point-source of light is projected on the retina in complete darkness (this being the significance of the so-called “autokinetic” phenomenon). There is no correspondence between an external pictorial form and the phenomenal form when the picture is slanted; there are only the puzzling anomalies of the tendency to “form constancy.” The sensation of extensity, or “retinal size,” is a myth; therefore the experience of object-size cannot be reduced to retinal size however hard experimenters try to do so. (This explains why experiments on the size of the moon in the sky are unrevealing: it is nonsense to ask an observer about the experienced object-size of the moon.)
The last three of the listed expectations have to do with temporal variation. Perhaps there are sensations of abrupt “on” and “off,” but psychophysical correspondence is spoiled by the interaction between time and intensity, and by the “fusion” of successive pulses of light. Attempts to link physical duration with phenomenal duration, the “time sense,” have not been convincing and the experiments are full of anomalies. Lastly, the supposed correspondence between the notion and velocity of a stimulus on the retina and sensations of notion and velocity simply does not exist. Efforts to discover it by treating the retina as if it were the skin have failed. The supposed correspondence is violated every time the eye moves (Gibson, 1968).
What are the implications? If simple and reliable correspondences cannot be established between “proximal” retinal stimuli and visual sensations there seem to be two alternatives for the theory of sense perception. One can cling to the assumptions stated above in the first paragraph and suppose that sense impressions would correspond to stimuli except for the intervention of a process of sensory organizationñ a process that explains not only the puzzling interactions among sense impressions but also the formation of percepts. This is the road of Gestalt theory. It partly accepts and partly rejects the expectations of sensory psychophysics (what Koffka called the “constancy hypothesis”). The other alternative, more radical, is to question the generality of all three of the primary assumptions, to suppose that the results of psychophysical experiments applying stimuli to a stationary retina are irrelevant to perception, and to begin the study of perception as an autonomous subject based on ecological optics and the notion of stimulus information.
J. J. Gibson. What gives rise to the perception of motion? Psychol. Rev., 1968,75, 335-346.
Koffka. Principles of Gestalt Psychology, 1935.