Note on the “Topography” of the Visual Stimulus
J. J. Gibson, Cornell University
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F. Klix has just written a book on the psychophysics of space perception, partly based on the 1950 hypothesis that the terrestrial environment is mapped into the retinal image as gradients of size, density, spacing, proportion, and the like. The “topography” of the stimulus, he suggests, is crucial, for it is related to the topography of the world by rules of transformation known to geometers.
This program for a psychophysics of space perception will undoubtedly be pushed to its limits, both at Cornell and elsewhere. What are its limitations, if any? Promising as it is, I do not see how it can handle all those types of perception involving sequential stimulation. Some of these are (1) the perception of the surrounding world by head and eye turning, (2) the perception resulting from (and controlling) locomotion, and (3) the perception of external events.
Topography is the accurate description of a permanent terrain by map-making. By analogy, stimulus topography could apply to a sort of frozen stimulus. Ultimately, the available energy for arousing vision is not completely analyzable as a map, or by the operation of mapping. For a complete analysis we shall have to consider (1) the spherical array of ambient light, (2) the continuous change in the projection of light to a non-stationary point, and (3) the continuous transformation of a “figure” without motion of the “ground”. Even if “topography” were adequate for the description of ambient light, accordingly, it would still be necessary to analyze changes in stimulus topography, one type being (2) propriospecific and the other being (3) exterospecific.
A projective transformation, that is a mapping of one surface on another or a correspondence between one form and another should not be confused with a sequence-transformation, that is, a transmutation, a change, or “motion”, or continuous process. These two have an entirely different status for stimulus-geometry. the first is a relation between object and stimulus; the second is a enduring stimulus in its own right. I have not been clear about this distinction in the past.
Consider the effect of Kohler-spectacles on perception; they cause a projective transformation of the entering array relative to the ambient array. But they also cause a visible sequence – transformation (an elastic deformation) of the phenomenal world when the head is moved. Adaptation to such spectacles is not simply a matter or “remapping” the picture delivered by the retinal image, but something much more comprehensive. The problem of space perceptions includes not only the question of how we see the correct arrangement of things but also how we see the rigidity of this layout during exploration and locomotion.