Note on the "Topography" of the Visual Stimulus
J. J. Gibson, Cornell University; Jan. 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 References to these essays must cite them explicitly as unpublished manuscripts.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.
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.
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