November 1957
Note on the Concept of “Stimulus”
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 References to these essays must cite them explicitly as unpublished manuscripts.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.
The distinction between the proximal and the distal stimulus (Heider, Koffka, Brunswik) was useful, but did not go into the problem deeply enough. Psychology (vs. physiology) needs to consider stimuli which excite the organism (vs. the receptor cell). For vision such “perceptual” stimulation can be analyzed at different stages and by different mathematical methods. The various levels at which “the” optical stimulus can be analyzed seem to be as follows:
1. The orthodox retinal image, which is defined with reference to the sensitive portion of the eye’s interior surface. Actually there is a pair of them. Neither has a definite boundary as does the projection-screen image with which it is so often falsely compared.
2. The pattern of the cone of light entering an eye, that is of focusable light, which may be analyzed in terms of rays or of transitions. Actually there are two such cones. (Each cone is a sector of the total optic array projected to that point.)
3. The pattern of the optic array projected to a station point in an environment, including the changes of pattern corresponding to moving objects.
4. (a) The transformations of pattern of an optic array to a moving station-point. (b) The disparity of pattern between two stationary optic arrays projected to slightly different station points (for animals with coordinated forward-pointing eyes).
5. The families of transformations of pattern from various points in an illuminated environment to other station points. (Is it true that the analysis of all stages above can be made in terms of two variables?)
6. The pattern or structure of the substances of an environment which radiate or reflect light. This is the “object”, or the “distal” stimulus. Its structure is analyzed in terms of three variables. (Now we can entertain the hypothesis that this tridimensional “ecological” structure is specified by combinations of variables in the bidimensional optical structure.)
7. Still higher properties of the substance of an environment, biophysical ones such as reflectivity, absorption spectra, shape, motility, viscosity, spontaneity of movement, etc. Which in turn specify such properties as being edible, mate-with-able, walk-on-able, etc.