January 1971
Concerning Onset and Cessation of Stimulation
and Disturbances in an Array of Stimulation
J. J. Gibson
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 References to these essays must cite them explicitly as unpublished manuscripts.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.
For all modes of stimulation, acoustic, cutaneous, olfactory, gustatory, and visual, the beginning and the ending of a stimulus at the receptive surface causes the beginning and the ending of a sensation. Only the latency of delay of the sensation differs. A train of sound waves begins and ends and so does a deformation of the skin, an odor at the olfactory membrane, a solution on the tongue and an incidence of light on the retina. Only these, properly speaking, are stimuli. A sound, a touch, a smell and a taste similarly have an onset and a cessation which correspond fairly well (even if not abrupt) to the beginning and end of the energy applied to the receptor. So does a sensation of “light” if it is caused by the rising and setting of the sun, or a flash of lightning, or the heating and cooling of a lamp filament. But the unique characteristic of vision as compared to the other senses is that its supposed stimulus, the retinal image, has no beginning and no end, given an ordinary environment. The ambient optic array of which the retinal image is a sample, is continuous during daylight, and the retina is continuously stimulated by the light in this array no matter how the head-and-eyes move.
The beginnings and endings that really count in vision, then, are disturbances of structure or perturbations of the array, not the on and offs of stimulation. These various disturbances specify various events in the world such as object displacement, changes of occlusion, and changes of material state. The events that animals need to see are in general ones that invoke a change in reflected light, not ones that involve a change in radiant light, although, to be sure, fires and fireflies and stars do exist.
The unique and neglected fact about light as compared with propagating sound, diffusing odor, and mechanical or chemical contact is that illumination reaches a steady state in the terrestrial world and thereby fills the air with optic arrays. In contrast to this, fires, collisions, and volatile sources broadcast light, sound, and odor which carry information about their sources. The changes of stimulus energy correspond to changes at the source, i.e., to events. The episodes of stimulation correspond to episodes in the world. But space-filling illumination goes beyond the limitations of changes in stimulation to changes in the structure of an array of stimulation, and these perturbations are invariant under changes of stimulation.
Sound, color, taste, and touch, I suggested provide only discontinuous or episodic stimulation. But there is an exceptional kind of mechanical stimulation which is continuous and endless — the pull of gravity on the vestibular statoliths and the push of the substratum on the body. For this kind of mechanical stimulation, as for ambient light, there are modulations of stimulus intensity but no episodes of stimulation. How ridiculous that we should still cling to the concept of the discrete stimulus as a causal agent in psychology!