July 1973
Note on the Perception of Displacement
J. J. Gibson, Cornell University
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It is tempting to assume that phenomenal variables must correspond to physical variables even in the face of mounting evidence against the general truth of this psychophysical formula. An example is the assumption that the experience of visual “velocity” must correspond to optical stimulus velocity (cf. Sverker Runeson, Uppsala report #140, 1973) at least for the frontal angular velocity of a moving spot. This takes for granted that “an optical motion is a projection in two dimensions of a physical motion in three dimensions” as I once put it. But this whole approach now seems to me mistaken, and the alternative is to study the optical information for the perception of ecological events.
We should have realized all along that the Newtonian laws of motion are not a good guide to the psychology of event perception. Motions in space can be analyzed in terms of instantaneous velocity, acceleration, and even higher derivatives if necessary, by calculus, but these variables, however useful in physics, are not ones to which the phenomenal qualities of an event correspond.
Consider a displacement, defined as a repositioning of a detached object in a layout of surfaces. This is a true event, having a beginning and an end. It should not be called a “motion” as this term is used in physics for that leads to endless confusion about the relativity of motion. A displacement begins and ends, and is absolute. It often starts and stops with a jerk, not with a measurable acceleration and deceleration. It can go a long way or a short way, and last a long time or a short time but it is perceived as a unit and the event does not have speed or velocity. The repositioning is seen as long or short and as fast or slow, but to ask about the seen rate of change of position with time, ds/dt, is simply to confuse the observer.
There is no optical information to specify the analytic variables of the event in physical terms, but there is optical information as a disturbance of structure in the optic array to specify the features of the event.
Various kinds of beginnings and endings are well specified in light. The start and stop are one kind. Becoming revealed and becoming concealed at an occluding edge are another. Coming into and going out of existence are another. If psychologists want to display the optical information for the perception of events they must take into account these different kinds of beginning and ending.
I now argue, therefore, that it is hopeless to try to isolate and control the stimulus variable for the phenomenal impression of velocity, ds/dt, since it is not something that could be specified in light, or represented in an optic array. And if this is so, optical disturbances cannot be reduced to the velocities of space.