SEPARABLE AFTEREFFECTS OF CHANGING-SIZE AND MOTION-IN-DEPTH - DIFFERENT NEURAL MECHANISMS

The following two aftereffects resulted from inspecting a square whose vertical edges moved towards each other with a ramping waveform: 1. (1) a static test square appeared to be continously expanding along the horizontal direction, and 2. (2) a static test square appeared to be moving in depth. We quantified each aftereffect by measuring the real rate of size change required to cancel it. Both aftereffects decayed exponentially. No. 1 rapidly with a time constant between 6.4 and 9.9 sec. and No. 2 more slowly with a time constant between 21 and 54 sec. Because of the different decay rates, the two aftereffects could be seen separately. We propose that the existence of these two aftereffects is consistent with a psychophysical model in which unidirectional motion filters feed a changing-size filter that in turn feeds a motion-in-depth stage. We tentatively suggest that this model might help to explain how the visual system resolves the ambiguity of a retinal image whose magnification is changing. © 1979.