Abstract

Subtractive mechanisms have been clearly established as playing an important role in light adaptation. Unlike gain control mechanisms, subtractive mechanisms remove the steady state signal from an adapting background without attenuating the response to incremental flashes. Little is known about the time course or neural basis of such mechanisms. Previous measurements¹ indicated that a large component of the subtractive change occurred effectively instantaneously at light onset but the rest of the change was quite slow, taking a second or two to complete. In this experiment we extended the measurements to get a more complete description of the time course of the subtractive component. Several different adapting background intensities and a small step, comparable to that in normal scenes, were examined. We also manipulated the spatial configuration of the display to test whether some component of the rapid subtractive change resulted from lateral inhibition. The experiment was performed in foveal vision, measuring threshold for a brief red incremental probe at the onset of a white flashed field. The adaptational state was manipulated by an additional white field. With large fields, there was no subtractive change for several hundred milliseconds unlike previous findings, and the process took 10-15 s to asymptote. It seems likely that the fast component previously observed results from center-surround antagonism.

© 1989 Optical Society of America