Abstract

It is well know in visual neurophysiology that units of the retinocortical pathway are selective for luminance and contrast polarity. The so-called ON and OFF pathways are comprised of cells responding either to increments in luminance or contrast (ON pathway) or decrements (OFF pathway)1. Although many psychophysical studies have manipulated stimulus polarity, there are relatively few papers that convincingly link polarity manipulation to selective activation of ON or OFF mechanisms. For example, there have been many demonstrations of threshold asymmetries, which typically show greater sensitivity to luminance decrements than increments2. But such a polarity asymmetry could occur in a generic visual pathway that had a nonlinear luminance or contrast response, and this result does not demand the existence of ON and OFF pathways in humans. More convincing evidence for selective ON and OFF pathway activation comes from studies of aftereffects of temporal sawtooth adaptation3, in which viewing of one polarity of sawtooth (either rapid on or rapid off) raises threshold for test stimuli of the same polarity more than for test stimuli of opposite polarity. This asymmetry suggests that polarity-sensitive visual mechanisms can be differentially adapted, and the result is consistent with a two-pathway ON-OFF model of luminance or contrast processing in the human visual system.

© 1998 Optical Society of America