The Effects of Temporal Modulation on the Orientation Channels of the Human Visual System

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Abstract
The orientation specificity of spatial adaptation was quantified by using an equivalent-contrast transformation. The half-width at half-amplitude of the adaptation effect was 6·5° when the sinusoidal gratings were stationary, but was increased to 11° when the gratings drifted laterally. Another type of temporal modulation, flashing the gratings on and off repetitively, also increased the half-width, but only at low spatial frequencies where these stimuli actually appeared to be flickering back and forth at threshold. At higher spatial frequencies the flashing stimuli appeared to be stationary at threshold and the half-width was as small as that for truly stationary gratings. It is suggested that there are two types of channel in the visual system: they differ in their orientation specificities, in their temporal properties, and in their roles in the analysis of a spatiotemporal stimulus.