Hue substitution: Wavelength latency effects

Abstract

Temporal response of the chromatic system in isolation from the achromatic system was investigated under conditions of hue substitution (no luminance transients occur during stimulus presentation). The method of constant stimuli in conjunction with a two alternative forced choice judgement of simultaneity of chromatic stimulus onset was used to measure relative differences in temporal processing as a function of wavelength. Results indicate a non-linear relation between wavelength and latency. A red of 621 nm has the shortest latency of the stimuli tested. Relative to 621 nm as zero, the latencies for shorter wavelengths increase to a maximum 33 msec at 504 nm and return to a latency near zero at the shortest wavelength of 452 nm. These results form a pattern which can be correlated with observed electrophysiological activity of an opponent chromatic mechanism at the level of the lateral geniculate. The functions defined by the psychophysical and electrophysiological results show similar proportional changes at the same wavelengths, and support the conclusion that chromatic latency differences are mediated by opponent channel activity. It is also shown from appropriate comparison of increment and substitution results that the latencies obtained under this method closely approximate an additive system.