Latencies for chromatic and achromatic visual mechanisms

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Relative visual onset latency and offset latency (visual persistence) were measured (using perceived simultaneity methodsl for pulsed chromatic stimuli presented under two conditions: as luminance increments of 0.2 or 0.3 log unit above a prevailing achromatic background field, or in hue substitution, matched in luminance to the achromatic field so as to eliminate spatio-temporal luminance transients. In the first experiment, for the hue substitution condition, both onset and offset latencies were found to be slow relative to latencies for the increment condition, and varied with wavelength according to a function resembling trichromatic saturation discrimination (slowest latency at 570 nm). Latencies to increment pulses of equal luminance did not vary with wavelength. In a second experiment, offset latency was measured as a function of pulse duration and pulse luminance level. The observed decrease in offset latency with pulse duration was markedly steeper at short pulse durations for chromatic increments than for hue substitution stimuli. Offset latency decreased with increasing pulse luminance for incremental stimuli but not for hue substitution stimuli. These results are interpreted as reflecting the temporal response properties of chromatic neural mechanisms (the hue substitution data) and the achromatic mechanism (the increment data).