Temporal summation and its limits were studied at subjective threshold by varying the pulse duration t of (a) constant-luminance pulse changes of wavelength Δλ, and (b) constant-wavelength pulse changes of luminance ΔL. Threshold depends on Δλ and ΔL for pulse durations longer than about 150–300 and 60 ms, respectively. Psychophysical sensitivity to a pulse of wavelength change improved as pulse duration was increased for short pulses, but was unaffected by pulse duration for long pulses. Although there are departures from full temporal summation for pulse durations longer than roughly 20 ms, a formal analog of Bloch’s law tΔλ = constant gives a fair description of the wavelength-pulse data for short pulses. The critical duration for wavelength pulses (tcw) is much longer than the critical duration for luminance pulses (tcL) at any given mean wavelength. Although tcL does not depend on mean wavelength, tcw is roughly twice as long in the blue-green as in the red. These data bear on the question of what part is played in color vision by the neural encoding of information in the time domain. They can provide fresh empirical tests of current theories of color vision. The findings are tentatively interpreted (a) in terms of different critical events in Walraven’s luminosity and chromaticity channels; these critical events are supposed to limit temporal summation for luminance pulses and wavelength pulses, respectively, and (b) from the viewpoint that central neural processing plays an important part in (task dependent) summation.
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