Mechanisms subserving temporal modulation sensitivity in silent-cone substitution

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Abstract
Temporal contrast sensitivity data were collected with sine-wave-modulated lights for achromatic, chromatic, and silent-cone-substitution stimuli. Achromatic (556- and 642-nm lights in phase) and chromatic (556- and 642-nm lights in counterphase) modulation sensitivities were measured at a constant time-average retinal illuminance of 1256 trolands (Td) and chromaticity of 595 nm. These data were considered to represent isolated temporal responses of luminance and red–green chromatic channels, respectively. Silent cone substitution was achieved with counterphase modulation of the 556- and the 642-nm lights and by suitable adjustment of the modulations or the radiances of the two lights. (1) The peak modulation depth of the 642-nm light was reduced to silence the long-wavelength-sensitive (LWS) cone, and the peak modulation depth of the 556-nm light was reduced to silence the middle-wavelength-sensitive (MWS) cone. These protocols maintained the time-average retinal illuminance and chromaticity as for the control conditions. (2) The luminance of the 642-nm light was decreased to silence the LWS cone and was increased to silence the MWS cone. In this procedure the time-average retinal illuminance and chromaticity differ for the silenced-LWS-cone (1047 Td and 589.5 nm) and the silenced-MWS-cone (4358 Td and 622 nm) conditions. The response modulation of the achromatic and the chromatic channels was calculated for the silent-substitution conditions. The chromatic channel is more sensitive at low frequencies, with a transition to greater achromatic channel sensitivity near 13 Hz for the silenced-LWS-cone condition and near 6 Hz for the silenced-MWS-cone condition. Thus the postreceptoral channels postulated to mediate sensitivity under silent-substitution conditions change with temporal frequency, and the transition frequency from chromatic to luminance processing differs between the two silenced-cone conditions.