Temporal structure of chromatic channels revealed through masking

The human color and luminance-driven systems appear to be mediated by mechanisms with distinct spatio-temporal tuning properties, with the iso-luminant color-driven system comparatively less sensitive to high rates of temporal modulation. While color and luminance defined speed and rate discrimination studies indicate perceptual access to the outputs of multiple, overlapping temporal frequency-selective mechanisms (channels), a detailed functional characterization of their response is currently restricted to the luminance-driven domain. Threshold elevations for 1- and 10-Hz targets were measured as a function of the temporal frequency of a spatio-temporally overlaid masking stimulus (1–30 Hz). Target and masking stimuli were iso-oriented gratings spatio-temporally modulated along each axis of human color space: L – M (red–green), S + Sj (violet–yellow; S-cone isolating), and L + M + S (achromatic). Qualitatively equivalent patterns of temporal frequency-dependent masking were observed when measured within and, with one exception (red–green target, achromatic mask), between cardinal axes of human color space: fitted by one low-pass and one bandpass Gaussian function. That both low-pass and higher bandpass masking functions were evident between iso-luminant cardinal color axes (L – M and S + Sj) suggests that both low and high temporal frequency masking may be cortically mediated.