Abstract
The human chromatic contrast sensitivity measured with psychophysics has a lowpass shape. Human neurophysiology results, in contrast, have revealed extensively spatially-tuned bandpass population responses in the early visual cortex (e.g., Rabin, Switkes, Crognale, Schneck & Adams, 1994; Nunez, Shapley & Gordon, 2018). In monkey single-neuron studies, it has been demonstrated that there is a dynamic transformation from non-spatial tunning to spatially-tuned color responses in V1 (Johnson, Hawken & Shapley, 2001). However, such a transformation has never been identified in humans. Here, we investigated the spatial tuning of color in human observers (N = 16), by recording steady-state visual evoked potentials (SSVEPs), which originate from early visual cortex especially the V1. We tested stimuli in red-green, blue-yellow and luminance (i.e., L-M, S, and Luminance axis in DKL space), at different spatial frequencies (0.2, 1, 2, 3, 4 and 8 cycles/deg), at both low and high temporal frequency (3 Hz and 15 Hz). The results reveal a dynamic change in spatial selectivity from high to low temporal frequency: color SSVEP is low-pass at 15Hz and band-pass at 3Hz. The low-pass and band-pass color SSVEP resembles neural responses of single-opponent and double-opponent V1 neurons, respectively, in macaque monkey (Johnson, Hawken & Shapley, 2001). In addition, in the band-pass responses at 3Hz, the preferred spatial frequency for S is lower than those for L-M and luminance, which is also consistent with monkey single neuron results. SSVEP to luminance is band-pass at both 15Hz and 3Hz, suggesting that the transformation is specific to color processing. The current result reveals a spatiotemporal interaction in human color vision. The visual cortex adds spatial selectivity for color boundaries in the processing of temporal filtering.