Abstract
Chromatic detection experiments in humans have been instrumental in elucidating the post-receptoral mechanisms that mediate color vision. To investigate the neural substrates of these mechanisms, we trained Rhesus monkeys to perform a spatial 2AFC chromatic detection task. Our goals were two-fold: First, to assess the utility of Rhesus monkeys as a model for human chromatic detection, and second, to measure the quality of color signals available in cortical area V1 that might subserve detection of isoluminant stimuli.
Monkeys proved to be exquisitely sensitive psychophysical observers – matching or exceeding the sensitivity of human subjects performing the detection task under identical stimulus conditions. The sensitivity of humans and monkeys depended similarly on the spatial frequency and chromaticity of the stimulus: sensitivity was low-pass for isoluminant stimuli and bandpass for achromatic stimuli. When stimuli were equated for cone-contrast, sensitivity was greater for L-M modulations than for S-cone modulations.
In a subset of these experiments, we recorded from individual V1 neurons in monkeys performing the detection task. Stimuli in these experiments were tailored to each neurons' preferred orientation and spatial frequency. Neuronal and psychophysical sensitivities were compared directly via an ideal observer analysis of firing rates during the stimulus presentation period. Although the sensitivity of individual neurons varied considerably, the most sensitive V1 neurons were roughly as sensitive as the monkey. Accordingly, detection thresholds of V1 neurons varied with color direction in qualitative agreement with the monkey's psychophysical behavior. These data demonstrate the existence of individual V1 neurons that are exquisitely sensitive to chromatic stimuli and attest to the value of Rhesus monkeys as a model for human chromatic detection.
This work was supported by an NIH (NIGMS) Training Grant (CH), the ARCS Foundation (CH), the McKnight Foundation (GH), and NIH grant RR000166 (GH).