Chromatic Detection Mechanisms

Detection psychophysics has been a valuable tool in developing models of how cone signals are transformed into color percepts. Several results hypothesize the existence of two linearly independent postreceptoral “cardinal mechanisms” (see figure 1) that transmit the signals necessary for chromatic detection. We aim to investigate the neural substrates of chromatic detection by recording from individual neurons in the primary visual cortex of rhesus monkeys trained to perform detection tasks. Our goals include validating the use of non-human primates as a model for human color detection, comparing the psychophysical sensitivity of monkeys to the sensitivity of individual V1 neurons, and to forge relationships between the discharge of V1 neurons and theoretically defined detection mechanisms.

Figure 1

The Red-Green mechanism receives opponent input from the L- and M-cones; the Blue-Yellow mechanism receives inputs from the S-cones in opposition to the sum of L- and M-cone inputs.

On each trial, a gabor stimulus is presented in one of two mirror symmetric locations on a computer monitor. Macaque monkeys are trained to fixate their gaze (indicated by the blue dot) on a central fixation point during the stimulus presentation. Monkeys are then rewarded for making an eye movement to the remembered location of the stimulus. At the end of the trial, a tone would indicate a correct choice.