Contrast affects speed tuning, space-time slant, and receptive-field organization of simple cells in macaque V1.

We measured speed tuning of V1 cells in alert macaques to high- and low-contrast stimuli. Most V1 cells tested, both simple and complex and directional as well as nondirectional, shifted their speed tuning to slower speeds for lower contrast stimuli. We found that the space-time slant of the receptive field of directional simple cells differed for high- and low-contrast stimuli, with the space-time slant predicting higher optimum speeds for the higher-contrast stimuli; i.e., there was a larger spatial shift of the receptive-field organization per unit time. Not only did the space-time maps of directional simple cells show different slants between high- and low-contrast stimuli, but they also showed a different organization, because for high-contrast stimuli, the maps tended to show a complete inversion of the receptive-field spatial organization at long delays after stimulus onset, with initial excitation followed by suppression and initial suppression followed by excitation, but for low-contrast stimuli the receptive-field organization showed only a quadrature shift over time. We show that a simple modification of earlier models for the generation of direction-selective simple cells can account for these observations.