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

Abstract

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.

FIG. 1

Speed tuning at low- and high-contrast in alert macaque V1. A: speed tuning of a V1 directional simple cell measured using high (thick lines) and low (thin lines) contrast stimuli. Dotted lines are the log-Gaussian fit to the data; the optimum speed tuning, taken from the center of the log-Gaussian, is indicated for each curve. B: same as A for a V1 complex cell. C: scatter plot of the speed tuning of 21 directional simple cells (open triangles), 22 directional complex cells (filled triangles), and 19 nondirectional complex cells for high- and low-contrast stimuli. D: scatter plot of direction indices of direction-selective simple and complex cells to phi motion stimuli (flashed sequential pairs of bars in a sparse noise stimulus) for high- and low-contrast stimuli.

FIG. 2

Space-time maps, relative response phase as a function of time after stimulus onset, and speed tuning for 4 V1 directional simple cells. Each row shows data for 1 cell. Left 2 columns: space-time maps (light minus dark responses) as a function of time after stimulus onset and stimulus position along the stimulus range, for high-contrast (far left) and low-contrast (2nd column) stimuli. Each map is normalized to the maximum average response (either light excitatory, or on, or dark excitatory, or off). Third column: phase of a Gabor, fit to the response (relative to response onset), as a function of time after stimulus onset. Right: speed tuning of each cell at high and low contrasts; dotted lines are the log-Gaussians fit to the data. The optimum speeds, taken from the centers of the log-Gaussians are indicated for each curve.

FIG. 3

Simple-cell spatial and temporal response characteristics for high- and low-contrast stimuli. A: tilt direction index for 21 simple cells for high- and low-contrast stimuli. The tilt direction index reflects the degree of asymmetry in the space-time map. B: optimal spatial frequency, derived from the spatial frequency peak of the 2-D Fourier transform of the space-time map, for 21 simple directional cells. C: optimal temporal frequency, derived from the temporal frequency peak of the 2-D Fourier transform of the space-time map, for 21 simple directional cells. D: slant of the space-time maps at high and low contrast, measured from the slope of phase vs. time (■) or from the maximum spatial-frequency/temporal-frequency orientation in the fast Fourier transform (FFT). Most of the simple cells showed slants that predicted faster optimum speeds at high contrast than at low; i.e., they are above the x = y diagonal.

FIG. 4

Correlations between actual speed tuning (vertical axis) and different ways of predicting the optimal speed from the spatiotemporal characteristics of the space-time maps. Each pair of connected points corresponds to data for 1 cell. A: optimum speed vs. optimum speed predicted from the ratio of the optimum temporal frequency to optimum spatial frequency. B: optimum speed vs. optimum speed predicted from the space-time slant measured from the slope of the response phase as a function of time. C: optimum speed vs. optimum speed predicted from the peak orientation in temporal frequency/spatial frequency of the Fourier transform of the space-time map.

FIG. 5

Space-time maps and phase shifts for high- and low-contrast stimuli for 2 different models for generating direction-selective simple cells. See text for details.