Motion-Contrast Sensitivity: Visibility of Motion Gradients of Various Spatial Frequencies

The purpose of these experiments was to estimate basic sensitivity to motion gradients, and to evaluate the evidence for second-order integration and differentiation of motion signals. We measured sensitivity to spatially sinusoidal contrast modulation between two oppositely-moving bandpass-filtered noise images. The motion-contrast sensitivity function, defined as the inverse of threshold modulation amplitude as a function of modulation spatial frequency, was band-pass in shape with declines at both highest and lowest frequencies. The functions for three noise spatial frequencies were approximately the same shape when modulation frequency was expressed as a fraction of the noise frequency. We compared the data to a model in which linear motion filters, whose outputs are squared or rectified, are followed by a second stage of excitatory and/or inhibitory pooling. The data are consistent with a model in which 1) all excitatory pooling occurs at the linear stage, and 2) the second stage contains a large inhibitory pooling area, with a radius about 8 times that of the linear receptive field.