Laminar organization and contrast sensitivity of direction-selective cells in the striate cortex of the Old World monkey

The directional preference of neurons sampled from all layers of the striate cortex was determined using the responses to drifting grating stimuli of optimal spatial and temporal frequency. In addition, contrast sensitivity as a function of spatial frequency was measured and from the resulting spatial contrast sensitivity function the peak contrast sensitivity and optimal spatial frequency were obtained. The distribution of directionally selective cells showed a distinct laminar pattern. Upper layer 4 (4a, 4b, and 4c alpha) and layer 6 were the only cortical layers with neurons that showed a pronounced preference for the direction of stimulus motion. The directionally selective cells in these layers are among those with the highest contrast sensitivities but had optimal spatial frequencies that were confined to the low and middle range of the optimal spatial frequency distribution. These findings suggest that the directionally selective cells may fall into at least 2 distinct populations, which may be the first stages in the visual pathway that correspond to those channels, inferred from psychophysical experiments, that underlie the detection of movement.