The role of cortical directional selectivity in detection of motion and flicker

Contrast sensitivity for moving and counterphase gratings was measured over a range of temporal and spatial frequencies in normal cats and in cats with selective loss of cortical directional selectivity (strobe-reared cats). Sensitivity ratios (moving/counterphase) in normal cats were largest (mean ratio: 1.9) at lower spatial and at higher temporal frequencies. The sensitivity of strobe-reared cats was more similar for moving and counterphase gratings than was that of normal cats. Sensitivity ratios were lower in strobe-reared cats (mean ratios: 1.2-1.5) than in normal cats and were largely independent of spatial and temporal frequencies. Reduced sensitivity ratios in strobe-reared cats were not the result of greater sensitivity to counterphase gratings, as would be expected if counterphase gratings were detected by independent directionally selective mechanisms. Rather, the low ratios appear to be due largely to reduced sensitivity to moving gratings. These results suggest that the superior sensitivity of normal cats for moving gratings is due to the activity of cortical directionally selective neurons. Moreover, it appears unlikely that the detection of counterphase gratings is mediated by directionally selective mechanisms.