Directional tuning of complex cells in area 17 of the feline visual cortex

1. Directional tuning for motion of a field of static visual noise and for motion of bar stimuli against the same stationary noise field as background was compared in eighty-four deep-layer complex cells recorded from area 17 of cats, lightly anaesthetized with N(2)O/O(2) supplemented with pentobarbitone. Detailed comparisons of monocular tuning for the dominant eye receptive field were made for seventy-two cells. Interocular, and stimulus-dependent ocular dominance differences are reported elsewhere (Hammond, 1978c, and in preparation).2. Directional bias for preferred and opposite directions of motion was enhanced with moving noise fields, compared with moving bar stimuli. Two thirds of cells directionally biased for bars, were directionally selective for noise (33/49); ten of these showed suppression of firing to noise moving in the ;null' direction. An additional twenty-three cells were directionally selective for bar stimuli and for noise, nine with suppression of firing in the null direction.3. A majority of cells (52/62) showed significant differences in preferred directions for bar stimuli and for noise; in an exceptional case, tuning for the two classes of stimuli was orthogonal. Tuning for bar stimuli was usually sharper than for noise. In cells asymmetrically tuned for bar stimuli, tuning was broader for the flank of the tuning curve closest to the preferred direction for noise. Ten cells showed similar tuning for bars and noise; tuning for the remaining ten cells was rather variable or poorly defined.22/62 cells showed bimodal directional tuning to noise, with depression of sensitivity over the range of directions to which the cell responded well with a moving bar stimulus.4. Cells were frequently more responsive to noise-field motion than to bar motion. Cells with large receptive fields showed greater sensitivity to noise than those with small fields, and were relatively more sensitive to noise than to bar stimuli.5. The results are interpreted as evidence that the directional and orientational sensitivity of complex cells are mediated by different mechanisms.