Collinearity tolerance of cells in areas 17 and 18 of the cat's visual cortex: relative sensitivity to straight lines and chevrons.

Collinearity tolerance and length dependence of orientation tuning were compared in cells recorded from areas 17 and 18 of the lightly anaesthetised cat's visual cortex. Orientation tuning and interaction between receptive field halves of the same cells are reported in the preceding paper and elsewhere (Hammond and Andrews, 1978a, b). In confirmation of previous work, increase in stimulus length was associated with sharper orientation tuning in all simple and hypercomplex cells, and in most complex cells even in the absence of length summation. Cells in areas 17 and 18 were more sharply tuned for chevrons was noticeably skewed compared with tuning for straight lines. In area 17, the best response was always obtained with a straight line of optimal orientation. The two halves of the receptive fields of some cells in area 18 had dissimilar preferred orientations. Even in cells whose receptive field halves were similarly tuned, broadly tuned, or apparently untuned for orientation, simultaneous stimulation of both halves of the receptive field led to substantial sharpening of tuning. In cells with dissimilarly tuned half fields, the skew in chevron tuning was predictable from the orientation tuning of each half of the receptive field. Two area 18 cells responded consistently better to a chevron stimulus than to a straight line of any orientation.