Orientation tuning of cells in areas 17 and 18 of the cat's visual cortex

Sharpness and symmetry of orientation tuning were quantitatively investigated and compared in ninety-seven cells from areas 17 and 18 of the lightly-anaesthetised feline visual cortex. Halfwidths of orientation tuning at half-height ranged between 5 degrees and 73 degrees for long stimuli, with an extreme exception at 111 degrees (excluding untuned cells). There was a tendency for cells in area 18 to be more broadly tuned than those in area 17, due largely to the relatively sharp tuning of area 17 simple cells. Confirming previous work, simple cells were more sharply tuned than complex cells in area 17. In area 18, there was no clear distinction in sharpness of tuning between complex type 1 cells (equated with area 17 simple cells), complex type 2 cells (equated with area 17 complex cells), or hypercomplex cells. Approximately 60% of cells in both areas were asymmetrically tuned for orientation: ratios of half-widths to either side of the optimal orientation ranged from 1.0-3.0, exceptionally 5.8. Asymmetry of tuning was more marked in area 18 than in area 17, except that area 18 complex type 2 cells as a group were relatively symmetrically tuned for orientation. Occasional cells with different preferred orientations for opposite directions of motion, for each peak of a bimodal response to a single direction, or for each half of the receptive field were also observed. The latter are described in the following paper.