Cat striate cortex: monocular and interocular comparisons of spatial-frequency selectivity.

Spatial frequency and bandwidth characteristics were determined for neurones in cat striate cortex. Responses to drifting sine-wave gratings, optimized for orientation, direction and velocity, were determined over a range of spatial frequencies. Comparative measurements of spatial frequency tuning at constant velocity and at constant temporal drift frequency revealed that, overall, tuning derived by either method was similar. Results were evaluated in relation to neuronal class (simple or complex); complex cell subclass (standard, intermediate or special), defined by length summation; directionality; and velocity selectivity. Distributions of optimal spatial frequency for simple and complex neurones were comparable. By contrast, bandwidths of simple neurones were markedly narrower than for complex neurones. Standard complex neurones, in turn, had narrower bandwidths than special or intermediate complex neurones. Optimal spatial frequency correlated inversely with optimal velocity, directly with orientation selectivity. Thus, neurones tuned to high spatial frequencies tended to respond optimally to low velocities, and were more sharply orientation selective, than neurones tuned to low spatial frequencies. In binocular neurones, spatial frequency tuning characteristics of the two monocular inputs were compared. For either eye, spatial frequency tuning curves were reproducible over time. In a minority of neurones, spatial frequency characteristics were matched for the two eyes. A majority showed mismatch in spatial frequency characteristics between the eyes. Individual neurones were tuned to different bands of spatial frequencies through either eye; more sharply spatial-frequency selective through one eye than the other; or had both dissimilar bandwidth and spatial frequency. Changing input spatial-frequency resulted in profound, systematic shifts in ocular dominance. These were progressive in the case of spatial-frequency mismatch. In cases of bandwidth, or bandwidth and spatial-frequency mismatch, the eye associated with more sharply-tuned input exerted relatively greater influence at centre frequencies, the other eye relatively greater influence at extreme frequencies. There was a marginal tendency for the dominant (or contralateral) eye to be tuned to higher spatial frequencies than the more weakly driving (or ipsilateral) eye. By contrast, interocular differences in bandwidth were pronounced: in a majority of neurones the dominant eye was more broadly tuned than the more weakly driving eye. Related to the established preponderance of contralaterally dominated cortical neurones, the input from the contralateral eye was markedly more broadly tuned than that from the ipsilateral eye, consistent with the notion that stronger drive is associated with greater pooling of inputs. These differences have important implications for binocular vision and, potentially, for coding of visual perspective.