Subcortical origins of orientation and direction sensitivity in the cat's visual pathway

The aim of the first investigation of this dissertation was to study the orientation and direction sensitivity of cat LGNd cells using drifting gratings, bars and spots. Seventy-five percent of the LGNd cells studied were orientation sensitive and about one third were direction sensitive. In general, orientation tuning was strongest at high spatial frequencies, whereas direction tuning was strongest at low spatial frequencies. Thus, subcortical orientation and direction sensitivity may be important for the generation of the high degree of selectivity found in visual cortex. In the second study, cortical contributions to the orientation and direction sensitivities of LGNd relay cells was investigated by recording from LGNd cells in cats in which the visual cortex was inactivated. For comparison, the orientation and direction tuning of striate cortical cells was also studied. There were no significant differences in the orientation and direction tuning of relay cells in the LGNd of normal and decorticate cats. The orientation and direction sensitivities of cortical cells were also found to be stimulus dependent in a fashion qualitatively similar to that of LGNd cells. Many of the differences between LGNd cells and cortical cells are explained by their differences in spatial frequency tuning. These findings are consistent with the hypothesis that the orientation and direction biases present in the LGNd inputs to the visual cortex contribute to the strong orientation and direction selectivities found in cortical cells. Finally, direction sensitivity in retinal ganglion cells was studied. The visual responses of retinal ganglion cells were recorded intraocularly using the same visual stimuli employed to study LGNd and cortical cells. Most cat retinal ganglion cells exhibit orientation biases when tested with gratings of relatively high spatial frequency. About 20% of the X- and Y-type retinal ganglion cells exhibited a clear direction bias that was usually clearest when the test grating was of relatively low spatial frequency. The direction sensitive responses of cat retinal ganglion cells were similar to those of X- and Y-cells in the cat LGNd. This suggests that the direction sensitivity of LGNd cells is a reflection of their retinal inputs.