Linear and nonlinear contributions to orientation tuning of simple cells in the cat's striate cortex

Orientation selectivity is one of the most conspicuous receptive-field (RF) properties that distinguishes neurons in the striate cortex from those in the lateral geniculate nucleus (LGN). It has been suggested that orientatio n selectivity arises from an elongated array of feedforward LGN inputs (Hub el & Wiesel, 1962). Others have argued that cortical mechanisms underlie or ientation selectivity (e.g. Sillito, 1975; Somers et al., 1995). However, i solation of each mechanism is experimentally difficult and no single study has analyzed both processes simultaneously to address their relative roles. An alternative approach, which we have employed in this study, is to exami ne the relative contributions of linear and nonlinear mechanisms in sharpen ing orientation tuning. Since the input stage of simple cells is remarkably linear, the nonlinear contribution can be attributed solely to cortical fa ctors. Therefore, if the nonlinear component is substantial compared to the linear contribution, it can be concluded that cortical factors play a prom inent role in sharpening orientation tuning. To obtain the linear contribut ion, we first measure RF profiles of simple cells in the cat's striate cort ex using a binary m-sequence noise stimulus. Then, based on linear spatial summation of the RF profile, we obtain a predicted orientation-tuning curve , which represents the linear contribution. The nonlinear contribution is e stimated as the difference between the predicted tuning curve and that meas ured with drifting sinusoidal gratings. We find that measured tuning curves are generally more sharply tuned for orientation than predicted curves, wh ich indicates that the linear mechanism is not enough to account for the sh arpness of orientation-tuning. Therefore, cortical factors must play an imp ortant role in sharpening orientation tuning of simple cells. We also exami ne the relationship of RF shape (subregion aspect ratio) and size (subregio n length and width) to orientation-tuning halfwidth. As expected, predicted tuning halfwidths are found to depend strongly on both subregion length an d subregion aspect ratio. However, we find that measured tuning halfwidths show only a weak correlation with subregion aspect ratio, and no significan t correlation with RF length and width. These results suggest that cortical mechanisms not only serve to sharpen orientation tuning, but also serve to make orientation tuning less dependent on the size and shape of the RE Thi s ensures that orientation is represented equally well regardless of RF siz e and shape.