Image features selected by neurons of the cat primary visual cortex

The sensitivity of neurons in field 17 of the visual cortex in cats to cross-shaped, Y-shaped, and star-shaped figures flashing in the receptive field was studied. About 40% of the neurons studied (114 of 289) were found to generate large responses (with an average response factor of 3.06 +/- 0.32) to one of the figures flashing in the center of the receptive field, as compared with the responses produced to a single bar in the optimal orientation. Most of these neurons (72%) were selectively sensitive to the shape and orientation of figures; the remainder demonstrated some degree of tuning invariance to these properties. The latent periods of responses to figures were usually shorter than those of responses to bars. Tuning parameters for bars and figures were generally related: neurons with acute orientational tuning to a bar were usually highly selective to both the configuration and the orientation people figures. Separate or combined stimulation with crosses in the center and near periphery of the receptive fields demonstrated summation, antagonism, or the lack of any interaction between these zones in producing sensitivity to crosses. Local blockade of intracortical GABAergic inhibition by microiontophoretic application of bicuculline showed that in one third of the neurons studied, sensitivity to figures was generated or enhanced by inhibition in normal conditions, while one third of cells showed suppression by inhibition, and sensitivity in the remainder was independent of inhibition. These data show that reconsideration of existing concepts of the role of field 17 in selecting only first-order shape features of images (i.e., the orientations of single lines) is needed, since almost half the neurons in the cat primary visual cortex can efficiently detect second-order features (angles and line intersections).