Second-order features extraction in the cat visual cortex: selective and invariant sensitivity of neurons to the shape and orientation of crosses and corners.

About 1/3 of neurons (52/174) studied in the cat striate cortex (area 17 or V1) gave a larger (by 3.2 times on average) response to a flashed cross or corner centered in the receptive field (RF) than to a single bar of optimal orientation. Most such neurons (71.4%) were found to be highly selective both to shape (angle between the lines) and to orientation of these figures. In the studied neuronal selection we also found all possible types of invariance of sensitivity to orientation and/or shape of these figures. We found neurons with selectivity to form of the figure and invariance to its orientation and vice versa. Some cells were found invariant both to form and orientation of the cross or corner but highly sensitive to flashing of any such figure in the RF. The role of RF center and surrounding area in sensitivity to cross figures was also studied in 44 additional units. Separated and combined stimulation of these zones revealed in different units summation, antagonism and absence of interaction of these zones by the selectivity index (figure/bar response ratio). Possible mechanisms of the described effects are discussed as well as their functional implication for second-order feature extraction in the visual cortex: selective or invariant sensitivity of neurons to the shape and orientation of the line-crossings.