EFFECTS OF SUPERIOR COLLICULUS REMOVAL ON RECEPTIVE-FIELD PROPERTIES OF NEURONS IN LATERAL SUPRASYLVIAN VISUAL AREA OF THE CAT

The influence of inputs from the tectothalamic pathway on receptive-field properties of cells in the cat's lateral suprasylvian visual area of cortex (LS area) was investigated. Bilateral aspiration lesions of the superior colliculus were made, and single-cell recording was carried out in the LS cortex 10-115 days later. Following removal of superior colliculus, many of the receptive-field properties of LS cortex cells appeared normal. The cells continued to respond to small stimuli and to show spatial summation to increases in stimulus size. Many of the cells also had internal or surround inhibitory mechanisms that limited the optimal stimulus size, and the proportion of cells with these properties was normal. About 80% of the cells were direction selective, just as in normal LS cortex, and their directional tuning was normal. In addition, most of the LS cortex cells were binocularly driven. Receptive-field size and visuotopic organization also were unaffected by the superior colliculus lesions. On the other hand, three important response properties were altered by removal of the tectothalamic inputs: a) There was an increased strength of response to stationary flashing stimuli. This was most evident among direction-selective cells, which normally respond poorly or not at all to such stimuli. Following the superior colliculus lesions, 42% of the direction-selective cells responded as well to stationary flashing stimuli as to moving stimuli. b) There was an increased response to slowly moving stimuli among movement-sensitive and direction-selective cells. In normal cats, only about 20% of these cells respond to stimulus movement below 5°/s, whereas over 60% of the cells did so following superior colliculus removal. On the other hand, the ability of the cells to respond to a broad range of velocities, including movement in excess of 200 °/s, was not changed following the lesions. c) Although the proportion of cells with internal and surround inhibition did not change following removal of superior colliculus, the spatial distribution of the inhibition differed from normal for many of the LS cortex cells. These cells showed strong inhibition along one axis of the receptive field and little or no inhibition along the orthogonal axis. As a result, they were orientation selective to elongated stimuli. About 30% of the LS cortex cells displayed these properties following superior colliculus removal, whereas we have seen no cells with this type of asymmetrical internal or surround inhibition in normal cats.