Spatial summation processes in visually driven neurones of cat's pretectal region

The spatial summation processes of single neurones of cat's pretectal region were investigated with moving and stationary visual stimuli. The results indicate that the majority of the investigated neurones changed their responses essentially at the gradual increase of size of the applied stimuli (i.e. showed negative or positive summation). Particularly, direction non-sensitive neurones showed symmetrical changes of spatial summation curves in response to two opposite directions of movement. By contrast, in some direction sensitive neurones different characteristics of responses for the two opposite directions of movement were observed. Thus the number of discharges in the responses to the preferred direction could increase or decrease at the gradual increase of the moving stimulus size, while the responses to the null direction could remain stable or vice versa. The same was observed for the "ON" and "OFF" responses in the ON-OFF neurones. Thus, it appears that the pattern of responses of a given neurone to different directions of movement and to the "on" and "off" periods of stationary stimulation are shaped by independent mechanisms.