Directionality of cat striate cortical neurones: contribution of suppression.

Direction-selective or direction-biased striate cortical neurones were assessed for absence or incidence of suppression of firing, maximal at 90 degrees or 180 degrees ("null" suppression) to the optimal direction, in 327 neurones recorded from the striate cortex of cats anaesthetized with N2O/O2/halothane. Stimuli were light or dark bars moving over uniform or stationary textured backgrounds; or square-wave gratings of optimal spatial frequency and velocity. Five identified directionality groups were correlated with neuronal class and a range of other receptive field properties. Suppression maximal at 90 degrees to optimum was common amongst direction-biased neurones, rare amongst direction-selective neurones. In the latter group, null suppression (maximal at 180 degrees to optimum) was more prevalent than at 90 degrees. Standard complex cells constituted the majority of complex neurones. They were more commonly direction-biased and less commonly showed suppression than special complex cells. The latter comprised the majority of direction-selective neurones with 180 degrees suppression. Endstopping was seen more frequently in special complex cells, but for each functional class was similarly distributed between the different directionality groups. Based on the mean and mode of partially overlapping distributions, for all neuronal classes direction-selective neurones were more broadly tuned than direction-biased neurones. Special complex neurones were appreciably more broadly tuned than standard complex neurones; those with suppression at 180 degrees were the most broadly tuned neurones in the cortex. Direction-biased neurones with suppression at 90 degrees to optimum were more sharply tuned than those lacking such suppression. Direction-selective neurones had larger receptive fields than direction-biased neurones.(ABSTRACT TRUNCATED AT 250 WORDS)