Directionality of cat striate cortical neurones: contribution of suppression

Summary Direction-selective or direction-biased striate cortical neurones were assessed for absence or incidence of suppression of firing, maximal at 90° or 180° (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 back-grounds; 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° to optimum was common amongst direction-biased neurones, rare amongst direction-selective neurones. In the latter group, null suppression (maximal at 180° to optimum) was more prevalent than at 90°. 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° 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° were the most broadly tuned neurones in the cortex. Direction-biased neurones with suppression at 90° to optimum were more sharply tuned than those lacking such suppression. Direction-selective neurones had larger receptive fields than direction-biased neurones. In both groups receptive fields decreased in size in the sequence: intermediate complex > standard complex > special complex > simple. Resting discharge was highest amongst direction-selective neurones with 180° suppression, lower in those with 90° suppression or those lacking it, and lowest amongst direction-biased neurones. With the possible exception of the minority of neurones that were silent, low levels of resting discharge have not seriously prejudiced either neuronal categorization or comparisons of tuning selectivity. The pattern which emerges is that suppression maximal in directions orthogonal to the preferred direction/orientation is more commonly associated with sharp tuning and directionbias, whereas null suppression, in the direction opposite that preferred, is associated with broad tuning, direction-selectivity, high resting discharge levels and strong texture sensitivity.