An in vitro study of horizontal connections in the intermediate layer of the superior colliculus

Some models propose that the spatial and temporal distributions of premotor activity in the intermediate layer of the superior colliculus are shaped by neuronal ensembles that give rise to local excitatory and distant inhibitory connections. One function proposed for these connections is to mediate a "winner-take-all" network; the short-range excitatory connections build up the activity of neighboring cells that command orienting movements in one direction, whereas the wide-ranging inhibitory projections attenuate the activity of remote cells that command incompatible movements. We used in vitro photostimulation and whole-cell patch-clamp recording to test these models by measuring the spatial extent of synaptic interactions within the rat intermediate layer. Uncaging glutamate over whole-cell patch-clamped cells in the intermediate layer elicited long-lasting inward currents, resulting from direct activation of glutamate receptors expressed by the cells, and brief synaptic currents evoked by activation of presynaptic neurons. The synaptic responses comprised clusters of excitatory and inhibitory currents. The size of these responses depended on the location of the stimulus with respect to the clamped cell. Large responses were commonly evoked by stimuli within 200 mu m of the soma in the intermediate layer; smaller responses could occasionally be evoked from sites as distant as 500 mu m. Responses evoked by stimulation beyond this distance were rare. Although the results demonstrated powerful local excitatory and inhibitory connections, they did not support the pattern of short-range excitation and widespread inhibition predicted by the winner-take-all hypothesis.