Organization of the intermediate gray layer of the superior colliculus. I. Intrinsic vertical connections

A pathway from the superficial visual layers to the intermediate premotor layers of the superior colliculus has been proposed to mediate visually guided orienting movements. In these experiments, we combined photostimulation using "caged" glutamate with in vitro whole cell patch-clamp recording to demonstrate this pathway in the rat. Photostimulation in the superficial gray and optic layers (SGS and SO, respectively) evoked synaptic responses in intermediate gray layer (SGI) cells. The responses comprised individual excitatory postsynaptic currents (EPSCs) or EPSC clusters. Blockade of these EPSCs by TTX confirmed that they were synaptically mediated. Stimulation within a column (approximately 500 microm diam) extending superficially from the recorded cell evoked the largest and most reliable responses, but off-axis stimuli were effective as well. The EPSCs could be evoked by stimuli 1,000 microm off-axis from the postsynaptic neuron. The dimensions of this wider region (approximately 2 mm diam) corresponded to those of the dendrites of superficial layer wide-field neurons. SGI neurons differed in their input from SGS and SO; neurons in the middle of the intermediate layer (SGIb) were less likely to respond to visual layer photostimulation than were those in sublayers just above and below them. However, focal stimulation within SGIa did evoke responses within SGIb, indicating that SGIb neurons may receive input from the visual layers indirectly. These results demonstrate a columnar pathway that may mediate visually guided orienting movements, but the results also reveal spatial attributes of the pathway which imply that it also plays a more complex role in visuomotor integration.