Visual stimuli excite the retina and often produce a shift of attention, we recorded the activity of neurons in the superior colliculus while monkeys performed various tasks. We discovered that neurons in the representations of the peripheral visual fields respond uniformly to targets, regardless of the direction of the animal's attention. This occurs whether attention is shifted by visual cues or by more cognitive processes. In contrast, neurons within parietal cortex are modulated by the direction of the animal's attention. Neurons within the foveal representation of the colliculus respond differentially to fixation targets, depending on the behavioral task. While the animal is simply fixating, these same cells. We injected the colliculus with muscimol, a GABA-agonist, and discovered that it produced a slowing of responses to all targets within the injected visual field. The neurons recorded at the injection site had increased spontaneous activity and were also more responsive to visual stimuli. These data are some of the first to demonstrate an attentional contribution of the colliculus, independent of eye movements. The results suggest that the superior colliculus provides a visual trigger signal for shifts of attention.
