Complex visual scenes require that a target for an impending saccadic eye movement be selected from a larger number of possible targets. We investigated whether changing the probability that a visual stimulus would be selected as the target for a saccade altered activity of monkey superior colliculus (SC) neurons in two experiments. First, we changed the number of possible targets on each trial. Second, we kept the visual display constant and presented a single saccade target repeatedly so that target probability was established over time. Buildup neurons in the SC, those with delay period activity, showed a consistent reduction in activity as the probability of the saccade decreased, independent of the visual stimulus configuration. Other SC neurons, fixation and burst, were largely unaffected by the changes in saccade target probability. Because we had monkeys making saccades to many locations within the visual field, we could examine activity associated with saccades outside of the movement field of neurons. We found the activity of buildup neurons to be similar across the SC, before the target was identified, and reduced when the number of possible targets increased. The results of our experiments are consistent with a role for this activity in establishing a motor set. We found, consistent with this interpretation, that the activity of these neurons was predictive of the latency of a saccadic eye movement and not other saccade parameters such as end point or peak velocity.

The superior colliculus is one of the most well-studied structures in the brain, and with each new report, its proposed role in behavior seems to increase in complexity. Forty years of evidence show that the colliculus is critical for reorienting an organism toward objects of interest. In monkeys, this involves saccadic eye movements. Recent work in the monkey colliculus and in the homologous optic tectum of the bird extends our understanding of the role of the colliculus in higher mental functions, such as attention and decision making. In this review, we highlight some of these recent results, as well as those capitalizing on circuit-based methodologies using transgenic mice models, to understand the contribution of the colliculus to attention and decision making. The wealth of information we have about the colliculus, together with new tools, provides a unique opportunity to obtain a detailed accounting of the neurons, circuits, and computations that underlie complex behavior.

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