Target probability modulates neuronal activity in the primate saccadic system

The brain has a limited capacity to process information, so perceptual discriminations made when viewing natural visual scenes require that individual stimuli be singled out as targets for further analysis. Motor systems are similarly challenged since goal directed behaviors-by definition-require identification of a single goal. The current report focuses on whether neuronal activity within structures related to the generation of rapid reorienting movements of the eyes, saccades, show modulations related to the probability of selecting one target from among many. We recorded from neurons in the Superior Colliculus (SC) and the Substantia Nigra Pars Reticulata (SNr) while monkeys performed a task in which the number of possible targets was manipulated. We found that neurons in both regions were modulated with changes in target probability. When the probability of a particular saccade target was high, the activity of SC neurons was also high. When the probability was low, the activity was low. Neuronal activity in the SNr was modulated in a manner similar but not identical to that seen in SC. The results suggest that neuronal elements in these regions reflect task demands similar to that seen in cortical regions involved in visual perception.