Modulation of neuronal activity by target uncertainty.

Visual scenes are composed of many elements and although we can appreciate a scene as a whole, we can only move our eyes to one element of the scene at a time. As visual scenes become more complex, the number of potential targets in the scene increases, and the uncertainty that any particular one will be selected for an eye movement also increases. How motor systems accommodate this target uncertainty remains unknown. The activities of neurons in both the cerebral cortex and superior colliculus are modulated by this selection process. We reasoned that activity associated with target uncertainty should be evident in the saccadic motor system at the final stages of neural processing, in the superior colliculus. By systematically changing the number of stimuli from which a selection must be made and recording from superior colliculus neurons, we found that as the target uncertainty increased, the neural activity preceding target selection decreased. These results indicate that neurons within the final common pathway for movement generation are active well in advance of the selection of a particular movement. This early activity varies with the probability that a particular movement will be selected.