MODULATION OF NEURONAL-ACTIVITY BY TARGET UNCERTAINTY

Visual scenes are composed of many elements and although we can apprec iate a scene as a whole, we can only move our eyes to one element of t he scene at a time. As visual scenes become more complex, the number o f potential targets in the scene increases, and the uncertainty that a ny 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(1-5) and super ior colliculus(6-8) are modulated by this selection process. We reason ed 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(9,10). By systematically changing the numb er of stimuli from which a selection must be made and recording from s uperior colliculus neurons, we found that as the target uncertainty in creased,the neural activity preceding target selection decreased. Thes e results indicate that neurons within the final common pathway for mo vement generation are active well in advance of the selection of a par ticular movement. This early activity varies with the probability that a particular movement will be selected.