A visual stimulus appearing in the periphery causes a shift of attention. This change is not localized to the site of the visual stimulus, but facilitates adjacent regions. Parietal cortex neurons are modulated during this attentional behavior: cells with visual receptive fields at the focus of attention are suppressed, a mechanism thus leading to facilitation of the responses of cells not at the focus of attention. When animals respond to visual targets that alternate between locations in a predictable way, reaction times to expected targets are faster than to than unexpected targets. Parietal cells respond best to the unexpected stimulus. These observations are consistent with the parietal cortexes containing a signal to shift attention. We have tested a group of Parkinson's disease patients on complex reaction time tasks. Although the disease produces a general impairment on all tasks, treatment with L-dopa selectively improves perfor- mance on only one. Responses which are """"""""compatible"""""""" with the visual stimulus signaling them are speeded with L-dopa; responses 'arbitrarily' associated with the triggering stimulus are unaffected by this treatment. These studies suggest that the basal ganglia, using dopamine, function in the initiation of movement. We have discovered that when a brainstem region involved in the initiation of head movements is electrically stimulated, the animal makes a brisk head movement. The size and amplitude of this movement depends on the intensity of the current and the starting position of the head. Injecting chemical tracers, we discovered that areas sending information to this brainstem region include the deep layers of the superior colliculus, the primary motor cortex, and the premotor cortex. The axons of cells in these areas reach the cervical spinal cord as well as other premotor, brainstem sites. These studies help explain the organization of systems for initiating head movements.
