Covert orienting of attention in the rat and the role of striatal dopamine.

Attention can be directed to a location in the absence of overt signs of orienting, a phenomenon termed "covert orienting." The ability to orient attention covertly has been well documented in humans, but recent progress has been made with the operational definition of the processes involved in covert orienting. Reaction times to visual targets are quickened when attention is drawn to the location of the subsequent target, and processes such as disengagement, maintenance, and movement of attention can be dissociated by using this method. The possible involvement of striatal dopamine in covert orienting is disputed, with conflicting reports of deficits in covert orienting in patients with Parkinson's disease. To examine the significance of dopamine in the striatum in attentional processes, a test of covert orienting, analogous to that used in humans, was devised for the rat. Unilateral dopamine-depleting lesions of the striatum resulted in increases in mean reaction times contralateral to the side of the lesion, but reaction times did not change differentially as a function of the requirements to maintain, disengage, or shift attention. These findings add additional support to the hypothesis that the deficit that appears as hemineglect observed after striatal damage reflects a motor impairment rather than damage in neural systems underlying mechanisms for directing attention.