Local Infusion of Scopolamine Into Intraparietal Cortex Slows Covert Orienting in Rhesus Monkeys

Abstract

There is accumulating evidence to suggest that cholinergic neurotransmission may play an important role in visuospatial attention, but the brain sites at which acetylcholine modulates attention are not well understood. The present work tested the hypothesis that the cholinergic influences within the intraparietal cortex are necessary for normal attentional shifting (covert orienting) in nonhuman primates. Two rhesus monkeys were trained to perform a visual, cued target detection task for liquid reinforcement. The animals pressed a lever to produce a visual display in which a central fixation point was flanked by two circles. Shortly after fixation was established, one of the circles brightened (cue), and a target appeared subsequently within one of the circles. Detection was signaled by a manual response and the reaction time to the appearance of the target was recorded. Four types of trials were presented. For valid cue trials, the cue and target were at the same spatial location; for invalid cues, cue and target were in opposite hemifields; for double cues, both cues were brightened but the target appeared in either the left or right circle; in no-cue trials, the cue was omitted. We localized the intraparietal region by recording attention-related, cellular activity with intracerebral microelectrodes. Among visually responsive cells in this area, valid cues presented to the receptive fields of visual neurons enhanced the responses to target stimuli in about half the cells and inhibited those responses in the remainder. In addition, some cells showed longer response latencies to invalid cues than to valid cues. We then infused scopolamine into attention-related activity sites and assessed its effect on performance. Scopolamine produced a dose-dependent increase in reaction times and decrease in performance accuracy that lasted more than 1 h. Neither vehicle injections in the same locations nor scopolamine outside the physiologically defined area produced any significant change in behavior. Under our conditions of measurement, we conclude that activity mediated by muscarinic cholinergic receptors within the intraparietal cortex is necessary for normal covert orienting.