Alteration of brain noradrenergic activity in rhesus monkeys affects the alerting component of covert orienting.

Experiments were conducted to elucidate the role of the noradrenergic neurotransmitter system in arousal and the orienting of attention. Rhesus monkeys were trained to perform a peripherally cued, covert orienting task for juice reward, and their manual reaction times (RTs) to visual stimuli were measured. The effects of parenteral injections of the alpha-2 adrenergic agonists clonidine and guanfacine, and normal saline were compared on the covert task. We assessed 1) overall error rates, 2) the difference in RTs between validly and invalidly cued trials (validity effect), 3) the difference in RTs between neutral and no-cue trials (alerting effect), 4) target location (visual field), and 5) cue-target interval. Changes in noradrenaline levels produced by clonidine (and to a lesser extent guanfacine) significantly decreased the alerting effect, and lowered RTs to stimuli in the left visual field, but did not change the validity effect, suggesting that noradrenaline is involved in maintaining non-spatial, sensory readiness to external cues but not in the shifting of the attentional focus.