Effects of altering brain cholinergic activity on covert orienting of attention: comparison of monkey and human performance

Experiments were conducted to elucidate the role of the cholinergic neurotransmitter system in arousal and the orienting of attention to peripheral targets. Rhesus monkeys and humans fixated a visual stimulus and responded to the onset of visual targets presented randomly in two visual field locations. The target was preceded by a valid cue (cue and target at the same location), an invalid cue (cue and target to opposite locations), a double cue (cues to both spatial locations, target to one), or, the cue was omitted (no-cue, target to either location). Reaction times (RTs) to the onset of the target were recorded. For monkeys, systemic injections of nicotine (0.003-0.012 mg/kg) or atropine (0.001-0.01 mg/kg), but not saline control injections, reduced mean RTs for all trials, indicating general behavioral stimulation. In addition, nicotine significantly reduced RTs for invalid trials but had little additional effect on those for valid, double, or no-cue trials. Virtually identical effects were observed for human chronic tobacco smokers in performing the same task following cigarette smoking. Injections of atropine in monkeys had no effect on RTs for valid or invalid trials but significantly slowed RTs in double-cue trials that did not require the orienting of attention. These results suggest that in both species, the nicotinic cholinergic system may play a role in automatic sensory orienting. In addition, the muscarinic system may play a role in alerting to visual stimuli in monkeys.