HUMAN AND MONKEY P3-LIKE RESPONSES IN A MIXED MODALITY PARADIGM - EFFECTS OF CONTEXT AND CONTEXT-DEPENDENT NORADRENERGIC INFLUENCES

Previous studies in non-human primates have reported that noradrenergi c agents, such as the alpha 2 agonist clonidine, affect auditory but n ot visual P3 responses. One explanation for these effects may be that distinct modalities are differentially susceptible to noradrenergic in fluences - a modality-dependent effect. Another possibility is that no radrenergic effects are modality-independent but context-dependent. Tw o separate experiments, in humans and monkeys, examined the role of st imulus modality, context and noradrenergic activity in an attempt to e lucidate the nature of the relationship between P3 and the action of t his catecholamine neurotransmitter. In Experiment 1, human subjects we re exposed to two versions of a mixed modality oddball paradigm. In on e version, the context or background activity was auditory, while in t he other it was visual. In both versions of the paradigm, the same aud itory and visual rare targets occurred. The results indicated that N2 and P3 to visual targets were more sensitive to changes in context tha n those to auditory targets. In both cases, N2 and P3 amplitudes to ta rgets were enhanced when the eliciting event and context differed. A m odality-specific N2b, on the other hand, was enhanced when the context matched the modality of the eliciting event. In Experiment 2, monkeys received systemic injections of a saline placebo or one of three dose s of the adrenergic antagonist L657,743 prior to presentation of a mix ed modality oddball paradigm in a visual context. Drug effects were ob served on earlier components such as N1, P2 and N2 but not on later P3 -like responses. The combined findings complement previous ones from o ur laboratory and suggest that P3 reflects context-dependent processes and specifically context-dependent, not modality-specific, noradrener gic activity. (C) Elsevier Science B.V.