The role of basal forebrain neurons in tonic and phasic activation of the cerebral cortex

The basal forebrain and in particular its cholinergic projections to the ce rebral cortex have long been implicated in the maintenance of cortical acti vation. This review summarizes evidence supporting a close link between basal foreb rain neuronal activity and the cortical electroencephalogram (EEG). The anatomy of basal forebrain projections and effects of acetylcholine on cortical and thalamic neurons are discussed along with the modulatory input s to basal forebrain neurons. As both cholinergic and GABAergic basal forebrain neurons project to the co rtex, identification of the transmitter specificity of basal forebrain neur ons is critical for correlating their activity with the activity of cortica l neurons and the EEG. Characteristics of the different basal forebrain neurons from in vitro and in vivo studies are summarized which might make it possible to identify dif ferent neuronal types. Recent evidence suggests that basal forebrain neurons activate the cortex n ot only tonically, as previously shown, but also phasically. Data on basal forebrain neuronal activity are presented, clearly showing th at there are strong tonic and phasic correlations between the firing of ind ividual basal forebrain cells and the cortical activity. Close analysis of temporal correlation indicates that changes in basal fore brain neuronal activity precede those in the cortex. While correlational, these data, together with the anatomical and pharmacol ogical findings, suggest that the basal forebrain has an important role in regulating both the tonic and the phasic functioning of the cortex. (C) 199 9 Elsevier Science Ltd. All rights reserved.