Tonic and phasic influence of basal forebrain unit activity on the cortical EEG.

Changes in arousal levels are normally accompanied by modification of gross electrical activity (EEG) in the cortex, with low amplitude fast waves characterizing high levels and large slow waves low levels of arousal. These changes in cortical EEG patterns depend mainly on two factors: on the input from the thalamus and on the state of various membrane channels in the cortical pyramidal cells, which are both regulated by ascending modulatory systems. Several lines of evidence indicate that of the activating systems the cholinergic is the most effective in activating the cortex. Its blockade with atropine induces large slow waves in the EEG, while inhibition of other systems has no such profound effect. The effect of atropine can be mimicked by lesioning the basal forebrain. Neurons in this area show very close tonic and phasic correlation with the cortical EEG, further supporting the suggestion that projections of these neurons have a special role in the regulation of cortical activity. However, there is a discrepancy between the effects of excitotoxic and selective cholinotoxic lesions of the basal forebrain. The immunohistochemical diversity of the corticopetal basal forebrain projection and the electrophysiological heterogeneity of the neurons also indicate that, in addition to cholinergic cells, other types of neurons do also participate in the regulation of cortical activity from this area. To understand the intimate details the activity of identified basal forebrain neurons must be recorded and correlated with cortical events.