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 cha racterizing high levels and large slow waves low levels of arousal. These c hanges in cortical EEC patterns depend mainly on two factors: on the input from the thalamus and on the state of various membrane channels in the cort ical pyramidal cells, which are both regulated by ascending modulatory syst ems. Several lines of evidence indicate that of the activating systems the cholinergic is the most effective in activating the cortex, its blockade wi th 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 ton ic and phasic correlation with the cortical EEG, further supporting the sug gestion that projections of these neurons have a special role in the regula tion of cortical activity. However, there is a discrepancy between the effe cts of excitotoxic and selective cholinotoxic lesions of the basal forebrai n. The immunohistochemical diversity of the corticopetal basal forebrain pr ojection and the electrophysiological heterogeneity of the neurons also ind icate that, in addition to cholinergic cells, other types of neurons do als o participate in the regulation of cortical activity from this area. To und erstand the intimate details the activity of identified basal forebrain neu rons must be recorded and correlated with cortical events. (C) 2000 Elsevie r Science B.V. All rights reserved.