CHOLINERGIC MODULATION OF CORTICAL OSCILLATORY DYNAMICS

1. The effect of cholinergic modulation on cortical oscillatory dynami cs was studied in a computational model of the piriform (olfactory) co rtex. The model included the cholinergic suppression of neuronal adapt ation, the cholinergic suppression of intrinsic fiber synaptic transmi ssion, the cholinergic enhancement of interneuron activity, and the ch olinergic suppression of inhibitory synaptic transmission. 2. Electroe ncephalographic (EEG) recordings and field potential recordings from t he piriform cortex were modeled with a simplified network in which cor tical pyramidal cells were represented by excitatory input/output func tions with gain parameters dependent on previous activity. The model i ncorporated distributed excitatory afferent input and excitatory conne ctions between units. In addition, the model contained two sets of inh ibitory units mediating inhibition with different time constants and d ifferent reversal potentials. This model can match effectively the pat terns of cortical EEG and field potentials, showing oscillatory dynami cs in both the gamma (30-80 Hz) and theta (3-10 Hz) frequency range. 3 . Cholinergic suppression of neuronal adaptation was modeled by reduci ng the change in gain associated with previous activity. This caused a n increased number of oscillations within the network in response to s hock stimulation of the lateral olfactory tract, effectively replicati ng the effect of carbachol on the field potential response in physiolo gical experiments. 4. Cholinergic suppression of intrinsic excitatory synaptic transmission decreased the prominence of gamma oscillations w ithin the network, allowing theta oscillations to predominate. Coupled with the cholinergic suppression of neuronal adaptation, this caused the network to shift from a nonoscillatory state into an oscillatory s tate of predominant theta oscillations. This replicates the longer ter m effect of carbachol in experimental preparations on the EEG potentia l recorded from the cortex in vivo and from brain-slice preparations o f the hippocampus in vitro. Analysis of the model suggests that these oscillations depend upon the time constant of neuronal adaptation rath er than the time constant of inhibition or the activity of bursting ne urons. 5. Cholinergic modulation may be involved in switching the dyna mics of this cortical region between those appropriate for learning an d those appropriate for recall. During recall, the spread of activity along intrinsic excitatory connections allows associative memory funct ion, whereas neuronal adaptation prevents the spread of activity betwe en different patterns. During learning, the recall of previously store d patterns is prevented by suppression of intrinsic excitatory connect ions, whereas the response to the new patterns is enhanced by suppress ion of neuronal adaptation.