The conversational brain: fronto-hippocampal interaction and disconnection

The paper promotes the view that the alert brain alternates between operating in an action mode, based on frontal lobe function, and a receptive mode, involving cholinergic system activity. Their alternation forms a conversation with the environment. It is hypothesized that competition between the modes centers on control over excitability of neurons in the CA1 field of the hippocampus. Increased excitability enhances the flow of hippocampal output through the subiculum resulting in support for frontal lobe function and the action mode. Decreased excitability, on the other hand, reduces this output and that support, leading to a disconnection between frontal lobes and hippocampus. At the same time, correlated cholinergic activity enhances receptive mode processes, indicated by the occurrence of the hippocampal theta rhythm. It is suggested that the hypothesis provides a conceptual framework for considering various phenomena including REM sleep, schizophrenia, and hypnosis. In REM sleep the receptive mode remains dominant as cholinergic activity supports the hippocampal integration of experience into a composite view of reality. In schizophrenia, the action and receptive modes are not properly coordinated because of a dysfunction in anterior hippocampal output. And hypnosis might be seen as a process in which conditions and suggestions are able to induce in some people a prolonged occurrence of the receptive mode allowing a normal view of reality to be altered.