The origins of neocortically monitored theta rhythm

Includes bibliographical references (pages 93-100)The hippocampus has been implicated in a variety of neural mechanisms which modulate behavior. A sinusoidal-like wave form, the theta rhythm has been traditionally stressed as the principle bioelectric activity of this structure. Evidence for this view has come from studies demonstrating the existence of theta generators within the hippocampus, as well as neurons which fire in theta-like patterns. Recent reports have pointed to the existence of extrahippocampal generators of this rhythmic slow activity located specifically in neocortical regions. The thrust for such a contention lies mainly in indirect evidence of observed dissociations between neocortically and hippocampally monitored EEG. This paper is concerned with elucidating the location and nature of these proposed neocortical generators by the use of laminar analysis and the method of cortical spreading depression. Results indicate that multiple waves of spreading depression which disrupt the neocortically monitored theta also result in disruption of RSA recorded within hippocampus. In addition, discrete laminar profiles through neocortex failed to detect any evidence for theta generators while intrahippocampal profiles suggest that RSA may emanate from mulitple sources with the hippocampus. Implications\ud for the significance of this RSA in modulating behavior are also discussed