Towards a Neuronal Model to Account for the Laminar Waveforms of Theta Activity

Abstract

The data presented so far, on the pacemaker and on the control and laminar waveforms of theta rhythms in the various structures from which they can be recorded, are quite complex. The task of theory building is in part to reduce complex data to simpler relationships which account for as much as possible of the data. One way of reducing the data presented is to consider the possible relationships of neurons which best account for the evidence considered above, in other words to construct a neuronal model of the hippocampus during generation of the theta rhythm. This is a major objective of the present chapter (which will not be achieved competely). In part this involves a deeper discussion of the laminar profiles of theta potentials described above. However, a variety of other evidence is drawn upon, which is also relevant to building a neuronal model. A related aspect of this model building, also dealt with below, is the relationship between theta potentials recordable in the hippocampus, and those recordable in isocortical regions, based on evidence reviewed above. A second way of bringing parsimony to the data has been hinted at several times already. In both Chapters 6 (Sect. 6.3.2.2) and 7 (Sect. 7.3.1.6) we obtained suggestions of a promising relationship: Theta rhythms, in some circumstances at least, may involve resonant circuits between the hippocampal formation where the rhythms originate, and neocortical loci which may be entrained to the hippocampal rhythm. However, the evidence considered so far does not compel this interpretation of the data.