Differential distribution of beta and theta EEG activity in the entorhinal cortex of the cat

The basic characteristics of the electroencephalograms (EEGs) of the entorhinal cortex (EC) were compared to those of the olfactory bulb (OB) and prepiriform cortex (PPC) in the awake cat, using methods that take into account linear (coherence functions) and non-linear relationships (mutual information). The frequency spectra of EC signals differ from those of EEG signals recorded from OB or PPC, but in the beta frequency band (35-40 Hz) coherent activity between these latter two areas and the EC could be put in evidence. Taking the values of coherence as a measure of linear coupling between populations of neurons, this coupling is strong between PPC and the rostral part of the EC, and becomes weaker towards more lateral, caudal and medial areas. This decrease in coupling is confirmed by non-linear analysis methods. On the basis of intracortical depth profiles it was shown that the beta activity is locally generated in layer II of the EC. Another rhythmic activity was also recorded in EC in the theta frequency range. Theta rhythm is generated mainly in the caudal part of EC. This area has the strongest coupling with the hippocampal formation. These analyses lead to the conclusion that in the EC olfactory and limbic domains have different topographical distributions but with a certain degree of overlap.