Independent generation of theta rhythm in the hippocampus and posterior cingulate cortex

Theta rhythmicity of field potentials recorded in the posterior cingulate cortex is thought to have a septo-hippocampal origin, principally because phase reversal of this theta occurs in the dorsal hippocampus, but not in the posterior cingulate cortex. In the current study, theta activity of cue-elicited field potentials and multiple unit activity in posterior cingulate cortical areas 29b and 29c/d, and in the associated anterior ventral (AV) thalamic nucleus, was monitored while rabbits underwent discriminative avoidance conditioning. Theta activity in the field potentials occurred during training in areas 29b and 29c/d, and was severely attenuated by electrolytic lesions in the dorsal hippocampus. Significant theta rhythmicity was not evident in multiple unit activity recorded from area 29c/d, either in lesioned or sham-operated subjects. However, theta-like modulation was evident in the multiple unit activity of area 29b, and in the AV thalamic nucleus, occurring in synchrony with the field potential oscillations. Theta rhythmicity of unit activity in these areas was unaffected by dorsal hippocampal lesions. These results suggest that theta-frequency oscillations apparent in posterior cingulate cortical field potentials are volume-conducted from the dorsal hippocampus, but that theta-like unit activity in posterior cingulate cortical area 29b and in the AV thalamic nucleus occurs independently of hippocampal theta.