Electrocorticograms of hippocampal and dorsal cortex of two reptiles: comparison with possible mammalian homologs.

To compare the ongoing electrical activity in possibly homologous structures of reptiles and mammals, the electrographic activity (micro-EEG) from major parts of the cortex of unanesthetized turtles (Pseudemys) and geckos (Gekko) was recorded with and without acute and chronic stimuli, physostigmine and atropine. Electrodes were placed in the medial cortex (MC) and in the dorsal cortex (DC), the possible homologs of the mammalian hippocampus and transitional or/and isocortex, respectively. The resting corticograms (1-50 Hz) are different in the two cortical areas. Both are wide-band; power falls steadily with frequency above a single maximum about 2 Hz. The MC has a nonrhythmic, low-voltage activity with occasional superimposed large sharp waves (LSWs), generally biphasic, 100-300 microV and lasting 0.25-0.75 s. The DC has smaller amplitudes (ca. 3-6 dB) at all frequencies and fewer LSWs. Reptilian LSWs are reminiscent of mammalian hippocampal sharp waves or spikes, a correlate of decreased arousal. The immobility-related rhythmic slow activity (theta), so characteristic of the hippocampus in a number of mammals, was not found in the cortex of either species of reptile under a variety of conditions. We cannot exclude the possibility of movement-related theta waves. Physostigmine injection does not produce theta, although it acts like an arousing stimulus, producing a disappearance of the LSWs and a substantial increase in the amplitude of the frequencies 12-24 Hz; these changes were more obvious in the DC. Atropine reversed the effects of physostigmine. Theta may represent a trait of the more highly differentiated hippocampal field of mammals. The condition represented by these reptiles, in which the EEG differs between parts of the pallium but without theta or reciprocal changes in the MC and DC, may be an earlier evolutionary stage. A distinctive reptilian EEG is not recognizable in Pseudemys and Gekko, but a number of differences from the EEG in familiar mammals are shared by these two neurologically quite different reptiles.