Hippocampal electrical activity in rabbits during exploration of a semi-natural environment.

The hippocampal EEG shows characteristic profiles during different behavioural items. A class of hippocampal neurones is active when the animal is in specific places of an environment (place units). However, there are only a few neurones and the role of the hippocampal EEG in spatial information processing remains to be explained. In the present experiment we studied the exploratory behaviour of rabbits (implanted with chronic electrodes in the hippocampus) in an open-air, semi-natural environment. Hippocampal EEG was recorded by telemetry and related to exploratory behaviour by superimposition onto video. Six rabbits were introduced singly to the semi-natural environment and each remained there for 3 days. Their behaviour and EEG was analysed for three 30 min sessions (at 0, 6 and 56 h). Exploration was the main behaviour in the first observation and decreased progressively in the following sessions. The environment was divided into three zones which were explored differently through time. The hippocampal EEG, recorded during exploration, presented an increase of frequency from the first to the second observation. However, since EEG frequency increased when the animal explored the central area of the enclosure, a response in the place domain cannot be excluded.