Changes in neuronal activity related to the repetition and relative familiarity of visual stimuli in rhinal and adjacent cortex of the anaesthetised rat.

Employing the same techniques as have been used with conscious rats, this study describes neuronal responses signalling information concerning the prior occurrence of visual stimuli in unconscious rats. Recordings of the activity of 387 neurons were made while anaesthetised rats were shown objects. Changes in neuronal responses related to stimulus repetition and the relative familiarity of visual stimuli were sought. The areas sampled were lateral occipital cortex, area TE of temporal cortex, perirhinal cortex and the hippocampal formation. The response to the first presentations of unfamiliar objects was significantly different from that to their second presentations for 30 (35%) of 86 visually responsive neurones; for 23 of the neurones the response was smaller when the stimulus was repeated, whereas for 7 it was larger. For all of these neurones the response change was maintained across intervening trials on which other stimuli were shown. For 4 (25%) of 16 neurones so tested, the response decrement persisted across at least 10 intervening trials. The activity of 63 neurones was recorded while rats were shown highly familiar as well as unfamiliar objects. The response to unfamiliar objects was significantly different from that to highly familiar objects for 3 (23%) of 13 visually responsive neurones. The types of neuronal response and their incidence expressed as a proportion of the number of visually responsive neurones were similar to those found in unanaesthetised rats (though the proportion of visually responsive neurones encountered in the anaesthetised rat was lower). The results indicate that information concerning the prior occurrence of stimuli is processed even under anaesthesia.