Medial septal area modulation of sustained negative shifts in awake rabbits

Experiments were performed on rabbits with chronically implanted Ag-AgCl electrodes over occipital cortex and chrome-nickel stimulation electrodes in the medial septal area. Visual evoked potentials (VEP) were elicited by means of randomly occurring flashes. After measuring the N1-amplitude of the VEPs, responses with N1-1 amplitudes below a previously determined discriminative value were reinforced by a weak footshock 1.5 S1 after the flash. In the group of high-amplitude VEP we found longer negative shifts than in the group of low-amplitude VEP, starting 600-700 ms after flash and lasting up to the end of the observed time range. The stimulation of medial septal area (0.5 mA, 6 imp/s) during the normal flash application led to a delay of the maximum of this negative shift which returned now to baseline within the observed range. The amplitude of these sustained negative shifts was larger during septal stimulation. Larger negative shifts followed high-amplitude VEP than the low-amplitude ones. Repetition of usual flash application without septal stimulation after a 10 min break turned the negative shift to normal. After electrolytic destruction of medial septal area this late negative shift vanished without any restitution.