Modification of sensory cortical evoked potentials by hippocampal stimulation

Abstract

In the cat, repetitive stimulation was applied to the hippocampus and resulting changes observed in the evoked potential elicited in the visual or auditory cortical areas by single shocks to the optic chiasm or to the medial geniculate body. These changes were contrasted with those produced by stimulation in the mesencephalic reticular formation. In the encéphale isolé preparation, hippocampal stimulation caused a decrease in the amplitudes of all components of the visual and auditory potentials. This was the opposite of the effect of reticular formation stimulation. Hippocampal stimulation also caused a reduction in amplitude of auditory sensory after-discharges in 33% of trials, whereas reticular formation stimulation abolished them completely. In the cerveau isolé preparation, hippocampal stimulation caused a marked increase in the amplitude of the visual evoked potentials. The observations were rationalized in terms of an inhibitory influence exerted by the hippocampus on the brain-stem reticular activating system, combined with facilitatory influence exerted on some more rostral structures, the latter influence being subordinate when the brain-stem is intact, but manifest clearly after midbrain transection.