Responsiveness of neurones in the hippocampal region of anaesthetised and unanaesthetised cats to stimulation of sensory pathways.

The responses to sensory stimuli and to shocks to the optic pathways have been studied for 399 units in the hippocampal region of 19 cats. 172 units were recorded from anaesthetised cats and 229 from 7 unanaesthetised cats. In the unanaesthetised animals the proportions of units responding were high (60%) and did not differ significantly between regions for each type of stimulation investigated: visual, auditory and optic chiasma shocks. The proportion of units responsive to chiasmatic stimulation was related to the background firing rate. The latencies and duration of the responses were relatively long and variable. Both increases and decreases in responsiveness occurred. In a number of units response were erratic, sometimes being brisk and at other times totally absent. Responses during one series of stimuli were not always similar to those during a subsequent series. A higher proportion of units responded decrementally to a series of sensory stimuli than to a series of shocks to the optic chiasma. Movement by the animal was wihtout consistent effect on unit activity. Responses were markedly influenced according to whether or not the cat looked intently at an object. An important determinant of HR unit responses was the state of arousal, the effect being regionally distributed. In anaesthetised animals the proportions of responding units were substantially lower and the responses were weaker than those in the unanaesthetised cats.