The mammalian hippocampus is proposed to constitute part of the neural machinery necessary for the generation of a brain process which is functionally equivalent to Pavlovian internal inhibition. Relevant neuroanatomical and electrophysiological data are presented in support of this proposal. The major behavioral effects of hippocampal lesions in situations in which internal inhibition is presumed to occur are discussed. These situations include habituation to novelty, discrimination reversal, overtraining, extinction, and spontaneous alternation. 285 1968 by the American Psychological Association, Inc.

The behavior of 10 rats with bilateral hippocampal lesions was compared with that of 11 cortically damaged and 12 unoperated control Ss in 4 experimental situations. Compared to other groups, hippocampal Ss (a) entered more squares of an open field (p < .01); (b) took more trials to reach criterion on a successive brightness discrimination (p < .01), but showed no deficit on a simultaneous discrimination using the same stimuli; and (c) made more errors on both of 2 Hebb-Williams mazes (p < .01). Both hippocampal and cortically damaged Ss showed less "passive avoidance" following shock (p < .05). Conclusions are: (a) neither "short-term memory" deficit nor "emotional disruption" hypothesis adequately explains these data, and (b) monolithic hypotheses concerning behavioral significance of the hippocampus are likely to be oversimplifications.

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