Retention impairment produced by unilateral amygdala implantation: Reduction by posttrial amygdala stimulation

This experiment examined the amygdala sites at which posttrial electrical stimulation was effective in enhancing later retention performance. Rats were trained in a one-trial inhibitory (passive) avoidance task. Following training, animals received no stimulation or were stimulated 30 sec or 4 hr after training. Implanted-unstimulated rats, as well as those stimulated 4 hr after training, had retention performance that was impaired relative to that of unimplanted rats; the impairment was observed primarily in those animals whose electrodes damaged the lateral two-thirds of the amygdala. Animals stimulated 30 sec after training had retention performance significantly better than that of the implanted-nonstimulated animals. Examination of electrode tip placements indicated that stimulation of the amygdala enhanced retention if administered to most areas other than the basomedial nucleus. This nucleus is an area in which stimulation is an effective region from which to produce amnesia under other behavioral conditions. Thus, memory enhancement and impairment produced by electrical stimulation of the amygdala may be mediated by different anatomical systems.