Single pulse electrical stimulation of the hippocampus is sufficient to impair human episodic memory

We have used the single pulse electrical stimulation (SPES) technique to investigate whether more localized stimulation of the hippocampus can affect human episodic memory. A recognition memory test including words, object drawings, abstract drawings and unfamiliar faces was performed without stimulation (baseline) or synchronized with single 1 ms electrical pulses applied to the left, right or both hippocampi in 12 epileptic patients investigated with bilateral depth electrodes. No differences were found in memory performance between baseline and unilateral stimulation, either in the total score or in material-specific scores. In contrast, bilateral stimulation was associated with a pronounced decrease in the median of total memory scores (57%), and of material-specific sub-scores for words (38%), geometrical drawings (81%) and faces (100%). Additional study of stimulation at presentation of stimuli (encoding) versus the recognition memory (retrieval) test phase, showed reduction in memory only at encoding. The results provide causal evidence that the hippocampi are necessary for supporting episodic memory. The induction of memory deficits by bilateral stimulation with parameters that do not induce effects when applied unilaterally suggests that recognition memory can be processed independently by the hippocampus on either hemisphere.