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dr Michal Kucewicz

Mayo Clinic, Rochester MN, USA

Mapping and stimulation of memory in the human brain – can engrams be electrically enhanced?

Direct electrical stimulation of the human brain can elicit sensory and motor perceptions as well as recall of memories. Stimulating higher order association areas of the lateral temporal cortex in particular has been reported to activate visual and auditory memory representations of past experiences. We hypothesized that this effect could be utilized to modulate memory processing. Recent attempts at memory enhancement in the human brain have focused on the hippocampus and other mesial temporal lobe structures, with a few reports of memory improvement in small studies of individual brain regions that have not been reproduced. We investigated the effect of stimulation in four brain regions thought to support declarative memory: hippocampus, parahippocampal neocortex, prefrontal cortex and temporal cortex. A classic verbal memory task was used to assess the effect of bipolar 50 Hz stimulation during encoding of word lists on subsequent free recall in human patients implanted with intracranial electrodes. We found enhanced recall of words from lists with electrical stimulation in the lateral temporal cortex, but not in the other three brain regions tested2. This selective enhancement was observed on the level of individual subjects, subjects stimulated in the temporal cortex, and across the four brain regions studied. In effect, more words were remembered with than without stimulation in the lateral temporal cortex. Stimulation targets in the other brain regions had a negative effect on memory compared to targets in the temporal cortex. These differential behavioral effects were paralleled by modulation of neural activities in the high gamma frequency band (60-120 Hz) during memory encoding3. These activities were proposed reflect coordinated firing of neuronal assemblies – the hypothetical neural substrate for engrams. We conclude that electrical stimulation in specific brain areas can modulate neural processes induced during encoding of word engrams and enhance their recall. Gamma frequency activities provide a useful biomarker to map memory engrams and guide human brain stimulation.