Gamma and theta-burst stimulation of the hippocampus and entorhinal area transiently disrupts memory

Fifty-hertz and theta-burst stimulation of the hippocampus and entorhinal area in two male macaque monkeys transiently disrupt associative memory, with 50-Hz entorhinal impairment and 27/21 percentage-point drops.

• Why it matters: Prior patient studies show inconsistent memory effects, leaving uncertainty about whether stimulation parameters or within-person variability drive enhancement versus disruption.
• What they did: Two male macaque monkeys with hippocampal/entorhinal depth electrodes and prefrontal grid performed an associative memory task while receiving 50-Hz or theta-burst stimulation during or before encoding, repeatedly across at least five sessions.
• The result: Stimulation during encoding significantly impaired subsequent retrieval (Monkey E: 50-Hz hippocampal, 50-Hz entorhinal, and theta-burst entorhinal, P < 0.001; Monkey O: theta-burst entorhinal, P < 0.05), did not rescue scopolamine impairment, and during continuous stimulation 50-Hz hippocampal reduced performance by 27 and 21 percentage points (P < 0.001) with no post-stimulation effect.