Electrical stimulation of the amygdala may boost declarative memory without influencing an emotional response, according to a study published in Proceedings of the National Academy of Sciences.
Cory S Inman, PhD, of the department of neurosurgery at Emory University in Atlanta, and associates sought to determine if the amygdala could improve declarative memory for specific images and neutral objects without stimulating any emotional response.
The study consisted of 14 volunteers (9 men) with drug-resistant epilepsy. Standard stereotactic depth electrode arrays were neurosurgically applied to the participants. No seizure activity was recorded during electrical impulse administration.
The volunteers underwent a series of 10 electrical stimulatory trials and 10 false trials (control), randomly. Following each test, individuals were asked to assess which type of trial was conducted. Electrical stimulation was administered to the basolateral complex of the amygdala in pulses for 1 second at a current of 0.5mA (pulse frequency, 50 Hz; train frequency, 8 Hz). During stimulation, patients were presented an arbitrary selection of images of nominal objects. Memory was assessed directly following the study and the following day.
The researchers found stimulation to have no significant effects on same-day memory testing. Next-day memory testing resulted in significantly enhanced memory in participants with electrical stimulation. Additionally, none of the patients reported experiencing any electrical stimulation, regardless of the presence of an actual pulse.
“To date no other invasive or noninvasive human brain-stimulation studies have shown temporally specific memory enhancement across periods longer than a few minutes after initial stimulation and encoding,” the investigators reported.
“In conclusion, our study constitutes direct evidence for a causal role of the human amygdala in the initiation of temporally specific memory enhancement and consequent functional changes in the hippocampus and perirhinal cortex without eliciting a subjective emotional response.… Such work will reveal the basic mechanisms of endogenous memory prioritization while also advancing translational interventions to treat human memory impairment.”
- Inman CS, Manns JR, Bijanki KR, et al. Amygdala-mediated memory enhancement in humans. PNAS. 2018 Jan. doi:10.1073/pnas.1714058114