Brain Stimulation Using Responsive Neurostimulation Improves Verbal Memory: A Crossover Case-Control Study

The effects of brain stimulation on memory formation in humans have shown conflicting results in previous studies. We hypothesized that direct cortical stimulation using an implanted responsive neurostimulation (RNS) system will improve memory.

To evaluate whether direct cortical stimulation using RNS improves memory as measured with recall scores of a list-learning task.

During outpatient visits, a list-learning task (Hopkins Verbal Learning Test-Revised) was administered to 17 patients with RNS implants. Patients were read a list of 12 semantically related words and asked to recall the list after 3 different learning trials. True or sham stimulations were performed for every third word presented for immediate recall. Most patients had frontotemporal network stimulation-one patient each had insular and parietal stimulations. After a 20-min delay, they were asked to recall the list again, first freely and then through a "yes/no" recognition paradigm. A crossover design was used in which half the patients had true stimulation during the initial visit and half had sham stimulation-followed by crossover to the other group at the next visit.

The Hopkins Verbal Learning Test-Revised delayed recall raw score was higher for the stimulation condition compared with the nonstimulation condition (paired t -test, P = .04, effect size d = 0.627).

Verbal memory improves by direct cortical stimulation during a list-learning task. The RNS system can be effectively used in memory research using direct cortical stimulation. This study has implications in the development of neurostimulation devices for cognitive enhancement in conditions such as epilepsy, dementia, and traumatic brain injury.