Brain Electrical Microstimulation Enhances Memory Recall

Individuals with epilepsy who received electrical microstimulation pulses showed a significant improvement in their ability to recognize specific faces and ignore similar ones, new UCLA research reports.

Eight of nine patients’ ability to recognize the faces of specific people improved after receiving low-current electrical pulses to the right side of the brain’s entorhinal area, which is critical to learning and memory. However, electrical stimulation delivered to the left side of the region, tested on four other people, resulted in no improvement in the patient’s recall.

The study builds on 2012 UCLA research demonstrating that human memory can be strengthened by electrically stimulating the brain’s entorhinal cortex.

Ultrafine Wire Implants

Researchers followed 13 people with epilepsy who had ultrafine wires implanted in their brains to pinpoint the origin of their seizures. The team monitored the wires to record neuron activity as memories were formed, then sent a specific pattern of quick pulses back into the entorhinal area.

Using the ultrafine wires allowed researchers to precisely target the stimulation but use a voltage as low as one-tenth to one-fifth as strong as had been used in previous studies.

The person-recognition task

(A) The person-recognition task consisted of three phases.
In the encoding phase, participants were shown portraits of people for 4 s each (black bars in timeline), preceded by a 4.2–5.2 s fixation dot (gray bars). Half of the pictures were randomly selected for stimulation. For these, one second of theta-burst stimulation (red bar) was applied beginning 2.2–2.7 s before picture onset, during viewing of the fixation dot.
To ensure that participants were viewing the portraits, they were asked to report whether the person in each image was male or female.
The encoding phase was followed by a 30 s distractor task in which single digits were presented once per second and the participant was asked to identify each digit as odd or even.
Finally, during the test phase, participants were shown a mixture of images they had seen during the encoding phase (targets) and portraits of people who looked similar but had not been previously viewed (lures). Participants were asked to report whether each image was ‘old’ or ‘new.’
(B) Theta burst stimulation consisted of 5 sets of current pulses, separated by 200 ms, in which each set included four biphasic stimulation pulses presented at 100 Hz.
(C) Representative pairings of target and lure images. All images used in the task were adapted with permission from the book Exactitudes (Versluis and Uyttenbroek, 2002).
© 2017 eLife Sciences Publications Ltd. Via Creative Commons

The study suggests that even low currents of electricity can affect the brain circuits that control memory and human learning. It also illustrates the importance of precisely targeting the stimulation to the right entorhinal region. Other studies that applied stimulation over a wide swath of brain tissue have produce conflicting results.

Electrical stimulation could offer promise for treating memory disorders like Alzheimer’s disease.

The study was funded by the National Institute of Neurological Disorders and Stroke, the A.P. Giannini Foundation, the Defense Advanced Research Projects Agency, the G. Harold and Leila Y. Mathers Charitable Foundation and the Swiss National Science Foundation.

Ali S Titiz, Michael R H Hill, Emily A Mankin, Zahra M Aghajan, Dawn Eliashiv , Natalia Tchemodano, Uri Maoz, John Stern, Michelle E Tran, Peter Schuette, Eric Behnke, Nanthia A Suthana, Itzhak Fried
Theta-burst microstimulation in the human entorhinal area improves memory specificity
eLife 2017;6:e29515

Top Image: The red crosshair denotes the location of the microstimulation electrode in the right angular bundle. © 2017 eLife Sciences Publications Ltd. Republished via Creative Commons.