Deep brain stimulation (DBS) of areas in the brain known to be involved in making memories does not improve memory performance, according to a new study.

Deep brain stimulation is an invasive technique that involves the implantation of electrodes deep into the brain and then running current through them to electrically stimulate nearby neurons.

In the recent study, no participants showed memory improvement. There was instead a range of memory impairment of 5 percent to 20 percent with stimulation.

The study negates earlier findings from 2012 that suggested a benefit but the silver lining is that if this specific form of stimulation diminishes memory performance, another approach may still improve it. The 2012 study reported in the New England Journal of Medicine (DOI: 10.1056/NEJMoa1107212) found a 64% improvement in memory performance for memories formed while subjects were experiencing brain stimulation. The findings were encouraging, but they ran counter to scientific evidence suggesting that the kind of stimulation used in the study would inhibit neurons and potentially impair memory.

Both studies presented subjects with similar memory tasks. A spatial task involved remembering the location of an object in a 3D virtual space. Participants would navigate through the space to find an object. During control sessions, they received no stimulation. In stimulation sessions, they would experience 50 pulses of electrical current per second for 5 seconds. A verbal task involved recalling words on a list. Electrical pulses were delivered for 4.6 seconds while participants studied the list.

The new study differed, says lead investigator Joshua Jacobs, assistant professor of biomedical engineering at Columbia University, in that they observed observed subjects as they performed 48 memory retrievals per session, 8 times as many as in the previous study. He also enrolled 49 subjects. The previous study included 7. "It's quite a bit bigger and more statistically powered, so it's more likely to produce accurate results," says Jacobs.

None of the participants showed statistically significant improvement. On average across both tasks, stimulation of the entorhinal cortex resulted in a reduced accuracy in memories of 9% compared to non-stimulated memories. Stimulation of the hippocampus resulted in 8% impairment on average. Impairment ranged from 5% to 20% across all regions stimulated for both tasks.

The study's measurement of spatial memory also differed from the earlier study. To measure spatial memory, participants revisited the 3D virtual space--this time, without the object being present in the space and without stimulation--and navigated to the place they recalled to be the object's location. Jacobs' team then calculated how far off the remembered locations were from actual locations.

In the previous study, the objects remained visible in the virtual space during the recall test, potentially skewing results in the positive direction for participants lucky enough to spot the object while navigating the space.

Such confounders often occur in early studies of new techniques, say Jacobs. Follow-on studies are important because they improve methods. "Theirs was a first study of its kind," says Jacobs. "It's important to improve the protocol in a way that quantifies spatial memory more precisely."