Direct electrical stimulation of the amygdala, a region of the brain associated with memory and emotional behavior, can enhance next-day recognition of images when applied immediately after the images are viewed the first time, according to a new study published in the journal Proceedings of the National Academy of Sciences (PNAS).
“We were able to tag specific memories to be better remembered later,” said co-first author Cory Inman, Ph.D., a postdoctoral fellow in the Department of Neurosurgery at Emory University.
“One day, this could be incorporated into a device aimed at helping patients with severe memory impairments, like those with traumatic brain injuries or mild cognitive impairment associated with various neurodegenerative diseases. However, right now, this is more of a scientific finding than a therapeutic one.”
Deep brain stimulation (DBS) involves an electrical current delivered continuously through an implanted device. The procedure is an established clinical method for the treatment of movement disorders such as Parkinson’s disease and is being tested for psychiatric conditions such as depression.
The current study involved 14 epilepsy patients at Emory University Hospital who were undergoing intracranial monitoring, an invasive procedure used for the diagnosis of seizure origin, during which electrodes are introduced into the brain.
The results show that 79 percent of the participants (11 out of 14) improved on overnight memory tests, while the remaining 21 percent showed no improvement or impairment. Study participants’ recognition only increased for the stimulated images and not for the control images shown in between the stimulated images.
When compared to no stimulation, the increase in the number of images accurately recognized ranged from around eight percent up to several hundred percent.
Some of the patients already had impaired memory as a result of their epilepsy; in general, these patients saw a greater effect of DBS. For example, one patient essentially forgot all of the control images but maintained good memory for the stimulated images. However, a substantial effect was also observed in people who had an average memory to start with.
“The average was like having a ‘B’-level memory performance move up to an ‘A’,” said senior author Jon T. Willie, M.D., Ph.D., assistant professor of neurosurgery and neurology at Emory University School of Medicine, who performed surgeries on patients in the study.
“We see this as a platform for the further study of memory enhancement. The time specificity enables a lot of other experiments since we know that there’s not a carry-over effect from one image to the next.”
The key role the amygdala plays in emotional responses and fear-associated learning has been well-studied. The scientists made sure that amygdala stimulation stayed at low levels of current (0.5 milliamps) and did not result in emotional responses, an elevated heart rate, or other signs of arousal. Patients reported that they did not notice the stimulation at any point in the study.
The researchers also avoided direct stimulation of the hippocampus, believing that would be too close to the machinery of memory itself, like introducing a live wire into a computer’s motherboard.
“We chose the amygdala because of decades of research in rodents, showing that it interacts with several other memory structures in a modulatory role,” said co-first author Joseph Manns, Ph.D., associate professor of psychology. “We wanted to stimulate its endogenous function, which we think is to signal salience — something standing out — so that specific experiences are remembered in the future.”
In previous research, Manns had already found that electrical amygdala stimulation increases rodents’ ability to recognize images later. The human experiments were designed to closely resemble how his lab’s tests with rodents were set up.
During the experiment, the patients were shown 160 neutral objects (emotional faces were excluded, for example) and were asked to tell whether the objects belonged indoors or outdoors.
For half of the images, participants received stimulation for one second after each image disappeared from the screen. They were tested on half the stimulated images and half the unstimulated images immediately, and the other half the next day. A total of 40 new images were used as decoys. The effect of stimulation on immediate recognition was not statistically strong. However, the next day, the effects on stimulated images were clear.
“This makes sense because the amygdala is thought to be important for memory consolidation — making sure important events stick over time,” Manns says.
Source: Emory Health Sciences