Neurons produce rhythmic patterns of electrical activity in the brain. One of the unresolved questions in neuroscience is what primarily drives these rhythmic signals, called oscillations. Researchers from the University of Arizona have found that simply remembering events can trigger them, even more so than when people experience the actual event.
The researchers, whose results are published in the journal Neuron, specifically focused on what are known as theta oscillations, which occur in the hippocampus region of the brain during activities such as exploration, navigation and sleep. The hippocampus plays a crucial role in the brain’s ability to remember the past.
Prior to this study, the external environment was thought to play a more important role in driving theta oscillations, said Arne Ekstrom, professor of cognition and neural systems in the UArizona Department of Psychology and senior author of the study. But Ekstrom and his collaborators found that memory generated in the brain is the main driver of theta activity.
“Surprisingly, we found that in humans, theta oscillations are more prevalent when someone is just remembering things compared to directly experiencing events,” said lead study author Sarah Seger, a graduate student in the Department of Neuroscience.
The results of the study could have implications for treating patients with brain damage and cognitive impairments, including patients who have experienced seizures, strokes and Parkinson’s disease, Ekstrom said. Memory could be used to create stimulations from within the brain and drive theta oscillations, potentially leading to improvements in memory over time, he said.
UArizona researchers collaborated on the study with researchers from the University of Texas Southwestern Medical Center in Dallas, including neurosurgeon Dr. Brad Lega and research technician Jennifer Kriegel. The researchers recruited 13 patients who were monitored at the center in preparation for epilepsy surgery. As part of the monitoring, electrodes were implanted into the patients’ brains to detect occasional seizures. The researchers recorded the theta oscillations in the brain’s hippocampus.
Patients participated in a virtual reality experiment in which they were given a joystick to navigate to shops in a virtual city on a computer. When they arrived at the correct destination, the virtual reality experiment was paused. The researchers asked the participants to imagine the place where they started their navigation and instructed them to mentally navigate the route they just passed through. The researchers then compared theta oscillations during the initial navigation with the participants’ subsequent recollection of the route.
During the actual navigation process using the joystick, the oscillations were less frequent and shorter in duration compared to oscillations that occurred when the participants were just imagining the route. So the researchers conclude that memory is a strong driver of theta oscillations in humans.
One way to compensate for decreased cognitive function is by using cognitive training and rehabilitation, Ekstrom said.
“Basically, you take a patient who has memory impairments and you try to teach them to be better at remembering,” he said.
In the future, Ekstrom plans to conduct this research in ambulatory patients as opposed to bedridden patients and find out how free navigation compares to memory in terms of brain oscillations.
“Being able to directly compare the oscillations that were present during the original experience and during a later retrieval of it is a major step forward in the field in terms of designing new experiments and understanding the neural basis of memory ,” Seger said.