A recent study shows that neurons generate rhythmic patterns of electrical activity, or oscillations, in the brain that are primarily driven by memory. Contrary to previous belief, the study shows that these theta oscillations in the hippocampus are more prevalent when people recall events than when they experience the event, highlighting memory as a key driver of theta activity and providing a potential avenue for treating brain injury and cognitive disorders.
These findings may lay the foundation for treating cognitive disorders and help improve memory.
Neurons produce rhythmic patterns of electrical activity in the brain. A pressing question in neuroscience is the primary driver of these rhythmic signals, known as oscillations. Researchers at the University of Arizona found that simply recalling the event can trigger these oscillations, even more than experiencing the event itself.
The study, published in the journal Neuron, focused specifically on theta oscillations. Such oscillations occur in the brain's hippocampus during activities such as exploration, navigation and sleep. The hippocampus plays a vital role in the brain's ability to remember the past.
Before this study, it was thought that the external environment played a more important role in driving theta oscillations, said Arne Ekstrom, the study's senior author and a professor of cognition and neural systems in the Department of Psychology at the University of Arizona. But Ekstrom and his collaborators found that memories created in the brain are the main driver of theta activity.
"Surprisingly, we found that theta oscillations in humans are more prevalent when a person is simply recalling events than when they are directly experiencing the event," said study lead author Sarah Seger, a graduate student in the Department of Neuroscience.
Ekstrom said the findings could have implications for treating patients with brain injuries and cognitive impairment, including those with seizures, stroke and Parkinson's disease. Memory could be used to generate stimulation from within the brain and drive theta oscillations, potentially improving memory over time, he said.
Researchers at the University of Arizona conducted the study in collaboration 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 being monitored at the center in preparation for epilepsy surgery. As part of the monitoring, researchers implanted electrodes in the patients' brains to detect occasional seizures. The researchers recorded theta oscillations in the brain's hippocampus.
Patients participated in a virtual reality experiment in which they navigated to stores in a virtual city using a joystick on a computer. When they reach the correct destination, the VR experiment pauses. The researchers asked participants to imagine where they were when they began their navigation and instructed them to mentally navigate the route they had just taken. The researchers then compared theta oscillations during initial navigation with participants' subsequent recall of the route.
During actual navigation using the joystick, the oscillations were lower in frequency and shorter in duration than when participants were simply imagining the route. Therefore, the researchers concluded that memory is a powerful driver of human theta oscillations.
One way to compensate for impaired cognitive function is through cognitive training and rehabilitation, Ekstrom said. "Basically, you can teach patients with memory impairment how to remember better," he said.
In the future, Ekstrom plans to conduct this study with patients who are walking freely rather than lying in bed to see how free navigation differs from memory in terms of brain oscillations.
"Being able to directly compare oscillations during original experience and later retrieval is a major step forward for the field in designing new experiments and understanding the neural basis of memory," Sager said.