New studies from researchers at the University of California, Berkeley, support the theory that a person’s memories are networked throughout many regions of the brain, rather than stored in specific areas. Stroke or damage to one area alone thus may not result in permanent loss.
“It’s not just specific regions, but a whole network, that’s supporting memory,” said lead author Bradley Voytek, Ph.D., a UC Berkeley postdoctoral fellow in the Helen Wills Neuroscience Institute.
Scientists have known for years that if damage occurs to a brain region in charge of movement, language or sensation, then other parts of the brain can take over the job, often as well the original region.
The UC studies, using electroencephalograpy (EEG) and tests with stroke victims, show that these processes also come to the rescue for memory and attention as well. However, these substitute regions only activate memories when necessary; otherwise, they carry on their usual duties.
“The view has always been, if you lose point A, point B will be on all the time to take over,” said co-author Dr. Robert Knight, UC Berkeley professor of psychology and head of the Wills Institute. “[This new research] has shown that’s not true. It actually only comes on if it’s needed.”
“Most of the time, it acts like a normal piece of brain tissue,” continues Knight. “It only kicks into hyper-drive when the bad part of the brain is particularly challenged, and it does it in less than a second. This is a remarkably fluid neural plasticity, but it isn’t the standard ‘B took over for A,’ it’s really ‘B will take over if and when needed.’ ”
In one particular study, Voytek placed electrodes on the scalps of six stroke patients who had lost some function in the prefrontal cortex, a region of the brain that controls attention and memory. The same was done to six control subjects with normal prefrontal cortex function.
The participants were then shown a series of pictures to check the person’s visual working memory — the ability to remember images for a short time. Visual working memory is used when comparing two objects; it allows us to hold one object in memory while we view the other object. For example, choosing the freshest fruit at a grocery store would involve visual working memory.
“We presented each subject with a really quick flash of a visual stimulus and then showed them a second one a little while later, and they had to say whether it was the same as the first,” Voytek explained.
“The idea is that you’re building a representation of your visual world somehow in your brain — and we don’t know how that happens — so that later you can compare this internal phantom representation you’re holding in your mind to a real world visual stimulus, something you actually see. These patients can’t do that as well.”
In the study, when pictures were shown to the eye on the opposite side of the injury (left eye output goes to the right hemisphere, and vice versa), the damaged prefrontal cortex did not respond, but the intact prefrontal cortex on the same side as the image reacted within 300 to 600 milliseconds.
“EEG, which is very good for looking at the timing of activity in the brain, showed that part of the brain is compensating on a sub-second basis,” Voytek said.
“It is very rapid compensation: Within a second of challenging the bad side, the intact side of the brain is coming online to pick up the slack.”
“This has implications for what physicians measure to see if there’s effective recovery after stroke,” Knight said, “and suggests that you can take advantage of this to train the area you would like to take over from a damaged area instead of just globally training the brain.”
Voytek and Knight also tested visual working memory in participants who had damage to both the prefrontal cortex as well as the basal ganglia. A pair of regions associated with motor control and learning, the basal ganglia is often impaired in people with Parkinson’s disease.
Those with stroke damage to the prefrontal cortex had a hard time when the pictures were shown to the eye on the opposite side of the lesion. Patients with basal ganglia damage, however, struggled with visual working memory no matter which side the image was shown.
“[B]asal ganglia lesions cause a more broad network deficit, whereas the prefrontal cortex lesions cause a more within-hemisphere deficit in memory,” Voytek said. “This demonstrates, again, that memory is a network phenomenon rather than a specifically regional phenomenon.”
Knight hopes to see further studies that use direct recordings from electrodes in the brain so that they can explore even further the brain regions involved in visual memory, and other types of memory and attention controlled by the prefrontal cortex.
“Cognition and memory are the highest forms of human behavior,” Knight said. “It is not just about raising or lowering your hand, or whether you can or cannot see. These are the things that make us human, and that is what makes it so interesting for us.”
Source: University of California