Restoring the memories of people who suffer memory loss due to brain damage, aging itself or because they have a neurodegenerative disease such as Alzheimer’s could one day be a reality. This is what emerges from a study that has been published in Frontiers in Human Neuroscience .
And how would such a feat be achieved? According to the new study, through a “memory prosthesis” , an electrode that is implanted in the brain and that mimics what the hippocampus does naturally. This region of the brain plays a crucial role in memory in that it not only helps us create short-term memories, but also appears to direct them to other regions of the central nervous system for long-term storage.
The memory prosthesis appears to work in people with memory disorders, and is even more effective in people with poor memory. More advanced versions of prosthetics could make the leap and help Alzheimer’s patients or those suffering from memory loss due to brain damage.
How to imitate the process of memory?
For more than ten years, the team of researchers has been searching for ways to mimic the memory process. His idea is to use brain electrodes to understand the patterns of electrical activity that occur when memories are encoded , and then use those same electrodes to trigger similar patterns of activity. They have tested them on animals and some humans who had already been implanted to treat epilepsy.
The scientists tested two versions of the memory prosthesis on 24 epileptic patients who had already had electrodes implanted in their brains. Some of these people also had brain injuries.
The first version, called the memory decoding model, or MDM , mimics the patterns of electrical activity that occur naturally in the hippocampus when each volunteer successfully forms memories. The MDM model takes an average of these patterns in each individual and then triggers this pattern of electrical stimulation.
The second version is called multiple-input multiple-output, or MIMO , and it better mimics how the hippocampus works. In a healthy hippocampus, electrical activity flows from one layer to another before spreading to other regions of the brain. The MIMO model is based on learning the patterns of electrical inputs and outputs that correspond to memory encoding, and then mimics them.
To test the two models, the researchers put the participants through memory tests . In one set of tests, each participant was shown an image on a computer screen. After a short period of time, the same image was shown to him again along with others. The person had to say which was the image that they had already seen before. Each volunteer completed between 100 and 150 of these short tasks, designed to test short-term memory. Between 15 and 90 minutes later, they were subjected to a different test that consisted of choosing the image that was most familiar to them from a set of three. This time, it was about measuring long-term memory.
The volunteers underwent both rounds of memory tests twice: once to record from the hippocampus and once to stimulate recorded patterns associated with successfully stored memories. What the researchers saw was that the recordings were unique, different from one person to another.
They also found that the memory prosthesis improved participants’ performance on the tests. They scored better if they had received the correct stimulation pattern when the images were first presented to them. This suggests that the memory prosthesis may help encode memories in the brain, the researchers say, pointing to improvements ranging from 11% to 54% .
As for the two versions of memory prostheses, the MIMO model, which more faithfully mimics the functioning of the hippocampus, was the one that obtained the best results, especially in those participants who had poorer memory at the beginning of the experiment.
All participants had their electrodes removed after about two weeks, when their epilepsy doctors deemed it appropriate. The researchers hope that the improvements in their memories will be long-lasting. In theory, the stimulation each person received could have strengthened the wiring of neurons in the hippocampus.
The future of memory prosthetics
Those responsible for the study hope that, in the future, their memory prosthesis will help people suffering from memory disorders to recover their memories. The first to receive this help would be those with brain injuries . Those that affect the hippocampus would be the easiest to treat, in contrast to, say, Alzheimer’s, which often involves damage to many brain regions.
The electrodes used in the study are only capable of recording between 40 and 100 neurons. According to the researchers, any prosthesis to be used to treat memory disorders will require brain electrodes with hundreds of contact points, allowing hundreds or thousands of neurons to be recorded and stimulated. Its use would also have to be perfected : if it would be convenient to have the device active all the time, if it should be used in conjunction with other technology that would alert the exact moment in which it would have to be activated, if it would also work at night… To the prosthesis memory still has some way to go for clinical use, but the researchers believe that, in principle, it could work. Time will tell.
Referencia: Roeder, B. et. al. 2022. Patterned Hippocampal Stimulation Facilitates Memory in Patients With a History of Head Impact and/or Brain Injury. Frontiers in Human Neuroscience. DOI: https://doi.org/10.3389/fnhum.2022.933401