According to new research at Massachusetts Institute of Technology (MIT), memories that have “vanished” through amnesia might now possibly be restored. The study, which was published in the journal Science, challenged the current understanding of how memory functions and revealed that past memories might simply be ‘out of reach’ rather than ‘erased’. The scientists even managed to successfully reactivate past memories in amnesia-stricken mice using the latest light-based brain tracking techniques.
Retrograde amnesia is the inability to recall previously established memories, often the result of head injury or neurological conditions such as Alzheimer’s. However, the question of “are these memories permanently erased or simply out of reach?” has remained open. Now, a finding by researchers from the RIKEN-MIT Center for Neural Circuit Genetics appears to provide an answer by demonstrating that memories may be retrieved from the amnestic brain. Furthermore, the team was able to show that the cellular pathways underlying these memories may be reactivated to rediscover past memories.
Initially, the team of scientists – led by Susumu Tonegawa, Director of the RIKEN Brain Science Institute in Saitama, Japan – were attempting to understand how stable memories may be formed in the human brain and how the storage of these memories might be affected by inducing amnesia. “Brain researchers have been divided for decades on whether amnesia is caused by an impairment in the storage of a memory, or in its recall,” explained Tonegawa.
In making the mice amnestic, they were taught to associate a small electrical stimulus to the foot with a particular environment to create a ‘freezing’ response. So, whenever the mice entered that environment, they learned to freeze, eventually without the electrical stimulus. The memory neurons activated during this procedure were genetically labelled with a blue light-sensitive protein, channelrhodopsin, to enable their visualisation and enable reactivation. A sample group of mice were then administered with anisomycin – a compound that inhibits the synapses involved in memory encoding. A control group was given a saline solution. Just as the team hypothesised, these mice appeared to ‘forget’ to freeze when they returned to the environment suggesting that these mice were unable to recall that particular memory association.
The next question was about whether the stored memory from the mild electrical stimulus was absent from the amnestic mice or still there – just inaccessible. In order to investigate, the team used an innovative technology called optogenetics, which may selectively activate the neurons that were labelled earlier. The mice were then placed in a neutral environment where scientists stimulated the cells involved in the foot shock memory by shining a blue light. Unexpectedly, both the control and the amnestic mice showed a freezing response, showing that the amnestic mice still retained the memory, however were just unable to recall it on their own.
To explain the phenomenon, the team suggests that there appears to be one neurological process for memory encoding and a second for recall.
“Our conclusion,” says Tonegawa, “is that in retrograde amnesia, and past memories may simply be inaccessible for recall. These findings provide striking insight into the fleeting nature of memories, and [may] stimulate future research on the biology of memory and its clinical restoration.” This study led to a monumental, significant discovery in neurology – memories are hardly gone forever, they are mostly just placed out of reach. While the research suggests past memories might hide somewhere in the brain, the implications for human amnesia patients may be further away. Co-author and MIT neuroscientist Tomas Ryan explained, “It’s very [challenging] to be doing this in humans, partly for the ethical reasons — the work is invasive — [and] also because we tag the memories in the brain before they’re learned.” This means researchers might need to be present while a memory was formed and then forgotten in order to help restore it.
How might treating amnesia be used to improve the brain’s ability to absorb information in otherwise healthy individuals?