Amnesia remains a controversial subject in the field of neuroscience, with some researchers believing that it occurs when cells are damaged and memory cannot be stored, while others argue that the memories are simply blocked and cannot be recalled. A new study has indicated that memories do in fact remain, but are simply unable to be recollected. Researchers at Massachusetts Institute of Technology and the Riken Brain Science Institute in Japan have gained new understanding on the workings of amnesia through research that used blue light to revive lost memories in mice.
Researcher Susumu Tonegawa of the Massachusetts Institute of Technology said, "The majority of researchers have favored the storage theory, but we have shown in this paper that this majority theory is probably wrong. Amnesia is a problem of retrieval impairment."
Memories are triggered when 'memory engrams', the neurons that are activated as memories are formed, are activated in normal day-to-day life by stimuli such as an image, smell or taste. During the study, researchers used blue light pulses to stimulate 'memory engrams'. They attached a protein to these neurons to enable them to be activated by light.
One change that was thought to occur in memory engrams during the formation of memory was the strengthening of their synapses, structures that allow the neurons to send signals to each other. Therefore, the researchers set out to see what would happen if the synapses did not strengthen, by using a compound called anisomycin to prevent that process from happening in mice. The study mice were placed in a chamber where they were given an electric shock to the feet, which elicits a 'freezing' response in the rodents.
It was observed that mice that did not receive the compound would exhibit the same freezing response when returning to the chamber where the shock was administered, but those that were given anisomycin did not freeze, having clearly forgotten the shock. The researchers then activated the neurons involved in the foot-shock memory in the treated mice, by using blue light pulses. Even when placed in a different chamber, the treated mice would demonstrate the freezing response, indicating that they were paralyzed with fear from the memory, which still existed.
MIT researcher Tomas Ryan, who co-authored the study, said, "The study allowed scientists to separate memory storage mechanisms from those allowing an organism to form and recover the memory. The strengthening of engram synapses is crucial for the brain's ability to access or retrieve those specific memories. The research indicated that past memories may not be erased, but could simply be lost and inaccessible for recall. The findings will stimulate future research on the biology of memory and its clinical restoration."
The study has been published in the Science