By Yue Liu
A 2004 film, 50 First Dates, depicted a romantic story about how the hero won the love every day from the heroine, who suffered from a fictional form of anterograde amnesia and lost her previous day’s memories after every single night. In reality, a unique form of human amnesia, sharing great similarities with that afflicting the heroine in 50 First Dates, was reported in 2010: after a car accident, the patient FL could recall events that had happened before the accident and remember things from the same day after the crash, but she could not register memories in her brain from the previous day after a night’s sleep. What had happened to those memories during her sleep?
The role of sleep in memory has been stated in a “two-stage” model: During the day, we temporarily store a remarkable amount of information in the hippocampus, a brain area named for the structural resemblance to the seahorse. While we sleep, the hippocampus gradually gets disengaged, and memories are handed over to the neocortex for long-term storage. In brief, we consolidate our reminiscences during sleep by transferring them from the hippocampus to the neocortex. If the transferring process during sleep is disrupted, as may be the case of the patient FL, temporary memories will be lost, whereas permanent memories that are already stored in the neocortex will remain intact.
In this month’s issue of Nature Neuroscience, Michaël Zugaro’s lab in France provided the first direct evidence for this two-stage model of memory. They observed a fine temporal coupling of oscillating activities between the hippocampus and neocortex in animals during deep sleep. When the animals’ learning periods (20 minutes) were long enough to trigger memory consolidation, the oscillatory coupling between the hippocampus and neocortex during sleep became stronger. However, when the learning periods (3 minutes) were too short, the strength of the hippocampo-cortical coupling did not increase; thus, the memories could not be consolidated. Interestingly, in the latter animals, boosting the hippocampo-cortical dialogue during sleep promoted memory consolidation, which otherwise would not have happened due to the short learning period.
This study offered the first causal link between the hippocampo-cortical dialogue during sleep and memory consolidation. It may also invigorate a fantasy: Can we learn much more quickly (in 3 rather than 20 minutes)? Can we study less during the day and receive a special electrical therapy during the night that can selectively enhance the hippocampo-cortical oscillatory coupling? Someday, an electrical device may be hooked up to a human brain to monitor and record electrical activities associated with various experiences. We may program the device to tighten the hippocampo-cortical coupling during the night for a specific experience, to strengthen that particular memory.
How about erasing a particular memory during sleep? In another 2004 film, Eternal Sunshine of the Spotless Mind, finding their relationship did not work out, a couple turned to a special procedure, which wiped out their memories about each other during sleep while their romantic episodes replayed. The basis of this fantasy procedure may be the vulnerability of memories while they are replayed during sleep. Human imagination may propel scientists to develop a strategy that can make erasing memories possible in reality. Someday, we may ease some insufferable emotional pain, such as that resultant from posttraumatic stress disorder (PTSD), by disrupting the replay of fear, stress, or anxiety associated memories.
When science and technology can make it possible to easily save and delete our memories, we may escape from laborious learning and unpleasant memories just by clicking “save” or “delete” on electrical devices connected to our brains. But remember, our memories sculpt who we are. After this technological intervention, will you still be you?