By assessing how well-rested and sleep-starved people retain information, psychologists and neuroscientists are making headway in understanding how sleep bolsters memory. What happens inside the sleeping brain to enable longer-lasting, sharper recollection of the facts and faces we absorb during waking hours? A lot, it seems.
Most neuroscience research on sleep and memory has focused on brain activity during different phases of sleep, as it enhances memory consolidation. But a new study from neuroscientists at Scripps Research Institute in Florida, published in Cell, looks at the flipside of remembering: not forgetting.
In other words, scientifically speaking, remembering is not the same thing as not forgetting.
Let’s say you type in the wrong pin number at the ATM. Then, in your head, you spend a few seconds, maybe longer, cycling through your other possible four-digit codes. When the right pin pops into your head, is that a feat of remembering? Or did you simply no longer forget? Does it even matter?
Yes, it does matter, at least when it comes to the brain mechanisms. In fact, it may be more accurate to think of forgetting not as a cognitive failure, but as a “biologically regulated function of the brain allowing optimal adaptability to an ever-changing environment,” according to the study.
The notion that sleep facilitates memory by suppressing forgetting (rather than bolstering consolidation) first showed up in early 20th-century psych studies, explained Scripps researchers in their paper. Sleep stops the brain from responding to environmental stimuli — as it normally does during wakefulness — thus enabling the consolidation of older, already-formed memories without interference.
The Scripps team is now grounding the psychological theory in neuroscience by artificially inducing arousal and sleep in fruit flies, and seeing how their neurons dance.
The cognitive process of forgetting, their research suggests, hinges on activating neurons that release dopamine, the stimulating neurotransmitter behind cocaine binges and manic episodes. In addition to inducing euphoric focus, dopamine is involved in plasticity, a blanket term to describe neural changes directly sparked by learning new information. Forgetting falls under the plasticity umbrella.
Dopamine revs up immediately after the formation of new memories (which means both learning new facts and processing smells and other environmental stimuli). It’s during this state of arousal — when dopamine neurons are highly active — that forgetting happens. (When it comes to fruit flies, “forgetting” means not responding to a smell.) Conversely, dopamine simmers down during periods of prolonged quietude, aka, sleep.
In a release from the Scripps Institute, lead study author Ron Davis said:
“Our findings add compelling evidence to support the model that sleep reduces the forgetting signal in the brain, thereby keeping memories intact,” Davis said. “As sleep progresses to deeper levels, dopamine neurons become less reactive to stimuli and this leads to more stable memories."
The findings don’t undermine the importance of sleep for memory consolidation. Instead, the ongoing research suggests that a previously overlooked “not-forgetting” brain mechanism works in a parallel, or perhaps even dependent, fashion.
Solid sleep habits may not turn everyone into record-holding memory champions. But the next time you blank on your ATM password, stop for a minute and think — have you forgotten the code? Or just not remembered? And, are you giving your brain enough downtime to block out interfering noise and consolidate the millions of tiny data points that populate our lives?