I’ve often wondered if any good could possibly come from a city full of the constant hustle of urban life. Cars always seem to be coming and going, zipping by on the streets below my window. Then the ambulance speeds past, its siren wailing, as it seeks the nearby hospital. Suddenly I am thrust into memory from last week.
Cars honk to one another as if speaking their own language. Smaller and more agile mopeds cut between them acting like they own the road. Firemen have positioned themselves along the sidewalk and are passing out fliers to anyone who will listen. The wail of a siren stuck in traffic was the centerpiece of a small Parisian intersection near the Bastille. My friends and I paused for a moment, mesmerized by the sounds, lights, and the notion that an ambulance with siren wailing could possibly be halted on its life-saving journey. Our stomachs growl in contempt of our delay so we continue shuffling along the sidewalk seeking nourishment after the morning’s academics, the smell of the boulangeries wafting invitingly towards us.
The cool breeze from the window brings me back to present. I now wonder how it is that I could remember that instant so clearly, yet there is nothing to say of its significance. As far as I could tell, there was no reason for this memory to be so strong.
The answer lies in the recent work of James Cousins and his colleagues (2014) regarding cued memory reactivation during slow-wave sleep. In his experiment, Cousins subjected his participants to a specific cognitive task and simultaneously played a series of tones. The researchers then put the participants to sleep while monitoring their brain activity. During slow-wave sleep, some of the participants were played the series of tones from the test, while others listened to brown noise (notably different than the “brown note”). Participants were woken up in the morning, allowed to gather their senses, and then retested on the cognitive task.
Cousins and his colleagues discovered that while the control participants who listened to brown noise all night slightly improved after having learned the task, the participants who were played the tone series improved significantly more. The researchers concluded that, during slow-wave sleep, auditory stimulation enhances the consolidation of related memories by the hippocampus.
Now lets get back to my ambulance example. After experiencing the piercing cry of the ambulance stuck in traffic on that small back road, my brain had begun creating a memory of this experience. That night as I drifted into slow-wave sleep, the sirens from the ambulances on the street below wailed past, causing my hippocampus to replay that particular memory. Over the course of the night, unbeknownst to me, this seemingly irrelevant memory became a recurrent experience.
I can no longer remember what I did end up eating for lunch that day, nor what we discussed in class. But thanks to my hippocampus and the sleepless city, I will long remember that ambulance stuck in traffic on a sunny morning in downtown Paris.
Cousins, J.N., El-Deredy, W., Parkes, L.M., Hennies, N. & Lewis, P.A. (2014) Cued Memory Reactivation during Slow-Wave Sleep Promotes Explicit Knowledge of a Motor Sequence. The Journal of Neuroscience, 34, 15870-15876.