Sleep can often feel like a fall into annihilation and rebirth. One moment you have all the memories and aches of a long day behind you, the next you wake up from nothingness into the start of something new. Or: a rush from some fictional slumber world into an entirely different waking world. What is it that’s happening inside your head? Why this rest?
Generally the answer that we are given is some sort of spring cleaning and consolidation, a removal of cruft and a return to what is important (baseline, learned). There is certainly plenty of evidence that the brain is doing this while you rest. One of the most powerful of these ‘resetting’ mechanisms is homeostatic plasticity. Homeostatic plasticity often feels like an overlooked form of learning, despite the gorgeous work being done on it (Gina Turrigiano and Eve Marder’s papers have been some of my all-time favorite work for forever).
One simple experiment that you can do to understand homeostatic plasticity is to take a slice of a brain and dump TTX on it to block sodium channels and thus spiking. When you remove it days later, the neurons will be spiking like crazy. Slowly, they will return to their former firing rate. It seems like every neuron knows what its average spiking should be, and tries to reach it.
But when does it happen? I would naively think that it should happen while you are asleep, while your brain can sort out what happened during the day, reorganize, and get back where it wants to be. Let’s test that idea.
Take a rat and at a certain age, blind one eye. Then just watch how visual neurons change their overall firing rate. Like so:
At first the firing rate goes down. There is no input! Why should they be doing anything? Then, slowly but surely the neuron goes back to doing what it did before it was blinded. Same ol’, same ol’. Let’s look at what it’s doing when the firing rate is returning to its former life:
This is something of a WTF moment. Nothing during sleep, nothing at all? Only when it is awake and – mostly – behaving? What is going on here?
Here’s my (very, very) speculative possibility: something like efference copy. When an animal is asleep, it’s getting nothing new. It doesn’t know that anything is ‘wrong’. Homeostatic plasticity may be ‘returning to baseline’, but it may also be ‘responding to signals the same way on average’. And when it is asleep, what signals are there? But when it is moving – ah, that is when it gets new signals.
When the brain generates a motor signal, telling the body to move, it also sends signals back to the sensory areas of the brain to let it know what is going on. Makes it much easier to keep things stable when you already know that the world is going to move in a certain way. Perhaps – perhaps – when it is moving, it is getting the largest error signals from the brain, the largest listen to me signals, and that is exactly when the homeostatic plasticity should happen: when it knows what it has something to return to baseline in respect to.
Hengen, K., Torrado Pacheco, A., McGregor, J., Van Hooser, S., & Turrigiano, G. (2016). Neuronal Firing Rate Homeostasis Is Inhibited by Sleep and Promoted by Wake Cell, 165 (1), 180-191 DOI: 10.1016/j.cell.2016.01.046