If I were to win the lottery today, I would certainly remember today much, much better than yesterday. I may even remember what I ate for breakfast today, or how my exam went. I would certainly not remember those things about yesterday.
Biological Imperatives
Hippocampus: the structure in the brain responsible for storing episodic memory, or memory that happen. Specifically, events that happen to you.
When awake, the hippocampus builds a cognitive map of the day. It does this by strengthening certain connections between Neurons.
During sleep, it replays and solidifies those connections. The big question is how it accomplishes this.
Hippocampus while sleeping
The body produces a sharp wave of noise and inputs to the hippocampus during sleep. This activates neurons in the cognitive map that the hippocampus made during the day. We don’t want to put too much signal into those neurons, obviously, so there are preventative measures by the brain in place. There are called inhibiting inter-neurons.
This creates a feedback loop. Excitation builds up, and then inhibition kicks in. The ripples of activity are known as Sharp Wave Ripples.
When the excitation sweeps the brain, many neurons want to fire. Yet, because of the inhibitory neurons, all neurons, now, cannot fire at the same time. So, they compete. Specifically, groups of interconnected neurons that represent events of the day compete.
Neocortex while sleeping
During the day, the neocortex is responsible for many other functions. But, during sleep, it becomes receptive to signals from the hippocampus. This means that the hippocampus can give it signals to strengthen the connections between the neurons. This consolidates memories.
Rat Experiment regarding Memory During Sleep
Setup:
- Rats run a maze, and then sleep
- Researchers analyze their neuron activity patterns.
- Even though there are so many neurons, they can group neurons as events to make it simple to analyze. They found that as the rat slept more, it got better and better at solving the maze, and the activity pattern that lit up (in the neocortex) actually changed over time, meaning that the sleep influenced the neocortex.
Sharp-Wave Ripples while Awake
Researchers also noticed that Sharp-Wave Ripples happen when awake at lower rates. They thought this didn’t make sense, as the neocortex is not receptive when one is awake. This may be because those ripples can “tag” certain events, strengthening the connections between them to later consolidate them during sleep.
Key takeaway
The brain really needs to be sleeping for memories to consolidate and solidify. Moreover, events are governed by how interconnected the neurons that define those events are - the more interconnection and more neurons, the more likely a particular event is to win the competition during the sharp-wave ripples during sleep and get consolidated in the neocortex. Overall, this means that sleep is really fucking important for memory consolidation.