Dr Matt Jones, The University of Bristol
How we form memories is a complicated system as many different things can disturb the process, from drug use to emotional state. But there’s one thing that we now know is critically important for memory, and that is sleep. Connie Orbach went to meet Dr Matt Jones in Bristol to find out more...
Connie - It sounds quite other worldly doesn’t it? Well actually, this is the sound of our brain’s orchestra. It’s the patterns of activity of different parts of our brains as we sleep, whirring away, organising, filing, sorting our memories and it was recorded by Dr Matt Jones’ team at the University of Bristol…
Matt - Well, it’s funny stuff sleep because we tend to equate it, of course, with rest but, in fact, parts of the brain are more active during sleep than they are during wakefulness. In particular, a part of the brain called the hippocampus, which is a central hub for integrating learning and memory, becomes very active during certain phases of sleep. As you probably know, human sleep is subdivided into non-REM (non-rapid eye movement) sleep, sometimes called slow wave sleep and REM (rapid eye movement) sleep, and these different phases are associated with different types (patterns) of hippocampal activity. Now the hippocampus is activated during wakefulness, so neurons in the hippocampus, for example, code information about new places that we visit and is then reactivated during subsequent sleep when those neurons that encoded new memories come to life again. So you replay your waking experience whilst you’re offline during sleep and it’s this process which is thought to support the strengthening of memories, the sourcing of memories: kind of sorting the wheat from the chaff - which things we need to remember, which things are less important and can, therefore, afford to forget.
Connie - Wow. So parts of the hippocampus are being activated specifically by location as we’re moving around and then, as we sleep, we’re literally replaying that in exactly the same order of brain cell activation, as it was in the day?
Matt - That’s right. We’re replaying those patterns of activity that encoded new memories during the day but we’re doing it at a different time scale. So, in fact, the patterns of activity that are replayed during sleep are replayed on a compressed time scale (about 10 times faster than they were during wakefulness), and that short time scale activity is thought to be important for driving changes in the strength of connections between brain cells that cooperatively encode these memories. Now we know it’s important for memory function because, if you disrupt that replay activity, then you slow formation of memories.
Connie - So is that all that’s happening at this level of hippocampus play cells or is there more than that?
Matt - So, no neuron and not brain structure is an island and the hippocampus has to share all this information it’s laying down memory with other parts of the brain. For example, certain parts of our environment might be particularly rewarding. So, if you wander down a new corridor and stumble upon a big pile of chocolates and think woohoo!, then dopamine signalling in the midbrain, for example, is activated and that dopamine signaling is also replayed during subsequent sleep in a way that coordinates with the hippocampus. That might, for example, enable the brain selectively strengthen particularly important memories about chocolate. Similarly, we know that over time as memories mature, then they’re integrated into other parts of the brain beyond the hippocampus into the neocortex, for example. So the neocortex is important for storing memories long term and that integration of memories across the hippocampal system and the neocortical system is supported by coordinated activity during sleep. We suspect that the way the brain deals with this is to brain waves or neural oscillations as a kind of clocking mechanism to coordinate activity. The brainwaves are acting almost like the conductor of an orchestra so the hippocampus, when we’re asleep, doesn’t necessarily know when to do its stuff and the neocortex is being told when to do its stuff by external stimuli. So instead, we have this internal conductor mechanism (these slow waves), which modulate the time of the hippocampal activity and the timing of the cortical activity and, therefore, bring the two into line allowing the hippocampus to tune in to the neocortex and vise versa.
Connie - In the day, we’re processing information and our brain cells in the hippocampus are firing, dependent on specific things that are happening. At night, we’re then recoding that at a quicker pace to give us a strong memory and that’s happening whilst we’re asleep. But on top of all of that are our hippocampus needs to go talking to other parts of the brain to link all of these bits of information and, in order for it to know when to do that, we’ve got these huge ripples of brainwave activity which are giving it a second by second, like a conductor coordination, telling it where to go.
Matt - That’s exactly right… yes.
Connie - Wow! That sounds incredibly complicated but amazing. It sounds really awe inspiring, I think, is probably where I want to go with that.
Awe inspiring indeed. I just can’t get over the complicated patterns of activity that are happening through our brains all the time. It’s so much more than I can even… take in. Maybe I should be giving sleep a bit more credit.