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1. What makes a mind wander? In our new perspective paper with @jmacshine and @WalpolaIshan we consider how ongoing neuromodulatory tone shapes the mind-wandering brain state, with a special role for hippocampal sharp-wave ripples https://doi.org/10.1098/rstb.2019.0699
2. Large-scale networks involved in mind-wandering are well characterised. Some great examples are captured by the dynamic framework proposed by @KalinaChristoff, Jess Andrews-Hanna & co. https://www.nature.com/articles/nrn.2016.113 and work from @the_mindwanders’s group e.g., https://www.nature.com/articles/s41467-019-11764-y
3. But we don’t know much about the endogenous brain rhythms and neuromodulatory systems that promote the mind-wandering brain state…
4. Work in large-scale networks tells us the default network is important for mind-wandering, and the hippocampus is an important player. This is especially clear in the changes that occur in mind-wandering with HC damage e.g., https://www.jneurosci.org/content/38/11/2745?etoc= & https://www.pnas.org/content/116/8/3316.short
5. We got to thinking about special features of the hippocampus that might lend themselves to mind-wandering. Something that occurs spontaneously, when we’re not dealing with demanding tasks, and can engage those large-scale networks that are implicated in mind-wandering…
6. Turns out hippocampal sharp-wave ripples (SWRs) are a perfect candidate for a brain state that could promote mind-wandering and they are released under particular patterns of neuromodulatory tone.
7. SWRs occur in quiescent behavioural states, are associated with memory + simulation, and can engage widespread neuronal targets across the brain. We suggest this spontaneously evoked activity, propagated via the default network, can act as a trigger for mind-wandering episodes
8. Most of the amazing stuff we know about SWRs is from rodents; we don’t know much re their associated phenomenological content (if any) in humans. But some tantalising work links SWRs to spontaneous recall https://science.sciencemag.org/content/365/6454/eaax1030.abstract suggesting they may influence conscious thought
9. Other cool work in humans links SWRs to organising abstract knowledge https://www.sciencedirect.com/science/article/pii/S0092867419306403 – it’s possible that an adaptive benefit of SWR-provoked mind-wandering is to augment such knowledge structures throughout our lifespan…
10. Together, a bunch of cross-species work suggests SWRs are well placed to trigger a mind-wandering brain state. Turns out SWR occurrence is strongly dependent on hippocampal neuromodulatory tone: we identify the neuromodulatory “sweet spot” where they are likely to occur
11. We tried to pull together work linking rodent SWRs to neuromodulators e.g., https://www.nature.com/articles/nn.3843/ , https://www.nature.com/articles/nn.3998, with more zoomed out ideas around neuromodulatory systems e.g., https://www.annualreviews.org/doi/abs/10.1146/annurev.neuro.051508.135535 – it all fits together under an inverted-U curve right?
12. There’s lots in the paper about how these neuromodulators might influence SWRs. TL;DR: lower hippocampal cholinergic and serotonergic tone + higher dopaminergic seems to promote SWRs; phasic bursts in the context of relatively low noradrenergic activity also conducive to SWRs
13. How (and if) an SWR-triggered mind-wandering episode evolves will further depend on the relative balance of these neuromodulatory systems and their role in constraining brain-wide dynamics in the epoch following an SWR…
14. …we have some hints from systems neuroscience about these ideas (e.g., https://macshine.github.io/publications/2019_natureneuro.pdf) but there’s still a lot to learn
15. Our framework generates predictions about how neuromodulators influence mind-wandering which can be tested. Hopefully we help draw together some cross-species insights about how neuromodulators shape the SWR brain state & underpin rich, complex behaviours like mind-wandering
16. Thanks to Peter Fazekas, @BenceNanay & @ProfJoelPearson for inviting us to this special Phil Trans B @RSocPublishing issue which prompted us to try and unpack these ideas. Check out the rest of the issue here for lots of other cool papers https://royalsocietypublishing.org/toc/rstb/2021/376/1817
