Circuit Reuse
Many neural circuits are reused across behaviours---the brain likes to recycle. This sharing of neural infrastructure means that changing one routine can spill into others.
Neural pathways are not single‑use roads from point A to point B. The same circuits are pressed into service for many different jobs. Middle segments of a route tend to be shared and reused, so changing one behaviour’s route can nudge its neighbours---sometimes helpfully, sometimes not.
To be clear, any given neural circuit is likely to be involved in many different mappings of perception to action. Some people go further and argue that certain circuits could, in principle, be involved in almost anything we do. That helps explain why “just change the habit” can be so stubborn---what looks like one behavioural routine is, under the hood, a web of shared sub‑routines. This is made even more complicated because the more we practice something, the more entrenched in the brain it becomes. If there is lots of overlap with other things, then practicing one thing can entrench all the things that share those same resources!
Viewed this way, practicing one behavioural routine strengthens segments other routines also rely on, which is why learning something new can jostle an old pattern, and nudging an old pattern can produce side‑effects elsewhere. The pragmatic move is to add an alternative route first---give the system a fresh pathway to run---and only then taper the old one once the new route is established. Separating contexts makes the hand‑over cleaner.
Where this leaves us is simple but useful: expect spillover when you change entrenched behaviour, and plan for it. The brain is economical. It recycles.
How can you think with this?
These heuristics help you apply this neural system:
Ways to think with this
Practical ways to use this neural mechanism in understanding behaviour
WIP: One route, many functions
The same neural circuits are used for many different perception-action mappings. This means that when you try to change one input-output relationship, you're almost certainly affecting others that share segments of the same circuitry. Strengthening one pathway strengthens the shared segments, which can inadvertently strengthen neighbours. Weakening one can destabilise others that rely on the same infrastructure.
So what can you do? Don't expect to change one thing in isolation. If a behaviour is proving stubborn, check whether other behaviours are keeping the shared circuits active. Sometimes the issue isn't that the target behaviour is hard to change; it's that adjacent behaviours are reinforcing the infrastructure it relies on. You might need to alter the neighbours first, or build entirely new circuitry for the target rather than trying to repurpose shared segments.
WIP: New routes prefer existing infrastructure
The brain is economical---it builds on what's already there. This means new pathways will tend to form where infrastructure already exists to support them, reusing segments from established routes. This is why learning something new is easier when it overlaps with something you already know, and why novel behaviours that don't fit anywhere in your existing repertoire are harder to establish.
So what can you do? When you're learning something new, look for overlaps with things you already do well. Leverage the shared infrastructure rather than building from scratch. And when you're trying to establish a genuinely novel behaviour, expect it to take longer because the system doesn't have existing routes to scaffold it. You're not just training the pathway; you're building the infrastructure to support it.
WIP: Expect side-effects when you intervene
Because circuits are reused, changing one behaviour can produce unexpected effects elsewhere. Learning something new can jostle an old pattern. Altering an old pattern can affect seemingly unrelated routines. The nervous system is a web of overlapping routes, not a stack of independent modules, which means interventions ripple.
So what can you do? Plan for spillover. When you're changing entrenched behaviour, monitor for side-effects in other areas and be ready to adjust. Sometimes the spillover is positive---practising one thing improves others that share infrastructure. Sometimes it's not---changing one thing destabilises its neighbours. Either way, the reuse is inevitable. You might as well expect it and make it work for you where you can.
Referenced by
Sources
- analects/what-are-neurons.md
- analects/brain-regions-to-networks.md
- analects/repressed-memories.md