The Spike by Mark Humphries;

Author:Mark Humphries; [Humphries;, Mark]
Language: eng
Format: epub
ISBN: 9780691195889
Publisher: PrincetonUP
Published: 2020-11-23T00:00:00+00:00

WHAT DO I DO NOW?

For your brain there’s a far grander challenge than just moving your arm: how do you know it is safe to move your arm now? Something else more important could already be happening, or need to happen instead. Be it you’re in flip-flops sprinting ungainly away from an angry squirrel, or you imminently need to hit the high F in “Let It Go” thanks to a drunken sign-up to the X Factor auditions, the last thing you need to add to your deep regret is to spasm your hands randomly in the air.

And that’s why our spike is at the same time sent down to the basal ganglia, the sullen outcropping of neurons underneath the frontal cortex, to ask: can I move my hand now? Our work and the work of many others has shown how the basal ganglia are the brain’s stern parent.8 Endless spikes pouring out of the basal ganglia are constantly stopping you from doing what you want. They inhibit everything they touch. No, you can’t do that. No. No. No. In order to move your arm, our spike must navigate the twisted pathways of the basal ganglia to reach their output and momentarily turn off the endless stream of spikes.

We land first in the striatum, gateway to the basal ganglia. The route we took here is one of uncountably many possible ones: first a jump between the pyramidal neurons of layers three and five of premotor cortex, then, instead of taking the branch of axon through the white matter and on to motor cortex, this time we cling to the cloned spike that turns down the branch to the striatum. Among the other routes, next door to us in layer five was the type of pyramidal neuron that sends its axon all the way to the spine; it too dispatched a cloned spike to the striatum. Indeed, all layer five neurons that send axons within the cortex seem to send a branch down to the striatum. And all layer five neurons that send axons destined for the brain stem or the spine also dispatch a clone of every spike to the striatum. Which means that as we arrive on a spike from premotor cortex, we’re joined by millions of other spikes, coming from all over the cortex, from all over the prefrontal and parietal cortices, the memory buffers, the evidence accumulators, and more, from all parts of motor cortex, from all types of sensory cortex, from touch, sound, and many stops along Highway What and Highway Do. All sending information about what is in the world, and what could be done about it. Striatum knows all.9

And we have compelling evidence that the striatum uses that knowledge as votes for different courses of action. Electrically stimulate a small group of neurons in the striatum, and you’ll make a body part move.10 More precisely, stimulate a group of cortical neurons sending axons to the striatum, and you’ll bias behavior toward whatever those neurons encode;

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