Dr. Christof (Christoph) Koch

So the circuits that convey experience in us, I'm not saying it's the same in other species, particularly, you know, non-mammals, but in us,

that grew up with the normal brain.

Again, I'm not talking about people who never, you know, unencephalic individuals.

That's very different.

So for most of us, we grew up with the normal brain.

And I think there the relevant circuits are the corticothalamo circuits.

And we can, in fact, we can exploit this knowledge now to test whether someone is conscious.

Because in principle, so what you can do

You can knock the brain using a technique called transcranial magnetic stimulation, right?

And then you listen to its echo using a high-density EEG net, okay?

And you can see if you knock here or here, depending where exactly you knock, you get these up and down states.

And if they last for, let's see, 200, 300, 400 milliseconds, and they occur at different places, you can formally compute what's called brain complexity using Lempel-Ziv complexity.

And you can show when...

Everyone who's either awake like us or we sleep in a dream state or we're on ketamine where we dissociate it, in all those cases, the brain complexity is high.

It's above a threshold.

However, when you're in a non-REM state, when you're in a state of deep sleep or you're anesthetized or you're, of course, in the most extreme case, your brain dead, then the brain complexity is very low.

And in animals, we've even done at the Allen Institute, we've done this experiment where we can systematically manipulate the corticothalamocortical circuits to really show it is this circuit that is really, that is the one that's critically involved

In fact, what we discovered over the last 10 years is this very abrupt threshold in brain complexity defined using this technique.

There's a thing called perturbational complex index.

It's a single number, PCI, between zero and one.