New Scientist recently published an article talking about how research on an epileptic patient has found the brain's "on-off switch" for consciousness. Like most claims in popular science articles, they're probably going a bit overboard, but the findings are extremely compelling. Unfortunately the original paper is behind a paywall (damn you, Elsevier!) so we have to rely on the second-hand article for details.
Doctors use a technique called electrical stimulation mapping to identify where exactly in the brain seizures originate from in epileptic patients. This is important since severe cases of epilepsy can require that part of the brain be removed, and you don't want to take out any more than you need. The technique involves surgically implanting electrodes into various areas of the patients brain and running current through them to see what happens. (This sounds a bit reckless, but apparently it works). In our case, the doctors placed an electrode near a small region of the brain known as the claustrum. Compared to other brain areas, relatively little is known about it. In fact, it wasn't even probed using electrical stimulation mapping until now. The doctors were quite surprised by what they found:
Huh. That's pretty weird. If we ignore for a moment the fact that this effect has only been observed in one epileptic patient with some other brain damage, this definitely implicates the claustrum as a critical part of conscious processing. But what does it do?
In 2004, just before Francis Crick died, he collaborated with Cristof Koch to try and identify a region of the brain where information from multiple senses could be bound together to form unified perceptions. This is an important part of most modern theories of consciousness because one of the defining features of consciousness is its apparent one-ness: when you see someone moving their mouth and hear sound hitting your eardrum, you somehow combine these streams of data into a single idea of reality in which the sound is coming from the person. Crick and Koch identified the claustrum as a great candidate for the location of this process in the brain. It has many connections both to and from many sensory regions of the cortex, making it an ideal spot for a global workspace of neurons.
(I also imagine the fact that its function was otherwise a mystery played largely into their selection of this region. Mysterious things are much sexier to researchers.)
A PET scan study of cross-modal reasoning provided evidence for the idea. In the study, subjects held an object in their hand and then asked to identify an identical object by sight among many similar objects. However, the object in their hand was blocked from view by a screen, and this required integration of touch and sight data. Sure enough, the scans showed increased activity in the claustrum.
Well, probably. Unfortunately PET scans are sufficiently low resolution that they couldn't say for certain whether the activity was actually in the claustrum, but it's certainly a compelling pointer.
In a convenient coincidence, the journal Frontiers in Systems Neuroscience just published a collection of papers about the claustrum. In one, Brian Mathur casts doubt on the Crick/Koch theory for three reasons:
- When Crick and Koch wrote their paper, it was thought that the claustrum was connected to the thalamus, a region of the brain thought to be involved in conscious processing, and this was an important part of their argument. However, later research (also by Mathur) cast doubt on the connection.
- Since the binding together of sensory data is presumably quite a complex task, we would assume that the brain region responsible for it would need to be highly structured. However, the claustrum shows none of the layered features we typically associate with more sophisticated processing in the cortex.
- Neurons in the claustrum are suspiciously quiet. They seem to fire mostly when presented with sudden sensory changes, such as a clap. If the claustrum really were the seat of conscious integration, we would expect to see it active whenever we are conscious.
Brian Mathur has an alternative hypothesis: instead of combining sensory data for conscious perception, the claustrum acts as a sort of "saliency filter" - it acts as a filter to only send data up to the cingulate cortex when there's something that's a big deal that needs to be acted on right now.
All the articles on the electrical stimulation study take it as evidence of the Crick/Koch theory, but the effect seems more in line with Mathur. Namely, if you shut the claustrum off completely, it'll stop telling the cortex that anything important is happening, and consciousness drifts away as it's apparently no longer needed. In contrast, shutting off the site of sensory integration seems like it would lead more to a form of sensory processing disorder, which is a well-known phenomenon where the one-ness of consciousness is disrupted. I am still confused about how either idea could explain the patient's amnesia about the whole event.
On another front, researcher Klaus Stiefel makes an interesting connection between the claustrum and salvia. Salvia is a psychedelic drug traditionally used in Mexico. Essentially, salvia works by stimulating a particular type of receptor known as a kappa-opiate receptor. The claustrum has the highest density of kappa-opiate receptors per neuron of any brain region. Furthermore, it can seriously mess with your conscious experience. Here's some reports cited in the study:
“It’s as if I have no body, but I can still feel that I’m connected by the roof of my mouth to this slab thing. It’s completely absurd. Then I start to feel like the slab is being held up vertically by somebody. I can’t see him, but it’s as if he has been carrying the slab around for eternity, and that I have always been here attached to it.”
“The Salvia spirits (so I believed at the time) begun pulling me with fuzzy green arms covered in eyes to take me into their dimension. Their vague form was a green loop functioning as both head and arms, with a translucent body in between. They were playful, unthreatening yet determined…The most powerful part of the hallucination was my belief that the spirits were real.”
Suffice to say, things can get a little weird. Stiefel takes this as evidence of Koch and Crick's thesis, but I'm not entirely sold. While the claustrum does have the highest density of kappa-opiate receptors, it is far from being the only region that uses them (see this table), so the effects of salvia are likely distributed about the brain. Additionally, Stiefel is forced in his paper to explain the strange body sensations by resorting to a different brain area that the claustrum may or may not be connected to. And then the comparison of the reports between LSD users and salvia users are somewhat suspect- I have no reason to believe that the reports weren't cherry-picked to confirm the researchers' hypothesis.
However, in another way this is quite exciting. It is extremely hard to do research on most psychotropic drugs due to absurd drug laws that prevent the progress of neuroscience and medicine*, but the use of salvia is still legal in many US states. Given that the legal situation may change soon, we should try to get as much data as possible on its effects in the claustrum, to see what's really going on. Of course, since salvia causes extremely uncomfortable changes in bodily and spatial perception (claustrophobia, as it were), there probably won't be too many people lining up to take it and then get into an fMRI machine.
Needless to say, the field of neuroscience is about to become very claustrophilic.
* Since the original paper is paywalled, you can find the relevant quotes on this crazy drug forum.