Jul 08

Comments on Koubessi et al.: Have we learned anything new about the role of the claustrum in consciousness?

This is a brief comment on a new paper that presents an observation that might be relevant to the role of the claustrum in consciousness.  The paper is:

Koubeissi, Mohamad Z., et al. “Electrical stimulation of a small brain area reversibly disrupts consciousness.” Epilepsy & Behavior 37 (2014): 32-35.

To summarize:  the authors are neurologists who treated a woman with epilepsy.  As part of the treatment they implanted several electrodes in her brain, and they observed that stimulating one particular electrode, which was located adjacent to the claustrum, caused her to transiently lose consciousness.  The loss of consciousness began and ended simultaneously with the electrical stimulation.

This report has drawn attention because it seems to fit in with a proposal made by Francis Crick and Christof Koch in a 2005 paper, titled What is the function of the claustrum? (open access).  Crick and Koch proposed that the claustrum plays an essential role in consciousness by integrating information from various parts of the brain and distributing that information broadly.

The claustrum, for background, is a thin layer of neural tissue that lies within a region of white matter located between the basal ganglia and the cerebral cortex.  It has long been rather mysterious, largely for technical reasons.  Its anatomical structure makes it impossible to lesion without extensive damage to other structures, and its thinness makes it difficult to record from using implanted electrodes.  Numerous ideas have been put forth concerning its function.  If you are interested in seeing a thorough review, you can find one in a paper called The claustrum in review (open access) by Brian N. Mathur.  That review is part of a “Research Topic”  in Frontiers in Systems Neuroscience, called The Claustrum: charting a way forward for the brain’s most mysterious nucleus.

The main point that I would like to make is that the Koubeissi paper, although thought-provoking and deserving of further exploration, is not actually a huge advance.  There is a good reason why it appeared in a specialized journal such as Epilepsy and Behavior rather than one of the “sexy” journals such as Science or Nature.

To begin with, the data were obtained from a single human subject, using a single electrode location.  Furthermore, that subject suffered from epilepsy.  The paper states that there were no signs the stimulus produced epileptiform activity, so it is unlikely that this was merely a set of absence seizures.  But even so, the fact that the subject had some level of brain pathology needs to be taken into consideration.  Also, it is not yet solidly established that the claustrum was the crucial brain area for the effect:  electrical stimulation at a point in the brain activates axons passing nearby as well as neurons located nearby, and can sometimes exert its effects on neurons located far away.  Thus it is important to replicate the observations before assigning deep significance to them.

More importantly, there are lots of brain areas where electrical stimulation can disrupt consciousness for reasons that are not very interesting.  There are even areas outside the brain:  if you electrically stimulate the heart in a way that induces fibrillation, consciousness will vanish within a few seconds—if you return the heart to a normal state, consciousness will rapidly reappear (presuming the disruption doesn’t last too long, of course).  At a less extreme level, there are a number of areas of the brainstem and thalamus, whose functions involve arousal, in which electrical activation can produce a rapid and transient loss of consciousness.

The root of the issue here is that the word “consciousness” has multiple meanings.  To neurologists, consciousness is defined by alertness, arousal, purposeful behavior, and verbal responsiveness.  To philosophers, consciousness is defined as “consciousness of” events of various sorts—it implies awareness and experience.  (I have discussed this distinction elsewhere.)  The Koubessi paper applies to consciousness in the neurological sense, but Crick and Koch were talking about the claustrum with respect to consciousness in the philosophical sense.  It is going to take a lot more information to justify leaping from one to the other.

 

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