(Disponível em Português)
Is consciousness generated by multiple areas throughout the brain or does it arise in one specific brain region? Some scientists believe in the existence of a “seat” of consciousness.
Do not read this article – at least, not until you’ve taken a moment to stop. Grab a book, a newspaper, a magazine. Open it randomly. Run through it. Now, try to really think about every sensation reaching your brain: visual (through your eyes) – colours, shapes, letters, numbers; auditory (through your ears) – the rustling of pages turning; olfactory (through your nose) – the characteristic smell of paper.
Somehow, your brain manages to combine all this information, together with memories, emotions, and thoughts, to create a unified conscious experience. But how is this happening? The “hard problem” of consciousness, as it is often referred to, has occupied the minds of neuroscientists over the last century, and will probably continue to do so for the foreseeable future.
However, the actual concept of “consciousness” is yet to be clearly defined. Most scholars consider consciousness to have two components: wakefulness and awareness. Wakefulness is fairly easy to define and measure experimentally, through EEG (electroencephalography, a technique used to measure electrical activity in the brain), because the pattern of activity shown by EEG is different in brains of awake subjects, compared to subjects who are asleep.
Awareness, on the other hand, is neither easy to assess, nor to define – what exactly does it mean to be “aware” of your surroundings? Are there different stages or levels of awareness? Are there any other more accurate and objective ways of assessing awareness other than through questionnaires filled in by subjects, which is how it is often assessed?
But the most crucial and interesting question yet to be answered is the following: is consciousness generated through the orchestrated activation of multiple brain areas, or is there one particular area responsible for it?
In 2005, famous British scientist Francis Crick, co-discoverer of the molecular structure of DNA – who in the later stage of his life became very much interested in neuroscience –, and Christof Koch, at the time a professor at the California Institute of Technology, suggested there was a “seat of consciousness”. Consciousness, they claimed, arose in one particular structure in the brain: the claustrum.
What exactly is the claustrum, and could it really be the sole brain area responsible for consciousness? The claustrum is a thin sheet of neuronal cell bodies located deep in the brain. It has reciprocal connections with most areas of the brain, including the visual, auditory and somatosensory cortices. This would make the claustrum particularly well-positioned to integrate sensory information, which argues for the idea that it could be in charge of generating the conscious experience.
The evidence for this “master-role” of the claustrum is, however, only correlational and therefore insufficient to demonstrate a causal role for the claustrum in consciousness . Just because the door of a lift opens almost every time a toddler shouts “Open up!”, it doesn’t mean it is those words that are causing the door to open – correlation doesn’t imply causation.
In neuroscience, a good way of finding a cause and effect relationship between any brain region and a brain function is to lesion or activate it selectively, and then assess how that affects function. While this is, unsurprisingly, extremely hard to do in humans, as the idea of being voluntarily subject to brain damage is not appealing to most people, there is some anecdotal evidence we can examine.
In fact, there are several cases suggesting that lesions in the claustrum can disrupt consciousness; for example, in 2011, a 21-year-old man diagnosed with bilateral claustrum lesions (as a result of an infection of mumps virus) suffered confusion and visual and auditory hallucinations, which disappeared after he recovered from the lesion. This suggests a link between claustrum lesions and a disruption of awareness, one of the components of consciousness, indicating that a functional claustrum might be required for normal consciousness.
The other side of the coin in this search for causality involves stimulation studies. These are, equally, rare (would you let someone stick a few electrodes in your brain to stimulate it electrically?), but there are a few case studies of this sort. One of them involved the electrical stimulation of the claustrum in a 54-year-old woman. Stimulation was found to immediately impair consciousness in all 10 trials, with a recovery of consciousness as soon as the stimulation stopped.
Therefore, it seems that the reciprocal connections of the claustrum with the rest of the brain, lesion studies, and electrical stimulation studies, all point towards the idea that the claustrum could be involved in consciousness. But is all this enough to demonstrate that the claustrum is the only region responsible for consciousness, that is, the “seat of consciousness”?
In reality, some of the evidence provided is actually rather ambiguous. The lesion and stimulation studies mentioned do seem to show a causal relationship between the claustrum and consciousness, but they are not particularly accurate. In fact, the claustrum is such a small region that it is almost impossible to guarantee that the temporary bilateral lesion in the 21-year-old and the stimulation in the 54-year-old woman were exclusively affecting this area, since most of the currently used brain imaging techniques are unable to combine a sufficiently high temporal and spatial resolution to detect it. This means that the consciousness impairment caused could be due to changes in the activity of nearby regions. Therefore, no exclusive causal link has been found between the claustrum and consciousness.
In short, while it is possible for the claustrum to play some role in consciousness, no studies so far have conclusively shown it to be the seat of this mysterious phenomenon. Therefore, it seems that the claim that “the claustrum is the seat of consciousness” might be an over-simplistic answer to a rather complex question.
In fact, even Koch, who initially worked with Crick on the idea that the claustrum might be the “seat of consciousness”, later seemed to reconsider this position. As Koch put it, “[taking] a piece of the brain and [trying] to press the juice of consciousness out of [it]” is almost impossible – in other words, consciousness is likely to be too complex a phenomenon to be generated by a single brain area. It is highly probable that other brain regions (such as the thalamus, which also integrates sensory information) may be involved in or responsible for consciousness. In fact, it may even be that consciousness is actually generated by the orchestrated activation of multiple brain areas.
This is the idea behind an alternative theory of consciousness, the global workspace theory, initially proposed by Bernard Baars (currently an Affiliated Fellow at the Neurosciences Institute in California) in 1988 as a theory of human cognitive architecture, and later applied to consciousness. This theory suggests that the conscious experience results from coherent neuronal activity between widely distributed brain regions, with fronto-parietal associative cortices as key elements. If that’s the case, then maybe a different study approach will be required if we are to ever understand consciousness.
One alternative approach could be something like the integrated information theory, developed by neuroscientist and psychiatrist Giulio Tononi, from the University of Wisconsin, and colleagues, which starts with consciousness itself, and tries to work backward to understand the physical processes that give rise to this phenomenon. While this will definitely be challenging to be put into practice and test experimentally, it may be a much-needed change in perspective and mindset, which will hopefully bring some insight into this field.
One thing seems, however, certain: consciousness will continue to fascinate scientists for a very long time.
1. Bartolomei F, Naccache L. The global workspace (GW) theory of consciousness and epilepsy. Behav Neurol 2011;24(1):67–74.
2. Chau, A., Salazar, A.M., Krueger, F., Cristofori, I., and Grafman, J. (2015). The effect of claustrum lesions on human consciousness and recovery of function. Conscious. Cogn. 36256-264.
3. Crick, F.C., and Koch, C. (2005). What is the function of the claustrum? Philos. Trans. R. Soc. Lond. B. Biol. Sci. 360(1458), 1271-1279.
4.Goll, Y., Atlan, G., and Citri, A. (2015). Attention: the claustrum. Trends Neurosci. 38(8), 486-495.
5. Ishii, K., Tsuji, H. and Tamaoka, A., 2011. Mumps virus encephalitis with symmetric claustrum lesions. American Journal of Neuroradiology, 32(7), pp.E139-E139.
6. Koubeissi, M.Z., Bartolomei, F., Beltagy, A., and Picard, F. (2014). Electrical stimulation of a small brain area reversibly disrupts consciousness. Epilepsy & Behavior 3732-35.
7. Ross, L.E., Murray, B.J. and Steiner, M., 2005. Sleep and perinatal mood disorders: a critical review. Journal of Psychiatry and Neuroscience, 30(4), p.247.
8. Spector, I., Hassmannova, J., and Albe-Fessard, D. (1974). Sensory properties of single neurons of cat’s claustrum. Brain Res. 66(1), 39-65.
9. Stevens, C.F. (2005). Consciousness: Crick and the claustrum. Nature 435(7045), 1040-1041.
10. Stiefel, K.M., Merrifield, A., and Holcombe, A.O. (2014). The claustrum’s proposed role in consciousness is supported by the effect and target localization of Salvia divinorum. Front. Integr. Neurosci. 820.
Elisa Clemente is a PhD student at University College London
Edited by: Ana Gerschenfeld(Science Communication office). Photo credit: Greg Dunn, Brian Edwards and Will Drinker