Can a single brain area be the seat of consciousness?
Written by: Elisa Clemente
Art by: Roween Rawat
Before reading this, take a moment to stop. Look around you. Inhale. Exhale. Listen. Think. Feel. You are now aware of what’s around you. Essentially, it’s your brain that’s allowing you to experience this – but how is this happening?
The ‘hard problem’ of consciousness has occupied the minds of neuroscientists over the last century, and the minds of philosophers for more than a millenia. How is it that a mass of cells combines a multitude of sensory information, memories, and emotions into a unified conscious experience? Is it a matter of the orchestrated activation of multiple brain areas, or is there one particular area responsible for this? A simple answer to all these questions was suggested by Francis Crick (co-discoverer of the molecular structure of DNA) and his colleagues: ‘the claustrum is the seat of consciousness’.
The claustrum is a thin, irregular sheet of neuronal cell bodies buried deep in the brain. It has reciprocal connections with most areas of the brain, including the visual cortex (vision), auditory cortex (hearing), and somatosensory cortex (touch). Additionally, there is some evidence suggesting that neurons in the claustrum are multimodal, i.e. capable of responding to stimuli of different sensory modalities. All in all, this seems to make the claustrum the perfect structure to integrate sensory information, thus making it the seat of consciousness.
However, this evidence is mostly correlational. If you want to find a cause and effect relationship between a brain region and a brain function, you need to either lesion it selectively or activate it selectively, and then see what effect that has on function. This is unsurprisingly extremely hard to do in humans (would you let someone stick a few electrodes in your brain to stimulate it electrically?). Fortunately, there is some anecdotal evidence we can examine. In fact, there are several cases showing the effects of lesions in the claustrum; for example, in 1996 a 12-year-old girl had temporary bilateral claustrum lesions, causing her to experience a loss of vision and hearing, as well as epileptic seizures, during which there is often a loss of consciousness. More recently, in 2011, a 21-year-old man diagnosed with an infection of mumps virus that resulted in bilateral claustrum lesions suffered visual and auditory hallucinations. This suggests a link between claustrum lesions and loss of consciousness.
The other side of the coin involves stimulation studies; while rare, there are a few case studies like this. One of these looked at the effects of smoking or ingesting Salvinorin A, a psychotropic molecule found in the Mexican plant Salvia divinorum, which is used by native Mazatec shamans to evoke spiritual experiences. Salvinorin A was more potent than LSD in inducing spatial hallucinations, with frequent loss of awareness of the subjects’ current surroundings (a disturbance of consciousness). This drug acts by activating κ-opioid receptors, a specific subtype of opioid receptors (receptors which are bound and activated by opioids). Importantly, the brain area where the density of κ-opioid receptors is highest is the claustrum.
Another study involved the electrical stimulation of the claustrum in a 54-year-old woman with a history of intractable epilepsy. Stimulation was found to immediately impair consciousness in all 10 trials. The patient returned to baseline as soon as the stimulation stopped and had no recollection of events during the stimulation period. Stimulation of the adjacent electrode contacts did not elicit the same phenomenon.
So, it seems that evidence from anatomical studies (including the reciprocal connections of the claustrum with the rest of the brain, the multimodality of its cells, and lesion studies), electrical stimulation studies, and chemical stimulation studies (Salvinorin A), all point towards the idea that the claustrum is the seat of consciousness.
But is it really? In reality, the studies that argue for the multimodality of neurons in the claustrum are quite old (over 50 years-old); more recent studies, employing more accurate techniques, have actually contradicted these previous findings. And while the claustrum does seem to be connected to multiple brain areas, this in itself isn’t conclusive evidence for a role in consciousness, let alone the idea that it would be the hub of consciousness. Lesion and stimulation studies do seem promising, but they are not particularly accurate either. In fact, the claustrum is such a small region that it is almost impossible to guarantee that the temporary bilateral lesion in the 12-year-old and the stimulation in the 54-year-old woman were exclusively affecting the claustrum. As for the Salvironin A experiment, these κ-opioid receptors are also expressed in other areas of the brain. In any case, it’s challenging to formulate a valid, all-encompassing theory of consciousness if there are only two or three anecdotal case studies to work with. As a matter of fact, the actual concept of ‘consciousness’ is still yet to be clearly defined. Most people consider consciousness to have two components, wakefulness and awareness; while wakefulness is fairly easy to measure experimentally, this isn’t the case for awareness. So maybe some of these questions need to be answered before taking experiments any further.
In short, while the claustrum does seem to play some role in consciousness, no studies so far have conclusively shown it to be the seat of this mysterious phenomenon. Maybe a different approach will be required if we ever are to understand consciousness. As Koch put it, “[taking] a piece of the brain and [trying] to press the juice of consciousness out of [it]” is almost impossible. One alternative could be something like the integrated information theory, developed by neuroscientist Giulio Toni and colleagues, which starts with consciousness itself, and tries to work backward to understand the physical processes that give rise to the phenomenon. One thing seems, however, certain: consciousness will continue to fascinate scientists for a very long time.