A diagram from Andreas Vesalius’s De humani corporis fabrica libri septem (1543). Image: Vesalius/Wikimedia Commons

Humanity’s Long Search for the Soul in the Brain

How philosophers and neuroscientists have tried and failed to nail down the precise location of human consciousness.

|
Jun 8 2015, 2:25pm

A diagram from Andreas Vesalius’s De humani corporis fabrica libri septem (1543). Image: Vesalius/Wikimedia Commons

The history of Western neuroscience may seem a dry topic to an outsider, a litany of impenetrable Latinisms and anatomical diagrams. It is, in fact, a series of fascinating battles between different models of and metaphors for the way we think—foremost among them the concept of the "soul", broadly defined in Christian thought as the incorporeal essence of a human being, and the basis for conscious thought.

The idea that the soul exists within a specific part of the human brain is, of course, no longer the subject of widespread investigation in the secular field of neuroscience. "There is no hypothesis in that question, nothing that you can test—it is much too broad and not 'scientific' as such," I was told by Sylvia McLain, a lecturer in biochemistry at the University of Oxford and prolific Guardian columnist.

The quest for the resting place of the soul served, however, as an important motivator for earlier generations of scholars, and continues to spur if not guide enquiry about the nature of our minds and bodies today. "Asking broad philosophical questions like 'what is a soul?' can eventually lead to scientific investigation," McLain noted. "Many of the early naturalists studied plants, animals, and the Earth to understand 'God's plan' or 'God's creation.'"

A 17th-century illustration of the human soul by Johannes Amos Comenius. Image: Wellcome Images/Wikimedia Commons

Once upon a time, the brain was thought of not as a biological computer matrix, made up of hundreds of billions of "electrically excitable" neuron cells, but as a sort of psychic refinery, pumping alchemised fluids through the body at the behest of the soul.

Writing in 1567 with reference to trends in medical practice initiated by the Greek physician Galen of Pergamon centuries before, the French scientist Jean Fernel declared that the body was suffused with three "spirits": "natural spirits" that arose from the liver and were transformed in the furnace of the heart to "vital spirits", which were then distilled into "animal spirits" by the brain—specifically, within the intricate bundles of nerve cells known as the choroid plexus, which we now understand to be a source of cerebrospinal fluid.

These animal spirits were then discharged back into the body by the soul to act as its "servants and porters", carrying orders to lower organs. It's a fabulously arcane model of the soul's supposed relationship to the flesh that, among other things, conveniently parallels the church-sanctioned idea of a monarch, ordained by God to sit in judgement over the organs of society. As with other thinkers of his time, Fernel was mindful of the consequences scientific speculation might have for the social order.

The idea of a host of specialised fluids under the direction of the soul remained popular well into the 17th century. The French philosopher René Descartes authored one of the more controversial theories as to how the soul determined the distribution of these spirits, contending that it manipulated the body via the pinecone-shaped pineal gland in the middle of the brain.

A diagram of the brain by René Descartes, showing the pineal gland in the centre. Image: Wellcome Images/Wikimedia Commons

In a letter from 1640, Descartes justified this by commenting, "I cannot find any part of the brain, except this [gland], which is not double. Since we see only one thing with two eyes, and hear only one voice with two ears, and in short have never more than one thought at a time, it must necessarily be the case that the impressions which enter by the two eyes or by the two ears, and so on, unite with each other in some part of the body before being considered by the soul."

The pineal gland was, he suggested, suited to this purpose thanks to its central vantage point, suspended in whirling brain fluids "like a balloon captive above a fire". It served as both the recipient of sensory impressions and the motive force for bodily action. The spurting of animal spirits through nerve channels from the senses traced patterns on the gland's surface, Descartes proposed, giving rise to the experience of heat, pain and so forth. The soul could then move the pineal gland to affect the flow, like a rudder—sending spirits gushing through the nervous system to other bodily organs.

Descartes' theories were widely disseminated, but divisive—among other complaints, it was objected that as the pineal gland was present in the brains of dogs, cats and other animals, it could not be the vessel of the immortal human soul. In 1713, the Italian anatomist Giovanni Maria Lancisi suggested an alternative in his Dissertatio Physiognomica: the soul must lie somewhere in the corpus callosum, a fat platter of white matter fibres that connects the brain's hemispheres.

Spirits, he explained, flowed along certain of these fibres to the front and rear of the brain, joining the soul and consciousness to the rest of the body. This theory was ultimately disproved when it was demonstrated that the corpus callosum could be sectioned, dividing the brain's hemispheres, without depriving a patient of consciousness.

An image of the brain showing the corpus callosum in the centre. Image: Henry Gray, Anatomy of the Human Body ()/Wikimedia Commons

As recounted in Marco Catani and Stefano Sandrone's book Brain Renaissance, by the late 18th century scientists had begun to abandon the notion of a specific site for the soul or consciousness. It was increasingly argued that the mind was a product of distributed networking, its faculties divided across organs that work in tandem.

Exactly how the impression of a singular consciousness is created when different aspects of the process are handled by separate parts of the brain remains a matter for debate. While talk of a set of bodily coordinates for an incorporeal human essence has died down, some neuroscientists maintain that a particular organ of the brain serves as a hub, unifying the mechanisms that give rise to conscious thought.

"Trying to look at consciousness may be like looking at the wind."

According to the neuroscientist Joseph Bogen, who died in 2005, the thalamus, a two-part structure on the brain's midline, may host the neurons in question. In a speculative paper from 1995, Bogen noted that the formation of small lesions in the intralaminar nuclei of the thalamus "typically" brings about a coma. By contrast, it is possible to infract upon or even remove portions of other brain organs, such as the cerebral cortex, without causing the patient to lose consciousness.

Bogen theorised that the intraluminar nuclei might control the inhibition or release of motor commands that originate elsewhere in the brain, via the striatum—that's to say, they play a role in the consideration of an action before it is performed—but was unable to show how this occurs. "Trying to look at consciousness may be like looking at the wind," he wrote. "We see only the effects of the wind."

Another study published in 2014 by Mohamad Koubeissi at the George Washington University in Washington DC suggests that the claustrum, a thin sheet of neurons attached to the underside of the neocortex, may help interweave the processes that make up conscious experience.

While medical imaging technology has advanced in leaps and bounds over the past few decades, the brain remains a mostly-misunderstood organ of daunting complexity. According to researchers at the Stanford University School of Medicine, its hundreds of trillions of synaptic connections vastly outnumber the stars in our galaxy. Unravelling the origins of conscious thought may seem a fool's errand, but this is unlikely to deter future generations of scholars—each unpicking the errors of their predecessors in hopes of finally uncovering the neurological functions that spark self-awareness.

My thanks to Michael Trimble, author of The Soul in the Brain: The Cerebral Basis of Language, Art, and Belief, for his insights on my research.

Jacked In is a series about brains and technology. Follow along here.