Two Ghosts in the Machine

This post is in part a response to CGP Grey’s video “You Are Two”

Our bodies are symmetric. We have two legs, two arms, and even one side of our face is, more or less, a mirror image of the other. Symmetry can also be found within our body: we have paired kidneys, paired ovaries/testicles, paired limbs, and importantly, our brain is structured in two halves – the hemispheres.

This remarkable structural property of the brain has provoked the curiosity of medics and neuroscientists for decades, if not centuries. Is one hemisphere dominant over the other? Do men and women use their hemispheres differently? And: How do our hemispheres communicate?

Communication between hemispheres

In healthy humans, it is hard to study one hemisphere in isolation: the groove between them – the longitudinal fissure – is bridged by the corpus callosum, a structure of nerve fibres that connects the two halves of our brain. As long as the callosum is intact, the left and right hemispheres are a functional unit. Information from the left hemisphere easily travels to the right just as information travels within the hemisphere itself. It is this cooperation that allowed the developement of different specializations in each hemisphere in the first place.

White Matter Connections Obtained with MRI Tractography
This image shows the nervous fibres of the brain. We see here that the brain can be separated in two halves and that these halves are bridged by a structure called the corpus callosum. The connections within the hemispheres are the same cell types that also bridge the two halves. Xavier Gigandet (CC BY 2.5)

The special case of split-brains

Hemispheres as distinct entities can only be studied under the rare circumstances that the callosum is cut or missing. Cutting it has long been a treatment for severe epilepsy. As presented in this review, each hemisphere controls the contralateral part of the body and receives visual input from the corresponding visual field (left for the right hemisphere and right for the left hemisphere). After undergoing a surgical intervention where the callosum is cut, these split-brain patients are not capable of transferring information from one hemisphere to the other. How does this influence a human’s perception, actions and thoughts?

Hemispheres are specialized

Even though the brain looks symmetric, it isn’t. Each hemisphere is in unique control of some cognitive functions such as speech, facial recognition, or processing of numbers. This is called “lateralization”. Without the callosum, functions of the two sides cannot be combined, which results in different behavioural observations for each. This fact still seems to come as a surprise to many people, including the popular YouTube educator CGP Grey.

You Are Two – Are You?

In case you haven’t seen it, below is CGP Grey’s short video on split brain patients.

The first minutes of this video provide a good and precise overview of most aspects of split brain patient behaviour. But CGP Grey’s analysis leads him to an interesting conclusion: “In your head, there is a separate ‘something’”. This sounds a lot like your brain is inhabited by a ‘something’, a “ghost in the machine” that drives your actions and thoughts. CGP Grey takes this view even further, arguing for an individual consciousness in each hemisphere, making one ghost in the machine into TWO ghosts in the machine.

In your head, there is a separate something.

I cannot prove the existence or non-existence of non-observable minds within the brain and I won’t try.  Instead, I would like to provide an alternative explanation for the behaviour of split brain patients. My line of thought follows the idea of the brain as a network of functions and explains altered self-perception in terms of a “left-brain-interpreter”.

Duality or Modality?

Studies on lesions and brain physiology together with modern imaging techniques have allowed humans to localize some functions of the brain such as memory, attention, perception, thought, and language.  These functions are associated with areas in the cerebral cortex – the outer layer of cells covering both hemispheres. In the past, research tried to disentangle these functions, looking for specific regions exhibiting the “language” or “memory” function. Only recently has this idea of the brain as a puzzle of individual functions been modified to include a view of the brain as a dynamic network.

This approach describes the brain as a system of subunits with highly complex interactions where functionality is essentially influenced by these interactions. This view can be used to study the question of our “self”-concept and may provide an alternative explanation for the behaviour of split-brain patients.

Modules in the brain are connected to one another. The connections are not restricted to one hemisphere – they also cross the corpus callosum and bridge the left and the right half of the brain. S-B Hong et al (CC BY 3.0)

The great strength of thinking about the brain as a network is that by doing so we acknowledge the fact that the brain is a physical unit as long as all its modules are connected. Additionally, we can explain differences in cognition and behaviour in terms of the strength of connections between functional units of the brain. So instead of assuming a “default” duality in the brain, we can also explain the behaviour of split-brain patients by the fact that one network can easily become two networks when its connections are cut.

Self-perception in the network

Even though the brain is a modular construct in which functionality arises from connections between the modules, we perceive ourselves as a whole. This is only possible if the brain also possesses the capacity to make sense out of all – conscious and unconscious – changes in the state of each module. Such a function was proposed by the godfather of split-brain research – Prof. Dr. Gazzaniga. He introduced the concept of a “left-brain interpreter” – a module of the brain’s network located in the left hemisphere that evolved simultaneously with functions that make inferences and draw causal conclusions.

If the left-brain interpreter is informed of changes in the right hemisphere, it will integrate them into the concept of “self-experience”. If not, it relies on information from the left hemisphere only – possibly misinterpreting the actions of the other half, as described at minute 1:15 of “You Are Two”.

The self is not defined by the longitudinal fissure

The alternative explanation

CGP Grey’s video on its own is coherent and logical, but he takes a wrong turn when he tries to allocate the “self” to one of the two hemispheres (minute 2:20). The failure to define one part of the brain’s network as “self” leads him to the conclusion that each hemisphere must possess its own, inherent and independent consciousness. It would be interesting to take this view further – e.g. to a scenario where all connections between the visual cortex and the motor cortex are cut. Would we have to assume the presence of a consciousness in each of these cortical areas?

Viewing the brain as a dynamic network, in which changes in connectivity can alter behaviour and the self-concept, provides a physical and physiological explanation for split-brain patients’ behaviour. It applies to the isolation, loss and acquisition of brain functions and can even be extended to the nervous network in our limbs and intestines. The resulting “self” is a dynamic concept, one that is not defined by the longitudinal fissure.

Who Are You? You Are a Network!

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