Guy Leschziner: ‘Reality is entirely a construct of our nervous system’

Guy Leschziner is a consultant neurologist at Guy’s and St Thomas’ hospitals. His areas of expertise are epilepsy and sleep disorders. He has presented two BBC radio series, one on sleep and another on the neurology of our sensory world. His latest book is The Man Who Tasted Words, which explores the relationship between our sensory perception and the reality it constructs.

A common phrase in cognitive neuroscience is “perception is nothing more than controlled hallucination”. What exactly does that mean?
What we think of as a hallucination is an experience that is devoid of reality or removed from reality. And the term “controlled hallucination” implies that our experiences are constructed by the brain. As I point out in the book, sometimes what we perceive to be reality is very different from the cold, hard reality or what we assumed to be the cold, hard reality around us. So it is essentially stressing the point that what we term reality is entirely a construct of our nervous system.

There is a debate in neuroscience between the orthodox viewpoint that sees our sensory system as a kind of flawed means of perceiving reality and those who suggest it is designed to conceal reality. What are your thoughts on those debates?
Intuitively as a human being, rather than as a neurologist, I find it very difficult to get my head around those views that say that our brains are designed to conceal reality. I’m fascinated by these theories. Do I, in my heart of hearts, like to accept them? No, I don’t. I’m much more comfortable with the view that there is some relationship between the world that we inhabit and what we experience rather than them being completely removed from each other.

The title of your book refers to a man with synaesthesia, in which one sense triggers another. He can taste words, while someone else sees colours in music. What does this tell us about our sensory system?
The most widely cited statistic regarding the prevalence of synaesthesia is that it affects about 4-5% of individuals. The thing that it really tells us is that there are people out there with essentially normal brains – these are not individuals with pathology, they’re not people with brain injury or damage – who experience reality in a very different way from most of us. It highlights that our perception of reality is so defined by how our brains work and illustrates that it’s entirely dependent upon our nervous system.

One of your areas of expertise is sleep. The neuroscientist David Eagleman argues that the brain is a Darwinian battlefield between neurons and that dreaming is a means of maintaining vision when we sleep. What do you think about that?
It’s true that humans have evolved to be more dependent upon their vision rather than the other senses. But there are lots of theories as to why we dream. The one that particularly appeals to me is that in order for our nervous system to function properly, for us to understand the world, we constantly need to make predictions. In order to make predictions, we need to have some sort of internal model of the world as we understand it. And dreaming may represent an integration of our experiences over the course of the day and, indeed, over the course of our lifetimes into that model. And that dreaming sleep is that stage in our lives every night where we tweak that internal model of the brain.

The issue of pain features prominently in your book. One of your subjects experiences no pain at all. Why do you think pain is quite so painful?
We know that pain has got many components to it. A major component is the sensory discretion that tells you where in the body that pain is. Another feature that everyone will be aware of is what is termed the affective component of pain. In addition to knowing that I’ve just hit my finger with a hammer, it’s that sort of overwhelming unpleasantness, that dread of pain. I think that that’s a very important evolutionary mechanism. Pain is a very strong driver to avoid damage to oneself.

You touch on Covid-19, particularly in relation to loss of smell. Do you expect the research into long Covid will produce neurological findings?
Undoubtedly, it is the case that we do see a lot of people in whom long Covid has had a significant psychological impact. For many people, it’s the first time they have been faced with their own mortality. But it’s clear that there appears also to be a physical or biological component to it. From a neurological perspective, one recent study has looked at individuals with the brain fog of long Covid. What they clearly demonstrated in this cohort of individuals is that 70% of them have abnormalities in their cerebrospinal fluid. I think that what we will find out is that there is a combination of factors that leads to long Covid. And to group it into one condition is probably erroneous.

If you had a spare billion pounds to spend on medical research, where – from a neurological perspective – would you like it to go?
From an entirely selfish perspective, my academic background is in epilepsy and sleep. Sleep was the subject of my first book. And I think that given that we spend a third of our lives asleep and yet we don’t fully understand the impact of what sleep does, I think that that is an area that remains very much underresearched and probably has very far-reaching implications for us all.

In a sense, you ask the reader to re-evaluate the senses, particularly the overlooked senses of taste and smell. How would you rank the senses in order of importance to you?
I would have said vision first, then hearing, then touch, then smell and then taste. By the end of writing the book, I’m not sure it changed significantly, but I certainly appreciated smell a lot more. It has far-reaching implications in terms of memory, in terms of emotion, in terms of lots of hidden aspects of our lives, for example, the attraction towards another individual. I think it’s much underappreciated that smell is a very important mode of communication.

In the time that you’ve been practising, what is the medical breakthrough that has had the greatest impact on your patients?
When I was a medical student, it was said that neurology was the speciality with 1,001 diagnoses, but only one treatment. And that treatment was steroids. Whereas our understanding of immunology, in particular, has caused an explosion in terms of treatments that are available for some very serious conditions such as multiple sclerosis or other autoimmune conditions that cause devastating neurological damage. That is a huge step forward therapeutically.