One of the most widely known aspects of the brain is hemispheric specialisation. The notion that one side is better at a given function, compared to the other. Most people have heard the oversimplified dichotomy of the logical and analytical left side, and the creative and intuitive right side. To what extent is this true? And how can we know?
The upper and largest part of your brain is called the cerebrum which facilitates a wide range of things, most famously is its involvement in conscious thought and action. It is this anatomical part that gives the brain its characteristic shape of two hemispheres. Each side is roughly the same size, although the right is slightly larger on average than the left. By examining patients with brain damage on one side in the 1800s, they understood that speech production and comprehension were primarily left-side functions. Although both located in the same half, they are found in separate places. This means that you can lose your ability to comprehend speech without losing your ability to speak (and vice versa). Speech production is one of the most well-established lateralised functions, with around 95% of right-handed people and about 70% of left-handed people having it localised on the left side.
The two hemispheres are primarily connected by a highway of white-matter fibres called corpus callosum. It is unique to placental mammals’ brains, and takes up greater space in relation to brain size in humans compared to the other animals. It consists of more than 200 million nerve fibres, which facilitate a flow of information between the two halves. What would happen if this path were to be disconnected? How would a human function with two stand-alone (ish) hemispheres?
A surgical procedure to do just that was first tried on humans in 1940, in a bid to alleviate severe epileptic symptoms. Rather surprisingly, despite having had their brains’ largest fibre tract cut off, the split-brain patients came across as largely normal post-surgery. The hemispheres’ ability to “talk” to each other was severely limited, but the personality of their owner seemed to be left remarkably unchanged. So too was the impression of their cognitive abilities, senses and movements. It left the world of science puzzled; what was each half up to? How could they function when not exchanging messages? To find answers, researchers needed a way to communicate with one side at a time. More than 20 years after the first surgery was carried out, neuroscientist Michael Gazzaniga came up with a way to investigate this. He cleverly took advantage of the crossing of the optic tract. This crossing sends visual information captured in the left visual field to be initially processed in the right hemisphere. By showing only one side of the brain information they could test it independently, establishing a method still used today to investigate hemispheric specialisation.
Today, we know that the two sides are both structurally and functionally different from each other. In general, both sides have the capacity to carry out a task, but one side is usually superior at performing it. One of my favourite (and most humbling) findings from the split-brain patient research on lateralisation is that the two sides use different cognitive strategies to draw conclusions. Gazzaniga coined the term ‘The Interpreter’ to describe the left side’s tendency to find explanations for why things happen. If the left side witnesses a behaviour initiated by the right, it will attempt to explain the reasons for it, even though it has no idea what the right side was thinking. And crucially, it will be certain about the accuracy of this explanation despite its lack of insight.
The right side is usually superior in perceptual processing and understanding of nonverbal information. It excels at things such as facial recognition, spatial understanding, and piecing together unified visual representations. And while the left tends to be dominant in language processing, the right is usually better at understanding someone’s tone of voice (the emotional aspect of communication). Recent research in the development of lateralisation suggests that facial recognition might initially exist as a bilateral capacity, only to get pushed to the right as children learn to read and the need to recognise words increase.
As you can see, while the left and right side function differently, they have evolved to be interconnected and work together, allowing us to be unaware of which part is doing what. Rather than thinking about people as being left or right dominant in a simplified dichotomous manner, we need to view hemisphere lateralisation as a way for the brain to operate and seamlessly integrate highly specialised abilities. Something that is central to how our minds work.