Your brain is not symmetrical: a horizontal MRI scan will show you that it’s a bit wonky – as if it’s been twisted anticlockwise slightly inside the skull (the technical term for this is Yakovlevian anticlockwise torque). The right frontal lobe, as a result, is bigger and wider than the left and it often protrudes forwards beyond the left frontal lobe; while the left occipital lobe is wider than the right and protrudes further backwards.
The curious thing about these neuroanatomical asymmetries is that they are not random variations between individuals: these are distinct patterns in anatomical design that appear to have some advantage. While most parts of your body – your eyes, your lungs, your arms – are more or less symmetrical with a few idiosyncratic exceptions, something in the development of your brain causes these systematic adjustments. Indeed, patterns in hemispheric symmetry and asymmetry that deviate from this norm are associated with schizophrenia, dyslexia, and a number of other disorders. Why? What causes wonky brains?
Vocalisation is lateralised to the left hemisphere – from monkeys to marmosets to mice
Many of these asymmetries are closely related to one of the most uniquely human abilities: language. In keeping with this twisted Yakovlevian appearance, the Sylvian fissure (which lies, roughly speaking, underneath the main auditory and language processing areas) is longer and less steep in the left hemisphere, making way for a larger planum temporale. Language (or other kinds of vocalisation) is lateralised to the left hemisphere – not just in humans, but a variety of other animals from monkeys to marmosets to mice (as well as song birds and frogs). In humans, the planum temporale is heavily implicated in auditory processing and the other structural differences between the left and right frontal lobes match the areas that are associated with language and speech. Indeed, in most autistic children it is the right hemisphere that dominates speech processing (unlike most other people, who show left lateralisation), and this is reversed as they improve their language skills and the left hemisphere gradually comes to dominate.
Asymmetry affords flexibility
It’s reasonable to assume that these asymmetrical quirks have some reason for being – nature doesn’t deviate from a pattern without good reason. While language lateralisation is the most studied, visuospatial ability, attention, music perception, and mathematical ability have all been found to be dominated by one or another hemisphere.
The best reason is, simply, that asymmetry affords flexibility. Having one part of the brain specialised for a particular task or function means that information can be processed more efficiently, freeing up other parts for other things – much the same as how most modern computers have dual-core processors for efficiency. It also reduces the amount that the hemispheres interfere or conflict with each other by granting dominance to one over the other.
Bisazza and Dadda (2006) demonstrated this by breeding two strains of the same species of fish, one strain with asymmetrical brain structure, and the other with symmetrical brain structure. When they were required to carry out one task (catching a shrimp), the two strains of fish performed equally well. But when they were required to do two tasks simultaneously (catch a shrimp while avoiding a predator), those with symmetrical brains took twice as long to catch the shrimp. Those with asymmetrical brains were barely affected by the distraction of the predator.
Translate this into humans, and lateralisation for speech means that your left hemisphere can perform the function of holding a conversation while your right hemisphere can get on with other cognitive tasks. As for some of the other asymmetrical quirks – the fact that Alzheimer’s disease progresses faster in the left hemisphere, for example; or that the left also dominates the perception and expression of positive emotions; or the observation that many of these asymmetries are most pronounced in right-handers – the answer is not as obvious.
Dadda, M., & Bisazza, A., (2006). Does brain asymmetry allow efficient performance of simultaneous tasks? Animal Behaviour, 72, 523-529.
Ehr, G. (2006). Hemisphere dominance of brain function – which functions are lateralized and why? In 23 Problems in Systems Neuroscience, van Hemmen, J. L., & Sejnowski, T. J. (Eds.). Oxford University Press: New York.
Petty, R. G. (1999). Structural asymmetries of the human brain and their disturbance in schizophrenia. Schizophrenia Bulletin, 25, 121-139.
Toga, A. W., & Thompson, P. M. (2003). Mapping brain asymmetry. Nature Reviews Neuroscience, 4, 37-48.