What are the ‘two brains’ believed to do?

This is an idea of two halves (pun intended): firstly that there is a difference between left brain thinking and right brain thinking, and secondly that individuals differ in the extent to which they favour either the ‘left brain’ or the ‘right brain’. According to this view, the left brain is the analytical, logical, verbal half while the right brain is the creative, emotional, visuo-spatial half; individuals who have one side more active than the other are believed show corresponding cognitive styles and personalities.

True or false?

As with most ideas in this series there is a certain degree of truth here. Looking at the gross anatomy of the brain one of the most striking features is that there are two very distinct hemispheres, which are joined together by a tract of connective tissue called the corpus collosum. There is also substantial regional specialisation, and in many cases processing is partly or even fully hosted by one hemisphere. This lateralisation (or hemispheric specialisation) was first seen in the mid 19^th Century through the observation of the behaviour of brain-damaged patients who were later autopsied. From the 1970s this idea was explored by behavioural experimentation on patients with severed corpus collosums (see Further resources). Only recently, with the use of neuroimaging, have researchers been able to trace the structure and function of lateralisation, as well as understand how the hemispheres work together and how their interaction changes over the lifespan.

The evidence on brain lateralisation

Meta-analyses, which combine findings across multiple studies, have shown that on the whole language is lateralised to the left hemisphere and visual-spatial processing is lateralised to the right. Creativity also seems to be right dominant, possibly due to a dependence in creative thinking on global processing and taking context into account[i]. However, even language is not exclusively left-brained. Only processing the sounds of words (phonological processing) seems to depend exclusively on the left hemisphere[ii], with the right being important in things like understanding the meaning and context of language, and aspects of speech such as intonation. In reality, for most tasks the two hemispheres work in tandem.

Does lateralisation differ between individuals?

It’s also true that lateralisation differs between individuals. Individuals vary both in structural lateralisation (the extent to which brain tissue is different in each hemisphere), and functional lateralisation (the extent to which activity is different across hemispheres)[iii]. Handedness is an obvious example.

No general lateralisation preference

However, people don’t show a general dominance of one or other hemisphere. In 2013 a large study was conducted at the University of Utah[iv] where they analysed brain scans from over 1,000 individuals while at rest. The team found some lateralisation in regions associated with different tasks- those associated with language in the left hemisphere and with controlling attention in the right. However they found no evidence to support the notion that people are intrinsically left or right sided in how their brains are wired.

The impact of this myth on teaching

The implication of the left brain/right brain myth is that some people are better than others, or more suited than others, to certain tasks as a consequence of brain structure: this is not true. Degree of lateralisation only relates to behaviour in very specific areas; for example, degree of behavioural handedness is related to degree of lateralisation of the area of the brain that controls the hand[v].

In the classroom the left brain/right brain myth has resulted in the development of the ‘whole-brain learning’ approach, which involves trying to balance processing in the two hemispheres by including both analytical and creative aspects in a task. In a way this is all very well, as varying teaching methods may keep students attending longer, providing more opportunities to learn material, but the approach is not based on good science. The verdict? Definitely a neuro-myth, even though its origins are clear.

Further resources

The history of work with split brain patients is fascinating. These individuals have usually had their corpus collosums cut as part of a last-resort surgery to treat epilepsy. Researchers including Roger Sperry and Michael Gazzaniga were the pioneers in this area and an extensive literature exists, plus a number of videos of patients on YouTube, e.g., https://www.youtube.com/watch?v=SaZcIVvLmeM

For an excellent review of the role that neuroimaging has played in our understanding of hemispheric specialisation see Herve, P.Y., Zago, L., Petit, L., Mazoyer, B., & Tzourio-Mazoyer, N. (2013). Revisiting human hemispheric specialization with neuroimaging. Trends in Cognitive Sciences 17, 69–80.