Broca (1865) famously stated “We speak with the left hemisphere.” and decades later Hughlings-Jackson (1874) proposed a complementary role for the right hemisphere; visual-spatial functions. After such statements, the idea that the left and right hemispheres play different roles in mental functioning has become one of the most well-known and researched scientific principles about the human nervous system.
Such a phenomenon was termed hemispheric specialisation. It is the idea that each hemisphere has specialised functions, or that each exerts greater control over a particular function; for example, language. Both hemispheres are linked via the corpus callosum, through which they communicate and coordinate. However, even though both the left and right hemispheres are involved in nearly all functions, they do appear to have some separate functions. For example, Ehrenwald (1984) considered the left hemisphere to specialise in verbal and analytical functions such as writing, reading, speech and breaking down and solving mathematical problems. Whereas, the right hemisphere is thought to be more dominant in nonverbal functions that are not dependent on language skills. These functions are typically spatial and visual thinking such as reading maps, facial recognition and creativity. In addition, it has been discovered that the left hemisphere controls the right side of the human body, and the right hemisphere controls the left side.
Today, both hemispheres individually as well as the connection between them, receive intense scrutiny from researchers representing a range of disciplines. What accounted for hemispheric specialisation? Such a question is no less challenging for the right hemisphere than for the left hemisphere functions. But given the greater roles specified to the left hemisphere for the ‘higher’ functions of intellect and as there is a far larger body of evidence for the left than the right hemisphere specialisation, the search for explanations is focussed mainly on the left hemisphere.
A few theories have been put forward in attempt to explain hemispheric specialisation. A widely accepted theory is rooted in evolution. It is thought that the human brain is a collection of neurological adaptations established through natural selection. These adaptations can be lateralised to specific regions of networks in the brain. Throughout the animal kingdom however, capacities are generally not lateralised; instead particular functions or skills tend to be found in both hemispheres to roughly equal degrees although some monkeys show signs of hemispheric specialisation (Ghazanfar & Hauser 2001). Due to this, it has always appeared that the hemispheric specialisation in the human brain was an evolutionary add-on, suggesting abilities or mechanisms were laid down in one hemisphere only. More recently however, it is speculated that some lateralised phenomena may arise from a hemisphere losing an ability not gaining one. By this, it is suggested that there would have been competition for cortical space, and the evolving brain would have been pushed to gain new facilities without losing old ones. Therefore hemispheric specialisation would have been the answer. Due to the two hemispheres being connected, mutational altering with a homologous cortical region could give rise to a new function. This would not affect the human as the other hemisphere would remain unaffected.
In an attempt to advance on the evolutionary theory of hemispheric specialisation, it has been considered that with the increase in brain size came an increase in the distance over which neural signals had to travel, which leads to an increased amount of time required for transmission. As there is no evidence for the increase in transmission speed, it is considered logical to look at the distances involved. This is proposed to be the reason behind hemispheric specialisation; the need for reduction in distance of transmission. Evidence supporting this is the observation that larger brains have proportionally speaking, a smaller corpus callosum, resulting in fewer trans-hemispheric connections and therefore increased specialisation. This can be supported when researching sex difference and hemispheric specialisation according to brain size. Fausto Sterling (1992) found in a study of over 4000 participants that the average difference between brain mass and sex was 9.8% across all ages. Males having larger brain mass, and therefore a small corpus callosum correlated positively with the findings from Bourne (2005) that males are more strongly hemispheric specialised. However, Sommer, Aleman, Somers, Boks & Kahn (2008) found no sex difference in hemispheric specialisation. Furthermore, Obleser, Eulitz, Lahiri & Elbert (2001) research showed contrasting results to the scientific explanation of brain mass and hemispheric specialisation. Obleser et al (2001) found that females, having smaller brain mass’ than males, were more strongly hemispheric specialised than males. Nevertheless, this shows that brain size and mass is not a reliable theory to explain hemispheric specialisation.
In a different light, the analytic-synthetic theory of hemispheric specialisation suggests that there are two fundamentally different modes of thinking; analytic and synthetic. Therefore the neural circuitry for each is primarily different. For example, the left hemisphere is thought to operate in an analytical, logical and sequential manner, whereas the right hemisphere is thought to work in a holistic fashion, and therefore makes immediate, overall, synthetic judgements. This theory for hemispheric specialisation is surrounded with research, providing support for both hemispheres. Broca (1861) was the first to provide evidence for language being linked to the left hemisphere after studying the brains of two patients with speech impairments after they had died. Broca’s first patient, ‘Tan’ could understand everything Broca would say to him, but found he could not respond back. In addition, over time Tan’s right side of his body became weak and eventually paralysed. Broca found after performing an autopsy on Tan’s brain that there was extensive softening throughout the left frontal lobe. Broca’s second patient, Lelong, was also found to have a lesion in the same areas as Tan. Although, Broca had not associated this speech impairment with the left hemisphere itself, he had noted in numerous patients that when he found a lesion on the left side of the brain, the patient showed a speech problem. This condition became known as Aphasia. This shows strong evidence that the left hemisphere is associated with language, speech in particular.
More early support for the analytic-synthetic theory was provided by Wernicke (1874). Wernicke discovered a different speech disorder to Broca, whereby patients articulated normally and fluently, but the speech was nonsensical, without coherency and contained lexical selection and grammatical errors. Once brain autopsies were performed, Wenicke found lesions in the left temporal lobe in the left hemisphere were the reason behind the speech problems. This again provides support that the left hemisphere is dominant for aspects of language.
Sperry (1968) investigated hemispheric specialisation, specifically language by using those who had had their corpus callosum severed as treatment for epilepsy. Sperry used a tachistoscope and tested each of the eleven split brain patients. They were to focus on a dot in the middle of a screen. When an image was flashed on the screen to the right of the dot, the visual information about the image travelled to the left hemisphere and when asked, the patient was able to state verbally what the flashed image was. However, when Sperry flashed an image to the left of the dot, the visual information travelled to the right hemisphere, and when the patient was asked about the flashed image, the patient had no knowledge that their was an image flashed onto the screen. What’s more, Sperry investigated further and asked the split brain patients to pick out an object using their left hand, when the patient did so the touch information from the left hand travelled to the right hemisphere, the side that ‘saw’ the image, but was unable to verbally state what the object was. Therefore, this shows that the right hemisphere has very little ability for language and the left hemisphere is completely dominant when it comes to language. In addition, one of the greatest findings from the split brain experiments was the insight into the knowledge that the right vision field is connected to the left hemisphere and the left vision field is connected to the right hemisphere, which in turn suggests that the function of vision is not specialised but is in both hemispheres. This also supports the analytic-synthetic theory.
A different way to show evidence for hemispheric specialisation was shown by Levy (1972) with chimeric figures. This is a figure whereby there was the left side of a woman’s face on the left, and a right side of a man’s face on the right, joined together by a dot in the centre. When the participant was asked to focus on the dot in the middle, the visual information about the woman’s face on the left will go to the right hemisphere and the visual information about the man’s face on the right will go to the left hemisphere. When a split brain patient is asked to point out which face they saw, the woman’s face from the left of the image is usually picked out. However, if the patient is asked to verbally state whether the picture was that of a man or a woman, the patient will state it was of a man. Therefore, this shows that depending on what the patient is required to do, either the right or left hemisphere will dominate. In this case, when speech is not required, the right hemisphere will dominate for recognition of faces. When language is needed the left hemisphere will dominate.
A different function that is thought to be dominant to the left hemisphere is arithmetic. Dehaeane & Cogen (1997) found that brain damaged patients with Aphasia generally exhibited calculation deficits. Aphasia is associated with left hemisphere lesions. In addition, neuroimaging studies have found that overlapping left parietal regions are involved in the recall of arithmetic facts Chochon, Cohen, van de Moortele & Dehaene (1999), and in addition, the fMRI scan found activity on the left and right hemispheres during subtraction tasks and just left hemispheric activity during multipication tasks, but in turn, no significant difference was found in cortical activity. However, such findings led Deheane to put forward the theory that both language-dependent and language-independent components contribute to arithmetic processing, and a model of mathematical processing was created. It was considered that the difference between the left and right hemispheres’ abilities within arithmetic was that the right hemisphere will be able to independently perform subtraction and approximate calculation, due to the processes relying on the quantity representations modules represented in both hemispheres. Therefore, the left hemisphere will be able to execute all of the arithmetic operations and perform exact and approximate calculations due to it having access to both language-dependent and language independent modules. As arithmetic skills might not be completely dominant by the left hemisphere, Stanescu-Cosson, Pinel, Moortele, Bihan, Cohen & Dehaene’s study (2000) involved an ERP and fMRI experiment comparing cortical activity during exact and approximate arithmetic operations and it was found that exact arithmetic obtained a more left hemispheric pattern of activity than approximate arithmetic. Similarly, Dehaene & Cohen (1991) reported Aphasic patients with impaired exact addition but preserved approximation. This in turn suggests each hemisphere is linked with arithmetic, but different areas of arithmetic seem to be specialised by each hemisphere. Earlier research showed that the left hemisphere was able to add, subtract, multiply and divide, but that the right hemisphere was unable to perform any of these operations Gazzaniga & Smylie (1984). However, there was a large limitation to the methodology in this study regarding linguistic ability. Due to the arithmetic problems being presented to the participant verbally with the exception of one digit that was briefly presented to the right or the left visual field, with the left hemisphere being dominant to the right in language, a hemispheric difference in comprehension of the verbally presented information could account for the hemispheric difference observed in calculation performance. Therefore, with all the recent research within arithmetic skills on board, it appears that this supports the analytic-synthetic theory; that each hemisphere has a different mode of thinking. It can be suggested further that the whole function of arithmetic is not individually hemispherically specialised, but certain aspects of arithmetic are dominant by the left or by the right hemisphere.
More recent research by Anes & Kruer (2004) linking emotive words and face recognition to hemispheric specialisation showed some interesting findings. A similar methodology to Levy’s (1972) chimeric study was used and participants were asked to determine whether lateralized images of human faces were showing an angry or a happy facial expression. Alongside each image were words such as ‘happy’ ‘angry’ ‘blank’ which served as distracting stimuli. It was found that there was greater stroop interference in identification accuracy with incongruent displays of facial expression in the left visual field and emotion words in the right visual field. Therefore, both males and females showed interference effects when their task was to identify the facial expression and an incongruent word was directed to the opposing visual field. This shows that hemispheric specialisation does occur. In contrast, Kavcic & Clarke (2000) found in their earlier research that they didn’t find hemispheric specialisation whilst using a face-word stroop task.
As it appears there is a lot of evidence showing hemispheric specialisation of the left hemisphere, there is also a significant amount that focuses on the right hemisphere within the region of emotion. Mills (1912) observed that damage to the right hemisphere caused a decrease of emotional expression. Similarly, Babinski (1914) found that patients with lesions to the right hemisphere became manic or emotionally different. Such early studies led to the right hemisphere hypothesis, which states a dominant role of the right hemisphere is emotion. More recently, Sackheim, Gur & Saucy (1978) found that facial expressions are more intensely expressed in the left side of the face, suggesting a greater involvement of the right hemisphere in the production of emotional displays. In turn, Adolphs, Damasdio, Tranel & Damasio (1996) provided even more support as they found patients with right hemisphere damage were more impaired in recognising facial expressions than patients with left hemisphere damage. This shows strong evidence for hemispheric specialisation of the right hemisphere for emotion.
However, even though there is such a strong database of support for right hemisphere dominance for emotion, there is empirical evidence showing otherwise. Goldstein (1939) showed that damage to the left hemisphere was more likely to cause a catastrophic-depressive reaction in psychiatric patients than damage to the right hemisphere. In addition, Sackheim et al (1982) reviewed 109 cases of pathological laughing and crying, and found evidence suggesting a differential hemispheric specialisation for positive and negative effects. For example, damage to the left hemisphere led to the onset of depressive symptoms, whereas damage to the right hemisphere was associated with a pathological laughing condition. Therefore, this shows possibility that there is not a specific hemisphere for emotion, but there is for negative and positive emotions; the left hemisphere linking with negative emotion and the right hemisphere linking with positive emotion.
In addition to right hemispheric dominance, the function of colour detection is also thought to be right hemisphere dominant. From research by Sasaki, Morimoto, Nishi & Matsuura (2007) it was strongly suggested that there is right hemisphere superiority for detection of colour among right-handed individuals by using reaction times and an achromatic target which was presented either on the right or the left visual field horizontally from a fixation point. These results were also consistent with previous research such as that of Scotti & Spinnler (1970) who reported that deficits in colour detection in the contralateral visual field are more frequently observed in patients with a lesion of the right hemisphere. It is interesting that Sasaki et al (2007) study used ‘normal’ participants, not those with brain damage or split brains, which leads to a definite hemispheric specialisation for colour detection in the right hemisphere. Furthermore, Albert, Reches & Silverberg (1975) reported patients with impairments of the left visual field have cortical colour blindness. These findings imply further that the right hemisphere is dominant in the detection of colour. There is no significant research known that disputes the theory of colour detection being heavily linked to the right hemisphere.