The human brain is made up of a number of very complex parts that can be broadly divided into the left and right hemisphere. To a casual observer these two hemispheres can seem identical, containing much the same sub-components. They are joined together and communicate through four bundles of axons known as commissures. The most important of these inter hemispheric channels is the corpus collosum, which connects the neo-cortex of each hemisphere together.
The first theories concerning the hemispheres of the brain concerned the two being identical, with each controlling the functions and actions of the contralateral side of the body (e. g. the right hemisphere would control the sensory functions of the left visual field and the motor functions of the left limbs). This idea and others like it have since been superseded by findings that have shown that there are a number of fundamental differences, both in form and function, between the two hemispheres. There have been a number of different ways in which the physiology and functional specialisation of the hemispheres have been studied.
The more basic physiological tests include direct cortical stimulation and various forms of brain scans such as PET, EEG and cortical recordings. Functional testing has employed both mentally healthy and brain-damaged participants (or those who have had psychosurgery such as a commissurotomy). These tests have included lateralised presentation of visual or auditory stimuli, and the anaesthetising of one or other hemisphere during various functional tasks. The most fundamental difference that exists between the two cerebral hemispheres is that of structure.
Although as far as basic components are concerned the two can be seen to be symmetrical, the size and in some cases the shape of these parts can be seen to vary greatly. Firstly, the Planum Temporale (the upper part of the temporal lobe) is, in the majority of cases, larger in the left hemisphere. Area Tpt (in the posterior third of the temporal gyrus) is also commonly larger in the left hemisphere. Broadman’s area 44 (part of Broca’s area, situated in the prefrontal cortex) is commonly larger in the left hemisphere.
Also in the left hemisphere the Sylvian fissure is often longer and shows a more horizontal orientation than that of the right hemisphere. On the right hemisphere, Heschl’s gyrus (part of the superior temporal gyrus) is larger, while its counterpart on the left hemisphere is often more oblique (Porac and Coren, 1981). Moreover, there is evidence that the occipital horn is larger in the left ventricle (on the left hemisphere) which in turn suggests that there is more occipital tissue on the right side.
Finally, the motor tracts on the right side are often larger, and the patterns of vascularisation also often differ (Galebunda et al, 1978). What is very important to note about these physiological differences is that they are far from consistent between all human brains. Instead, they are seen to be probabilistic, with many people showing some but not all of the differences, and then to greater or lesser extents. Moreover, the occurrences of these structural differences also seem to be directly related to handedness in the subjects (and so, by implication to the hemispherical dominance of the individual).
The asymmetries as stated seem to remain relatively constant among right handers, but are far less generalisable among those who show a left-hand preference. In these left handers, when a physiological asymmetry is apparent between the cerebral hemispheres it is often the mirror of those previously described as common among right handers (e. g. it is the right hemisphere that contains a larger Planum Temporale). What seems far more common in left-hand dominant individuals is a much greater symmetry between the hemispheres.
Where this is seen, however, the symmetry commonly follows a double left (the left hemispherical feature is replicated on the right side) rather than a double right pattern (for example a replica of the left Sylvian fissure is located on the right, not vice versa) (Porac and Coren, 1981). One question that has arisen from the study of these structural differences between hemispheres is whether the asymmetries correlate to the lateralisation of function that seems to exist.
There is currently no definitive answer to the question, but evidence from recent studies would suggest that there is almost definitely some sort of connection between the two. What this leads on to is the question of whether the two cerebral hemispheres are identical in function, again to which the answer seems to be a definitive ‘no’. In many of the functions of the human brain (especially those involving higher-order reasoning) there can often be seen a specialisation by one hemisphere or the other.
It is difficult to make generalisations due to the many, varied conditions that can effect hemispheric form and function. The most general statement that can be made is that the specialisations of the right hemisphere seem to be centred around non linguistic functions that involve complex visual or spatial processes (Springer and Deutsch, 1998). In an area of basic functioning such as visual processing, the hemispheric activity seems to be rather symmetrical, with little evidence for specialisation by either hemisphere.
If however we look at more complex functions within this category, such as part-whole relationships or spatial recognition we can quickly see proof that specialisation does indeed take place (Witelson, 1976). In one study carried out using patients who had undergone commissurotomies (had the commissures that link the two hemispheres surgically severed so they subsequently functioned unilaterally), the patients had arcs presented to either their left or right visual field. After each presentation the patients were offered a choice of three circles of various sizes, of which they had to select the one that was formed by the arcs they had seen.
The patients performed significantly more accurately on the judgements where the arcs were presented to their left visual field (Nebes, 1978). According to the aforementioned theory of contralateral relations these results would be indicative of right hemisphere dominance in this area. This theory is further supported by the findings that split-brained patients (those who had undergone a commissurotomy) lost relatively little ability to draw a cube with their left hand, but found considerable difficulty in doing so with their left (Gazzaniga and LeDoux, 1978).
Another complex function that is related to visual processing is that of facial recognition, and again this can be seen to be subject to lateralisation with areas of the right hemisphere showing much higher levels of activity during this action. This was explored using PET scanning of cerebral blood flow to the various areas of the two hemispheres during face recognition tasks (compared to levels during control conditions and baseline conditions) (Sergent et al, 1992).
What was shown was that during basic visual processing tasks blood flow increased (indicating increased activity) in the striate cortex of the occipital lobe (the primary visual cortex) in both hemispheres equally. However, in the face recognition tasks there was also activation of the posterior regions just outside the visual cortex of the right hemisphere, as well as activation of the right temporal lobe regions which extended deep into the right temporal lobe and down towards the hippocampus.
This increased activity on specific areas of the right hemisphere was not observed in the left hemisphere. What further studies did suggest is that the left hemisphere is much more involved with the recognition of specific features, as was shown by experiments involving inverted face perception (Springer and Deutsch, 1998). A second area of functioning in which hemispheric dominance is apparent is that of auditory perception. In this, even the basic function can be seen to be clearly asymmetric, as demonstrated by the concept of the ‘right ear advantage’.
This is the concept that auditory information taken in by either ear is sent to receptors in both hemispheres simultaneously. However, each ear sends the information to the contralateral hemisphere over the relatively strong contralateral pathways, but to the hemisphere on the same side to the relatively much weaker ipsilateral pathways. Kimura (1967) pointed out that if two different pieces of aural information were presented simultaneously (one to each ear), whichever was processed more accurately would indicate which hemisphere was dominant in auditory processing.
Implication of this dichotic listening experiment resulted in the finding that the participants only reported hearing the information presented in the right ear suggesting left hemisphere dominance in this field. Further studies have focused on the higher-order function of music perception, rather than basic auditory perception. In these tests, ‘in vivo magnetic resonance morphometry’ and PET scanning were used to measure the activity of the planum temporale during perfect pitch tests (Schlaug et al, 1995). The results were similar to those in the previous tests, showing strong dominance of activity in the left hemisphere.
Studies into language functioning have also resulted in support for the lateralisation of the two hemispheres. Studies that have employed the various methods of scanning previously seen have concluded that there is much more activity in the left hemisphere during the majority of language tasks. Split-brain studies (those using commissurotomy patients) have also demonstrated very poor language comprehension abilities in the right hemisphere, demonstrating minimal comprehension with a very limited lexicon and a lack of grammatical understanding.
Moreover, studies into acquired aphasias (language disorders) have showed that they are almost exclusively the result of damage to the left hemisphere (in particular to Broca’s area, Wernicke’s area or the angular or supermarginal gyri) (Springer and Deutsch, 1998). Lesion studies of memory have also produced evidence that suggests that the cerebral hemispheres are not identical. They have shown that lateralised lesions such as unilateral temporal lobectomies result in memory deficits, and in particular in specific semantic memory skills.
There is also a reported difference in the kind of memories that are disrupted after left and right temporal lobe lesions, which indicates that each hemisphere is unilaterally responsible for different aspects of the memory function. Left temporal lobectomies have been seen to disrupt learning and retention of verbal material. In contrast, right temporal lobectomies lead to difficulty with non-verbal materials, as well as other related tasks such as maze learning (Blakemore and Falconer, 1967). These findings from lesion studies of memory have been supported by evidence from the analysis of memory abilities in split-brain patients.
When the two hemispheres have been tested separately there is a clear difference observed in what kinds of information each can learn and remember. Language information is retained far more effectively by the left hemisphere, whereas visuo-spatial information is retained much better in the right hemisphere. Short term memory has also been proven to be hemispherically asymmetrical. Short term phonological memory store is impaired by damage to the lower region of the posterior left parietal lobe.
Short term visuo-spatial memory is impaired by damage to the posterior right hemisphere (Springer and Deutsch, 1998). Thus it can be seen that it totally unrealistic to try and claim that the two cerebral hemispheres are identical. Firstly, they are different in their physiology, with many of the fundamental areas of each showing marked asymmetry. It is not yet clear why such asymmetry exists, and it is as yet impossible to accurately generalise as to what form it will take, but it is undeniable that it is common and results in an inability to claim identical structure.
Moreover, many various studies (such as various forms of brain scans, studies of hemispheric damage, and studies of split-brain patients) have shown that the two hemispheres differ fundamentally in their functions. The Right hemisphere is seen to be dominant over many actions that involve visuo-spatial relations and non-verbal actions or reasoning. The left hemisphere seems to focus more on verbally based functioning and reasoning as well as much of what is considered to be ‘basic’ functioning.