Brain lateralization refers to the structural and functional differences between the hemispheres of the cerebrum. Functional differences crystallize a left-hemispheric dominance in language processes. In childhood brain lesions, the hemispheres compensate for the damage entirely.
What is brain lateralization?
Brain lateralization refers to the structural and functional differences between the hemispheres of the cerebrum. The cerebrum has two distinct halves. These so-called hemispheres of the cerebrum are separated by the fissura longitudinalis cerebri and connected by a thick nerve cord called the corpus callosum. Functionally, the two cerebral hemispheres are not identical in structure. The division of processes between the left and right hemispheres of the brain is described by the medical term ‘lateralization’ and thus corresponds to a neuroanatomical inequality in the specialization of the cerebral hemispheres. The brain of higher organisms is usually bilaterally symmetrical. Although the symmetry points to a similar structure, observations and experiments have long ago brought to light the spatial specialization of brain functions. Partial functions are preferentially performed in one of the cerebral hemispheres. The structural differences between the hemispheres are called anatomical asymmetries and manifest themselves, for example, in different volumes or with reference to the length, depth, or shape of the cerebral grooves. The hemispheres also differ with regard to the occurrence of certain cell types and the interconnectedness of the cells. Important asymmetries concern, for example, the Sylvian groove, the Heschl’s gyrus, the planum temporale, and the sulcus centralis. For example, the Sylvian groove is more extensive in the left hemisphere, so especially in right-handers. The left hemisphere tends to have a greater total weight, a greater proportion of gray matter, a larger inferior temporal lobe, and a larger nucleus lateralis posterior in the thalamus.
Function and task
In addition to the structural asymmetries of the left and right cerebral hemispheres, the two cerebral hemispheres also exhibit functional differences. The materialization of the brain corresponds to both structural and functional differences. Early studies in functional specialization correspond primarily to neurological or neuropsychological studies of brain injury showing an effect on cognitive abilities. For example, these studies compared patients with lesions of different hemispheres and thus inferred functional lateralization through the principle of double dissociation. In the 1960s, experimental studies of functional lateralization also took place using epilepsy patients who had the connection between the two hemispheres removed. Thanks to technological advances, imaging techniques such as magnetic resonance imaging (MRI) can now also be used to study functional lateralization. So far, studies concerning cognitive abilities have demonstrated a functional asymmetry of the hemispheres for speech production, for example. In this context, there is talk of a dominance of the left hemisphere in linguistic processes, which has been demonstrated in 95 percent of right-handers and 70 percent of left-handers. Research has proven that stimulus processing in the right hemisphere does not allow for speech-expressive utterance. The left hemisphere is also considered the dominant hemisphere in word recognition and mathematical operations. Medicine assumes a dominance of the right hemisphere for example in face recognition and spatial perception. Annett described the so-called right-shift theory for the lateralization of the cerebral hemispheres, which attributes the language dominance of the left hemisphere to just one single gene. According to Annett, an extremely pronounced dominance of a single hemisphere is associated with disadvantages in cognitive and also motor performance. Annett’s right-shift theory remains controversial in contemporary research, as researchers such as Crow have been unable to find a link between extreme hemisphere dominance and cognitive or motor impairment.
Diseases and ailments
Lateralization of the brain is particularly prominent when a single cerebral hemisphere is damaged. For example, if the left hemisphere of the brain is affected by a cerebral infarction-related or inflammation-related lesion, speech difficulties may result. Word recognition disorders may also be the result of such a lesion. Depending on the extent of the damage, speech therapy measures can alleviate the symptoms. If, on the other hand, the right hemisphere is impaired in its function due to a lesion, disorientation and a disturbed sense of space are common symptoms. From a neurological point of view, such damage becomes particularly interesting if the lateralization of the brain has not yet been fully completed at the time of the damage. The lateralization of the brain is not completed until puberty and is considered difficult to change thereafter. For example, when children experience damage to the left hemisphere due to an accident or other cause, the unfinished lateralization can become a great advantage. For example, it has been found that children generally do not experience speech problems in adulthood despite a left hemisphere lesion. Before the completion of lateralization, the brain is apparently able to fully compensate for the damage. Thus, in the case of aphasic damage in the speech center of the brain, the undamaged right hemisphere apparently completely takes over the speech functions of the left hemisphere. The same may be true for damage to the right hemisphere, which should actually be accompanied by impaired spatial awareness. After the completion of lateralization, a complete transfer of functions between the left and right hemispheres is no longer possible. Compensation for the damage is thus much more difficult and is therefore often associated with permanent damage.
