Lateralization of the brain

With the Lateralization of the brain are the structural and functional differences between the hemispheres of the cerebrum. When it comes to functional differences, a left-hemispheric dominance in language processes emerges. In childhood brain lesions, the hemispheres fully compensate for the damage.

What is the lateralization of the brain?

The cerebrum has two different halves. These so-called hemispheres of the cerebrum are separated by the fissura longitudinalis cerebri and connected to one another by a thick nerve cord, the so-called corpus callosum.

Functionally, the two cerebral hemispheres are not structured identically. The division of processes between the left and right hemispheres of the brain is described using 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 constructed bilaterally symmetrically. The symmetry indicates a similar structure, but observations and experiments have long ago brought to light the spatial specialization of the brain functions. Partial functions are preferably carried out in one of the hemispheres of the brain.

The structural differences between the hemispheres are known as anatomical asymmetries and are expressed, for example, in different volumes or in relation to the length, depth or shape of the cerebral furrows. The hemispheres also differ with regard to the occurrence of certain cell types and the interlinking of the cells with one another.

Important asymmetries concern, for example, the Sylvian furrow, the Heschl gyrus, the planum temporale and the sulcus centralis. For example, the Sylvian groove is more extensive in the left hemisphere, especially in right-handed people. The left hemisphere usually has a greater total weight, a greater proportion of gray matter, a larger inferior-temporal lobe, and a larger posterior lateral nucleus in the thalamus.

Function & task

In addition to the structural asymmetries of the left and right cerebral hemispheres, the two halves of the brain also show functional differences. The materialization of the brain corresponds to both the structural and functional differences. Early studies in functional specialization correspond primarily to neurological or neuropsychological studies of brain injuries that show an effect on cognitive abilities. For example, the studies compared patients with lesions in different hemispheres and concluded that the functional lateralization was based on the principle of double dissociation.

In the 1960s, experimental studies of functional lateralization also took place on epilepsy patients who had had the connection between the two hemispheres removed.

Thanks to technical progress, imaging methods such as magnetic resonance tomography (MRT) can now also be used to examine functional lateralization. So far, studies on cognitive abilities have shown a functional asymmetry of the hemispheres, for example for speech production. In this context, there is talk of a dominance of the left hemisphere in linguistic processes, which could be demonstrated in 95 percent of right-handers and 70 percent of left-handers.

Studies have shown that stimulus processing in the right hemisphere does not allow for expressive speech. The left hemisphere is also the dominant hemisphere in word recognition and mathematical operations. Medicine assumes that the right hemisphere is dominant, for example in facial recognition and spatial perception.

Annett described the so-called right-shift theory for the lateralization of the hemispheres of the brain, which traces the dominance of language in the left hemisphere back to a single gene. According to Annett, an extremely pronounced dominance of a single hemisphere is associated with disadvantages in cognitive and motor performance. Annett's right-shift theory remains controversial in contemporary research, as researchers such as Crow could not find a connection between extreme dominance of the hemispheres and cognitive or motor impairment.

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Illnesses & ailments

The lateralization of the brain is particularly evident when a single hemisphere of the brain is damaged. For example, if the left side of the brain is affected by a cerebral infarction or inflammation-related lesion, speech difficulties may arise. Word recognition disorders can also result from such a lesion. Depending on the extent of the damage, speech therapy measures can alleviate the symptoms. If, on the other hand, the function of the right hemisphere is impaired by a lesion, disorientation and a disturbed sense of space are common symptoms.

Such damage becomes particularly interesting from a neurological point of view 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 then considered to be difficult to change. For example, if children experience damage to the left hemisphere as a result of an accident or otherwise, the incomplete lateralization can be a great advantage. For example, it has been found that children, despite a lesion of the left hemisphere, generally do not have any language disorders in adulthood. Before the completion of the lateralization, the brain is apparently able to fully compensate for the damage.The undamaged right hemisphere apparently completely takes over the speech functions of the left hemisphere in the event of aphasic damage in the speech center of the brain. The same can apply to damage to the right hemisphere, which should actually be accompanied by a disturbed sense of space.

After the completion of the lateralization, a complete takeover of functions between the left and right hemisphere is no longer possible. Compensation for the damage is therefore much more difficult and is therefore often associated with permanent damage.