Hemispheric asymmetries

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32 Hemispheric asymmetries

The two cerebral hemispheres are asymmetrical in certain respects. Some of the asymmetries have to do with handedness, language, and complex motor activities; other, more subtle differences come under the general rubric of cognitive style. (Limbic asymmetries are described in Ch. 34.)

Handedness and Language

Handedness often determines the hemisphere that is dominant for motor control. Left hemisphere/right-hand dominance is the rule. Advances in ultrasound technology have made it possible for motor behavior in the fetus to be observed, and it has been noted that handedness is already established before birth on the basis of the preferred hand used for thumb sucking during fetal life.

The best indicator available for population estimates of handedness is the preferred hand for writing: this criterion indicates a left hemisphere dominance for motor control in about 90% of the population.

In 90% of subjects, the left hemisphere is dominant for language. In a further 7.5%, the right hemisphere is dominant in both sexes, and in the remaining 2.5%, the two hemispheres have an equal share. Although the left hemisphere is dominant in respect of both motor control and language, the two features are statistically independent: many left-handers have their language areas in the left hemisphere.

Language areas

Although several areas of the cortex, notably in the frontal lobe, are active during speech, two areas are specifically devoted to this function.

Broca’s area (Figure 32.1)

The French pathologist Pierre Broca assigned a motor speech function to the inferior frontal gyrus of the left side in 1861. The principal area concerned occupies the opercular and triangular parts of the inferior frontal gyrus corresponding to areas 44 and 45 of Brodmann. Both areas are larger on the left side in people who are right handed. The main output of Broca’s area is to cell columns in the face and tongue areas of the adjacent motor cortex.

Lesions involving Broca’s area are associated with expressive aphasia (see Clinical Panel 32.1). Some workers believe that expressive aphasia requires that the lesion also includes the lower end of the precentral gyrus.

Clinical Panel 32.1 The aphasias

Aphasia is a disturbance of language function caused by a lesion of the brain. The usual cause is a stroke produced by vascular occlusion in the anterior cortical territory of the left middle cerebral artery.

Wernicke’s area (Figure 32.1)

The German neurologist Karl Wernicke made extensive contributions to language processing in the late 19th century. He designated the posterior part of area 22 in the superior temporal gyrus of the left hemisphere as a sensory area concerned with understanding the spoken word. The upper surface of Wernicke’s area is called the temporal plane (Figure 32.2). The volume of cerebral cortex in the temporal plane is larger on the left side in 60% of subjects. The horizontal part of the lateral fissure is longer in consequence – a feature readily identified on MRI scans. Lesions involving Wernicke’s area in adults are associated with receptive aphasia (see Clinical Panel 32.1).

Wernicke’s area is linked to Broca’s area by association fibers of the arcuate fasciculus curving around the posterior end of the lateral fissure within the underlying white matter (Figure 32.1). The two areas are also linked through the insula.

It is difficult to assess the significance of the asymmetry of the temporal plane. The 60% incidence of left-sided relative enlargement does not match the 95% left hemisphere dominance for speech. Moreover, the overall length of the lateral sulcus is much the same on both sides. The parietal plane of that sulcus is longer on the right side because the right supramarginal gyrus is larger than the left one. This feature has been advanced as an explanation for the shorter temporal plane on the right.

Maldevelopment of the left temporal plane is a significant feature in cases of schizophrenia (Clinical Panel 32.4).

Listening to spoken words

Figure 32.3 contrasts regional increases in blood flow during PET scanning when a volunteer listens to words (‘active listening’) vs random tone sequences (‘passive listening’). As expected, tone sequences activate the primary auditory cortex (bilaterally). Wernicke’s area (left side) also becomes active, probably in screening out this non-verbal material from further processing. Area 9 in the frontal lobe is thought to be part of a supervisory, vigilance system.

During active listening to words, areas 21 (middle temporal lobe), 37 (posteroinferior temporal lobe), and 39 (angular gyrus) all participate in auditory word processing. Area 39 identifies phonemes. Areas 21 and 37 identify words in the sound sequence and tap into lexicons (dictionaries) stored in memory in a search for meaning – a process called semantic retrieval.

Activity in the left dorsolateral prefrontal cortex (DLPFC) expands to include area 46. Engagement of Broca’s area is thought to signify ‘subvocal articulation’ of words heard (see Neuroanatomy of reading, later).

When listening to one’s own voice, the areas of the temporal lobe identified above become active. An important function being served here is meta-analysis (post hoc analysis) of speech, whereby ‘slips of the tongue’ can be identified. Speech meta-analysis is singularly lacking in cases of receptive aphasia (Clinical Panel 32.1).