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.

Figure 32.1 Broca’s and Wernicke’s language areas and the arcuate fasciculus.

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.

Figure CP 32.1.1 (A) Vascular lesion involving Broca’s area. (B) Vascular lesion involving Wernicke’s area.

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).

Figure 32.2 Views of the opened lateral sulcus, showing the upper surface of the temporal lobes.

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).

Right hemisphere contribution

During normal conversation there is some increase in blood flow in areas of the right hemisphere matching those of the left. These areas are believed to be concerned with melodic aspects of speech – the cadences, emphases, and nuances, collectively called prosody. Disturbances of the melodic function are called aprosodias (Clinical Panel 32.1).

Recovery of speech function – when it occurs – depends upon the age of the subject, and in adults upon the extent of the lesion. Occasional cases have been reported of recovery of near-normal speech in right-handed patients 7 years of age or less following complete removal of the left hemisphere as a treatment for intractable epilepsy. This can only be explained by language processing, including speech, being not fully lateralized at the time of operation. In adults, PET studies have shown increased activity in Broca’s and Wernicke’s equivalents on the right side following cerebrovascular accidents on the left. However, significant improvement is possible only if the left temporal plane is sufficiently viable to be able to process signals passed to it from the right side through the corpus callosum.

Angular gyrus

The angular gyrus (area 39) belongs descriptively to the inferior parietal lobule. The left angular gyrus receives a projection from the inferior part of area 19 (the lingual gyrus, shown in Figure 2.6), and itself projects to the temporal plane. It is commonly included as a part of Wernicke’s area.

The angular gyrus seems to contain a neural lexicon (dictionary) of words, syllables, and numerical or other symbols, which can be retrieved by visual inputs – or even by visual imagery – and forwarded in the form of impulse trains to Wernicke’s area within area 22. During reading, it is engaged in the conversion of written syllables (‘graphemes’) into the corresponding sound equivalents (‘phonemes’). The angular gyrus is also active during listening to spoken words.