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.

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.

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.

Motor (anterior) aphasia

Patients having a lesion that includes Broca’s area suffer from motor aphasia. These patients have difficulty in expressing what they want to say. Speech is slow, labored, and characteristically ‘telegraphic’ in style. The important nouns and verbs are spoken but prepositions and conjunctions are omitted. The patient comprehends what other people are saying and is well aware of being unable to speak fluently. There is usually an associated agraphia (inability to express thoughts in writing).

If the lesion involves a substantial amount of the cortical territory of the middle cerebral artery, there will be a motor weakness of the right lower face and right arm. Because the lips and tongue are affected on the right side, the patient will also have dysarthria (difficulty in speech articulation) in the form of slurring of certain syllables. In the example shown in Figure CP 32.1.1A, Broca’s aphasia would be associated with right-sided weakness of the lower face but not of the arm, together with some dysarthria.

Sensory (posterior) aphasia

A lesion in Wernicke’s area is accompanied by a deficit of auditory comprehension. If the lesion includes the angular gyrus, the ability to read will also be compromised. In addition to their difficulty in understanding the speech of others, these patients lose the ability to monitor their own conversation, and usually have difficulty in retrieving correct descriptive names. Speech fluency is normal but two kinds of abnormality occur in the use of nouns:

• Verbal paraphasia (use of words usually of allied meaning): instead of ‘use a knife’, ‘use a fork’.

• Phonemic paraphasia (use of made-up but similar-sounding syllables): instead of ‘knife and fork’, ‘bife and dork’.

The most striking feature of Wernicke’s aphasia is that, despite garbling to the point of being unintelligible (jargon aphasia), the patient may be quite unaware of making mistakes.

In the example shown in Figure CP 32.1.1B, Wernicke’s aphasia would be associated with alexia (angular gyrus), ideomotor apraxia (supramarginal gyrus) and probably with a right upper quadrant visual deficit (lower fibers of left optic radiation in the temporal white matter).

Aprosodia

Lesions of the right hemisphere may affect speech in subtle ways. Lesions that include area 44 (corresponding to Broca’s area on the left) tend to change the patient’s speech to a dull monotone. On the other hand, lesions that involve area 22 (corresponding to Wernicke’s area) may lead to listening errors, e.g. being unable to detect inflections of speech; the patient may not know whether a particular remark is intended as a statement or as a question.

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

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.