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.

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).

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.

Figure 32.3 Regions of increased blood flow during listening (A) to tones, and (B) to words.

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).

Modular organization of language

In alert subjects, electrical studies of the cortex exposed during neurosurgical procedures indicate the presence of a vast cortical mosaic for language. The mosaic of modules extends along the entire length of the frontoparietal operculum above the lateral sulcus and of the temporal operculum below the sulcus. The frontoparietal operculum is predominantly concerned with the motor functions of speaking and writing, and the temporal operculum with the sensory functions of hearing and reading.

It is well known that children have greater facility than adults in acquiring a second language. fMRI and other approaches have shown that the loci of second-language acquisition before the age of 7 years overlap extensively with those processing the native language. A second language learned in later years is non-overlapping with the first. One possibility is that in children, the syntactical systems for processing nouns, verbs, etc., are able to cope with two or more languages simultaneously.

It is of interest that following a small vascular lesion in an adult, either a late-acquired language or the native language may be lost, leaving the other relatively intact.

Neuroanatomy of reading (Figure 32.4)

Glossary

• Graphemes. Written syllables.

• Orthography (Gr. ‘correct writing’). Word and sentence construction.

• Phonemes (Gr. ‘sounds’). The sounds of syllables. ‘Cat’ is a single syllable containing three phonemes, [k], [a], and [t].

• Phonology. The rules governing the sounds of words. Testing could include: ‘How many of these words have two syllables?’ or ‘How many of these words rhyme with one another?’

• Retrieval. Matching words, phrases, and sentences with those previously entered into memory.

• Semantics (Gr. ‘meaning’). Meaning of words and sentences.

Figure 32.4 (A) Areas of increased cortical blood flow in the left hemisphere observed in PET scans during reading aloud. (B) Diagram representing input (blue), processing (green), and output (red) pathways active during reading aloud. DLPFC, dorsolateral prefrontal cortex.

Superior parietal lobule and the body schema

The term body schema refers to an awareness of the existence and spatial relationships of body parts, based on previous (stored) and current sensory experience. The reality of body schema has been established by the condition known as hemineglect, in which a patient with a lesion involving the superior parietal lobule ignores the contralateral side of the body.

Hemineglect is much more common following a right parietal lobe lesion than a left one. Under normal conditions, however, each parietal lobe exchanges information freely with its partner through the corpus callosum and the left and right hand are equally adept at distinguishing a key from a coin in a coat pocket without the aid of vision (stereognosis, Ch. 29).

Patients with a right hemisphere lesion involving the superior parietal lobule have difficulty in distinguishing between unseen objects of different shapes with the left hand. They have astereognosis. Patients with a comparable lesion in the left hemisphere are able to make this distinction using the right hand but they have difficulty in announcing the function of a selected object. The left supramarginal gyrus participates in phonological retrieval, as already noted, and the deficit, although a semantic one, may be related to interference with the ‘inner speech’ that usually accompanies problem-solving.

Both deficits are forms of tactile agnosia. The right hemisphere deficit has become known as apperceptive tactile agnosia (‘apperception’, awareness of perception), and the left one as associative tactile agnosia (failure to identify functional associations).

Handedness and balance

In Chapter 18 we noted that the pathway from the vestibular nucleus to the parieto-insular vestibular cortex (PIVC) is mainly ipsilateral. Formal testing of static and dynamic vestibular functions, under fMRI monitoring, has revealed that maximal activation of the PIVC is produced in the minor hemisphere, as illustrated in Figure 32.6. The limited data available indicate a tight linkage between handedness and vestibular cortical activation, to the extent that the ‘preferred’ vestibular hemisphere, in view of its phylogenetic antiquity and early ontogenesis (e.g. presence of postural vestibular reflexes in the newborn), may determine handedness.

Figure 32.6 Diagrams illustrating hemispheric dominance in response to warm water irrigation of the outer ear canals of pronounced right-handers and left-handers. (The head was tilted back 20° to put the lateral semicircular canals in the horizontal position.) Arrows indicate the slow phase of caloric nystagmus. Note pronounced minor hemisphere response to ipsilateral caloric irrigation (2 and 3), and less marked major hemisphere response (1 and 4).

(Based on diagrams in Dieterich et al., 2003.)

Parietal lobe and movement initiation

There are several sites for movement initiation in different behavioral contexts. The present context is the performance of learned movements of some complexity: examples would include turning a door knob, combing one’s hair, blowing out a match, and clapping. It is logical to anticipate a starting point within the dominant hemisphere because they can all be performed in response to a verbal command (oral or written). This notion receives support from the observation that, if the corpus callosum has been severed surgically, the patient can perform a learned movement on command using the right hand, but not on attempting it with the left hand.

Failure to perform a learned movement on request is called ideomotor apraxia, or limb apraxia. It has been repeatedly observed immediately following vascular lesions at the sites listed and described in Figure 32.7. In that figure, a vascular stroke at site 2 injuring corticospinal fibers descending from the hand area of the motor cortex, would cause clumsiness of movement of both hands, whereas a similar lesion on the right side would only compromise movements of the left hand.

Figure 32.7 Association and commissural pathways serving motor responses to sensory cues. (Adapted from Kertesz and Ferro, 1984.) The premotor cortex is under higher control by the prefrontal cortex.

Lesions at site 1 effectively sever the anterior part of the corpus callosum and produce ipsilateral limb apraxia (left lesion, left limb). Lesions at either site 2 (superior longitudinal fasciculus) or site 3 (angular gyrus) may produce bilateral limb apraxia. In practice, a large lesion may render right-limb apraxia impossible to assess because of associated right hemiplegia or receptive aphasia.

Ideomotor apraxia can be accounted for if the dominant parietal lobe is considered to contain a repertoire of learned movement programs which, on retrieval, elicit appropriate responses by the premotor cortex on one or both sides under directives from the prefrontal cortex. (The basal ganglia would also be involved, as described in Ch. 33.)

Ideomotor apraxia is a transient phenomenon. Because parietal blood flow increases almost equally on both sides during reaching movements, the right hemisphere seems to be able to assume a full role for the left arm when no longer overshadowed.

Clinical Panel 32.3 provides a brief account of parietal lobe dysfunction.

Figure CP 32.3.1 Angular gyrus.