The cerebral hemisphere is derived from the embryological telencephalon (Ch. 1). It is the largest part of the forebrain and it reaches the greatest degree of development in the human brain. Superficially, the cerebral hemisphere consists of a layer of grey matter, the cerebral cortex, which is highly convoluted to form a complex pattern of ridges (gyri) and furrows (sulci). This serves to maximise the surface area of the cerebral cortex, about 70% of which is hidden within the depths of sulci (Figs 13.1, 13.2). Beneath the surface, axons running to and from the cells of the cortex form an extensive mass of white matter. Figures 13.3-13.12 show successive coronal sections through the brain. The vast majority of those nerve fibres that pass between the cerebral cortex and subcortical structures are condensed, deep within the hemisphere, into a broad sheet called the internal capsule (Figs 13.4-13.11; see also Fig. 1.23). Between the internal capsule and the cortical surface, fibres radiate in and out to produce a fan-like arrangement, the corona radiata. Buried within the white matter lie a number of nuclear masses. Most notable among these are the caudate nucleus, putamen and globus pallidus, known collectively as the corpus striatum or basal ganglia (Figs 13.3-13.15). Within the cerebral hemisphere lies the large C-shaped cavity of the lateral ventricle, which is considered with the rest of the ventricular system in Chapter 6.
Figure 13.1 Lateral aspect of the cerebral hemisphere showing major gyri and sulci.
Figure 13.2 Median sagittal section of the cerebral hemisphere showing major gyri and sulci. The brain stem and cerebellum have been removed to show the inferomedial aspect of the temporal lobe.
Figure 13.3 Coronal section of the cerebral hemisphere.
Figure 13.4 Coronal section of the cerebral hemisphere.
Figure 13.5 Coronal section of the cerebral hemisphere.
Figure 13.6 Coronal section of the cerebral hemisphere.
Figure 13.7 Coronal section of the cerebral hemisphere.
Figure 13.8 Coronal section of the cerebral hemisphere.
Figure 13.9 Coronal section of the cerebral hemisphere.
Figure 13.10 Coronal section of the cerebral hemisphere.
Figure 13.11 Coronal section of the cerebral hemisphere.
Figure 13.12 Coronal section of the cerebral hemisphere.
Figure 13.13 Horizontal (axial) magnetic resonance image of the living brain.
(Courtesy of Professor A Jackson, Wolfson Molecular Imaging Centre, University of Manchester, Manchester, UK.)
Figure 13.14 Horizontal section of the brain.
Figure 13.15 Coronal magnetic resonance image of the living brain.
(Courtesy of Professor A Jackson, Wolfson Molecular Imaging Centre, University of Manchester, Manchester, UK.)
The two cerebral hemispheres are separated by a deep cleft, the great longitudinal fissure, which accommodates the meningeal falx cerebri. In the depths of the fissure, the hemispheres are united by the corpus callosum, an enormous sheet of commissural nerve fibres which run between corresponding areas of the two cortices (Figs 13.2-13.15; see also Figs 13.22, 13.23, 13.24).
Gyri, sulci and lobes of the cerebral hemisphere
Certain gyri and sulci on the surface of the hemisphere are consistently located in different individuals and form the basis of dividing the hemisphere into four lobes, namely the frontal, parietal, temporaland occipital lobes. Their principal topographical features and functional significance are described below. The most conspicuous and deepest cleft on the lateral surface of the hemisphere is the lateral fissure (Fig. 13.1). This separates the temporal lobe below, from the frontal and parietal lobes above. Within the depths of the lateral fissure lies a cortical area known as the insula (Figs 13.6-13.14). The parts of the frontal, parietal and temporal lobes that overlie the insula are called the opercula. Also on the lateral surface of the hemisphere, a single, uninterrupted sulcus can usually be identified, running continuously between the great longitudinal fissure and the lateral fissure. This is the central sulcus, which marks the boundary between the frontal and parietal lobes (Figs 13.1, 13.16). The central sulcus extends for a short distance onto the medial surface of the hemisphere, within the great longitudinal fissure (Figs 13.2, 13.16).
Figure 13.16 Major functional areas of the cerebral cortex: (A) Lateral aspect of left cerebral hemisphere; (B) Medial aspect of right cerebral hemisphere in sagittal section.
The frontal lobe constitutes the entire region in front of the central sulcus. Immediately in front of the sulcus, and running parallel to it, lies the precentral gyrus, which is the primary motor region of the cerebral cortex. In front of the precentral gyrus, the rest of the frontal lobe consists of a more variable pattern of convolutions, of which the superior, middle and inferior frontal gyri can usually be identified (Fig. 13.1).
Behind the central sulcus, and above the lateral fissure, lies the parietal lobe. Its most anterior part is the postcentral gyrus, which is the site of the primary somatosensory cortex. Behind the postcentral gyrus, on the lateral surface of the hemisphere, the intraparietal sulcus divides the rest of the parietal lobe into superior and inferior parietal lobules (Figs 13.1, 13.16).
The boundary between the parietal lobe and the posteriorly located occipital lobe is not coincident with a single sulcus on the lateral surface of the hemisphere; however, it is clearly marked by the deep parieto-occipital sulcuson the medial surface (Figs 13.2, 13.16). The occipital lobe does not bear any important landmarks on its lateral surface but, on the medial surface, the prominent calcarine sulcusindicates the location of the primary visual cortex (Figs 13.2, 13.16).
The temporal lobe lies beneath the lateral fissure, merging posteriorly with the parietal and occipital lobes. On its lateral surface the temporal lobe is divided into three principal gyri that run roughly parallel to the lateral fissure: the superior, middle and inferior temporal gyri (Fig. 13.1). The superior temporal gyrus includes the primary auditory cortex. Most of this functional region is situated on the superior bank of the gyrus, within the lateral fissure, where the transverse temporal gyri, or Heschl’s convolutions, provide a more precise localisation (Fig. 13.17).
Figure 13.17 Superolateral aspect of the left cerebral hemisphere. The frontal and parietal operculae have been removed to show the location of the transverse temporal gyri (Heschl’s convolutions) and the insula.
On the medial surface of the hemisphere, certain portions of the frontal, parietal and temporal lobes also constitute components of the limbic system. Curving around the corpus callosum, and running parallel to it, lies the cingulate gyrus (Figs 13.2, 13.16), separated from the rest of the hemisphere by the cingulate sulcus. The cingulate gyrus passes posteriorly and inferiorly round the posterior portion, or splenium, of the corpus callosum to become continuous with the parahippocampal gyrus of the temporal lobe. Deep to the parahippocampal gyrus, within the temporal lobe, lies the hippocampus (Figs 13.8-13.12). This structure is formed by an in-curling of the inferomedial part of the temporal lobe. The cingulate gyrus, parahippocampal gyrus and hippocampus are sometimes referred to as the limbic lobeof the cerebral hemisphere.
Gyri, sulci and lobes of the cerebral hemisphere
The cerebral hemisphere consists of:
The superficial cerebral cortex, convoluted to form gyri and sulci
Underlying white matter, consisting of cortical afferent and efferent fibres
Deep nuclear masses, the basal ganglia.
The two cerebral hemispheres are separated by the great longitudinal fissure and joined by the corpus callosum.
The hemisphere is divided into four lobes (frontal, parietal, temporal and occipital) on the basis of surface topography.
Principal landmarks that indicate the divisions between lobes are the lateral fissure, central sulcus and parieto-occipital sulcus.
The cerebral cortex forms the outer surface of the cerebral hemisphere. It consists of a layer, several millimetres in thickness, of nerve cell bodies, dendritic arborisations and synaptic interconnections. In the early part of the twentieth century, the Swedish anatomist Brodmann produced a numbered, cytoarchitectural map of the cerebral cortex based upon its regional histological characteristics. Although largely superseded by the elucidation of function, in some instances there is good correspondence between Brodmann’s areas and functionally defined regions of the cortex. In such cases, Brodmann’s numbers are retained in common use for descriptive purposes.
Long ago in evolutionary history, the cerebral cortex originally arose in relation to olfactory function. Phylogenetically old parts of the cortex (referred to as archicortex and paleocortex), such as the hippocampus and other parts of the temporal lobe, retain throughout evolution an association with the olfactory system and have a primitive, three-layered cytoarchitecture. These regions have important functions in the emotional aspects of behaviour and in memory. Together with other parts of the cortex and certain subcortical nuclei they constitute the limbic system (Ch. 16). However, most of the cerebral cortex is a more recent acquisition in phylogenetic terms and is referred to as the neocortex. Although its detailed cytological structure varies from region to region, it is generally recognised as consisting of six layers (Fig. 13.18):
Layer I, the most superficial layer, contains few nerve cell bodies but many dendritic and axonal processes in synaptic interaction.
Layer II contains many small neurones, which establish intracortical connections.
Layer III contains medium-sized neurones giving rise to association and commissural fibres.
Layer IV is the site of termination of afferent fibres from the specific thalamic nuclei.
Layer V is the origin of projection fibres to extracortical targets, such as basal ganglia, thalamus, brain stem and spinal cord. In the primary motor cortex of the frontal lobe, this layer contains the giant Betz cells, which project fibres into the pyramidal tract.
Layer VI also contains association and projection neurones.
Figure 13.18 The histological structure of the cerebral cortex.
(From Mitchell, G A G and Patterson, E L. Basic Anatomy. London: Livingstone; 1954. Courtesy of Churchill Livingstone.)
The cerebral cortex is necessary for conscious awareness and thought, memory and intellect. It is the region to which all sensory modalities ultimately ascend (mostly via the thalamus) and where they are consciously perceived and interpreted in the light of previous experience. The cerebral cortex is the highest level at which the motor system is represented. It is here that actions are conceived and initiated.
Focal cerebral lesions
Focal cerebral lesions, e.g. a stroke or tumour, produce three kinds of symptom:
1. Focal epileptic seizures. The repetitive discharges of groups of neurones in the cerebral cortex produce paroxysmal attacks lasting for brief periods and reflecting the functional properties of the neurones concerned. The patient experiences sudden attacks of abnormal movements or sensations (simple focal seizures) or brief alterations in perception, mood and behaviour (complex partial seizures). Focal seizures may trigger generalised (tonic–clonic) seizures.
2. Sensory/motor deficits. There is a loss of sensation or movement, detectable on clinical neurological examination.
3. Psychological deficits. There are breakdowns in psychological processes such as language, perception and memory, demonstrable on psychological evaluation.
If the focal lesion is space-occupying, the syndrome of raised intracranial pressure results (see p. 47).
A unilateral cerebral hemisphere lesion causes mental impairment (e.g. aphasia), a contralateral spastic hemiparesis, hyperreflexia and an extensor plantar response (upper motor neurone lesion), and contralateral hemisensory loss (Fig. 13.19; see also Fig. 1.41). A vascular insult to the internal capsule, such as an infarction or haemorrhage, leads to the rapid development of this syndrome, known as stroke.
Figure 13.19 Unilateral cerebral hemisphere lesion.
The posterior part of the cerebrum receives sensory information from the outside world in the primary sensory areas of the parietal lobe (somatosensory), occipital lobe (vision) and temporal lobe (hearing).
In adjacent cortical zones, the information is elaborated to permit identification of objects by touch, sight and hearing in a modality-specific act of perception. Areas of cortex at the junction of the three cerebral lobes, known as association cortex, are critical for the multimodal and spatial recognition of the environment.
The medial portions of the cerebral hemisphere (limbic system) enable the storage and retrieval of information processed in the posterior hemispheric regions.
The anterior part of the cerebrum (frontal lobe) is concerned with the organisation of movement (primary motor area; premotor and supplementary motor areas) and the strategic guidance of complex motor behaviour over time (prefrontal area).
In the majority of individuals, areas of association cortex in frontal, parietal and temporal lobes of the left hemisphere are responsible for the comprehension and expression of language. The left hemisphere is, therefore, said to be dominant for language.
The frontal lobe lies anterior to the central sulcus. Immediately anterior to the central sulcus, and running parallel to it, is the precentral gyrus. Functionally, this is known as the primary motor cortex (Figs 13.1, 13.2, 13.16). It corresponds to Brodmann’s area 4. Within the cortex of the precentral gyrus, the contralateral half of the body is represented in a precise somatotopic fashion (Fig. 13.20). The representation of the body is inverted, with the head area located in the most inferior part of the precentral gyrus, just above the lateral fissure. Progressing superiorly, successive areas of cortex represent the digits, hand, arm, shoulder and trunk. The lower limb is represented on the medial surface of the hemisphere, above the corpus callosum. The area of cortex devoted to a particular body part is proportional, not to its size, but to the degree of precision with which movements can be executed. Therefore, the larynx, tongue, face and digits of the hand are represented by relatively large regions.
Figure 13.20 Schematic coronal section through the cerebral hemisphere illustrating the approximate somatotopic representation of the contralateral body half in the motor and sensory cortices.
Stimulation of the primary motor cortex elicits contraction of discrete muscle groups on the opposite side of the body. The function of this region is the control of voluntary, skilled movements, sometimes referred to as fractionated movements; 30% of corticospinal (pyramidal tract) and corticobulbar fibres arise from neurones of the primary motor cortex, about 3% originating from giant pyramidal (Betz) cells. The principal subcortical afferents to the primary motor cortex originate from the ventral lateral nucleus of the thalamus (see Fig. 12.6), which in turn receives input mainly from the dentate nucleus of the cerebellum and from the globus pallidus of the basal ganglia.
The region immediately anterior to the primary motor cortex is known as the premotor cortex (Brodmann’s area 6) (Fig. 13.16). On the lateral surface of the hemisphere, this includes the posterior portions of the superior, middle and inferior frontal gyri. On the medial surface of the hemisphere, the premotor cortex includes a region referred to as the supplementary motor cortex. Here, like the primary motor cortex, there is somatotopic representation of the body although, unlike the primary motor cortex, representation appears to be bilateral in both hemispheres.
Stimulation of premotor cortical areas induces movements that are less focused than those elicited from the primary motor cortex and that involve groups of functionally related muscles. Movements evoked from the supplementary motor cortex tend to be postural in nature, involving axial and proximal musculature. Premotor cortical areas are thought to function in the programming of, and preparation for, movement and in the control of posture. The premotor cortex exerts its actions partly via the primary motor cortex, with which it is connected by short association fibres, and partly via corticospinal and corticobulbar fibres. About 30% of the latter originate in the premotor cortex although, unlike the primary motor cortex, giant Betz cells are absent from premotor areas. The principal subcortical input to premotor cortical regions, including the supplementary motor cortex, is the ventral anterior nucleus of the thalamus. This, in turn, receives fibres from the globus pallidus and substantia nigra.
Immediately in front of the premotor cortex, on the lateral surface of the hemisphere, are located two other important regions. In the middle frontal gyrus lies the frontal eye field (Brodmann’s area 8). This region controls voluntary conjugate deviation of the eyes, as occur when scanning the visual field. Unilateral damage to this area causes conjugate deviation of the eyes towards the side of the lesion. In the inferior frontal gyrus of the dominant hemisphere (usually the left) lies the motor speech area, also known as Broca’s area (Brodmann’s areas 44 and 45). Broca’s area has important interconnections with parts of the ipsilateral temporal, parietal and occipital lobes that are involved in language function.
Left frontal lobe lesions
Lesions of the left frontal lobe cause:
Focal seizures. Paroxysmal jerking movements of the contralateral limbs are termed ‘simple motor’ or ‘Jacksonian’ seizures.
Sensory/motor deficit. Weakness of the face and upper motor neurone signs in the limbs on the opposite side to the lesion occur (contralateral hemiplegia).
Psychological deficit. Speech is produced with great effort and poor articulation, in brief utterances with word errors (paraphasia). Repetition of words is impaired but powers of comprehension are relatively preserved. This is known as Broca’s aphasia. There is also impairment of reading and writing (alexia and agraphia).
Bilateral cortical disorders
Alzheimer’s disease, a common degenerative disorder of the elderly, leads to atrophy of the temporal and parietal lobes and the limbic system (Fig. 13.21). It causes disorientation in space, and loss of language (aphasia) and memory (amnesia).
Figure 13.21 Coronal section through the cerebral hemisphere of a patient dying with Alzheimer’s disease, showing enlarged lateral ventricles and atrophic cortical gyri.
(Courtesy of Professor D Mann, Clinical Neurosciences, Hope Hospital, University of Manchester, Manchester, UK.)
The common degenerative disorder of frontotemporal dementia leads to a total alteration of personality with loss of judgement, planning and insight, and the appearance of bizarre and uncharacteristic behaviour. Young people are affected.
The extensive regions of the cortex of the frontal lobe that lie anterior to premotor areas are referred to as prefrontal cortex. The prefrontal cortex has rich connections with parietal, temporal and occipital cortex through long association fibres running in the subcortical white matter. Subcortical afferents arise mainly in the mediodorsal and anterior nuclei of the thalamus. The prefrontal cortex has cognitive functions of a high order. These include intellectual, judgemental and predictive faculties and the planning of behaviour.
The parietal lobe lies behind the frontal lobe and is bounded posteriorly and inferiorly by the occipital and temporal lobes, respectively. The most anterior part of the parietal lobe is the postcentral gyrus, running parallel to the central sulcus (Figs 13.1, 13.2, 13.16). Functionally, this region is the primary somatosensory cortex (Brodmann’s areas 1, 2 and 3). It is here that thalamocortical neurones terminate; these constitute the third and final relay in the chain from peripheral receptors for general sensation to a conscious level. The thalamic origin of these neurones is the ventral posterior nucleus, which in turn receives fibres of the medial lemniscus (fine touch and proprioception), spinal lemniscus (coarse touch and pressure), spinothalamic tracts (pain and temperature) and trigeminothalamic tracts (general sensation from the head). Within the somatosensory cortex, the contralateral half of the body is represented in an inverted, somatotopic pattern that resembles that in the primary motor cortex of the frontal lobe (Fig. 13.20).
Once again, the area of cortex devoted to a particular body part is disproportionate to the size of the latter; in the case of the sensory cortex it reflects rather the richness of sensory innervation. Therefore, the pharynx, tongue, face, lips and the palmar surface of the hands and digits are particularly well represented. Adjacent to the mouth area is a region where taste is perceived.
The surface of the parietal lobe posterior to the primary somatosensory cortex constitutes the parietal association cortex. The superior parietal lobule is responsible for the interpretation of general sensory information and for conscious awareness of the contralateral half of the body. Lesions here impair the interpretation and understanding of sensory input and may cause neglect of the opposite side of the body. The inferior parietal lobule interfaces between somatosensory cortex and the visual and auditory association cortices of the occipital and temporal lobes, respectively, and in the dominant hemisphere it contributes to language functions.
The lateral surface of the temporal lobe is divided into superior, middle and inferior temporal gyri, which run parallel to the lateral fissure. Within the superior temporal gyrus is located the primary auditory cortex (Brodmann’s areas 41 and 42). More exactly, most of this functional zone lies in the superior bank of the gyrus, normally hidden within the lateral fissure. Its precise location is marked by the small transverse temporal gyri, or Heschl’s convolutions (Fig. 13.17).
Parietal lobe lesions
Left parietal lobe lesions cause:
Focal seizures – paroxysmal attacks of abnormal sensations, spreading down the contralateral side of the body (sensory seizures).
Sensory/motor deficit – a contralateral hemisensory loss and inferior visual field loss.
Psychological deficit – an inability to name objects (anomia) and a loss of literacy, with inability to read (alexia), to write (agraphia) and to calculate (acalculia).
Right parietal lobe lesions cause:
Focal seizures – paroxysmal attacks of sensory disturbance affecting the contralateral side of the body (simple sensory seizures).
Sensory/motor deficit – contralateral hemisensory loss and an inferior visual field loss.
Psychological deficit – an inability to copy and construct designs because of spatial disorientation (constructional apraxia).
The primary auditory cortex is responsible for the conscious perception of sound and within it there is so-called ‘tonotopical’ representation of the cochlear duct. The primary auditory cortex receives input from the medial geniculate nucleus of the thalamus. The ascending acoustic projection undergoes partial decussation in the brain stem on its way to the medial geniculate nucleus (Ch. 10). At the cortical level, therefore, the organs of hearing are bilaterally represented so that unilateral lesions of the primary auditory cortex cause partial deafness in both ears. Auditory information is further processed and interpreted in the auditory association cortex, which lies surrounding and immediately posterior to the primary auditory cortex. In the dominant hemisphere, this region is also known as Wernicke’s area. It is crucial for understanding of the spoken word and has important connections with other language areas of the brain.
The location of the cortical representation of the vestibular system is uncertain. There is evidence that it lies in the superior temporal gyrus anterior to the primary auditory cortex, or in the inferior parietal lobule.
The inferomedial part of the temporal lobe is curled inwards to form the hippocampus. This structure lies in the floor of the inferior horn of the lateral ventricle, deep to the parahippocampal gyrus (Figs 13.8-13.12, 13.16, see also Ch. 16). As part of the limbic system, the principal functions of the hippocampus are in relation to memory and the emotional aspects of behaviour. Close to the anterior end of the hippocampus and the temporal pole lies a mass of subcortical grey matter, the amygdala, which is also part of the limbic system. The amygdala and adjacent parts of the inferomedial temporal cortex receive fibres from the olfactory tract and are responsible for the conscious appreciation of the sense of smell. These connections receive further consideration in Chapter 16.
Left temporal lobe lesions
Left temporal lobe lesions cause:
Focal seizures – paroxysmal attacks of unresponsiveness (absences), purposeless behaviour (automatism), olfactory and complex visual and auditory hallucinations, and disturbances of mood and memory (déjà vu). These attacks are referred to as complex partial seizures.
Sensory/motor deficit – a contralateral superior visual field loss.
Psychological deficit – speech that is fluent and rapid but contains word errors (paraphasia) and is incomprehensible. There is profound word-finding difficulty, impaired repetition of words, and profound loss of comprehension. This is known as Wernicke’s aphasia.
The occipital lobe lies behind the parietal and temporal lobes. On the medial surface of the hemisphere, the boundary with the parietal lobe is marked by the deep parieto-occipital sulcus. Also on the medial surface, the calcarine sulcus marks the location of the primary visual cortex (Brodmann’s area 17; Fig. 13.16) which is responsible for visual perception. It occupies the gyri immediately above and below the calcarine sulcus, much of it being hidden in the depths of the sulci. This region receives fibres from the lateral geniculate nucleus of the thalamus by way of the optic radiation of the internal capsule. Each lateral half of the visual field is represented in the primary visual cortex of the contralateral hemisphere. The upper half of the visual field is represented below the calcarine sulcus, and the lower half is represented above the sulcus. The rest of the occipital lobe constitutes the visual association cortex. This region is concerned with the interpretation of visual images. Lesions of the primary visual cortex cause blindness in the corresponding part of the visual field, while damage to the visual association cortex causes deficits in visual interpretation and recognition. The visual system is considered in more detail in Chapter 15.
Occipital lobe lesions
Occipital lobe lesions cause:
Focal seizures – paroxysmal visual hallucinations of a simple, unformed nature, such as lights and colours (simple partial seizures).
Sensory/motor deficit – a contralateral visual field loss (contralateral homonymous hemianopia).
Bilateral occipital lobe lesions lead to cortical blindness, of which the patient is unaware (Anton’s syndrome). Bilateral occipitoparietal lesions can spare elementary vision but prevent the recognition and depiction of objects (apperceptive visual agnosia).
Language areas of the cerebral hemisphere
Certain higher cognitive functions are dealt with primarily, or even exclusively, by one of the cerebral hemispheres, which is then referred to as dominant for that function. In the great majority of people the left hemisphere is dominant for language and mathematical ability. The right hemisphere excels at spatial perception and musical proficiency. Cerebral dominance becomes established during the first few years after birth. During this formative period, both hemispheres exhibit linguistic ability and if one hemisphere sustains damage it may be compensated for by the plasticity of the developing brain and the child learns to speak normally. Later in life, this flexibility becomes greatly diminished and damage to the dominant hemisphere often causes loss of speech in addition to the other deficits produced by hemispheric lesions.
The language areas of the brain are organised around the lateral fissure of the cerebral hemisphere. In the frontal lobe, Broca’s area occupies the posterior part of the inferior frontal gyrus, adjacent to the motor cortical area for the head and neck. This region is concerned with expressive aspects of language (articulation). In the temporal lobe, the auditory association cortex, or Wernicke’s area, is responsible for comprehension of the spoken word.
Nearby regions of the temporal lobe and parietal lobe, most notably the angular gyrus and supramarginal gyrus of the inferior parietal lobule, provide a functional interface between auditory and visual association areas important in naming, reading, writing and calculation.
The cerebral cortex
The precentral gyrus is the primary motor region of the cerebral cortex and is located within the frontal lobe, immediately in front of the central sulcus. Anterior to this lie the premotor and supplementary motor cortices and, in the left hemisphere, Broca’s (motor speech) area. The prefrontal cortex is concerned with complex cognitive functions.
The postcentral gyrus is the primary somatosensory region of the cerebral cortex and lies within the parietal lobe, immediately posterior to the central sulcus. It receives afferents from the ventral posterior nucleus of the thalamus, which is the site of termination of the spinothalamic tracts, trigeminothalamic tract and the medial lemniscus. Behind this region lies the sensory association cortex, which is responsible for the interpretation of general sensory information.
The temporal lobe lies beneath the lateral fissure. On the superior surface of the superior temporal gyrus, the transverse temporal gyri (Heschl’s convolutions) mark the location of the primary auditory cortex, which receives input from the medial geniculate nucleus of the thalamus. Adjacent lies the auditory association cortex, which is responsible for the interpretation of auditory information and which, in the left hemisphere, constitutes Wernicke’s area.
The occipital lobe makes up the posterior part of the hemisphere. On the medial surface, the calcarine sulcus indicates the location of the primary visual cortex, which receives afferents from the lateral geniculate nucleus of the thalamus. The rest of the occipital lobe is the visual association cortex, which is responsible for the interpretation of visual information.
White matter of the cerebral hemisphere
Beneath the cortical surface lies an extensive mass of nerve fibres, all of which have their origin or termination, and sometimes both, within the cortex. The fibres are classified into three types, depending upon their origin and destination:
1. Association fibres, which interconnect cortical sites lying within one cerebral hemisphere
2. Commissural fibres, which run from one cerebral hemisphere to the other, connecting functionally related structures
3. Projection fibres, which pass between the cerebral cortex and subcortical structures such as the thalamus, striatum, brain stem and spinal cord.
Some association fibres (Figs 13.22, 13.23) are short and link nearby areas of cortex by arching beneath adjacent cerebral sulci (U fibres). Other association fibres are longer and travel through the white matter to link distant areas of cerebral cortex. The primary sensory areas in the parietal, temporal and occipital lobes are linked by long association fibres to the association areas of the cerebral cortex. These, in turn, are connected to each other.
Figure 13.22 Coronal section of the cerebral hemisphere. The diagram shows the location of the principal association, commissural and projection fibres.
Figure 13.23 Principal association and commissural fibres of the cerebral hemisphere projected onto a median sagittal section.
The large superior longitudinal fasciculus interconnects the frontal and occipital lobes. A subsidiary of this bundle, known as the arcuate fasciculus, links gyri in the frontal and temporal lobes that are important for language function.
The inferior longitudinal fasciculus runs from the occipital to the temporal poles and contributes to the function of visual recognition.
The uncinate fasciculus connects the anterior and inferior parts of the frontal lobe with the temporal gyri, which are important structures in the regulation of behaviour. The cingulum lies within the cingulate gyrus and courses around the corpus callosum, connecting the frontal and parietal lobes with the parahippocampal and adjacent temporal gyri.
Cerebral damage, such as that caused by carbon monoxide poisoning, can destroy the inferior longitudinal fasciculus bilaterally. In such cases, the individual has intact elementary vision but cannot identify the nature of objects (object agnosia) or individual faces (prosopagnosia), although they can depict and match them.
The major interhemispheric commissural fibres are the corpus callosum, the anterior commissure and the hippocampal commissure (or commissure of the fornix) (Figs 13.22-13.26).
Figure 13.24 Dissection of the brain from the superior aspect revealing the corpus callosum.
Figure 13.25 Diffusion MRI tractography reconstruction of the projection fibres passing through the internal capsule. Tractography measures the diffusion of water along axonal fibres and allows reconstruction of their trajectories in the living human brain.
(Courtesy of Dr Marco Catani, Institute of Psychiatry, London, UK.)
Figure 13.26 Horizontal section of the cerebral hemisphere showing the parts of the internal capsule.
The corpus callosum spans the two cerebral hemispheres and connects corresponding regions of neocortex for all but the temporal fields (these have their own connection, the anterior commissure). The major parts of the corpus callosum, from rostral to caudal, are named the rostrum, genu, body and splenium. The corpus callosum is shorter rostrocaudally than is the hemisphere; as a result, callosal fibres linking the frontal or occipital poles curve forwards or backwards as the anterior and posterior forceps, respectively. The splenium interconnects the occipital cortices and, therefore, contributes to visual functions.
The anterior commissure runs transversely in front of the anterior column of the fornix and interconnects the inferior and middle temporal gyri and the olfactory regions on the two sides.
The hippocampal commissure consists of transverse fibres linking the posterior columns of the fornix on each side.
Projection fibres (Fig. 13.25) consist of afferent fibres conveying impulses to the cortex and efferent fibres conducting impulses away from it. The fibres projecting to and from the cerebral cortex are distributed radially as the corona radiata and then converge downwards towards the brain stem. The fibres become concentrated in a narrow area, called the internal capsule, between the thalamus and caudate nucleus medially, and the lentiform nucleus laterally. The internal capsule is angulated to form an anterior limb, genu, posterior limb and retrolenticular part (Fig. 13.26, see also Fig. 1.23).
Damage to the corpus callosum
Patients with chronic epilepsy have undergone section of the corpus callosum to relieve their seizures. Such individuals betray few difficulties under normal circumstances. However, when these ‘split-brain’ patients undergo psychological testing, the two halves of the brain appear to behave relatively autonomously. For example, visual information directed to the right, non-dominant, hemisphere alone does not evoke a verbal response; as a result, individuals cannot name objects or read words presented solely to the left visual field.
Destruction of the splenium of the corpus callosum by stroke or tumour leads to the posterior disconnection syndrome of alexia without agraphia. Such individuals speak and write without difficulty but cannot understand written material (alexia). Disconnection of visual processing in the right hemisphere from the verbal processing of the dominant left hemisphere is thought to explain the syndrome.
The anterior limb contains connections between the mediodorsal nucleus of the thalamus and the prefrontal cortex, and also frontopontine fibres that project to the pontine nuclei in the basal portion of the pons.
The posterior limb contains corticobulbar and corticospinal motor fibres. Also within the posterior limb are thalamocortical projections passing from the ventral posterior nucleus to the primary somatosensory cortex, and from the ventral anterior and ventral lateral nuclei to motor regions of the frontal lobe.
Behind the posterior limb is a region referred to as the retrolenticular part of the internal capsule. This consists of fibres arising from the medial and lateral geniculate nuclei of the thalamus that pass to the auditory and visual cortices as the auditory and visual radiations, respectively. Visual thalamocortical fibres (also known as geniculocalcarine fibres) pass round the lateral ventricle and follow one of two courses to the visual cortex (Fig. 15.6). Those which represent the lower half of the visual field project to the upper part of the visual cortex (above the calcarine sulcus). They may be interrupted in their course by lesions of the parietal lobe. Fibres that represent the upper half of the visual field loop forwards over the inferior horn of the lateral ventricle (Meyer’s loop) and may be damaged by lesions of the temporal lobe.
White matter of the cerebral hemisphere
Nerve fibres within the subcortical white matter are classified on the basis of their origin and termination.
Association fibres link cortical regions within a single hemisphere. Important systems are: the superior longitudinal fasciculus, arcuate fasciculus, inferior longitudinal fasciculus and uncinate fasciculus.
Commissural fibres pass between corresponding regions of the two hemispheres. The principal commissural system is the corpus callosum.
Projection fibres run between the cerebral cortex and various subcortical structures. They pass through the corona radiata and the internal capsule. Particularly important fibres in this category are corticospinal, corticobulbar and thalamocortical projections.