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sum are derived from very extensive areas of the cerebral cortex; a midline section of the brain shows them tightly bundled in the corpus callosum. Once they have crossed over to the opposite hemisphere, the callosal fibers fan out again, in the so-called callosal radiation, to reach the cortical locations that correspond, in mirror-image fashion, to their sites of origin. This symmetrical linkage of homotopic cortical areas by commissural fibers is absent only in the primary visual cortex (area 17) and in the hand and foot areas of the somatosensory cortex.
The commissural fibers are interspersed in the subcortical white matter with the fibers of the corona radiata and the association bundles. As the corpus callosum is shorter than the hemispheres, the fibers at its anterior end (rostrum, genu) or posterior end (splenium) take a U-shaped course to link mirrorsymmetric cortical areas at the frontal or occipital poles. These curving fibers form the forceps minor (for the frontal pole) and the forceps major (for the occipital pole).
Functional Localization in the Cerebral Cortex
The earliest clinical neurologists and neuroscientists were already deeply interested in the question whether individual functions of the brain could be localized to particular brain areas. From the mid-nineteenth century onward, researchers answered this question through the painstaking study of brain lesions found at autopsy in patients who, during their lives, had suffered from particular types of neurological deficit. This patho-anatomically oriented functional analysis of cortical structures was supplemented, from 1870 onward, by experiments with direct electrical or chemical stimulation of the cerebral cortex, both in animals and in humans. Later techniques, including stereotaxy, electroencephalography, and microelectrode recording of potentials from individual neurons and nerve fibers, yielded ever more detailed functional “maps” of the brain (cf. Fig. 9.17). The original idea of the “localizability” of brain function remains valid after a century and a half of study, especially with respect to the primary cortical areas, which we will describe further below.
In the last 20 years, however, basic neurobiological research on the localization of cortical function has been largely transformed by the emergence of newer, more powerful techniques of investigation, particularly functional neuroimaging. Current thinking has turned away from the parceling out of functions to individual anatomical structures (as derived from the important studies of Brodmann, Penfield, and many others) and toward the concept of
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tects are strong enough that a three-dimensional image of field sources can be computed from them, including sources deep in the brain. Functional imaging of the brain with MEG can be performed with high temporal resolution but relatively low spatial resolution (as compared to fMRI).
Positron emission tomography, a scanning procedure involving radionuclides, is used to investigate metabolic processes in the brain. Oxygen and glucose consumption in the brain can be directly measured after the injection of the corresponding radioactively labeled substances into the body. Radioactively labeled drugs can also be used to visualize intracerebral synaptic activity and receptor distribution. The disadvantages of PET include the radiation dose to the patient, which is not always insignificant, and the technical difficulty and expense of the procedure. Some of the radioactive isotopes used in PET have very short half-lives and must be generated directly adjacent to the scanner, in an on-site cyclotron. Furthermore, the spatial resolution and temporal resolution of PET are relatively low.
Functional magnetic resonance imaging. Most of the problems associated with MEG and PET, as just described, do not affect fMRI. This technique is based on the different magnetic properties of oxyhemoglobin and deoxyhemoglobin. Regional cerebral activation is immediately followed not just by a change in blood flow but also by a change in the relative concentrations of the two forms of hemoglobin, which can be detected as a very small change in the MRI signal. fMRI is not known to have any harmful effect on the body, so that subjects can be examined at length or repeatedly. fMRI has now largely replaced PET for studies of cerebral activation, but it cannot yet be used reliably to visualize metabolic processes.
We will now describe some aspects of the new conception of functional localization in the cerebral cortex that has been obtained through the application of these new techniques.
Primary Cortical Fields
From the functional point of view, the cortex can be divided into primary cortical fields and unimodal (p. 384) and multimodal association areas (p. 385).
Most of the primary cortical fields have a receptive function: they are the final targets of the somatosensory and special sensory pathways (visual, auditory, etc.) in the CNS, and they receive their afferent input by way of a thalamic relay. The primary cortical fields serve to bring the respective sensory qualities to consciousness in raw form, i.e., without interpretation. The individual primary cortical fields have no distinctive gross anatomical features and do not
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correspond precisely to the pattern of convolutions on the brain surface. Rather, the extent of a primary cortical field is defined as the area of cortex in which the corresponding thalamic projection terminates.
In addition to the various primary receptive fields, there is also a primary motor area, which sends motor impulses through the pyramidal pathway to the spinal cord and, ultimately, to the muscles.
Primary Somatosensory and Motor Cortical Areas
Localization and function. The primary somatosensory cortex (areas 3, 2, and 1, Fig. 9.18) roughly corresponds to the postcentral gyrus of the parietal lobe and a portion of the precentral gyrus. It extends upward onto the medial surface of the hemisphere, where it occupies the posterior portion of the paracentral lobule. The primary somatosensory cortex is responsible for the conscious perception of pain and temperature as well as somatic sensation and proprioception, mainly from the contralateral half of the body and face. Its afferent input is derived from the ventral posterolateral and posteromedial nuclei of the thalamus (Fig. 6.4, p. 266). Even though some sensory stimuli, particularly painful stimuli, may already be vaguely perceived at the thalamic level, more precise differentiation in terms of localization, intensity, and type of stimulus cannot occur until impulses reach the somatosensory cortex. The conscious perception of vibration and position is not possible without the participation of the cortex.
The primary motor cortex (area 4) roughly corresponds to the precentral gyrus of the frontal lobe, including the anterior wall of the central sulcus, and extends upward into the anterior portion of the paracentral lobule on the medial surface of the hemisphere. The fifth cortical layer in area 4 contains the characteristic Betz pyramidal cells, which give off the rapidly conducting, thickly myelinated fibers of the pyramidal tract. Area 4 is thus considered the site of origin of voluntary movement, sending motor impulses to the muscles by way of the pyramidal tract and anterior horn cells of the spinal cord. It receives afferent input from other areas of the brain that participate in the planning and initiation of voluntary movement, particularly the ventro-oral posterior nucleus of the thalamus (cf. p. 265f.), the premotor areas 6 and 8, and the somatosensory areas.
Somatotopy and plasticity. The primary somatosensory and motor fields of the neocortex contain somatotopic, i.e., point-to-point, representations of the periphery of the body, taking the form of a homunculus (a “little man,” as it were, drawn on the surface of the brain; the Latin term is the diminutive of homo, man, in the sense of human being; cf. Fig. 9.19). The configuration of these
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Fig. 9.18 Primary cortical fields and premotor and prefrontal cortical areas (diagram). a Lateral view. b Medial view.
maps of the body on the cortical surface was originally determined by pathoanatomical study (Fig. 9.20). The findings were confirmed and refined by the intraoperative electrical stimulation studies of Penfield (Fig. 9.21), by the somatosensory evoked potential mapping studies of Marshall, and, more re-
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Foot |
Elbow flexion |
Thumb tapping |
Index finger |
Left fist |
Lip-pursing |
Fig. 9.22 The cortical representation of regions of the body as revealed by functional MRI (fMRI) in normal persons. fMRI data are shown projected onto a model of the brain surface. The data were obtained from 30 subjects who performed repetitive movements of the indicated body parts. Bright colors correspond to high levels of activation: i.e., brightly colored brain areas are activated during the respective movements. Localization, as determined by this technique, is in perfect accordance with the earlier findings of Penfield and Foerster (Fig. 9.21). fMRI is thus a noninvasive means of mapping the “homunculus” very reliably, either in normal persons or in patients. The images are reproduced with the kind permission of Professor Grodd. (From: Lotze M, Erb M, Flor H, et al.: Neuroimage 11 (2000) 473−481.)
cently, by PET, fMRI, and MEG studies (Fig. 9.22). fMRI enables visualization of the regions of the brain that are activated when normal, healthy subjects perform motor tasks.
These cortical maps are not metrically proportional representations of the body.Inthecorticalrepresentationofsuperficialsensation,forexample,partsof thebodythataredenselyinnervatedbysensoryfibers(suchasthetongue,mouth, andface)aremappedtodisproportionatelylargeareasofcortex,andlessdensely innervated parts (arm, thigh, back) are mapped to smaller areas (Fig. 9.19).
Furthermore, and despite earlier assumptions, these maps are not static: rather, the cortical representation of a given body part can enlarge or shrink, depending on the degree to which that body part is put to use. Thus, if a tactile
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discrimination task involving the thumb and index finger (such as the palpation of a die to explore its surface) is carried out repetitively for a long enough time, the representation of these two fingers in the primary somatosensory cortex will enlarge. Similar, or even more extensive, changes of cortical representation are found after the injury or amputation of a limb. In such cases, the somatotopic map of the body in the cerebral cortex can be shifted by as much as several centimeters. When an arm is amputated, for example, the cortical area previously responsible for sensory impulses from the (now missing) hand can change its function and instead process sensory impulses from the face. This change is brought about by neuronal reorganization in the brain.
Much current research concerns the potential connection between shifting cortical representations and the generation of painful conditions such as phantom pain. If a connection exists, then some type of therapeutic alteration or suppression of this form of cortical “plasticity” might be used to treat, or even prevent, these conditions.
Cortical columns. In addition to the somatotopic cortical representation of superficial sensation (touch and pressure), which involves impulses that are generated in cutaneous mechanoreceptors and then transmitted to the cortex along the pathways that have been described, there are also other cortical maps for the remaining somatosensory modalities (proprioception, temperature, pain), which lie deeper within the cortex but have a generally similar configuration. Thus, somatic sensation as a whole is represented by cortical columns: each column deals with a particular, small region of the body surface, and cells at different depths within the column respond to different somatosensory modalities. This structural property enables the brain to process impulses from all somatosensory modalities simultaneously and in parallel, even though they have reached the cortex through distinct neuroanatomical pathways.
A lesion of the primary somatosensory cortex impairs or abolishes the sensations of touch, pressure, pain, and temperature, as well as two-point discrimination and position sense, in a corresponding area on the opposite side of the body (contralateral hemihypesthesia or hemianesthesia).
A lesion in area 4 produces contralateral flaccid hemiparesis. Additional damage of the adjacent premotor area and the underlying fiber tracts is necessary to produce spastic hemiparesis, which reflects the interruption of nonpyramidal as well as pyramidal pathways. Focal epileptic seizures restricted to the somatosensory cortex are characterized by repetitive motor phenomena, such as twitching, or by paresthesia/dysesthesia on the opposite side of the body or face (motor or sensory jacksonian seizures).
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Fig. 9.23 Functional localization in the primary visual cortex as revealed by fMRI. Normal subjects viewed visual stimuli in the form of expanding rings, and the associated cortical activity is depicted, projected onto a model of the brain surface. There is activation of the primary visual cortex at the calcarine sulcus, as well as of the secondary visual areas. Images obtained by Professor Grodd. (From: Kammer T, Erb M, Beck S, and Grodd W: Zur Topographie von Phosphenen: Eine Studie mit fMRI und TMS. 3. Tübinger Wahrnehmungskonferenz (3rd Tübingen Conference on Perception), 2000).)
Primary Visual Cortex
Localization and retinotopy. The primary visual cortex corresponds to area 17 of the occipital lobe (Figs. 9.17, 9.18). It is located in the depths of the calcarine sulcus, and in the gyri immediately above and below this sulcus on the medial surface of the hemisphere, and it extends only slightly beyond the occipital pole (Fig. 9.23). It is also called the striate (“striped”) cortex because of the white stripe of Gennari, which is grossly visible within it in a perpendicular anatomical section. The visual cortex receives input by way of the optic radiation from the lateral geniculate body, in orderly, retinotopic fashion: the visual cortex of one side receives visual information from the temporal half of the ipsilateral retina and the nasal half of the contralateral retina. Thus, the right visual cortex subserves the left half of the visual field, and vice versa (p. 134). Visual information from the macula lutea is conveyed to the posterior part of area 17, i.e., the area around the occipital pole.
Columnar structure. The neurons of the primary visual cortex respond to stimuli having a particular position and orientation in the contralateral visual field. Neurons responding to similarly oriented stimuli are organized in vertical columns. Each column is 30100 microns wide. Neighboring columns are organized in “pinwheels” (Fig. 9.24), in which every direction of the compass is
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represented once. The orientation columns are interrupted at regular distances by the “blobs” (Fig. 9.24), which contain neurons primarily responding to color. Finally, the ocular dominance columns are the third major structural component of the primary visual cortex. Each ocular dominance column responds to visual stimulation of a single eye; the adjacent column responds to visual stimulation of the other eye.
These three major components of the primary visual cortex together form a hypercolumn occupying an area of about 1 mm2. The hypercolumns, in turn, make up a regularly repeating pattern on the surface of the primary visual cortex. They are interconnected through horizontal cells. The structural and functional organization of the visual cortex enables it to carry out an elementary analysis of visual stimuli for their shape and color. Direct electrical stimulation of the primary visual cortex (e. g., in awake patients undergoing brain surgery) induces the perception of flashes of light, bright lines, and colors.
A unilateral lesion of area 17 produces contralateral hemianopsia; a partial lesion produces quadrantanopsia in the part of the visual field that corresponds to the site of the lesion. Central vision is unimpaired as long as the lesion spares the posterior end of the calcarine fissure at the occipital pole.
Primary Auditory Cortex
Localization. The primary auditory cortex is located in the transverse gyri of Heschl (area 41), which form part of the upper surface of the superior temporal gyrus (see Figs. 9.10, 9.17, 9.18, and 9.25). It receives its afferent input from the medial geniculate body, which, in turn, receives auditory impulses from both organs of Corti by way of the lateral lemnisci. Thus, the primary auditory cortex of each side processes impulses arising in both ears (bilateral projection).
Tonotopy. The structure of the primary auditory cortex resembles that of the primary visual cortex in many respects. Its neurons are finely tuned for the detection and processing of tones of a particular frequency. The entire spectrum of audible sound is tonotopically represented: the cells for lower frequencies are found rostrolaterally, and those for higher frequencies caudomedially, along the sylvian fissure. The primary auditory cortex thus contains isofrequency bands running in a medial-to-lateral direction. Area 41 neurons preferentially respond not only to a particular frequency but also to a particular intensity of sound.
Columnar structure. The primary auditory cortex also appears to possess a columnar organization for the processing of stimuli from the two ears. Two types of neurons respond in different ways to binaural stimuli. One responds
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Auditory instruction: start
Auditory instruction: stop
Fig. 9.25 Functional localization of the auditory cortex and language centers by fMRI. Eighteen subjects were asked to listen to and repeat spoken words (names of months). Listening is associated with activation of the primary auditory cortex bilaterally in the area of the transverse gyri of Heschl. Repetition, on the other hand, is associated with activity in the left hemisphere only; specifically, in the angular gyrus of the parietal lobe (Wernicke’s area) and in the inferior frontal gyrus (Broca’s area). Images obtained by Professor Grodd. (From: Wildgruber D, Kischka U, Ackermann H, et al.: Cognitive Brain Research 7 (1999) 285−294.)
more strongly to stimuli delivered to both ears than to stimuli in a single ear (EE neurons), while the other is inhibited by simultaneous binaural stimulation (EI neurons). Columns of cells of these two types are found in alternation on the surface of the primary auditory cortex, like the ocular dominance columns of the primary visual cortex (Fig. 9.24). These columns lie tangential to the isofrequency bands. A further special property of neurons of the primary auditory cortex is that different neurons are excited by auditory stimuli of the same frequency but different duration.
Direct electrical stimulation of the auditory cortex induces the perception of simple sounds of higher or lower frequency and greater or lesser volume, but never of words.
Unilateral lesions of the primary auditory cortex cause only subtle hearing loss because of the bilateral projections in the auditory pathway. The impairment mainly concerns directed hearing, and the ability to distinguish simple from complex sounds of the same frequency and intensity.
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