LOCALIZATION

By: Jafar Mehvari, MDNeurologist and EpileptologistAssociative Professor of Isfahan University of Medical Science

General Anatomic and PhysiologicConsiderations of Cortical Function

Along strictly histologic lines, Brodmann distinguished 47 different areas of cerebral cortex

the cortex as a heterogeneous array of many anatomic systems, each with highly organized intercortical and diencephalic connections.

it has a surface extent of about 4000 cm2—about the size of a full sheet of newsprint (right and left pages).

Contained in the cortex are many billions of neurons (estimated at 10 to 30 billion) and five times this number of supporting glial cells.

Most of the human cerebral cortex is phylogenetically recent, hence the term neocortex

allocortex (“other cortex”), which comprises mainly the hippocampus and olfactory cortex

Histology of the neocortex, six layers (laminae) can be distinguished

the molecular (or plexiform), external granular, external pyramidal, internal granular, ganglionic (or internal pyramidal), and multiform (or fusiform) layers

Two main types of neocortex are recognized: (1) the homotypical cortex, in which the six-layered arrangement is readily discerned, and

(2) the heterotypical cortex, in which the layers are less distinct

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The association cortex—the large areas (75 percent of the surface) that are not obviously committed to primary motor or sensory functions—is generally of this latter type.

precentral cortex (Brodmann’s areas 4 and 6, mainly motor regionis dominated by pyramidal rather than granular cells, especially in layer V (hence the term agranular

primary sensory cortex—postcentral gyrus (areas 3, 1, 2), banks of the calcarine sulcus (area 17), and the transverse gyri of

Heschl (areas 41 and 42)—where layers II and IV are strongly developed for the receipt of afferent impulses, has been termed granular cortex

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certain regions of the cerebrum are committed to special perceptual, motor, sensory, mnemonic, and linguistic activities,

In area 17, the polar region of the occipital lobe, there are discrete, highly specialized groups of neurons, each of which is activated in a small area of lamina 4 by spots of light or lines and

transmitted via particular cells in the lateral geniculate bodies;

other groups of adjacent cortical neurons are essential for the perception of color

between the main unimodal receptive areas for vision, audition,and somesthetic perception are zones of integration called heteromodal cortices

Cortical-subcortical integrations Interregional connections Disconnection syndromes depend not

merely on involvement of certain cortical regions butalso on the interruption of inter- and intrahemispheric fiber tracts

the involved fiber systems include the corpus callosum, anterior commissure, uncinate temporofrontal fasciculus, occipito- and temporoparietal tracts

SYNDROMES CAUSED BYLESIONS OF THE FRONTALLOBES

30 percent of the cerebrum Brodmann’sareas 4, 6, 8, and 44 relate

specifically to motor activities primary motor cortex, i.e., area 4, is

directly connected with somatosensory neurons of the anterior part of the postcentral gyrus as well as with other parietal areas, thalamic and red nuclei, and the reticular formation of the brainstem.

Area 8 is concerned with turning the eyes and head contralaterally.

Area 44 of the dominant hemisphere (Broca’s area) and the contiguous part of area 4 are “centers” of motor speech and related functions of the lips, tongue, larynx, and pharynx

The medial-orbital gyri and anterior parts of the cingulate gyri, which are the frontal components of the limbic system, take part in the control of respiration, blood pressure, peristalsis, and other autonomic functions

all motor activity requires sensory guidance, and this comes from the somesthetic, visual, and auditory cortices and from the cerebellum via the ventral tier of thalamic

The most anterior parts of the frontal lobes (areas 9 to 12 and 45 to 47), sometimes referred to as the prefrontal areas,

initiation of planned action and executive control of all mental operations, including emotional expression.

The frontal agranular cortex (areas 4 and 6) and, more specifically, pyramidal cells of layer V of the pre- and postcentral convolutions provide most of the cerebral efferent motor system that forms the pyramidal or corticospinal tract

Areas 8 and 6 are connected with the ocular and other brainstem motor nuclei and with identical areas of the other cerebral hemisphere through the corpus callosum.

Electrical stimulation of the orbitofrontal cortex and cingulate gyrus has manifest effects on respiratory, circulatory, and other vegetative functions, as already mentioned.

Clinical Effects of Frontal Lobe Lesions

(1) motor abnormalities related to the prerolandic motor cortex

; (2) speech and language disorders related to the dominant hemisphere;

(3) incontinenceof bladder and bowel ; (4) impairment of certain cognitiv functions, especially

attention, concentration, capacity for sustained mental activity, and ability to shift from one line of thought or action to another—i.e., bot impersistence and perseveration;

(5) akinesia and lack of initiative and spontaneity (apathy and abulia);

(6) other changes in personality, particularly in mood and selfcontrol (disinhibition of behavior); and

(7) a distinctive abnormali tof gait

Destruction of Broca’s convolution (areas 44 and 45) and the adjacent insular and motor cortex of the dominant hemisphere result in a reduction or loss of motor speech, and agraphia, and apraxia of the face, lips, and tongue

Damage to the cortices anterior to areas 6 and 8—i.e., to areas 9, 10, 45, and 46—the prefrontal cortex, and also the anterior cingulate gyri, has less easily defined effects on motor behavior

An ataxia of the contralateral limbs has been attributed to prefrontal lesions, but careful anatomic verification is lacking

the resultant pattern is a slowed, slightly imbalanced, and short-stepped gait with the torso and legs not properly in phase when placed in motion, to which may be added the feature of “magnetic” gait,

In many instances with frontal lobe lesions are inclined to

manipulate objects placed before them (utilization behavior) or to imitate the gestures of others. extreme degrees of hyperactivity (“organic drivenness”) bilateral lateralorbital lesions

Incontinence is another manifestation of frontal lobe disease. Right- or left-sided lesions involving the posterior part of the superior frontal gyrus, the anterior cingulate gyrus

speech and language, a number of abnormalities other than Broca’s aphasia appear in conjunction with disease of the frontal lobes—laconic speech, lack of spontaneity of speech, telegraphic speech (agrammatism), loss of fluency, perseveration of speech, a tendency to whisper instead of speaking aloud, and dysarthria.

Cognitive and Intellectual Changes

the frontal lobes are injuredby disease, there was not only a general psychomotor slowing and easy distractibility but also an erroneous analysis of the abovelisted conditions of the problem.

“loss of the abstract attitude” (the patient thinks concretely, i.e., he reacts directly to the stimulus situation

Other Alterations of Behavior and Personality

A lack of initiative and spontaneity is the most common effect of frontal lobe

they tend to perseverate. Placidity is a notable feature of the

behavior Worry, anxiety, self-concern,

hypochondriasis, complaints of chronic pain, and depression are all reduced by frontal lobe disease

Extensive and bilateral frontal lobe disease is accompanied by a quantitative reduction in all psychomotor activity

The number of movements, spoken words, and thoughts per unit of time diminish.

Abulia akinetic mutism bilateral lesions in the ventromedial

frontal regions or frontal diencephalic

hyperactivity syndrome,or “organic drivenness,”combined frontal and temporal lobe lesions,

Some patients, particularly those with inferofrontal lesions, feel compelled to make silly jokes that are inappropriate to the situation—so-called Witzelsucht or moria

patients with lesions of either frontal lobe manifest a slight elevation and instability of mood, with increased talkativeness and a tendency to joke, lack of tact, inability to adapt to a new situation, and loss of initiative

More right

temporal

Most of the temporal lobe cortex, including Heschl’s gyri, has nearly equally developed pyramidal and granular layers

Unlike the six-layered neocortex, the hippocampus and dentate gyrus are typical of the phylogenetically older three-layered allocortex.

inferior or uncinate fasciculus connects the anterior temporal and orbital frontal regions

The arcuate fasciculus connects the posterosuperior temporal lobe to the motor cortex and Broca’s area

Clinical Effects of Temporal LobeLesions

Visual Disorders that lesions of the white matter of the central

and posterior parts of the temporal lobe characteristically involve the lower arching fibers of the geniculocalcarine pathway (Meyer’s loop).

upper homonymous quadrantanopia Visual hallucinations of complex form, including

ones of the patient himself (autoscopy), appear during temporal lobe seizures large (macropsia) or small (micropsia), too close or far away, or unreal

Cortical Deafness

Bilateral lesions of the transverse gyri of Heschl, while rare, are known to cause a central deafness

cortically deaf persons may seem to be unaware of their deafness, a state similar to that of blind persons who act as though they could see (the latter is called Anton syndrome

they are heard less well in the ear contralateral to the lesion.

areas 41 and 42).

Word-Deafness (Auditory Verbal Agnosia

Lesions of the secondary (unimodal association) zones of the auditory cortex—area 22 and part of area 21—

have no effect on the perception of sounds and pure tones

inability to recognize sounds, different musical notes (amusia), or words

In agnosia for sounds, auditory sensations cannot be distinguished from one another

Amusia proves to be more complicated, for the appreciation of music has several aspects: the recognition of a familiar melody and the ability to name it (musicality itself); the perception of pitch, timbre, and rhythm; and the ability to produce, read, and write music.

results from lesions in the middle temporal gyrus and not from lesions at the pole of the temporal lobe

In any case, the temporal lobe opposite that responsible for language (i.e., the right) is implicated in almost all cases

Word-Deafness (Auditory Verbal Agnosia

Worddeafnes is a failure of the left temporal lobe function in decoding the acoustic signals of speech and converting them into understandable words.

can occur by itself, without other features of Wernicke’s aphasia verbal agnosia may be combined with agnosia for sounds and music, or the two may occur separately

Auditory Illusions

sounds are perceived as being louder or less loud than normal

Sounds or words may seem strange or disagreeable, or they may seem to be repeated, a kind of sensory perseveration.

Elementary hallucinations and dreamy states have been reported with

lesions of either temporal lobe

Auditory Hallucinations

Elementary Complex sounds and musical themes are heard

more clearly than voices the superior and posterior parts of the

dominant or both temporal lobes were then involved

Vestibular Disturbances

In the superior and posterior part of the temporal lobe (posterior to the primary auditory cortex

If this area is destroyed on one side, the only clinical effect may be an illusion that the environment is tipped on its side or is upside down

Epileptic activation of this area induces vertigo or a sense of disequilibrium

Disturbances of Time Perception In a temporal lobe seizure originating on

either side, time may seem to stand still or to pass with great speed.

Certainly the most common disruptions of the sense of time occur as part of confusional states of any type

The patient with a Korsakoff amnesic state is unable to place events in their proper time relationship

Disturbances of Smell and Taste seizure foci in the medial part of the

temporal lobe (in the region of the uncus) often evoke olfactory hallucinations

posterior orbitofrontal, subcallosal, anterior temporal, and insular cortices,

Stimulation of the posterior insular area elicited a sensation of taste along with disturbances of alimentary function

Other (Nonauditory) Syndromes Amnesic dysnomia Prosopagnosia The loss of certain visual

integrative abilities, particularly face recognition

Disorders of Memory, Emotion, and Behavior

Attention must be drawn to the central role of the temporal lobe, notably

its hippocampal and limbic parts, in memory and learning and in

the emotional life of the individual.

I. Effects of unilateral disease of the dominant temporal lobe A. Homonymous upper quadrantanopia B. Wernicke’s aphasia (word-deafness—auditory verbal agnosia) C. Amusia (some types) D. Impairment in tests of verbal material presented through the auditory sense E. Dysnomia or amnesic aphasia F. Visual agnosia G. Occasionally amnesic (Korsakoff) syndrome II. Effects of unilateral disease of the nondominant temporal lobe A. Homonymous upper quadrantanopia B. Inability to judge spatial relationships in some cases C. Impairment in tests of visually presented nonverbal material D. Agnosia for sounds and some qualities of music

III. Effects of disease of either temporal lobe A. Auditory, visual, olfactory, and gustatory

hallucinations B. Dreamy states with uncinate seizures C. Emotional and behavioral changes D. Delirium (usually nondominant) E. Disturbances of time perception IV. Effects of bilateral disease A. Korsakoff amnesic defect (hippocampal

formations) B. Apathy and placidity

SYNDROMES CAUSED BY LESIONS OF THE PARIETAL LOBES

The postcentral gyrus, or primary somatosensory cortex, receives most of its afferent projections from the ventroposterior thalamic nucleus, which is the terminus of the ascending somatosensory pathways.

The primary sensory cortex projects to the superior parietal lobule (area 5), which is the somatosensory association cortex

Clinical Effects of Parietal Lobe Lesions

Cortical Sensory Syndromes

an impairment or loss of the sense of position and passive movement and the ability to localize tactile, thermal, and noxious stimuli applied to the body surfa

to distinguish objects by their size, shape, and texture (astereognosis)

to recognize figures written on the skin; to distinguish between single and double

contacts (two-point discrimination and to detect the direction of movement of a

tactile stimulus.

With anterior parietal lobe lesions there is often an associated mild hemiparesis, since this portion of the parietal lobe contributes a considerable number of fibers to the corticospinal tract

weakness, tend to remain hypotonic and the musculature may undergo atrophy of a degree not explained by inactivity alone

The Asomatognosias

The term asomatognosia denotes the inability to recognize part of one’s body

asomatognosia

with a dense hemiplegia, usually of the left side, may be indifferent to the paralysis or unaware

If told it is paralyzed, the patient may deny that this is so or offer an excuse:

cortex and white matter of the superior parietal lobule

Unilateral asomatognosia is seven times as frequent with right (nondominant) parietal lesions as with left-sided ones

Often there is a blunted emotionality the right parietal lobe is truly dominant

Gerstmann Syndrome

The characteristic features are inability to designate or name the different fingers of the two hands (finger agnosia),

confusion of the right and left sides of the body, and inability to calculate (dyscalculia) to write (dysgraphia). One or more of these manifestations may be

associated with word-blindness (alexia) and homonymous hemianopia or a lower quadrantanopia,

lesion is in the inferior parietal lobuleparticularly the angular gyrus or subjacent white matter of the left hemisphere

Ideomotor and Ideational Apraxia patients with parietal lesions of the

dominant hemisphere who exhibit no defects in motor or sensory function lose the ability to perform learned motor skills on command or by imitation

Visual Disorders with Parietal Lesions

A lesion deep to the inferior part of the parietal lobe, at its junction with the temporal lobe, involves the geniculocalcarine radiations and result in an incongruous homonymous hemianopia or an inferior quadrantanopia

An alexia or components of the Gerstmann syndrome may be associated

severe left-sided visual neglect results from a lesion in the right angular gyrus

Visual Disorientation andDisord ers of Spatial (Topographic)Localization

Patients with this disorder are unable to orient themselves in an abstract spatial setting (topographagnosia)

such patients have lost topographic memory

almost invariably caused by lesions in the dorsal convexity of the right parietal lobe.

I. Effects of unilateral disease of the parietal lobe, right or left A. Corticosensory syndrome and sensory extinction (or total hemianesthesia with large acute lesions of white matter) B. Mild hemiparesis (variable), unilateral muscular atrophy in children, hypotonia, poverty of movement, hemiataxia (all seen only occasionally) C. Homonymous hemianopia or inferior quadrantanopia (incongruent or congruent) or visual inattention D. Abolition of optokinetic nystagmus with target moving toward side of the lesion E. Neglect of the opposite side of external space (far more prominent with lesions of the right parietal lobe—see below) II. Effects of unilateral disease of the dominant (left) parietal lobe (in right-handed and most left-handed patients)—additional phenomena include A. Disorders of language (especially alexia) B. Gerstmann syndrome (dysgraphia, dyscalculia, finger agnosia, right-left confusion) C. Tactile agnosia (bimanual astereognosis) D. Bilateral ideomotor and ideational apraxia (

III. Effects of unilateral disease of the nondominant (right) parietal lobe A. Visuospatial disorders B. Topographic memory loss C. Anosognosia, dressing and constructional apraxias (these disorders may occur with lesions of either hemisphere but one observed more frequently and are of greater severity with lesions of the nondominant one) D. Confusion E. Tendency to keep the eyes closed, resist lid opening, and blepharospasm IV. Effects of bilateral disease of the parietal lobes A. Visual spatial imperception, spatial disorientation, and complete or partial Balint syndrome (optic apraxia, described

SYNDROMES CAUSED BYLESIONS OF THE OCCIPITALLOBES

Visual Field Defects

homonymous hemianopia A lesion confined to the pole of the

occipital lobe results in a central hemianopic defect that splits the macula and leaves the peripheral fields intact

Bilateral lesions of the occipital poles, as in embolism of the posterior cerebral arteries, result in bilateral central hemianopias

optokinetic responses are usually spared in

Cortical Blindness

bilateral lesions of the occipital lobes (destruction of area 17 of both hemispheres), there is a loss of sight and a loss of reflex closure of the eyelids to a bright light or threat

The pupillary light reflexes are preserved, The eyes are still able to move through a full range optokinetic nystagmus cannot be elicited Visual imagination and visual imagery in dreams are

preserve There may also be visual hallucinations of either

elementary or complex type Central vision is intact

Visual Anosognosia (Anton Syndrome

The main characteristic of this disorder is the denial of blindness by a patient who obviously cannot see

The lesions in cases of negation of blindness extend beyond the striate cortex to involve the visual association areas.

Visual Illusions (Metamorphopsias

deformation of the image, change in size, illusion of movement, or a combination of all three

lesions confined to the occipital lobes but are more frequently due to shared occipitoparietal or occipitotemporal lesions;

right hemisphere appears to be involved more often than the left

Visual field defects are present in many of the cases.

Visual Hallucinations

elementary or complex, and both types have sensory as well as cognitive aspects

They are indicative of lesions in the visual association areas or their connections with the temporal lobes.

The patient may realize that the hallucinations are false experiences or may be convinced of their reality.

Often they are associated with a homonymous hemianopia

Visual Object Agnosia

consists of a failure to name and indicate the use of a seen object by spoken or written word or by gesture.

Visual acuity is intact, the mind is clear, and the patient is not aphasic— conditions requisite for the diagnosis of agnosia

If the object is palpated, it is recognized at once, and it can also be identified by smell or sound if it has an odor or makes a noise.

visual object agnosia is usually associated with visual verbal agnosia (alexia) and homonymous hemianopia

Prosopagnosia (the inability to identify faces; see further on) is also present in most cases

The underlying lesions are usually bilateral

I. Effects of unilateral disease, either right or left A. Contralateral (congruent) homonymous hemianopia, which may be central (splitting the macula) or

peripheral; also homonymous hemiachromatopsia B. Elementary (unformed) hallucinations—usually due to irritative lesions II. Effects of left occipital disease A. Right homonymous hemianopia B. If deep white matter or splenium of corpus callosum is involved, alexia and color-naming defect C. Visual object agnosia

III. Effects of right occipital disease A. Left homonymous hemianopia B. With more extensive lesions, visual illusions (metamorphopsias) and hallucinations (more frequent with rightsided than left-sided lesions) C. Loss of topographic memory and visual orientation IV. Bilateral occipital disease A. Cortical blindness (pupils reactive) B. Anton syndrome (visual anosognosia, denial of cortical blindness) C. Loss of perception of color (achromatopsia) D. Prosopagnosia (temporo-occipital), simultanagnosia (parieto-occipital) E. Balint syndrome (parieto-occipital)