Section IV • Neuroscience
ofprimary visual cortex. Blob zones project to the inferior part ofthe temporal lobe in areas 20 and 21. Unilateral lesions here result in achromatopsia, a com plete loss ofcolorvision inthe contralateral hemifields. Patients see everything in shades ofgray. Additionally, these patients mayalso present with prosopagnosia, an inability to recognize faces.
Motion and depth are processed by the magnocellular system. This "rod stream'' originates in the peripheral part of the retina, relays through separate layers of the lateral geniculate, and projects to thick stripe zones ofprimary visual cortex. Striped areas project through the middle temporal lobe to the parietal lobe in areas 18 and 19. Lesions here result in a deficit in perceiving visual motion; visual fields, color vision, and reading are unaffected.
Clinical Correlate
Visual Agnosia
Damage to parts ofthe temporal lobes involving the cone stream produces a visual agnosia. Visual agnosia is the inability to recognize visual patterns (including objects) in the absence of a visual field deficit. For example, you
might show a patient with an object agnosia a pair of glasses, and the patient would describe them as 2 circles and a bar. Lesions in areas 20 and 21 of the temporal lobe that also include some destruction of adjacent occipital lobe in either hemisphere result in prosopagnosia, a specific inability to recognize faces. The patient can usually read and name objects. The deficiency is an
inability to form associations between faces and identities. On hearing the voice of the same person, the patient can immediately identify the person.
Alexia WithoutAgraphia
A principal "higher-order" deficit associated with occipital lobe damage is alexia without agraphia (or pure word blindness). The patients are unable to read at all and, curiously, often have a color anomia (inability to name colors). However, they are able to write. This is another example of a disconnect syndrome in which information from the occipital lobe is not available to the parietal or frontal lobes to either understand or express what has been seen.
(Recall that alexia with agraphia-inability to read or write-occurs with lesions encompassing the angular gyrus in the dominant parietal lobe. The cause ofthe syndrome is usually an infarction ofthe left posterior cerebral artery that affects not only the anterior part ofthe occipital lobe but the splenium ofthe corpus callosum. Involvement ofthe left occipital cortex results in a right homonymous hemianopsia with macular sparing. Involvement ofthe splenium ofthe corpus callosum prevents visual information from the intact right occipital cortexfrom reaching language comprehension centers in the left hemisphere. Patients can see words in the left visual field but do not understand what the words mean.
Chapter 10 • Cerebral Cortex
Table IV-10-3. CNS Blood Supply and Stroke-related Deficits
System |
Primary Arteries |
Branches |
Supplies |
Deficits after Stroke |
Vertebrobasilar |
Vertebral arteries |
Anterior spinal artery |
Anterior two-thirds |
Dorsal columns spared; all |
(posterior |
|
|
of spinal cord |
else bilateral |
circulation) |
|
Posterior cerebellar |
Dorsolateral |
See Brain-Stem Lesions in |
|
|
|
|
(PICA) |
medulla |
Chapter IV-5. |
|
Basilar artery |
Pontine arteries |
Base of pons |
|
|
|
Anterior inferior |
Inferior cerebellum, |
|
|
|
cerebellar artery (AICA) |
cerebellar nuclei |
|
|
|
Superior cerebellar artery |
Dorsal cerebellar |
|
|
|
|
hemispheres; |
|
|
|
|
superior cerebellar |
|
|
|
|
peduncle |
|
|
|
Labyrinthine artery |
Inner ear |
|
|
(sometimes arises from |
|
|
|
AICA) |
|
|
Posterior cerebral |
|
Midbrain, thalamus, |
|
arteries |
|
occipital lobe |
Internal carotid |
Ophthalmic: artery |
Central artery of retina |
Retina |
(anterior |
Posterior |
|
|
circulation) |
|
|
|
communicating |
|
|
|
artery |
|
|
|
Anterior c:erebral |
|
Primary motor and |
|
artery |
|
sensory cortex (leg/ |
|
|
|
foot) |
|
Anterior |
|
|
|
communicating |
|
|
|
artery |
|
|
|
Middle cerebral |
Outer cortical |
Lateral convexity of |
|
artery |
|
hemispheres |
|
|
Lenticulostriate |
Internal capsule, |
|
|
|
caudate, putamen, |
|
|
|
globus pallidus |
*If dominant hemisphere is affected (usually the left) tRight parietal lobe lesion
Contralateral hemianopia with macular sparing
Alexia without agraphia*
Blindness
Second most common aneurysm site (often with CN Ill palsy)
Contralateral spastic paralysis and anesthesia of lower limb
Frontal lobe abnormalities
Most common site of aneurysm
Contralateral spastic paralysis and anesthesia of upper limb/ face
Gaze palsy Aphasia*
Gerstmann syndrome""
Hemi inattention and neglect of contralateral bodyt
Section IV • Neuroscience
Table IV-10-4. Key Features of Lobes
Lobes |
Important Regions |
Frontal |
Primary motor and premotor |
|
cortex |
|
Frontal eye fields |
|
Broca speech area* |
|
(Areas 44, 45) |
Deficit After Lesion
Contralateral spastic paresis (region depends on area of homunculus affected), premotor: apraxia
Eyes deviate to ipsilateral side
Broca aphasia (expressive, nonfluent aphasia): patient can understand written and spoken language, but speech and writing are slow and effortful; patients are aware oftheir problem; often associated with right arm weakness and right lower face weakness.
|
Prefrontal cortex |
Parietal |
Primary somatosensory cortex |
|
Superior parietal lobule |
|
Inferior parietal lobule |
|
(Angular gyrus; Area 39) |
Temporal |
Primary auditory cortex |
Frontal lobe syndrome: symptoms can include poor judgment, difficulty concentrating and problem solving, apathy, inappropriate social behavior
Contralateral hemihypesthesia (region depends on area of homunculus affected)
Contralateral astereognosis/apraxia
Gerstmann syndrome (if dominant hemisphere): right/left confusion, alexia, dyscalculia and dysgraphia, finger agnosia, contralateral hemianopia or lower quadrantanopia; unilateral neglect (nondominant)
Bilateral damage -+ deafness
Unilateral leads to slight hearing loss
|
Wernicke area* |
|
(Area 22) |
|
Hippocampus |
|
Amygdala |
|
Olfactory bulb, tract, primary |
|
cortex |
|
Meyer loop (visual radiations) |
Occipital |
Primary visual cortex |
Wernicke aphasia (receptive, fluent aphasia): patient cannot understand any form of language; speech is fast and fluent, but not comprehensible
Bilateral lesions lead to inability to consolidate short-term to long-term memory
Kliiver-Bucy syndrome: hyperphagia, hypersexuality, visual agnosia
Ipsilateral anosmia
Contralateral upper quadrantanopia ("pie in the sky")
Cortical blindness if bilateral; macular sparing hemianopia
*In the dominant hemisphere. Eighty percent of people are left-hemisphere dominant.
Chapter 10 • Cerebral Cortex
ChapterSummary
•The external layer ofthe gray matter covering the surface ofthe cortex is characterized by numerous convolutions called gyri, separated by grooves called sulci. The cortex is divided into the frontal, parietal, occipital, and temporal lobes by several prominent sulci. Different areas ofthe cortex are concerned with sensory and motor functions. The frontal lobe contains the primary motor and premotor cortex, frontal eye field, and Broca speech area. The primary somatosensory and association cortex is found in the parietal
lobe. The temporal lobe contains the primary auditory cortex and Wernicke area. The primary visual cortex is at the posterior pole ofthe occipital lobe.
•The blood supply ofthe cortex is supplied by branches ofthe 2 internal carotid arteries and 2 vertebral arteries. On the ventral surface ofthe brain, the anterior cerebral and middle cerebral branches ofthe internal carotid arteries connect with the posterior cerebral artery, derived from the basilar arteryform the circle ofWillis. This circle ofvessels is completed by the anterior and posterior communicating arteries. The middle carotid artery mainly supplies the lateral surface ofthe frontal, parietal, and upper aspect ofthe temporal lobe. Deep branches also supply part ofthe basal ganglia and internal capsule. The anterior cerebral artery supplies the medial aspect ofthe frontal and parietal lobes. The entire occipital lobe, lower aspect oftemporal lobe, and the midbrain are supplied by the posterior cerebral artery.
•The homunculus of the motor and sensory cortex indicates that the upper limb and head are demonstrated on the lateral surface of the cortex. The pelvis and the lower limb are represented on the medial surface of the hemispheres. Therefore, the motor and sensory functions of the lower limb are supplied by the anterior cerebral artery while the motor and sensory functions ofthe upper limb and head are supplied by the middle cerebral artery.
•The primary language centers (Broca and Wernicke areas) are functionally located only in the dominant hemisphere, usually the left hemisphere. Both ofthese are supplied by the middle cerebral artery. Lesions ofthe Broca area result in motor or expressive aphasia (intact comprehension). Lesions of the Wernicke area produce receptive aphasia (lack of comprehension). Conduction aphasia results from a lesion ofthe arcuate fasciculus that connects the Broca and Wernicke areas.
•The internal capsule is a large mass ofwhite matter that conducts almost all tracts to and from the cerebral cortex. It is divided into an anterior limb, genu, and posterior limb. The anterior limb is supplied by the anterior cerebral artery, and the genu and posterior limb are supplied by the middle cerebral artery. The primary motor and sensory systems course through the posterior limb and genu.
