Occipital lobe and clinical effects of its dysfunction
Layers of cerebral cortex and its variations. Surface and functional anotomy of occipital lobe. Clinical effects of occipital lobar dysfunction. Differentiation between malingering and organic visual loss.
› One third of the cerebral cortex is on the exposed part of gyri remaining is buried in sulci and gyri.› 15 to 30 billion nerve cells.› Thickness of the cortex from 4.5 mm in the frontal area to 1.3 mm in the occipital area.
› The areas of where six layers can not be identified are called as heterotypical different from homotypical.› Supragranular cortex high level cortical functions.› Archicortex and paleo cortex.› Granular and agranular cortex.
› Superiolateraly occupies a small area behind the parieto occipital sulcus.› Medially area behind the occipetoparietal sulcus.› Inferiorly limited by collateral sulcus.› medially occipital lobe is devided in to cuneus and lingual gyrus.
Consists of Broadmann areas 17,18,19. Primary visual cortex or striate cortex or area 17 › Well devoloped layer 4 a thick layer of external band of ballirager. › Lips of calcarine with adjacent areas cuneus and lingual gyrus. › Perception of simple sensation color, size, shape, motion and illumination.
› Stimulation or ictal activity produces simple visal hallucinations.
Associative visual cortex: › Parastiriate [area 18] and peristriate [area 19] › receives and interprets impulses. › Have extensive connections. › Concerned with more complex visual functions of perception, spatial orientation, visual association, and visual memory. › Stimulation produces formed or complex visual hallucinations.
› In humans occipital eye field is present in the visual association cortex concerned with reflex eye movements.
Unclear . Dual representation of macula in each occipital pole. Collateral blood supply from anterior and middle cerebral artery. Extensive cortical representation at occipital pole and depths of anterior calcarine fissure.
Cortical blindness: › Bilateral lesions of the occipital lobe there is loss of sight and reflex closure of eyelids to threat and light with sparing of pupilary light reflex. › Most common cause bilateral pca infarcts. › Other causes are PRES, CJD, PML and gliomas.
Visual anasagnosia or Antons syndrome: › Denial of blindness. › Occurs with bilateral PCA infarcts. › Sparing of tiny islands of vision › Patient complains of fluctuation of images as the vision is captured in spared islands of cortex.
Visual illusions: › Size, shape, movement or combination of three. › Occipital lesion or in combination occipitotemporal and occipitoparietal areas. › Illusion of movement occurs with occipetotemporal lesions. › Polyopia with lesions in occipital lobe. › Palinopsia with lesions in occipitoparietal lesions.
Visual hallucination: › Elimentary or simple visual hallucination is the feature of striate cortex › Includes flashes of light, luminous lightened candles, multiple stars and geomatric forms. › Complex or formed visual hallucination is a feature of association cortex or its connections › Includes objects, animals, persons and scenes.
VISUAL OBJECT AGNOSIA: › Failure to recognize objects by vision with preserved ability to recognize them through touch or hearing and in the absence of impaired primary visual perception or dementia
Aperceptive visual agnosia: › Perceived elements of object are synthesized to whole image. › Pick out features of the object correctly such as lines, angles,colors or movement but fail to appreciate the whole object. › Examples : spectacles › Right hemisphere particularly lingual gyrus involved in global processing of the object
Left hemisphere occipital cortex invoved in more local processing.
ASSOCIATED VISUAL AGNOSIA: › Is more closely related to aphasia than primary disorder of vision. › Patients can copy and match the drawing of objects but can not name them. › They can be identified by tactile or auditary modality. › have associated color agnosia and prosagnosia.
• Bilateral posterior hemispheric lesions involving occipitotemporal gyrus some times lingual gyri and adjacent white matter.
Charecterised by › Simultagnosia is a disorder of visual attention especially to peripheral field associated inability to perform orderly visual scanning of the environment and attention to other sensory stimuli are intact. › Optic ataxia is the loss of hand eye co- ordination with difficulty in touching or reaching the objects under visual guidance.
• Optic apraxia is inability project gaze voluntarily in the peripheral field despite intact occulomotor movements.
› Inability to recognise familiar faces or pictures.› Associated with inability to memorise new faces.› Inability to name the species of birds, animals or car model.› Bilateral ventromesial occipital lesions.
The classic syndrome of pure alexia without agraphia is caused by a left posterior cerebral artery occlusion in a right-handed individual All visual information enters only the right hemisphere The right visual cortex perceives the written material but cannot transmit it to the left hemisphere because of the callosal lesion
The inferior parietal lobule in the dominant hemisphere (primarily area 39, the angular gyms) is the association cortex that combines the visual and auditory information necessary for reading and writing A second distinct type of alexia, classically called alexia with agraphia, results from damage to the inferior parietal lobule itself (angular gyrus and environs). This lesion renders the patient unable to read or write
› Differentiated between perceptual color disturbance or anomia› Congenital retinal color blindness is the most common type tested by using ishihara charts.› Acquired color blindness due to cerebral lesion is called central achromatopsia.› Associated visual field defects and prosopagnosia.
› Most often the lesions are bilateral and tend to affect the upper quadrants with lesions in bilateral ventro mesial temporal lobes and lower part of the striate cortex.
Color anomia can be a part of pure word blindness or anomic aphasia.
Signing Shmidt Rimpler test A functionally blind person ignorant of laws of reflection may have much improved vision reading an acuity chart held at his chest in a mirror 10 ft away than reading the chart 20 ft away. Optokinetic nystagmus. Photic drive on eeg and VER.
1. Effects of unilateral disease: › Contraletral homonymous hemianopia, which may be central or peripheral. › Visual hallucinations.2. Left occipital disease: › Rt. Homonymous hemianopia › Alexia with out agraphia, color anomia.
› Visual object agnosia.3. Rt. Occipital disease: › Lt. Homonymous hemianopia. › Visual illusinations and hallucinations › loss of topographic memory and visual orientation.