Auditory And Visual System

AUDITORY AND VISUAL SYSTEM VISUAL PATHWAY Optic Nerve & Optical Chiasm • Optic nerves send electrical signals from each eye meet in the brain at the optic chiasm. • Here, the left visual from one eye is combined with the other eye and the same goes for the right visual. • The signals split again. • The right visual heads for the left brain and the left visual makes its way to the right side of the brain. • Visual messages from both eyes will reach both halves of the visual cortex. • The brain then merges the image into one image.

Lateral Geniculate • From the optic chiasm, the signals reach the lateral geniculate, twin relay stations deep in each hemisphere of the brain.

Visual Cortex • Most visual impulses will travel directly from the lateral geniculate nuclei to the primary visual cortex. • In the visual cortex lies the ability to detect spatial organization of a scene, the shape of objects and their tone of shading. • Then from the cortex, some more nerve fibers connect it to the secondary visual cortex. • This area of the brain decodes visual messages a higher level than the primary visual cortex. • Any damage done to the visual sites in the temporal lobe can inhibit visual learning.

VISUAL DEFECTS The typical visual field defects in lesions at various sites in the optic pathways are summarized in Fig. · At the level of the optic nerve - unilateral blindness · At the middle of the optic chiasma - bitemporal hemianopia. · At the lateral aspect of the optic chiasma - unilateral nasal hemianopic defects; if the pressure is bilateral - bilateral nasal hemianopic defect.

· In the posterior part of the optic chiasma when the pressure is from the inferior aspect bitemporal scotoma. · At the level of the optic tract and geniculate body incongruous incomplete homonymous hemianopia. · The optic radiation in the temporal lobe - incongruous homonymous superior quadrantic defect. · The optic radiation in the temporal lobe - incongruous homonymous inferior quadrantic defect. · The optic radiation in the posterior portion congruous homonymous hemianopia with macular sparing. · In the mid-portion of the calcarine cortex contralateral loss of the temporal crescent. · At the tip of the occipital lobe - congruous homonymous hemianopic scotoma. · Anterior tip of calcarine fissure - contralateral loss of the temporal crescent with otherwise normal visual fields.

Must remember

Tip- Lateral geniculate body - Light - Optic or Visual Pathway Medial geniculate body - Music - Auditory Pathway

Note that pretectal nucleus is not involved in accommodation reflex. In lesions of the pretectal nucleus the light reflex is lost, but pupil contracts on accommodation. This is called Argyll Robertson pupil.

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