Visual pathway

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Perceiving an Image

Initial stimulus

  • An image is perceived
  • the part of the image in the left visual field reaches the right retinal fields of both eyes, whereas the part of the image in the right visual field reaches the left retinal fields of both eyes
  • Optic nerves transmit the retinal images from the retina
  • The orientation of the image in each optic nerve is identical to that perceived in the retina of each eye
  • temporal retinal field: left retinal field of the left eye and the right retinal field of the right eye
  • nasal retinal field: right retinal field of the left eye and the left retinal field of the right

Optic chiasma and tracts

  • Optic nerves travel a short distance to reach the optic chiasma
  • optic nerve fibres carrying the portion of the image perceived in the nasal fields decussate
  • Recombined fibres are named optic tracts
    • The left optic tract carries the portion of the image in the right visual field
    • the right optic tract carries the image of the left visual field

Optic radiations

  • The optic tracts enter the thalamus in the diencephalon
  • Fibres reach the lateral geniculate nucleus (LGN) and synapse with secondary neurons
  • Post-synaptic fibres travel from the LGN to the primary visual (striate) cortex
  • These fibres travel along the calcarine sulcus as geniculo-calcarine fasciculus or optic radiations

Visual (striate) cortex

  • The striate (lingual and cuneate) cortex of each lobe receives the portion of the image in the opposite visual field
  • Retinal signals near the fovea reach the posterior portion of striate cortex
  • Retinal signals further away from the fovea reach the anterior portion of the cortex

Corpus callosum

  • See Corpus callosum
  • Reciprocal projections between the visual cortices of the hemispheres allow for the cortical perception of a complete image
  • The projections decussate via the splenium portion of the corpus callosum

Visual field defects

Results of lesions in the optic pathway (Fig. 5-3)

Right optic nerve lesion

  • Results in blindness in the right eye and loss of both visual fields for that eye
  • It is still possible to perceive both visual fields through the left eye
  • but perception of the depth of the image is impaired (monocular)

Optic chiasma lesion

  • Results in loss of left visual field in left eye and right visual in right eye
  • This condition is known as bitemporal hemianopia
    • The term heteronymous hemianopia is used in this case to indicate that temporal fields in both eyes are impaired
    • Impairment of either both right or both left visual fields would result in a homonymous defect as described below

Lesion of the right optic tract, LGN, or right optic radiation

  • loss of both left visual fields, thus left homonymous hemianopia

Lesion of the right optic radiation to the cuneate gyrus or the right cuneate gyrus itself

  • loss of the left lower quadrant of the visual field

Lesion of the right optic radiation to the lingual gyrus or the right lingual gyrus itself

  • loss of the left upper quadrant of the visual field


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