Cranial Nerves: Visual System - Visual Fields

Sight — Visual Fields (VF) — CN II

• visual field = what you see

• picture is showing the visual field of each eye when both eyes are opened

• visual field is not the same as retina

Left vs Right VF

• individually, each eye has a right VF and a left vf

Left vs Right Nasal VF

• visual field for each individual eye can also be described as a temporal VF or a Nasal VF

Temporal VF

the “outside” (lateral) image for each eye

Nasal VF

the “inside” (medial) image for each eye

Left/Right VF vs Nasal/Temporal VF

• for the left eye:

  • its left side VF = temporal VF

  • right side VF = nasal VF

• right eye:

  • left side VF = nasal VF

  • right side VF = temporal VF

Visual Fields

• information from the right visual field of each eye will end in the left primary visual cortex of the occipital lobe

• information from the left visual field of each eye will end in the right primary visual cortex of the occipital lobe

• uses 2 neurons:

  • 1st neuron: from retina to the lateral geniculate nucleus (LGN) of the thalamus

  • 2nd neuron": from LGN to primary visual cortex

From Retina to Cortex

• 1st neuron axons in retina travel in CN II, through optic chiasm. optic tract then synapse in lateral geniculate nucleus (LGN) of thalamus

• 2nd neuron: axons travel form LGN through optic radiations to the primary visual cortex

• this two neurons pathway is also called the retinogeniculocalcarine pathway

Primary visual cortex

• processes conscious awareness of vision

• registers basic information about size, shape, edges, texture of objects

• it does not provide information about color (secondary visual cortex) what the object is or how to move your eyes to follow the object. that requires input to other cortical areas

Retina — Overview

• to reach the occipital lobe, the visual fields (light) need to first be processed by the retina

• information from the right VF of each eye will end on the left side of the retina

• information from the left VF of each eye will end on the right side of the retina

Remember

• visual fields for each eye can be described as temporal VF or Nasal VF

• the retina can also be described as having a nasal side and a temporal side

• information from the nasal VF of either eye will end on the temporal side of the retina (Green)

• information from the temporal VF of either eye will end on the nasal side of the retina (purple)

From optic to optic chiasm to LGN

• info from temporal retina will stay ipsilateral on its way to primary visual cortex

• info from nasal retina will cross over to the other side in the optic chias, and end in the contralateral primary visual cortex

From LGN to Primary Visual Cortex

• info from LGN reaches primary visual cortex via Optic radiations

• upper optic radiations

  • process lower visual field info from each eye

  • travel through parietal lobe then synapse in cuneus gyrus of primary visual cortex (occipital lobe)

• lower optic radiations (Meyer’s loop)

  • process upper visual field info from each eye

  • travel through temporal lobe then synapses in lingual gyrus of the primary visual cortex (occipital lobe)

Upper/Lower Visual field and upper/lower retina

• info from lower visual field goes to upper retina

  • once in LGN, axons use upper optic radiations (parietal lobe) to synapse in cuneate gyrus

• info from upper visual field goes to lower retina

  • once in LGN, axons use lower optic radiations (temporal lobe) to synapse in lingual gyrus

Upper/Lower Optic Radiation

• upper optic radiation = green

  • represent lower visual field

  • travels in parietal lobe

• Meyer’s loop (lower optic radiation) = red

  • represents upper visual field

  • travels in temporal lobe

Visual Tract

CN II — Visual Field Deficits

what happens to someone’s visual field if there are lesions/defects along the visual pathway?

Circle of Willis

different lesions along the visual pathway

1. R. Optic nerve

2. optic chiasm

3. R. Optic tract

4. R. Meyer’s loop (lower optic radiations)

5. R. upper optic radiations

6. R. optic radiations

R. Optic nerve: R. eye blindness

can see out of left eye

Optic Chiasm: Bitemporal hemianopsia

tunnel vision (can see medial half)

R. optic tract: Left homonymous hemianopsia

• can see through right visual field

R. Meyer’s loop: left homonymous superior quadrantic anopsia (quadranopsia)

pie shape, can see everything expect top left quarter of visual field

R. Upper Radiations: Left homonymous inferior quadrantic anopsia (quadranopsia)

pie shape, can see everything expect bottom left quarter of visual field

R. Optic Radiations: Left homonymous hemianopsia (same as lesion 3)

cannot see out of left visual field