Neuro Week 12

Key Structures and Layers of the Eye

• The cornea, located at the front of the eye, refracts and focuses light, acts as a protective barrier, maintains transparency, and provides an immune privilege environment.
• The iris, the colored part of the eye, sits behind the cornea and functions similarly to a lens; its color depends on individual genetics.
• Posterior to the iris, the lens regulates incoming light with the help of the ciliary muscle, which bends or compresses the lens to prevent damage from excessive light.
• The retina, located at the back of the eye, performs initial lower-level processing of light.
• The fovea, a unique feature within the retina, is critical for high-resolution vision tasks like reading.
• Rods, located in the periphery of the retina, are highly sensitive to light, offer resolution in low light, and primarily detect monochromatic colors.
• Cones, located closer to the medial aspect of the retina, are helpful for distinguishing colors and are most active in daytime.
• The optic disc, located towards the optic nerve, is where retinal ganglion cell axons converge to form the optic nerve, contains no photoreceptors, resulting in a blind spot.
• The optic nerve, the second cranial nerve at the back of the eye, transmits visual information to the brain. The optic nerve is surrounded by dura mater, arachnoid, and pia mater.
• Vision occurs as light enters the eye, gets focused by the cornea and lens onto the retina, gets processed by photoreceptor cells, and then gets transmitted to the brain via the optic nerve.

Pathway from Retina to Occipital Cortex

• Light passes through the cornea, lens, and pupil to reach the retina where rods and cones perform initial lower-level processing of visual information, including contrast, color, and movement.
• Visual information is transmitted from the retina via the optic nerve (second cranial nerve); retinal ganglion cell axons converge at the optic disc to form the optic nerve.
• The optic nerve from each eye travels to the optic chiasm, a crucial point where nasal and temporal visual field information is segregated.
• At the optic chiasm, fibers from the nasal portion of each retina cross over to the contralateral side, while temporal portion fibers remain on the ipsilateral side.
• Reorganized fibers form the optic tracts after the optic chiasm; each optic tract contains information from the contralateral visual field, originating from the temporal retina of one eye and the nasal retina of the other eye.
• Optic tracts travel to the lateral geniculate body (LGB) in the thalamus, which relays optic tract fibers to the next set of neurons in the visual pathway; this is the only synaptic relay before the cortex.
• Optic radiations carry visual information from the LGB through the temporal and parietal lobes towards the back of the brain.
• Optic radiations reach the occipital lobe, specifically the primary visual cortex (V1), where visual information is perceived and interpreted, leading to conscious sight; the image projected onto the retina is flipped upside down, and the occipital cortex reconstructs it.
• The visual pathway ensures that information from the left visual field is processed in the right occipital cortex, and vice versa.

Localizing Lesions Based on Visual Field Deficits

• Optic Nerve Lesion (#1): Total vision loss occurs in the affected eye since visual information cannot reach the brain.
• Optic Chiasm Lesion: Bitemporal hemianopsia (loss of temporal visual fields in both eyes) results from disrupted nasal fibers (interpreting temporal visual fields) crossing at the chiasm.
• Optic Tract Lesion: A homonymous hemianopsia (loss of the visual field on the same side relative to the lesion) occurs after the optic chiasm.
• A right optic tract lesion causes a left homonymous hemianopsia, resulting in loss of the left temporal visual field (right eye) and the left nasal visual field (left eye).
• Conversely, a left optic tract lesion causes a right homonymous hemianopsia, with loss of the right visual field.
• Optic Radiation Lesion: Homonymous hemianopsia can be caused by effects to the optic radiation, depending on the affected portion of the optic radiation or visual cortex.
• Occipital Cortex Lesion: Contralateral homonymous hemianopsia can be caused by damage to the occipital cortex.
• Localizing lesions requires assessing lost visual field parts in each eye; the pattern of loss directly corresponds to the damage location along the visual pathway.

Visual Pathway & Lesions - Questions and Answers

• The visual pathway from the orbit to the visual cortex includes:
• Retina (rods and cones for light detection and initial processing)
• Optic Nerve (transmits visual information)
• Optic Chiasm (nasal fibers decussate, temporal fibers remain ipsilateral)
• Optic Tract (information from contralateral visual field)
• Lateral Geniculate Body (LGB) (synapse in the thalamus)
• Optic Radiations (project to the occipital cortex)
• Occipital Lobe/Primary Visual Cortex (perception and interpretation)
• Visual field deficits based on the lesion location:
• Optic nerve lesion: Total vision loss in the affected eye
• Optic chiasm lesion: Bitemporal hemianopsia (loss of temporal vision in both eyes)
• Optic tract lesion: Homonymous hemianopsia (loss of the visual field on the opposite side of the lesion)
• Optic radiation: Homonymous hemianopsia (contralateral to the lesion)
• Occipital cortex lesion: Contralateral homonymous hemianopsia
• Examples:
  • Loss of vision in the left temporal and right nasal region: Left homonymous hemianopsia
  • Right homonymous hemianopsia: Impacts the nasal portion of the left retina and the temporal portion of the right retina
• Unilateral spatial neglect results in failure to attend to stimuli on the opposite side of the body.
• Consequences:
• Ignoring objects/people on the neglected side
• Difficulties with tasks requiring attention to both sides
• Potential safety issues
• May worsen visual deficits where present, even if visual pathway is intact.
• Visual field cut is a loss of visual information; unilateral spatial neglect is an attentional deficits where individuals do not process information on one side.