Visual System

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Eyes and Retina

  • Lens – as light enters the lense, the image is inverted and reversed (so information from the upper visual space is projected to the lower and vice versa; R-L inversion occurs as well)
  • Fovea – central fixation point; region of retina with highest visual acuity; central 1-2 degrees of visual space
  • Macula – surrounds fovea and has relatively high visual acuity; central 5 degrees of visual space
  • Optic Disc – 15 degrees medial to the fovea; where axons leave retina to form optic nerve = blind spot

Two Classes of Photoreceptors

  • Rods – more numerous by 20:1; poor spatial and temporal resolution, do not detect colors; main function is vision in low-level lighting conditions
  • Cones – less numerous (though more highly represented in fovea); high spatial and temporal resolution, detect colors
    • Unlike any other neurons, photoreceptors and bipolar cells do not fire action potentials. Instead, information is conveyed by passive electrical conduction and they communicate through “nontraditional” synapses that release neurotransmitters in a graded fashion

Optic Nerves, Optic Chiasm, Optic Tracts

Probably the most critical information in this chapter is contained in Figure 11.15.

  • Homonymous – visual fields of both eyes are affected
    • Optic nerve is not a true nerve – pathway of retinal ganglion cells (which send axons into optic “nerve”) lies entirely within CNS. Initial portion (anterior to chiasm) = optic nerve; proximal portion (posterior to chiasm) = optic tract
    • Partial crossing of fibers in optic chiasm; therefore, fibers from right hemiretinas (i.e., left visual field) end up in right optic tract (and vice versa)
    • Lesions of optic chiasm – bitemporal (bilateral lateral) visual field defects
    • Lesions of eye, retina, optic nerve – monocular visual field defects
    • Lesions proximal to optic chiasm (i.e. optic tracts, LGN, optic radiation or visual cortex) – homonymous visual field defects
    • Because of location (i.e., ventral surface of brain, beneath frontal lobes and in front of pituitary gland), chiasm is susceptible to compression by pituitary tumors and other lesions

Lateral Geniculate Nucleus and Extragenuclate Pathways

  • Retino-geniculo-striate pathway (See figure 11.6 for visual depiction of pathway)
    • Axons of retinal ganglion cells in optic tract – LGN of thalamus – primary visual cortex
    • Functions in visual discrimination and perception
    • A minority of fibers bypass this pathway to go to the one described below
  • Retino-tecto-pulvinar-extrastriate cortex pathway (i.e., Extrageniculate Visual Pathway)
    • optic tract – brachium of superior colliculus – pretectal area and superior colliculus – relays in the pulvinar and lateral posterior nucleus of the thalamus – numerous brainstem areas and association cortex (lateral parietal cortex, frontal eye fields of prefrontal cortex)
    • Functions in visual attention and orientation (may be involved in “blindsight”

Optic Radiations to Primary Visual Cortex

  • Optic Radiations (see figure 11.8) – axons leaving the LGN fan out over a wide area of white matter
    • Lesion of optic radiation – homonymous defect affecting the contralateral visual field
    • Meyer’s Loop – inferior optic radiation fibers; arc forward into temporal lobe; carry info from the inferior retina/superior visual
      • Temporal lobe lesions – contralateral homonymous superior quadrantopia (“pie in the sky”)
    • Upper optic radiations pass under parietal lobe
      • Parietal lobe lesions – contralateral homonymous inferior quadrantopia (“pie on the floor”)
  • Primary Visual Cortex (see figure 11.8)
    • Lies on banks of calcarine fissure in the occipital lobe
    • Is retinotopically organized (fovea – occipital pole; peripheral regions – anterior along the fissure)
    • Fovea’s cortical representation occupies 50% of the primary visual cortex (despite its small retinal area, b/c it is the region of highest visual acuity)
    • Cuneus (“wedge”) – portion of medial occipital lobe above calcarine fissure
    • Lingula (“little tongue”) – portion of medial occipital lobe below calcarine fissure
    • Upper portion of optic radiation projects to superior bank of calcarine fissure
      • Upper-bank lesions = contralateral inferior quadrant defects
    • Lower portion of optic radiation projects to inferior bank of calcarine fissure
      • Lower-bank lesions = contralateral superior quadrant defects

Visual Processing in the Neocortex

Primary visual cortex (area 17) = striate cortex

  • 2 main streams of higher-order visual processing (after primary and secondary visual cortex):
    • “Where?” – dorsal pathways projecting to parieto-occipital association cortex; analyze motion and spatial relationships between objects and between the body and visual stimuli
    • “What?” – ventral pathways projecting to occipitotemporal association cortex; analyze form; specific regions for colors, faces, letters, etc.

Assessment of Visual Disturbances

  • Visual Acuity:
    • reported using Snellen notation of 20/X
    • visual field defects do not typically affect visual acuity
  • Monocular vs. Binocular visual disturbance – essential for localization, but patient report is often unreliable; “blurred vision” is particularly difficult to interpret (e.g., patient will say they lost the vision in one eye, but they really mean one visual field)
  • Negative phenomena (patients may be unaware)
    • scotoma
    • homonymous visual field defect
  • Positive visual phenomena
    • simple visual phenomena (lights, colors, geometric shapes, etc.) – caused by disturbance anywhere from eye to primary visual cortex
    • fortification scotoma – jagged alternating light and dark zigzagging lines common to migraine
    • pulsating colored lights or moving geometric shapes – suspect occipital seizures
    • formed visual hallucinations (e.g., people or animals) – arise from inferior temporo-occipital visual association cortex; common causes are toxic/metabolic disturbance, alcohol/sedative withdrawal, focal seizures, complex migraine, neurodegenerative conditions (e.g., CJD, LBD), narcolepsy, midbrain ischemia, psychiatric disorders (though auditory more common than visual in latter)
    • release phenomenon – patients with deprivation in part or all of visual field have formed visual hallucinations, especially during early stages of deficit
    • Bonnet syndrome – visual hallucinations occurring in the elderly as a result of impaired vision
  • Terms to Describe Visual Disturbances
    • Scotoma – a circumscribed region of visual loss
    • Homonymous Defect – a visual field defect in the same region for both eyes
    • Refractive Error – indistinct vision improved by corrective lenses
    • Photopsias – bright, unformed flashes, streaks, or balls of light
    • Phosphenes – photopsias produced by retinal shear or optic nerve disease
    • Entopic Phenomena – seeing structures in one’s own eye
    • Illusions – distortion or misinterpretation of visual perception
    • Hallucination – perception of something that is not present

Localization of Visual Field Defects

Visual Field Testing

  • Confrontation testing – the examiner should test their own visual field simultaneously by holding a visual stimulus midway between self and patient
  • Extinction is a sign of visual neglect
  • Formal visual field testing can be done using Goldmann perimetry

Visual Field Defects

Figure 11.15 falls in this section, and is critical for understanding.

  • Retinal lesion – causes monocular scotoma or, if severe enough, monocular visual loss (common cause: retinal infarct)
  • Optic nerve lesion – causes monocular scotoma or monocular visual loss (may be partial or incomplete depending on severity) (common causes: glaucoma, optic neuritis, elevated ICP)
  • Optic chiasm lesion – bitemporal hemianopia (often caused by compression due to lesion; common cause: pituitary adenoma)
  • Retrochiasmal lesion (optic tracts, LGN, optic radiations, visual cortex) – homonymous visual field defects
    • Optic tract lesion (uncommon) – contralateral homonymous hemianopia
    • LGN lesion – contralateral homonymous hemianopia
    • Lower optic radiation lesion (through temporal lobe) – contralateral superior quadrantopia (“pie in the sky”) (common cause: MCA inferior division infarct)
    • Upper optic radiation lesion (through parietal lobe) – contralateral inferior quadrantopia (“pie on the floor”) (common cause: MCA superior division infarct)
    • Entire optic radiation lesion – contralateral homonymous hemianopia
  • Primary visual cortex lesion – common cause: PCA infarcts, tumors, hemorrhage, infection, trauma to occipital poles
    • Upper bank of calcarine fissure – contralateral inferior quadrantopia
    • Lower bank of calcarine fissure – contralateral superior quadrantopia
    • Entire primary visual cortex lesion – contralateral homonymous hemianopia
  • Macular sparing (see figure 11.16) – seen occasionally with partial lesions of visual pathways; occurs because fovea has relatively large representation for its size; term usually used in context of cortical lesions, but other injuries can cause this (e.g., increased ICP)

Blood Supply and Ischemia in Visual Pathways

  • Retina – ophthalmic artery
    • 3 main causes of impaired blood flow:
      • emboli
      • stenosis
      • vasculitis
      • Amaurosis fugax – transient ischemic attack of the retina; browning out or loss of vision in one eye for about 10 minutes (“like a window shade”); should be worked up like any other TIA
  • Optic tracts, optic chiasm, intracranial segment of optic nerves – small branches arising from proximal portions of ACA, MCA, and anterior and posterior communicating arteries
  • LGN – variable blood supply; infarcts may be associated with contralateral hemiparesis or hemisensory loss due to involvement of nearby posterior limb of internal capsule and thalamic somatosensory radiations
  • As optic radiations pass through the parietal and temporal lobes– may be damaged by infarcts to MCA
  • Primary Visual Cortex – PCA; basilar artery supplies both PCAs; bilateral altitudinal scotoma (see figure 11.17) strongly suggests vertebrobasilar insufficiency causing bilateral infarcts or TIAs
  • Inferior occipitotemporal visual association cortex (“what?” stream) – PCA
  • Lateral parieto-occipital visual association cortex (“where?” stream) – MCA-PCA watershed territory

Optic Neuritis

(additional detail on neuro exam and treatment on p. 445)

  • Optic Neuritis – inflammatory demyelinating disorder of the optic nerve that is epidemiologically and pathophysiologically related to MS
    • >50% of patients with an isolated episode of optic neuritis will eventually develop MS
    • typical onset – eye pain, monocular visual problems (monocular central scotoma, decreased visual acuity, impaired color vision)
    • onset acute or gradual (several days to weeks)
    • near complete recovery common in 6-8 weeks
    • may be permanent visual loss