For example, the inferior nasal retina reflects the superior temporal visual field, and vice versa.

• When light enters the eye, it passes through the lens, with the subsequent image becoming inverted and reversed: the right visual field projects to the left retina; the left visual field projects to the right retina.
  • Therefore, a monocular upper left visual field defect is likely due to a lesion impacting the lower right retina.
  • Lesions impacting only one eye are nearly always anterior to the optic chiasm (eg, optic nerve, retina, globe).
    • However, there is one notable exception:  Lesions that impact only one side of the lateral optic chiasm.
  • Images projected onto the retina are inverted and reversed.
• Goldmann visual field testing
• Photoreceptors of the eye are rods & cones
  • Cones in the retina are responsible for colour vision
    • Most concentrated in the fovea of the macula — has no rods
  • Rods are responsible for low light vision — rod dystrophy can lead to night blindness

Visual information from the nasal visual field of the left eye will be conveyed through the:

• left temporal hemiretina (light that enters the eye forms an image on the retina that is inverted and reversed).
• left optic nerve, which transmits information from one eye.
• left lateral portion of the optic chiasm. Information from the temporal hemiretina does NOT cross at the optic chiasm but travels along the lateral edge of the chiasm to stay in the left side of the brain.
• left optic tract. After the optic chiasm, information from the binocular right visual fields travels together in the left optic tract.
• left lateral geniculate nucleus of the thalamus. Neurons from the optic tract synapse on the lateral geniculate nucleus.
• optic radiations. The inferior optic radiations carry information from the superior visual field through the temporal lobe (Meyer loop), and the superior optic radiations carry information from the inferior visual field through the parietal lobe.
• primary visual cortex, which is located in the occipital lobe.

A. Monocular scotoma

• Central scotoma can occur due to a lesion in the retina or optic nerve
  • macular diseases such as age-related macular degeneration
  • optic neuritis common in multiple sclerosis

B. Anopia

• Monocular vision loss results from a lesion anterior to the optic chiasm, including lesions to the optic nerve, retina, or globe.
  • However, there is one notable exception:  Lesions that impact only one side of the lateral optic chiasm.

C. Bitemporal hemianopia

• In the optic chiasm, fibers from the nasal retina (which process the temporal visual field) cross so that the hemivisual fields from both eyes are processed in the same contralateral hemisphere.
• A lesion that affects these crossing fibers in the middle of the optic chiasm can produce bitemporal hemianopia.

D. Nasal Hemianopia (Unilateral)

• Optic nerve fibers from the temporal part of the retina travel laterally (ie, without crossing) through the optic chiasm to pass into the ipsilateral optic tract.
• Lesions involving the lateral aspects of the optic chiasm can cause dysfunction in only these uncrossed fibers from the ipsilateral temporal retina.  This can cause ipsilateral nasal hemianopia, which may occur with aneurysm of the internal carotid artery (eg, cavernous or ophthalmic segments)

E. Homonymous hemianopia

• Right homonymous hemianopia may occur with lesions to the left optic tract or the left occipital cortex.
  • It may also occur with lesions to the optic radiations, but only if they affect both the superior and inferior optic radiations.

F. Contralateral superior homonymous quadrantanopia — pie in the sky

• Meyer’s loop lesion; temporal lobe — pie in the sky

• After synapsing in the lateral geniculate nucleus (LGN), the optic radiations fan out and project to the occipital lobe. The inferior optic radiations, which carry information from the inferior retina/superior visual field, travel through the temporal lobe white matter.
  • Lesions in this area would lead to superior quadrant visual field defects (ie, contralateral homonymous superior quadrantanopia).

G. Contralateral lower homonymous quadrantanopia

• The superior optic radiations, which carry information from the superior retina/inferior visual field, travel through the parietal lobe white matter; lesions here lead to contralateral lower quadrantanopia.

H. Homonymous hemianopia with macular sparing

• A lesion in the left occipital lobe would impact the primary visual cortex. Because of the crossing fibers in the optic chiasm, the left occipital cortex receives information from the right visual fields of both eyes. A lesion in this area would result in right homonymous hemianopia (classically with macular sparing due to collaterals from the middle cerebral artery).

I. Homonymous central scotoma

• Lesions in the occipital pole