Physiology of vision and hearing

The pathways

Vision: light -> retina (rods - low light/peripheral, cones - colour/acuity, macula) -> optic nerve -> optic chiasm (nasal fibres decussate) -> optic tract -> lateral geniculate nucleus -> optic radiations -> primary visual cortex (occipital, V1)
Hearing: sound -> tympanic membrane -> ossicles (amplification) -> cochlea (hair cells, tonotopically organised - base=high frequency, apex=low frequency) -> CN VIII -> brainstem nuclei -> bilateral auditory pathways -> auditory cortex (temporal lobe)

Visual field defects localise precisely along this pathway - a core clinical localisation tool

Lesion site	Visual field defect
Optic nerve	Monocular vision loss
Optic chiasm	Bitemporal hemianopia (classically pituitary tumour)
Optic tract	Contralateral homonymous hemianopia
Optic radiation - temporal (Meyer's loop)	Contralateral homonymous superior quadrantanopia
Optic radiation - parietal	Contralateral homonymous inferior quadrantanopia
Occipital cortex (V1)	Contralateral homonymous hemianopia, macular sparing (dual PCA/MCA blood supply)

Bilateral auditory cortical projection from each cochlea means a unilateral cortical/brainstem lesion does not cause unilateral deafness - unilateral hearing loss localises to the ear, cochlea, or CN VIII, not higher

This pathway anatomy is the basis for using visual field testing to localise intracranial lesions without imaging (e.g. bitemporal hemianopia prompting pituitary imaging)

Pituitary adenoma (chiasmal compression), stroke (MCA/PCA territory field defects), CN VIII schwannoma (unilateral sensorineural hearing loss)

Occipital cortex macular sparing after PCA stroke reflects dual blood supply to the macular representation - a distinguishing feature from MCA-territory field loss

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