Optic radiations (geniculocalcarine tract)

● Lesion manifestations

▪ Temporal component (Meyer’s loop) injury:

◦ Contralateral superior quadrantanopia (“pie in the sky”)

▪ Parietal component (Baum’s loop) injury:

◦ Contralateral inferior quadrantanopia (“pie on the floor”)

▪ Complete unilateral optic radiation injury:

◦ Contralateral homonymous hemianopia

▪ Bilateral posterior visual pathway injury:

◦ Cortical blindness

● Clinical pearls

▪ Optic radiations carry visual information from the lateral geniculate nucleus to the primary visual cortex, with different fibres representing different parts of the visual field.

▪ The temporal lobe contains Meyer’s loop, which carries inferior retinal fibres representing the superior visual field; the parietal optic radiations (Baum’s loop) carry fibres representing the inferior visual field.

▪ Loss of visual input can occasionally produce visual release phenomena (deafferentation hallucinations), particularly in the setting of visual pathway injury or sensory deprivation. Earlier visual pathway deafferentation (e.g., retina, optic nerve, optic tract, or optic radiations) is more likely to produce simple or elemental phenomena (e.g., flashes, colours, shapes), whereas involvement of higher-order visual association networks (e.g., occipital association cortex, posterior parietal cortex, and ventral occipitotemporal regions) is more likely to produce complex formed hallucinations (e.g., objects, scenes, people).