Optic radiation

The optic radiation, also called the geniculocalcarine tract, is a major projection fibre system of the visual pathway that connects the lateral geniculate nucleus (LGN) of the thalamus with the primary visual cortex around the calcarine sulcus of the occipital lobe. Each optic radiation carries visual information representing the contralateral half of the visual field.

After arising from the lateral geniculate region, the optic radiation fibres spread through the deep white matter around the temporal horn, atrium and occipital horn of the lateral ventricle before reaching the visual cortex. They form part of the larger posterior thalamic projection fibres and become incorporated into the sagittal stratum as they pass posteriorly. Pescatori et al. identify optic radiation fibres as a particularly prominent component of the retrolentiform region of the internal capsule, while also describing portions passing beneath the lentiform nucleus before entering the sagittal stratum.

Three main bundles of option radiation fibres

For descriptive purposes, the optic radiation can be divided into three main bundles:

• Anterior bundle (Meyer loop): These fibres initially pass anteriorly into the temporal lobe around the roof of the temporal horn and then make a characteristic loop before turning posteriorly along the lateral aspect of the temporal horn. They ultimately project predominantly to the lower bank of the calcarine sulcus. They represent the contralateral superior visual field.

• Central bundle: These fibres initially pass laterally across the roof of the temporal horn and then turn posteriorly, coursing along the lateral aspect of the atrium and occipital horn toward the occipital cortex. Traditional descriptions associate this bundle particularly with central or macular vision, although the detailed retinotopic arrangement of the optic radiation is more complex than this simple three-bundle model suggests.

• Posterior (dorsal) bundle: These fibres take a more direct posterior course along the atrium and occipital horn and project predominantly to the upper bank of the calcarine sulcus. They broadly represent the contralateral inferior visual field.

Relationship with the sagittal stratum

As the optic radiation fibres pass posteriorly, they contribute to the sagittal stratum, a longitudinally arranged system of deep white-matter fibres in the temporal and occipital regions. Pescatori et al. describe an outer layer containing optic radiation fibres and a deeper layer containing parietopontine and occipitopontine fibres. The tapetum of the corpus callosum separates these fibres from the ventricular cavity. This close packing with other projection, association and commissural fibres explains why the individual fibre systems can be difficult to separate anatomically.

Clinical correlates

The course of the optic radiation is clinically important because different portions correspond broadly to different parts of the visual field. Damage to Meyer loop in the temporal lobe typically produces a contralateral superior homonymous quadrantanopia, whereas damage to the more superior fibres passing through the parietal region can produce a contralateral inferior homonymous visual field defect. More extensive involvement of the optic radiation may produce contralateral homonymous hemianopia.