retinogeniculate

Retinogeniculate, or the retinogeniculate projection, is the principal thalamic visual pathway that relays information from the retina to the lateral geniculate nucleus (LGN) of the thalamus. It constitutes the first major synapse in the cortical visual pathway and a primary conduit for transmitting retinal signals to the cortex, distinct from the retinotectal pathway that projects to the superior colliculus.

Anatomy and organization: Retinal ganglion cell axons form the optic nerve, partially cross at the optic chiasm, and continue as the optic tract to the LGN. In primates the LGN consists of six principal layers: magnocellular layers (M) 1–2, parvocellular layers (P) 3–6, with koniocellular (K) layers intercalated between them. Each eye contributes to specific LGN layers, and the arrangement preserves a retinotopic map so neighboring retinal regions project to neighboring LGN locations. The M, P, and K streams carry different feature information, with M pathways linked to motion and luminance, P pathways to high-resolution form and color, and K pathways to additional color signals.

Function and cortical relay: LGN neurons relay processed retinal information to the primary visual cortex (V1) via the geniculocalcarine tract (optic radiations). The retinogeniculate relay preserves spatial organization and transmits basic visual features such as contrast and luminance, providing the main input to cortical areas for further analysis. A secondary retinotectal pathway to the superior colliculus supports reflexive orienting.

Clinical relevance: Lesions affecting the optic tract or LGN disrupt the contralateral visual field, producing characteristic homonymous visual field defects. The exact pattern depends on lesion extent and specific LGN layers involved, with central vision often differently affected due to its representation.