The fibers exiting from the chiasm proceed circumferentially around the diencephalon lateral to the hypothalamus and in contact with the ambient cistern (see Fig 1-11). Just prior to the LGN, the fibers involved in the pupillary pathways exit to the pretectal nuclei; other fibers exit to the superficial layers of the superior colliculi (SC) via the brachium of the SC. These fibers, which originate from ipRGCs, are likely the sole source of pupillomotor input from the retina to the midbrain. The ipRGCs also project to the suprachiasmatic nucleus of the hypothalamus, which is probably responsible for light-induced diurnal rhythms.
The incongruous nature of optic tract visual field defects is explained by the lack of proximity between corresponding fibers from the right and left eyes. Most of the axons that originate in the retinal ganglion cells terminate within the LGN.
The LGN is located in the posterior thalamus below and lateral to the pulvinar and above the lateral recess of the ambient cistern. The LGN is a peaked, mushroom-shaped structure that is divided into 6 levels. The 4 superior levels are the termini of P-cell axons, which are the ganglion cells with smaller receptive fields; they are responsible for mediating maximal spatial resolution and color perception. The 2 inferior layers receive input from M-cell fibers, which are the ganglion cells with larger receptive fields; they are more sensitive to motion detection. Axons originating in the contralateral eye terminate in layers 1, 4, and 6 of the LGN; the ipsilateral fibers innervate layers 2, 3, and 5. As the fibers approach the LGN, the superior fibers move superomedially and the inferior fibers swing inferolaterally. Overall, the retinal representation rotates almost 90°, with the superior fibers moving medially and the inferior fibers laterally. The macular fibers tend to move superolaterally. Cortical and subcortical pathways may modulate activity in the LGN. In addition, the cortex, SC, and pretectal nuclei project back to the LGN.
Excerpted from BCSC 2020-2021 series: Section 5 - Neuro-Ophthalmology. For more information and to purchase the entire series, please visit https://www.aao.org/bcsc.