Tuesday, February 22, 2011

Visual Pathway

The visual pathway begins at the retina with the ganglion cells converging at the optic disc to form the optic nerve. Then the nerves enter the cranial cavity through the optic foramina and get together to form the optic chiasm. Fibers from the contralateral retina cross to the opposite side at the optic chiasm, joining the ipsilateral temporal fibers to form the optic tract. Most of the fibers then synapse at the lateral geniculate nucleus. The last relay of the visual pathway corresponds to fibers from the lateral geniculate nucleus that go to the calcarine cortex. These fibers, known as optic radiations, or Gratiolet radiations, constitute the geniculocalcarine tract, whose anterior part is called Meyer's loop.

At the optic radiation, the visual pathway is segregated into three bundles on the lateral ventricle: the upper, central, and inferior. The inferior, known as Meyer's loop, travels around the temporal horn. This bundle makes a wide anterior and lateral loop around the temporal horn of the lateral ventricle before curving around the posterior atrium to reach the occipital cortex. These inferior fibers pass into the uncinate region of the temporal lobe and are constituents of the uncinate fascicle located at the limen insula or temporal stem. At the temporal stem, the middle cerebral artery turns posteriorly and transitions between the sphenoidal or horizontal segment to the insular segment.

The upper and central bundles pass through the parietal and high temporal lobes, respectively, to synapse in the primary visual cortex (striate area, Brodmann area 17). The upper fibers carry information from the superior quadrants of the retina; the lower fibers carry information from the inferior quadrants of the retina. The central bundle contains macular fibers from central area of the retina. This anatomical configuration explains superior homonymous quadrantanopia--sparing of central, macular vision after temporal lobe surgery. This defect has been associated with both congruous and incongruous patterns after epilepsy surgery.