›

Visual Pathways

Visual Pathways

Sections







Overview

The visual projections span from the retinae to the occipital cortices.

Projections: 4 Key Rules

  • The LEFT visual field projects to the RIGHT occipital cortex.
  • The RIGHT visual field projects to the LEFT occipital cortex.
  • The SUPERIOR visual field projects to the INFERIOR visual cortex.
  • The INFERIOR visual field projects to the SUPERIOR visual cortex.

Visual Projection Anatomy

  • The visual fields represent the individual's view of the world.
  • The fovea is the center point of each eye: it is the area of maximal visual acuity. It lies in the center of the macula.
    • We remember this by recognizing the common clinical syndrome of macular degeneration, which causes central vision loss.
  • The retina subdivides into nasal and temporal hemiretinae.
  • The left visual field projects to the right temporal hemiretina and the left nasal hemiretina.
  • The right visual field projects to the left temporal hemiretina and the right nasal hemiretina.

Optic Projections

Key Structures

Here, we learn the projections to the LEFT primary visual cortex.

  • The left temporal hemiretina projects ipsilaterally to the left lateral geniculate nucleus.
  • The right nasal hemiretina sends crossing fibers through the optic chiasm to the contralateral lateral geniculate nucleus (the left lateral geniculate nucleus).
  • The left lateral geniculate nucleus sends optic radiations to the left occipital cortex.

The visual projections on the right half of the cerebrum are the mirror image of those to the left.

Terminology

  • The optic nerve the optic projects between the retina and the optic chiasm.
  • The optic tract projects between the optic chiasm and the lateral geniculate nucleus.
  • Optic nerve injuries are prechiasmatic and optic tract injuries are postchiasmatic.
  • Optic radiations are the projections from the lateral geniculate nucleus to the primary visual cortex.

Superior/Inferior Projections

The superior visual field projects to the inferior portion of the retina and that the inferior visual field projects to the superior portion of the retina.

  • The superior and inferior retinal projections bundle within the optic nerve.
  • The superior optic radiation bundle, which carries superior retinal input (from the inferior visual field), projects along the occipital horn through the superior temporal and inferior parietal lobes and terminates in the superior aspect of the primary visual cortex.
  • The inferior optic radiation bundle, which carries inferior retinal input (from the superior visual field), fans out in Meyer's loop over the temporal horn and projects back through the inferior temporal lobe to the inferior primary visual cortex.
    • Injury to the inferior bundle is more common than to the superior bundle, so superior visual field defects are more common than inferior field defects.

Clinical Correlation: PCA Stroke

Quadrantanopia
Injury to one optic radiation or the other is called quadrantanopia because it results in injury to a single visual quadrant with preservation of the other three quadrants.

  • For instance, a lesion to the left inferior radiation will affect vision from the right, superior visual quadrant, only.
  • The anterior extent of the inferior optic radiation is important because surgeons must be mindful of the optic radiations during anterior temporal lobe resection.

Primary visual cortex

Brain atlas: calcarine sulcus

The cortical representation of central (or macular) vision lies in the posterior calcarine sulcus and occupies a large cortical area relative to its small retinal expanse, whereas representation of peripheral vision lies in the anterior calcarine sulcus and encompasses a small cortical area relative to its broad retinal expanse.

  • Visual cortex is also called calcarine cortex because it lies within the dorsal and ventral banks of the calcarine sulcus, which separates the cuneus from the lingual gyrus.
  • The upper bank of the calcarine sulcus encodes the lower half of the visual fields whereas the lower bank encodes the upper half of the visual fields.

Hemianopias

Lateral Geniculate Body

Neuroscience of the lateral geniculate body

  • Central vision comprises the majority of the lateral geniculate body and lies posterior, whereas peripheral vision localizes within the most anterior portion of the lateral geniculate body.
  • The lateral geniculate body has both parvocellular and magnocellular components (it also koniocellular components but we will address only the parvocellular and magnocellular components, here, because the koniocellular components are less well understood.)
  • The posterior layers are parvocellular and comprise layers 3 through 6.
  • The anterior layers are magnocellular and comprise layers 1 and 2.
  • The parvocellular layers receive input from the cone layers of the retina and the magnocellular layers receive input from the rod layers.
  • Cones, which lie within the central retina (the macula), communicate with X retinal ganglion cells called midget cells, which have small fields and are responsible for visual acuity and color vision.
  • Rods, which lie in the periphery of the retina, communicate with Y retinal ganglion cells called parasol cells, which have large fields and are sensitive to motion.
  • The X and Y differentiation of the retinal ganglion output cells is preserved within the lateral geniculate nucleus as parvocellular and magnocellular regions, respectively.
  • Thus, the X and Y ganglion cells are synonymously referred to as P (for parvocellular) and M (for magnocellular) cells, respectively. Note, however, that the X-Y and P-M comparison is imperfect but the differences are beyond our scope, here.

Drawing Highlight

Assess your knowledge with this Visual Pathways drawing highlight.