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Visual Pathways

Notes

Visual Pathways

Sections

the visual pathways

The visual projections 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.

The optic projection: 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.

optic projection: 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.

the superior/inferior nature of the visual 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.

Retinotopic map of the 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.

Clinical Correlations: Visual Field Deficits - Hemianopias

neuroscience of the lateral geniculate body

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.

Full-Length Text

  • Here, we will draw the visual pathways, which are the visual projections from the retinae to the occipital cortices.
  • Before we draw them out, start a table.
  • Denote the following rules regarding the visual pathways:
    • 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.

Now, let's illustrate the anatomy that derives these rules.

First, let's draw the visual pathways in axial view.

In this diagram, we will learn about the laterality of the optic projections.

  • Draw the visual fields as follows:
  • Draw central vision, then the left visual field, and then the right visual field.
    • The visual fields represent the individual's view of the world.
  • Now, draw axial sections through the left eye and then the right eye.
  • Next, draw the fovea as the center point of each eye.
  • Now, draw a projection from central vision to the fovea of the left eye and then to the fovea of the right eye.
    • The fovea 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.
  • Next, using the fovea as the midpoint of the retina, subdivide the retinae into nasal and temporal hemiretinae.
  • First, show that the left visual field projects to the right temporal hemiretina and the left nasal hemiretina.
  • Then, show that the right visual field projects to the left temporal hemiretina and the right nasal hemiretina.

Before we illustrate the projections from the retina to the occipital cortices, let's establish the key structures along the optic projection path.

  • First, draw the optic chiasm, then the left lateral geniculate nucleus, and then the right lateral geniculate nucleus.
  • Then, draw the left primary visual cortex and then the right primary visual cortex.
  • Now, show that the left temporal hemiretina projects ipsilaterally to the left lateral geniculate nucleus, and then, show that the right nasal hemiretina sends crossing fibers through the optic chiasm to the contralateral lateral geniculate nucleus (the left lateral geniculate nucleus).
  • Next, show that 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; we leave them out for clarity.

Next, let's label the terminology for the optic projections.

  • Encircle the optic projection between the retina and the optic chiasm and label it as the optic nerve.
  • Then, encircle the projection between the optic chiasm and the lateral geniculate nucleus and label it as the optic tract.
    • Optic nerve injuries are prechiasmatic and optic tract injuries are postchiasmatic.
  • Finally, show that the projections from the lateral geniculate nucleus to the primary visual cortex are called optic radiations.

Next, let's draw the visual pathways in sagittal view.

In this diagram, we will learn about the superior/inferior nature of the visual projections.

  • First, draw a sagittal view of a cerebral hemisphere.
  • Then, draw an eye.
  • Next, draw the superior visual field and then the inferior visual field.
  • Now, indicate that 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.
  • Show that the superior and inferior retinal projections bundle within the optic nerve.
  • Now, draw the following structures: the lateral geniculate nucleus, temporal horn of the lateral ventricle, and the occipital horn of the lateral ventricle.
  • Next, show that the retina projects to the lateral geniculate nucleus.
  • Then, show that 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.
  • Then, show that 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.
    • 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.

END OF ESSENTIAL/START OF ADVANCED

  • Now, draw an enlarged retinotopic map of the primary visual cortex. Indicate that 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.

Now, let's address the neuroscience of the lateral geniculate body.

  • Indicate that 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 and also koniocellular components; we will address only the parvocellular and magnocellular components, here, because the koniocellular components are less well understood.
  • Indicate that the posterior layers are parvocellular and comprise layers 3 through 6.
  • Then, indicate that the anterior layers are magnocellular and comprise layers 1 and 2.
  • Now, show that the parvocellular layers receive input from the cone layers of the retina and the magnocellular layers receive input from the rod layers.
  • Indicate that 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.
  • Then, indicate that 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.