392 Neuroanatomy: Draw It to Know It
Visual Pathways: Sagittal View
Here, we will draw the visual pathways in sagittal view.
First, draw a sagittal view of a cerebral hemisphere and
include an eye. Indicate that the superior visual world
projects to the inferior portion of the retina and that the
inferior visual world projects to the superior portion of
the retina. Next, show that the retina projects to the lateral
geniculate nucleus. Th en, indicate that the stretch of the
pathway anterior to the optic chiasm is optic nerve, and
the stretch posterior to the optic chiasm is optic tract.
Now, draw an enlarged retinotopic map 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.
Next, let’s draw the optic radiations. First, defi ne the
occipital horn of the lateral ventricle and show that the
superior optic radiation bundle, which carries superior
retinal input (from the inferior visual world), projects
along the occipital horn through the superior temporal
and inferior parietal lobes and terminates in the superior
primary visual cortex. Th en, defi ne the temporal horn
of the lateral ventricle and show that the inferior optic
radiation bundle, which carries inferior retinal input
(from the superior visual world), fans out in Meyer’s
loop over the temporal horn and projects back
through the inferior temporal lobe to the inferior pri-
mary visual cortex. Injury to the inferior bundle is
more common than to the superior bundle, so superior
visual fi eld defects are more common than inferior fi eld
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 infe-
rior radiation will aff ect vision from the right, superior
visual quadrant, only. Th e anterior extent of the inferior
optic radiation is important because surgeons must be
mindful of the optic radiations during anterior temporal
lobe resection.^4
Lastly, draw an enlarged retinotopic map of the pri-
mary visual cortex. Indicate that the cortical representa-
tion 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 g yrus. Th e upper bank of the calcarine sulcus
encodes the lower half of the visual fi elds whereas the
lower bank encodes the upper half of the visual fi elds.
Now, let’s address the neuroscience of the lateral
geniculate body. Th e lateral geniculate body has both
parvocellular and magnocellular components and also
koniocellular components; we will address only the par-
vocellular and magnocellular components, here, because
the koniocellular components are less well understood.
Layers 3 through 6, the most posterior layers, are parvo-
cellular, whereas layers 1 and 2, the most anterior layers,
are magnocellular. Th e parvocellular layers receive input
from the cone layers of the retina and the magnocellular
layers receive input from the rod layers. Cones lie within
the central retina (the macula) and communicate with X
retinal ganglion cells called midget cells, which have
small fi elds and are responsible for visual acuity and color
vision. Rods lie in the periphery of the retina and com-
municate with Y retinal ganglion cells called parasol
cells, which have large fi elds and are sensitive to motion.
Th us, because the X and Y division of the retinal gan-
glion output cells is preserved within the lateral genicu-
late nucleus as parvocellular and magnocellular regions,
respectively, the X and Y ganglion cells are synonymously
referred to as P (for parvocellular) and M (for magnocel-
lular) cells, respectively. Note, however, that the X-Y
and P-M comparison is imperfect—the diff erences are
beyond our scope, here.^4 – 11