400 Neuroanatomy: Draw It to Know It
Cortical Visual Processing (Advanced )
Here, we will create a diagram for the organization of the
visual cortex. To create an overview of cortical visual
processing , draw a lateral cerebral hemisphere. First,
label the posterior, occipital region as the occipital area.
In this region, initial cortical visual processing occurs.
For our purposes, here, consider this region to encom-
pass the primary visual cortex (aka striate cortex) and
the secondary and tertiary visual cortices. Next, label
the inferior temporal lobe as the ventral stream and the
parietal lobe as the dorsal stream. Th e ventral stream
comprises the “what,” object recognition pathway (or
P pathway), and the dorsal stream comprises the “where,”
spatial localization (or M pathway). Note that the divi-
sion between these pathways begins within the photore-
ceptors of the retinae: the cone photoreceptors are
responsible for color detection and excite parvocellular
ganglion cells of the “what” pathway, and the rod photo-
receptors are responsible for motion detection and excite
magnocellular ganglion cells of the “where” pathway.
As visual information is fed forward within the cortex,
the “what” and “where” properties of vision separate
topographically into ventral and dorsal visual streams.
Within the ventral stream, components of objects are
integrated to allow for cohesive object identifi cation,
and within the dorsal stream, numerous diff erent visu-
ospatial processing centers exist. To complete this over-
view diagram, draw the frontal eye fi elds, which are
responsible for the cortical initiation of many diff erent
classes of eye movements.
Next, let’s label the most well-studied cortical visual
areas. Draw the posterior aspect of the medial and lateral
hemispheres. Th en, draw the following anatomic land-
marks. Within the medial hemisphere, draw the calcarine
sulcus and then the collateral sulcus (which separates the
parahippocampal g yrus, medially, from the fusiform
g yrus, laterally); then, within the lateral hemisphere,
label the Sylvian fi ssure and the temporo-parietal-
occipital junction. Now, show that V1 (the primary
visual cortex) lies along the calcarine sulcus of the medial
face of the occipital lobe, and also show that it lies at the
very tip of the lateral occipital pole. Th e primary visual
cortex is known as V1 because visual cortical stimuli fi rst
collect in this area. V1 is oft en referred to by its Brodmann
designation — Brodmann area 17. Also, it is commonly
referred to as the striate cortex because of the heavy
myelination of its fourth cytoarchitectural layer, which
produces a white stripe called the stria of Gennari.
Within layer 4, sublayer 4Ca is the primary recipient of
magnocellular input and sublayer 4Cb is the primary
recipient of parvocellular input. Th e primary visual
cortex processes the most basic visual properties: for
example, line orientation, motion direction, luminance
orientation, and color. Th e primary visual cortex in each
hemisphere encodes the visual fi eld from the opposite
half of the world: right V1 encodes the left visual fi eld
and left V1 encodes the right visual fi eld. Th e 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 repre-
sentation of the peripheral retina lies in the anterior
calcarine sulcus and encompasses a small cortical area
relative to its broad retinal expanse. Th e upper bank of
the calcarine sulcus encodes the lower half of the visual
fi eld and the lower bank encodes the upper half of the
visual fi eld. Interestingly, patients with injury to area V1
oft en report having blindsight — an unconscious utiliza-
tion of visual information from the visual fi eld in which
they are blind.^19 – 24