Neuroanatomy Draw It To Know It

(nextflipdebug5) #1

402 Neuroanatomy: Draw It to Know It


Cortical Visual Processing (Advanced ) (Cont.)


Next, let’s draw the secondary and tertiary visual corti-
ces. Secondary visual cortex is called V2 and corresponds
to Brodmann area 18; tertiary visual cortex is called V3
and corresponds to Brodmann area 19. Indicate that on
the medial face of the cerebral hemisphere, V2 lies above
and below V1, and V3 lies above and below V2. Note
that the ventral part of V3 is commonly referred to as VP
instead of V3 because controversy exists as to whether
the ventral part of V3 is a unique visual cortical area or
whether the dorsal and ventral parts of V3 are both part
of a common visual cortical area. Also note that the
dorsal and ventral parts of the visual cortices are oft en
denoted by the abbreviations “d” for dorsal and “v” for
ventral; for instance, the ventral part of V2 is oft en
denoted as “V2v.” Next, on the lateral surface of the
hemisphere, indicate that V2 comprises a small strip of
the posterior occipital lobe, just anterior to V1; and then
show that V3 lies in front of V2 — leave a small space
inferior to V3 for V4 (drawn next). Th e secondary and
tertiary visual cortices process simple visual properties
akin to those processed in V1; notably, however, V2 pro-
cesses illusory boundaries, contours that cannot actually
be visualized but that are implied by the context of a
larger scene. Stand in the doorway with one half of
your body in the room and the other half outside of view;
no one will misperceive that only half of you exists.
Th rough grouping principles of proximity, continuity,
and similarity, we naturally incorporate the hidden half
of your body into our Gestalt — our general makeup of
the world. We generate an illusory boundary of your
body based on the characteristics of you that we are able
to visualize.^19 – 24
Now, let’s label the color-processing area V4, which,
for practical purposes, can be considered a transitional
zone in the visual pathway wherein the ventral stream
(the “what” pathway) begins to topographically sepa-
rate from the dorsal stream; note that we take liberty in


designating V4 as part of the ventral stream — the ventral
stream is commonly considered to begin anterior to V4.
On the medial and lateral surfaces of the cerebral hemi-
sphere, in the ventral-occipital lobe, label the region
inferior to V3 as V4. Note that debate exists regarding
the anatomy of V4, and also note that an additional
visual area separate from V4, called V8, has also been
introduced into the literature; for this reason, the ven-
tro-occipital region is oft en referred to as the V4/V8
region. Also, note that a similar term for V8 has been
introduced called VO, which stands for the “ventral
occipital” cluster.^19 – 24
One important aspect of color processing found
within area V4 is that of color constancy, which is the
property of color vision wherein regardless of the illumi-
nation cast on an object, the object maintains its per-
ceived color. It has been shown that a red patch will
maintain its perceived red color and a white patch will
maintain its perceived white color even when the level
illumination cast on the two diff erent colors is adjusted
so that they should be perceived as the same color. To
demonstrate this for yourself, consider the color of your
pants. Now, look closely at them and see that your pants
are not a single color or even a discrete combination of
colors but, instead, they comprise a vast array of colors
from the diff erent levels of illumination cast onto them —
yet we are able to retain a uniform impression of our
clothing color due to the high-level color vision process-
ing that occurs within area V4. As a clinical corollary for
color processing, before functional MRI studies could
identify the ventral occipital cortical response to color
patterns, clinical-pathologic case studies demonstrated
that ventral occipito-temporal injury caused abnormali-
ties in color processing , called achromatopsia, which
results in the visual world appearing gray or drained of
color; note, however, that injury to area V4, alone, may
or may not be suffi cient to cause achromatopsia.^25
Free download pdf