406 Neuroanatomy: Draw It to Know It
Cortical Visual Processing (Advanced ) (Cont.)
Now, at the occipital-temporal-parietal junction, label
area V5, which is the motion-processing center and
which is oft en referred to as hMT because it represents
the human correlate to the macaque middle temporal
visual area. Visual perception of motion is processed in
this region; this visual area is part of our model for the
circuitry of smooth pursuit eye movements (see Drawing
23-5) — it plays a fundamental role in the tracking of a
moving target. Note that motion processing also involves
other brain regions, including area V5a, the human cor-
ollary for the macaque medial superior temporal visual
area, which lies adjacent (just superior) to V5. Note, as
well, that for practical purposes, just as we consider V4
to be the fi rst step in the topographic separation of the
ventral stream from the dorsal stream, so, too, here, we
take the practical liberty of considering V5 to be the fi rst
step in the separation of the dorsal stream from the ven-
tral stream — even though the dorsal stream, itself, is
commonly limited to the parietal visual areas (discussed
next). Finally, as a clinical corollary, patients with injury
to V5 demonstrate akinetopsia, which is an inability to
visualize moving objects despite a preserved ability to
accurately visualize stationary objects.
Next, label the lateral parietal lobe as P-DSA, which
stands for “parietal dorsal stream area.” Th e parietal lobes
contain numerous visual cortical areas dedicated to the
processing of spatial awareness, and they comprise the
“where” pathway. Some of the specifi c visual areas that
are identifi ed within the parietal dorsal stream area are
the parietal reach region in the intraparietal sulcus,
which activates during reach and pointing movements;
the anterior parietal area in the anterior parietal sulcus,
which activates during fi ne motor movements; the ven-
tral parietal area in the depths of the intraparietal sulcus,
which activates during multimodal motion detection of
moving stimuli; the lateral intraparietal area in the pos-
terior intraparietal sulcus, which activates during visually
guided saccadic eye movements; and the superior and
inferior parietal lobules, which function in attention
control and spatial awareness. We can think of the pari-
etal dorsal stream area as the “show me the way” area; it
instructs the motor cortex where to move.^22
As a fi nal point, consider that an important aspect of
visual processing is depth perception, which, as it turns
out, is more than simply a product of stereoscopic vision
(the reconciliation of binocular disparity); it also involves
high-level processing of numerous visual cues. Th is visual
processing allows for the preservation of depth percep-
tion in the setting of monocular (one-eye) vision. As a
prominent example of the capability of monocular vision
to provide depth perception, consider that Wiley Post
was a highly regarded pilot who won the 1930 Los
Angeles to Chicago Air Derby with the use of only one
eye.^33 – 35