294 Neuroanatomy: Draw It to Know It
Long Association Fibers ( Advanced )
Here, we will draw the long association fi bers. Begin
with a sagittal section through a cerebral hemisphere.
Th e fi rst fi bers we will draw are the superior longitudinal
fasciculus and the arcuate fasciculus. Note that histori-
cally and commonly these fi bers are classifi ed either syn-
onymously or as part of a single system. First, show that
the superior longitudinal fasciculus is a bidirectional
pathway that connects the parietal and frontal cortices —
classically it is more broadly defi ned as involving the
temporal and occipital cortices, as well. Th e superior
longitudinal fasciculus is further subcategorized into
three diff erent fasciculi, which, roughly, from rostral to
caudal are SLF I, II, and III. In accordance with its broad
anatomy, the superior longitudinal fasciculus is involved
in a wide variety of cognitive functions, but indicate that,
notably, it communicates spatial information between
the frontal and parietal lobes.
Next, show that the arcuate fasciculus originates in
the posterior superior temporal area and passes forward
to the frontal lobe. Th e ability to repeat language, which
is called language conduction, has historically been
assigned to the arcuate fasciculus (and still commonly
is). Language conduction requires the transmission of
language from the superior temporal reception area to
the frontal motor speech output area. Despite its histor-
ical assignment to the arcuate fasciculus, evidence sug-
gests that the arcuate fasciculus is actually more generally
involved in the spatial orientation of sound and that the
superior longitudinal fasciculus (specifi cally, SLF III),
the middle longitudinal fasciculus, and the extreme cap-
sule, instead, transmit language conduction.
Now, draw the uncinate fasciculus, which connects the
orbital prefrontal cortex and anterior cingulate g yrus with
the anterior temporal lobe. Indicate that the uncinate fas-
ciculus combines processed somatosensory information
(auditory, visual, gustatory) with emotional response reg-
ulation to assist in decision-making processes.
Next, we will draw the cingulum, which is the main
white matter bundle of the limbic system. Show that the
cingulum projects bidirectionally through the limbic
lobe: from the cingulate g yrus around the posterior end
of the corpus callosum back forward through the para-
hippocampal g yrus. Indicate that the cingulum plays an
important role in emotional processing. Interestingly,
cingulotomy (ie, cingulum fi ber transection) was unsuc-
cessfully used to treat psychosis in the mid-twentieth
century and was abandoned, but is now sometimes used
for the treatment of obsessive-compulsive disorder and
intractable pain.
Now, draw the middle longitudinal fasciculus from
the posterior end of the superior temporal g yrus to the
temporal pole (note that this is not the medial longitudi-
nal fasciculus, which lies in the brainstem and transmits
eye movements). Indicate that the middle longitudinal
fasciculus connects paralimbic, associative, and prefron-
tal cortices.
Next, draw the inferior longitudinal fasciculus from
the antero-inferior temporal lobe to the parieto- occipital
lobe. Th e inferior longitudinal fasciculus is the major
white matter bundle of the ventral occipitotemporal
visual pathway, which is responsible for object recogni-
tion and identifi cation — the so-called “what” pathway.
Next, draw the fronto-occipital fasciculus in the supe-
rior cortex from the parieto-occipital area to the pre-
frontal area. Th e fronto-occipital pathway is the “where”
pathway corollary to the ventral occipitotemporal “what”
pathway; it transmits visual spatial information to the
frontal motor areas to guide movement.
Lastly, show that the extreme capsule spans from the
middle temporal area to the ventral prefrontal cortex. It
is discretely situated between the claustrum and the
insula (as opposed to the external capsule, which is situ-
ated between the claustrum and the putamen) and is
most easily viewed on coronal radiographic imaging.
Indicate that the extreme capsule plays a role in language,
specifi cally in syntax and grammar, and as discussed pre-
viously, conduction aphasia may at least partially be due
to dysfunction within this white matter bundle.^3 – 8