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hello everyone today i'll be talking
about the arcuate fasciculus which is a
white matter fiber tract within the
human brain
we all know that one of our most
important cognitive specializations
is our capacity for language it's safe
to say that only humans can combine
thousands of arbitrary symbols
according to a defined set of rules
to create phrases with a nearly infinite
variety of meanings
so there must be some neurological
specialization that supports this
cognitive linguistic specialization
what is it that's different different
about the human brain
that allows us to be capable of language
well human brains we know are about
three to four times larger than the
brains of our closest living relatives
the chimpanzee
and so perhaps this explains why humans
are capable of language
whereas other primates are not perhaps
we simply have
more brain power because we have a
larger brain
well this hypothesis has been challenged
by the observation that
there are some human beings who have
chimpanzee-sized brains and we call
those people microcephalics
who have language capacities that exceed
those of language-trained chimpanzees
and so that suggests to us that there
must be some
qualitative differences in addition to
whatever quantitative differences there
are there must be something
about the internal composition of the
human brain that's different
so in the 19th century neurologists like
broca and wernicke identified cortical
regions of the human brain that were
involved in speech production in the
case of broca
and speech comprehension in the case of
of wernicke
homologs of both of these areas have
been been identified in non-human
primate brains
and here for example you can see on the
right side of the slide
the location of broca's and wernicke's
areas homologues
in the brain of a rhesus macaque
however a number of specializations
have been identified in human broca's in
workings areas that might help to
explain our capacity for language
so for example broca's area is larger in
the left than the right hemisphere in
humans but that's not true in non-human
primates
and
the cortical columns in wernicke's area
are also larger wider in the left
hemisphere than the right hemisphere
in humans and again that's not true
of non-human primates
but today i'm going to be talking about
uh the connections between
broca's and wernicke's area and that
connection is known as the arcuate
fasciculus pathway
uh in a now
classic publication from 1970
geshwan depicted the arc fasciculus as
you see here
and um he was right in so far as the arc
of fasciculus connects broca's and
warnicky's area
but this picture is actually a little
bit misleading because the arc of
fasciculus
extends well beyond classic wernicke's
area in the human brain
so back in 1895 the french neurologist
dejarine used postmortem dissection to
describe the arcuate fasciculus
from both his written description and
his illustration
it's clear that the arcus fasciculus
extended beyond wernicke's area and into
the ventral lateral temporal cortex
so here is where wernicke wernicke's
area would be
and you can see the arc
fasciculus projecting well beyond
wernicke's area
with modern neural imaging methods it's
become possible to revisit this question
specifically diffusion tensor
tractography
allows for the visualization of white
matter fiber tracts in the human brain
diffusion tensor tractography is based
on the fact that water will
preferentially diffuse
parallel to the direction in which axons
are oriented
thus by simply following the principal
direction of water diffusion across
voxels
we can estimate the trajectory of white
matter fiber tracts
diffusion tractography is not perfect we
know that it can be vulnerable to both
false positives and false negatives
nevertheless several studies have
reported good correspondence between
tractography results
and the gold standard anthro grade and
retrograde tracer studies
that have been done in
macaque monkeys for example
so in this study
by shmaman and all
you can see a close correspondence
between the tracer and tractography
reconstructions of the first second and
third branch of the superior
longitudinal fasciculus
reconstructions of the arctic fasciculus
using diffusion tractography agree with
dejarine and showing that it extends
well beyond
classic wernicke's area
so
here you can see
where wernicke's area is and there are
projections to wernicke's area but there
are also
a large number of projections ventral to
wernicke's area
in most humans
we know that language is lateralized to
the left cerebral hemisphere and so
therefore it's of interest that the
human arcuate fasciculus
on average tends to be
larger in the left than the right
hemisphere so this is one sort of piece
of evidence to suggest to us that
perhaps
it's important for language
and that leads to the question of what
exactly is the function of the arcus
fasciculus
our knowledge about the function of the
arcuate fasciculus is based heavily on
evidence from brain damage patients
this evidence has implicated
the arc fasciculus in multiple different
linguistic functions
originally back in the 1800s lesion
studies implicated the arc fasciculus in
word
repetition however more recent studies
have also linked it with a wide range of
different linguistic functions
including
naming so for example naming
scene objects
complex syntax
speech fluency
and word and sentence comprehension
some researchers have examined the
development of the arcuate fasciculus
across childhood
this study divided the arcuate
fasciculus into two different components
so there's this yellow component that
you see here
that you see here that terminates in the
pre-motor cortex
and then there's this purple component
that terminates in broca's area proper
now if we look at
what these pathways look like in newborn
infants
we can see that the yellow pathway
is visible and present and detectable in
newborns whereas the purple pathway has
not
um the group that has done this research
angela friederichi and her colleagues
has argued that
this yellow pathway is involved in
mapping of sound to articulation
so that it would be particularly
important for infants as they're
learning phonemes by
repeating the sounds that they hear
and this is probably a really important
for the the babbling phase of human
infant development
on the other hand
the purple pathway
is hypothesized to be more involved with
complex syntax which is of course
something that
newborn infants are not capable of
um and in fact the purple pathway is not
even fully developed in seven-year-old
children and that's interesting because
we know that seven-year-old children
can still make mistakes in terms of
processing syntactically complex
sentences
in fact
the arcuate fasciculus is not fully
mature even by age 9 or 10. so this is a
study that
looked at the the development of the
arcuate fasciculus in terms of its
white matter integrity or its degree of
myelination
and they showed that even by age nine or
ten
um the arctic fasciculus is not fully
myelinated
um and furthermore among the group of
children that they studied
the maturation of the arcuate fasciculus
as reflected by its degree of
myelination
was correlated with both the speed and
accuracy
in decoding complex syntax
and so here this is just showing
myelination of the arctic fasciculus
being positively correlated with
the accuracy in decoding this
these complex
syntactical sentences
so
the question the next question is what
does the arcuate fasciculus look like in
non-human primates if we think this
pathway is important for language and
humans have language but non-human
primates don't
we might expect the pathway to look
quite different in non-human primates
and perhaps even
non-existent
so back in 2008
we did a comparative diffusion
tractography study
in which we
tracked the arcuate fasciculus in human
brains chimpanzee brains and
rhesus macaque brains
and our
tractography results are shown on the
left
but i'll present our results uh
schematically to you on the right
so in humans we found as expected that
the arc fasciculus projected to
um classic wernicke's area but also well
beyond that into the ventral lateral
temporal cortex
including the superior temporal sulcus
the middle temporal gyrus and even into
the inferior temporal gyrus
and chimpanzees on the other hand
the arcuate fasciculus pathway is
less prominent
and it's focused primarily on the
homolog of wernicke's area in
chimpanzees
and there
were a small proportion of chimpanzees
that additionally showed weak
projections into the middle temporal
gyrus
but nowhere near
what we saw in humans
and then finally in rhesus macaques
there is a weak arc fasciculus pathway
that projects
primarily to
wernicke's area homolog
also
it's important again to emphasize that
the arcuate fasciculus is more prominent
in the left than the right cerebral
hemisphere in humans
and in 2012
we showed that that is also the case in
chimpanzees so the chimpanzee arcuate
fasciculus is more prominent in the left
than the right cerebral hemisphere
and this suggests to us
the possibility that that leftward
asymmetry
evolved prior to the time at which
humans and chimps diverged
approximately six to eight million years
ago
we can ask well what is this cortex
doing in the ventral lateral temporal
lobe that the arcuate fasciculus is
projecting to in the human brain
and this is a
a meta-analysis of human fmri studies of
of language
um showing you that this cortex on the
lateral surface of the temporal lobe
is involved in
semantics syntax and phonology so it's
involved in several different higher
level linguistic functions
and that means that
the the cortex that the arc of
fasciculus is reaching
in the
ventral lateral temporal cortex
is involved in higher level linguistic
functions
uh finally
there was an important follow-up study
to
the comparative work that we did which
was done by roger mars group
and i think it's a really interesting
and creative study
wanted to say a word about it so what
they did in this study is they
took the cortical surface
of chimpanzees and they warped that
cortical surface
to the cortical surface of the human
brain
and then they took that transformation
and applied that to the chimpanzee
tractography results in order to predict
what the arcuate fasciculus should look
like in humans
assuming that the arcuate simply
maintained connectivity between expanded
anterior and posterior language cortices
and
what they found so that the prediction
that they made when they did that was
what you see here in blue
um but that differed quite a bit from
what you see in actuality which is what
you see in red and so you can see that
the the actual
projection extends much further into the
temporal lobe than the prediction
and what this means is that the human
arc fasciculus
expanded
beyond these language areas that
were inter were innervated in
chimpanzees and it expanded into new
cortical territories in the human brain
and i think that's a very important
insight
finally an important caveat about all of
these comparative studies is that
this work has only examined the brains
of captive non-human primates
we don't know if the brains of non-human
primates from the wild
will look the same
and i happen to know that there are
researchers working on this question so
we'll have to
stay tuned for that
so in summary
the human arcuate fasciculus is
implicated in several different
linguistic functions including word
repetition
lexical semantics and complex syntax
the arc fasciculus is not fully
developed until late in childhood and it
tracks the development of syntactical
abilities
in contrast to chimpanzees and rhesus
macaques the human arcuate fasciculus is
much more prominent
and extends well beyond classic
wernicke's area into the lateral
temporal cortex involved in lexical
semantics and syntax
and finally in contrast to rhesus
macaques
the arcuate fasciculus
is leftwardly asymmetric in both humans
and chimpanzees
i'd like to thank all of the co-authors
who were involved in this work which is
really too many to name
but special thanks to matthew glasser
todd price and timothy behrens
and we're very grateful for the funding
from the national institutes of health
as well as the yerkes primate center
thank you very much
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you