The brain’s complexity arises from its connectivity — this
is highlighted by the disproportionate increase in white
matter volume throughout primate evolution1. The largest
connective structure in the brain is the corpus callosum;
it consists of over 190 million axons that transfer infor-
mation between the two cerebral hemispheres2 (FIG. 1).
The corpus callosum contains homotopic and hetero-
topic interhemispheric connections. Although there has
been debate about whether the connections are primarily
excitatory (integrating information across hemispheres)
or inhibitory (allowing the hemispheres to inhibit each
other to maximize independent function)3, they appear
to be primarily excitatory, and this is the focus of most
research on interhemispheric transfer (IHT).
Corpus callosum function in humans was first inves-
tigated in classic studies of ‘split-brain’ patients, whose
callosum is severed surgically for the treatment of epi-
lepsy4,5 (BOX 1). However, there is another population
that provides valuable insight about the functions of the
corpus callosum and the role of altered connectivity in
neurodevelopmental disorders: individuals with devel-
opmental absence (agenesis) of the corpus callosum
(AgCC) (BOX 2; FIG. 2).
AgCC encompasses complete absence as well as
hypogenesis (partial absence) of the corpus callosum
(BOX 3). This review covers a broad range of findings
from research into AgCC, including animal models of
callosal development, genetic and environmental con-
tributions to AgCC, neuroimaging in acallosal humans,
and neuropsychological outcomes in individuals with
primary AgCC. Therefore, the interdisciplinary nature
of this review provides a framework for bridging two
once largely non-overlapping domains of neuroscience:
genetics and neuropsychology.
AgCC is a complex condition, which can result from
disruption in any one of the multiple steps of callosal
development, such as cellular proliferation and migra-
tion, axonal growth or glial patterning at the midline.
We review the molecular mechanisms underlying these
processes. Later sections address behavioural and neuro-
psychological aspects of AgCC. We briefly examine
research on IHT and alternative hypotheses regarding
behavioural symptoms. Although the contribution of
AgCC to our understanding of callosal function is com-
plicated by concomitant anatomical changes (BOX 2),
we suggest that AgCC might be a powerful model
for understanding cortico-cortical plasticity in other
neurological and psychiatric populations.
Development of the corpus callosum
Corpus callosum formation involves multiple steps,
including correct midline patterning, formation of telen-
cephalic hemispheres, birth and specification of commis-
sural neurons and axon guidance across the midline to
their final target in the contralateral hemisphere. Much
of what we know about the stages of callosal development
comes from animal models6,7. Several principal mecha-
nisms have been proposed to regulate callosal formation.
*California Institute of
Technology, MC 228-77
Pasadena, California 91125,
USA. ‡Travis Research
Institute, 180 N. Oakland
Ave., Pasadena, California
91101, USA. §University of
Queensland, Department of
Anatomy and Developmental
Biology, Otto Hirschfeld
Building #81, St Lucia,
Queensland 4072, Australia.
¶Department of Neurology,
University of California San
Francisco, 350 Parnassus
Ave, Suite 609, California
Correspondence to L.K.P. or
(IHT). Transmission of
information between the
cerebral hemispheres, typically
assessed with laterally
Agenesis of the corpus callosum:
genetic, developmental and
functional aspects of connectivity
Lynn K. Paul*‡, Warren S. Brown‡, Ralph Adolphs*, J. Michael Tyszka*,
Linda J. Richards§, Pratik Mukherjee¶ and Elliott H. Sherr¶
Abstract | Agenesis of the corpus callosum (AgCC), a failure to develop the large bundle
of fibres that connect the cerebral hemispheres, occurs in 1:4000 individuals. Genetics,
animal models and detailed structural neuroimaging are now providing insights into the
developmental and molecular bases of AgCC. Studies using neuropsychological, electroen-
cephalogram and functional MRI approaches are examining the resulting impairments in
emotional and social functioning, and have begun to explore the functional neuroanatomy
underlying impaired higher-order cognition. The study of AgCC could provide insight into
the integrated cerebral functioning of healthy brains, and may offer a model for
understanding certain psychiatric illnesses, such as schizophrenia and autism.
NATURE REVIEWS | NEUROSCIENCE
VOLUME 8 | APRIL 2007 | 287
(dMRI). This broad term covers
both diffusion-weighted MRI
data acquisition and image
analysis of this data, including
diffusion tensor imaging. The
MRI signal is weighted by the
amount of water diffusion
within tissues. The weighting
can vary with direction,
allowing diffusion anisotropy
arising from microscopic
restrictions in biological tissues
to be observed.
These are axons that innervate
targets early in development
and form a substrate for the
guidance of later developing
Guidance by pre-existing axon tracts. The first axons
to cross the midline arise from neurons in the cingulate
cortex. In mice, these pioneer axons cross the rostral
midline at embryonic day 15.5 (REF. 8), providing a path
for the fasciculation of later-arriving neocortical axons.
In humans, pioneer axons express the guidance receptor
neuropilin 1 (REF. 9), which can guide the axons themselves
or the later-arriving callosal neurons from the neocortex.
Cingulate cortex neurons also project axons into the
rostrolateral cortex, perhaps to initially guide neocortical
axons towards the midline. In more caudal regions of the
corpus callosum, the hippocampal commissure, which in
mice is formed a day earlier than the corpus callosum,
may provide a growth substrate10,11.
Midline glial structures. Multiple glial structures includ-
ing the glial wedge, midline zipper glia and indusium
griseum (FIG. 3) are present at the developing midline
and are probably required for corpus callosum forma-
tion12–15. The glial wedge is a bilaterally symmetrical
structure composed of radial glial cells that reside ventral
to the corpus callosum at the corticoseptal boundary.
It prevents callosal axons from entering the ventrally
located septum, and once callosal axons have crossed
the midline, repels the axons away from the midline
into the contralateral hemisphere13,16. Guidance by
the glial wedge occurs through both SLIT–ROBO and
WNT–RYK signalling13,16–18. Midline zipper glia are
found ventral to the developing corpus callosum at the
septal midline. Their fusion at the midline has been sug-
gested to be necessary for subsequent callosal axons to
grow across the midline19.
The indusium griseum glia (IGG), which are dorsal
to the developing corpus callosum, also express SLIT2
and may help guide commissural axons towards their
site of midline crossing13. Recent work in conditional
fibroblast growth factor receptor / glial fibrillary acidic
protein (Fgfr1/Gfap) Cre knockout mice has shown the
importance of these glia in corpus callosum formation14.
When FGFR1 is selectively eliminated from glia (and
not neurons), the corpus callosum fails to form. Further
analysis showed that FGFR1 is required for the migration
of the IGG and for the development of this midline glial
structure14. However, when Fgfr1 is knocked out earlier
in development, all midline glial structures at the cortico-
septal boundary fail to develop, suggesting that FGFR1
has a signalling role at multiple stages of callosal devel-
opment15. Similarly, Nfia- and Nfib-knockout mice20,21,
whose IGG and glial wedge are absent or significantly
reduced in size, do not form a corpus callosum. However,
midline glia are not the only guidance mechanisms at the
midline. Fgfr1+/– heterozygotes15 and growth-associated
protein 43 (Gap43)-knockout mice22 do form midline
glial structures (and express Slit2), but callosal axons fail
to cross, suggesting that multiple mechanisms regulate
The subcallosal sling lies dorsal to the glial wedge and
ventral to the developing corpus callosum. When the
sling is severed experimentally, the corpus callosum fails
to form19, suggesting a role for this structure in callosal
guidance, although the first callosal axons cross prior
to the formation of the subcallosal sling. In mice, the
majority of cells that make up the sling prenatally are
neurons23, but in humans, whose subcallosal sling con-
tains a large number of glia, the cellular origins of this
structure are more complex9 (FIG. 3). Finally, additional
neurons that have been identified within the corpus
callosum24 may have a role in axon guidance.
Target recognition and selective pruning in the contra-
lateral hemisphere. After crossing the midline, callosal
axons grow into the contralateral hemisphere towards
their designated target region, usually homotopic
to their region of origin, and then innervate the appro-
priate cortical layer. Such processes probably involve
both molecular recognition of the appropriate target
region and activity-dependent mechanisms that regulate
axon targeting to the correct layer and the subsequent
refinement of the projection25. In cats and ferrets,
Figure 1 | Neuroanatomy of the corpus callosum. The human corpus callosum contains
approximately 190 million axons. a | Organization of a human corpus callosum based on
histological and neuroimaging findings. b | Diffusion MRI (dMRI) and tractography
modelling provide important information about the corpus callosum fibre tracts and the
cortical regions they connect. These dMRI data144 of transcallosal fibre tracts in normal
brains resulted in a new organizational scheme that describes corpus callosum structure,
and suggested that much more of the corpus callosum is involved in premotor and
supplementary motor coordination than previously thought. Fibres are coloured
according to their projection areas: prefrontal lobe (green), premotor and supplementary
motor areas (light blue), primary motor areas (dark blue), primary sensory cortex (red),
parietal lobe (orange), occipital lobe (yellow), and temporal lobe (violet). c | In monkeys,
researchers have been able to use chemical tracers to map the organization of cortical
fibres passing through the corpus callosum, providing a level of detail currently
unavailable in humans. BA23, Brodmann’s area; CC, corpus callosum; SMA,
supplementary motor area. Panel a modified, with permission, from REF. 145 © (2004)
American Society of Neuroradiology. Panel b reproduced, with permission, from REF. 144
© (2006) Elsevier Science. Panel c modified, with permission, from REF. 146 © (2006)
Oxford Univ. Press.
288 | APRIL 2007 | VOLUME 8
Surgical procedure that
involves severing the corpus
callosum as well as the anterior
commissures (can also include
severing of posterior and
This is the original ‘split-brain’
procedure as reported by
Surgical procedure that
involves severing only the
corpus callosum, either in part
or in its entirety, leaving other
commissures intact. This has
also been described by some
as a ‘split-brain’ procedure.
Impairment in the expression
of one’s feelings and mood
states. A dominant hypothesis
is that alexithymia arises from
between the language
processing in the left
hemisphere and affect
processing in the right.
refinement of callosal visual projections occurs through
the selective pruning of axons after eye opening25–27.
Correct pruning and stabilization at the border of areas
17 and 18 (but to a lesser extent in other areas) requires
visual input28. A similar refinement of developmentally
exuberant projections occurs in the somatosensory
cortex25. It is not yet clear whether defects in axonal
pruning may affect corpus callosum size and contribute
to callosal hypoplasia (BOX 3) in humans.
Animal models of AgCC
Animal models of AgCC provide a basis for identifying
genes that may be involved in human AgCC. The inacti-
vation of genes that causes AgCC in mice often also trig-
gers neurological deficits in other large-fibre tracts such
as the internal capsule, and consequently leads to death
at birth in many cases. These phenotypes, mostly result-
ing from gene deletions, may be too severe to model
human AgCC, and such gene deletions might also result
in embryonic or perinatal death in humans. However, a
number of mouse models exist in which AgCC is partially
or fully penetrant, but the animals have normal lifespans.
Strains such as 129 and BTBR have been used to map
quantitative trait loci that affect corpus callosum size29.
Recent studies have shown that the gene disrupted in
schizophrenia 1 (Disc1) is homozygously inactivated in all
129 strain mice30, and this genetic mutation may be caus-
ally linked to AgCC in these animals. Thus, inactivation
of Disc1 might be an important mechanistic link between
schizophrenia and AgCC31.
Finally, recent studies have used new tools for label-
ling and isolating functional subsets of neurons to
identify markers that are unique to callosal projection
neurons32. These studies have identified the gene LIM
domain only 4 (Lmo4) as a candidate transcription
factor for specifying callosal ‘identity’ to projection
neurons. This approach will potentially lead to a greater
understanding of how neurons acquire their functional
Causes of AgCC in humans
Genetic causes. The genetics of AgCC in humans are
variable, and reflect the underlying complexity of cal-
losal development. Current evidence indicates that a
combination of genetic mechanisms, including single-
gene Mendelian mutations, single-gene sporadic muta-
tions and complex genetics (which may have a mixture
of inherited and sporadic mutations) might have a role
in the aetiology of AgCC. Retrospective chart reviews and
cross-sectional cohort studies report that 30–45% of
cases of AgCC have identifiable causes. Approximately
10% have chromosomal anomalies and the remaining
20–35% have recognizable genetic syndromes33 (TABLE 1).
However, if we only consider individuals with complete
AgCC, then the percentage of patients with recognizable
syndromes drops to 10–15%, and thus 75% of cases of
complete AgCC do not have an identified cause.
One example of AgCC associated with a Mendelian
disorder is X-linked lissencephaly with AgCC and
ambiguous genitalia (XLAG), which results from a
mutation in the aristaless-related homeobox gene (ARX).
The first description of this disorder included only male
patients. However, females with mutations in ARX, can,
because of X-inactivation, have clinical symptoms that
range from none to spasticity, mental retardation and
seizures. MRI scans of these female patients are either
normal or show isolated AgCC with Probst bundles34
(BOX 3). Male Arx-knockout mice also have AgCC and
replicate many of the other clinical and anatomical find-
ings in XLAG35, including a significant reduction in cor-
tical interneurons, which probably explains the severe
and uncontrollable seizures experienced by patients with
Another syndrome caused by a single-gene muta-
tion with considerable overlap between the human
and animal phenotype is CRASH syndrome (corpus
callosum agenesis, retardation, adducted thumbs, spastic
paraplegia and hydrocephalus), which is accompanied
by diminutive corticospinal tracts within the brainstem.
CRASH is caused by mutations in the L1 cell adhesion
molecule (L1CAM) gene that codes for a transmem-
brane cell adhesion protein broadly expressed in the
CNS. Mice with L1cam inactivation show complete
or partial AgCC, hydrocephalus, small corticospinal
tracts, reduced neuron numbers and additional abnor-
malities in the elaboration of apical dendrites from
cortical pyramidal neurons37. Recent work suggests that
inhibition of L1CAM homophilic binding can cause
hydrocephalus, but that preventing corpus callosum
Box 1 | AgCC and the classic ‘split-brain’
Surgical commissurotomies (split-brain) are typically conducted in adulthood for the
treatment of intractable epilepsy, while AgCC is a brain abnormality present at birth. In
patients with commissurotomy, all cerebral commissures including the anterior
commissure are severed, whereas the anterior commissure is intact in almost all patients
with primary AgCC. In patients with callosotomy the anterior commissure is not
Individuals with commissurotomy manifest a ‘disconnection syndrome’ that includes
the absence of callosal transfer of sensory information121,122, and a deficiency in
bimanually coordinated motor activity123.
The presurgical existence of a seizure disorder complicates interpretation of higher
cognitive functions in split-brain cases. However, Roger Sperry comments that “speech,
verbal intelligence, calculation, motor coordination, verbal reasoning and recall,
personality and temperament are all preserved to a surprising degree in the absence of
hemispheric interconnection”4. Nevertheless, deficits have been noted in cognitive
processing time, arithmetic, abstract reasoning4 and short-term memory124.
Commissurotomized patients may also exhibit alexithymia125.
Overall, patients with AgCC have better, although limited, interhemispheric
integration than patients with commissurotomy on many forms of visual and tactile
information68,126. The relative importance of age at onset of AgCC versus
commissurotomy for interhemispheric transfer (IHT) is illustrated by the finding that
patients with early callosotomy and children with AgCC show little evidence of a
disconnection syndrome in IHT tests with simple tactile information, whereas adolescent
and adult callosotomy patients show marked transfer deficits127. This suggests that neural
plasticity in children may allow for reinforcement of alternative neural pathways and that
presence of the anterior commissure alone may not be sufficient to explain residual IHT
in AgCC. The extent to which this compensatory plasticity involves unique recruitment
of anterior commissure fibres remains unclear.
Despite the difference in functional interhemispheric connectivity, commissurotomy
and AgCC both result in impairments of reasoning in complex novel situations128. Social
situations require extremely rapid processing of very complex information that is
typically handled within lateralized regions (that is, lexical and affective processes) and
therefore may be particularly sensitive to corpus callosum abnormality.
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VOLUME 8 | APRIL 2007 | 289
A trait resulting from changes
in a single gene that has a
significant effect on the
phenotype and is inherited in a
simple pattern that is similar or
identical to those described by
Gregor Mendel. Also referred
to as monogenic.
Retrospective chart reviews
A retrospective analysis of
medical records of a group of
individuals with a particular
condition or disease, typically
used to study rare diseases for
identification and follow up are
Early embryonic inactivation of
the genes in each cell on one
of a female’s X chromosomes
(may also occur in males with
Klinefelter syndrome who have
more than one X chromosome).
The result is that dosage of X-
chromosomal gene products is
equivalent to those in typical
males (who ony have one X
formation also requires the disruption of heterophilic
interactions with other proteins, including integrins38.
Gene dosage effects have also been observed in mouse
knockout models for the genes deleted in colorectal
carcinoma (Dcc) and Gap43. Here, heterozygote mice show
partial AgCC whereas homozygote knockout mice have
complete AgCC with additional anomalies22.
Andermann syndrome, an autosomal recessive con-
dition prevalent in the Saguenay-Lac-St-Jean region of
Quebec, presents with callosal hypoplasia or AgCC, cog-
nitive impairment, episodes of psychosis and a progres-
sive central and peripheral neuropathy. It is caused by
mutation of the KCl cotransporter KCC3 (REF. 39). Kcc3-
knockout mice display neurodegeneration, and also have
hearing loss and progressive neuropathy40. However,
in contrast to ARX and L1 mouse mutants, they have
normally formed corpora callosa. Interestingly, some
patients with KCC3 mutations also have a fully formed
corpus callosum, and there is even phenotypic variabil-
ity within families, suggesting that additional genetic
influences are at work.
The variable effects of gene inactivation on callosal
development in mice and humans are also evident
in Meckel–Gruber syndrome (MKS3). In humans, the
mutation of meckelin (the gene in MKS3) causes
occipital encephaloceles, hepatic ductal cysts and
polydactyly. In mice, mutation of the same gene
causes AgCC, hydrocephalus and cysts within the
kidney41. TABLE 1 shows other disorders associated with
callosal agenesis that have a clear recessive pattern of
inheritance but for which the causative gene has
not been identified.
In spite of the progress of research into single-gene
Mendelian causes of AgCC, in most individuals with
AgCC there is no clearly inherited cause or a recognized
genetic syndrome, suggesting that AgCC can be caused
by sporadic (de novo) genetic events. One salient exam-
ple of this is Mowat–Wilson syndrome (MWS), which, in
addition to AgCC, causes Hirschsprung disease, congenital
heart disease, genitourinary anomalies, microcephaly,
epilepsy and severe cognitive impairment42. MWS is
caused by heterozygous inactivating mutations in the
gene zinc finger homeobox 1b (ZFHX1B) on chromo-
some 2q22, which codes for SIP1 (SMAD interacting
protein 1)43. AgCC is not observed in all MWS cases,
suggesting that haploinsufficiency or gene dosage of SIP1
interacts with other genetic polymorphisms to alter
Aicardi syndrome is another disorder prob-
ably caused by sporadic mutations, in this case on the
X chromosome. Only observed in females and XXY
males with Klinefelter syndrome, this disorder consists of
AgCC, infantile spasms and chorioretinal lacunae. Patients
with Aicardi syndrome show a constellation of additional
cerebral and ophthalmological abnormalities, so it is
likely that the mutation participates widely and early in
prosencephalic development. By inference, it is likely that
other cases of AgCC are caused by haploinsufficiency
at other genetic loci. This is supported by many reports
of patients with AgCC who have sporadic chromosomal
mutations, with particular loci identified repeatedly44.
Recent data obtained using microarray-based compara-
tive genomic hybridization demonstrate that patients with
AgCC have chromosomal deletions or duplications that
are smaller than those that can be detected using con-
ventional cytogenetics45. Indeed, in collaboration with
the California Birth Defects Monitoring Program, we
have shown that the risk of having a child with AgCC is
nearly threefold higher for mothers aged 40 and above,
which is consistent with causal sporadic chromosomal
changes (E.S., unpublished observations).
As noted for the single-gene disorders discussed
above, not every patient displays AgCC, indicating that
many cases of AgCC might be caused by polygenic and
other complex interactions. Moreover, the abundance of
case reports of AgCC associated with specific diseases
(partially listed in TABLE 1) probably also reflects complex
underlying mechanisms. This is exemplified by a recent
report examining the MRI findings of individuals with
Sotos syndrome, which is caused by haploinsufficiency
of the NSD1 gene. In this study, only one patient had
complete AgCC and the other 35 patients had either
diffuse hypoplasia or thinning of the posterior body of
the corpus callosum46. These findings suggest that some
genes, often referred to as modifier genes, only partially
contribute to callosal formation. Common disorders that
affect behaviour and for which the influence of many
modifier genes is the likely mode of inheritance include
autism and schizophrenia47,48. Moreover, as there are
many reports of AgCC or abnormally formed corpora
callosa in patients with autism and schizophrenia49,50,
Box 2 | Prevalence and features of AgCC
Agenesis of the corpus callosum (AgCC) encompasses a broad range of diagnoses. A
synthesis of recent neonatal and prenatal imaging studies suggested that AgCC occurs
in at least 1:4000 live births129,130, and other imaging studies131,132 demonstrated that 3–5%
of individuals assessed for neurodevelopmental disorders have AgCC.
Complete and partial AgCC probably result from disruption of the early stages of
callosal development, which could have genetic, infectious, vascular or toxic causes65,133–
135. Further heterogeneity in AgCC can arise from concomitant abnormalities in the
anterior commissure. A recent study reported that the anterior commissure was small or
absent in 60%, yet enlarged in 10% of AgCC cases136. The latter cases may provide insight
into brain plasticity, as it has been suggested that interhemispheric connections in AgCC
could be re-routed through the anterior commissure68,137. This idea is indirectly supported
by better clinical outcomes in individuals with a normal or large anterior commissure
(E.H.S., unpublished observations).
The contribution of AgCC to our understanding of callosal function is complicated by
concomitant anatomical changes, including colpocephaly and Probst bundles. It is
possible that cognitive and behavioural differences between AgCC and split-brain
patients arise from these other anatomical differences. Colpocephaly refers to the
dilatation of the posterior aspect of the lateral ventricles, frequently including the
temporal horns. This does not represent hydrocephalus138, but may signify the reduction
of ipsilateral cortical association tracts139. Probst bundles are the misrouted callosal
axons that run parallel to the interhemispheric fissure and can also be observed in cases
of partial AgCC. Apparently within the Probst bundles, a structure called the sigmoid
bundle has been recently identified in several cases of partial AgCC140 (FIG. 2). This long,
heterotopic commissural tract appears to connect the left frontal lobe with the right
Other brain malformations can also be associated with AgCC136. One AgCC autopsy
study documented a lack of pyramidal tract decussation, suggesting a more global
disorder of midline crossing141. This pattern is also observed in many animal models of
callosal agenesis7,142,143. All concomitant anatomical abnormalities, including changes in
commissural fibres outside the corpus callosum, may be relevant to clinical outcome.
290 | APRIL 2007 | VOLUME 8
(MKS3). Patients typically have
renal cysts, CNS
malformations, hepatic ductal
polydactyly. MKS3 is caused
by mutations in the gene
meckelin (also known as
A neural tube defect (NTD) that
results in a sac-like protrusion
of brain tissue and overlying
meninges. These NTDs are
frequently associated with
other brain or craniofacial
malformations, and clinically
it is possible that the modifier genes that affect callosal
development overlap significantly with those that cause
these neuropsychiatric disorders.
Environmental factors. Finally, it is important to note
that environmental factors might contribute to AgCC
as well. While much less is known about these than
the genetic factors we have reviewed above, one clear
example of environmental influences on callosal devel-
opment is provided by fetal alcohol syndrome (FAS).
Both clinical and experimental evidence indicates that
alcohol exposure in utero decreases gliogenesis51 and
glial–neuronal interactions52, processes that are vital for
corpus callosum development53. Additionally, a grow-
ing body of literature suggests that ethanol disrupts the
transcription and biochemical function of L1CAM54–56,
implicating an interplay of environment and genetics in
AgCC. The incidence of AgCC in FAS is approximately
6.8%57, with an even higher incidence of corpus callosum
malformations short of complete AgCC. In many FAS
cases, the corpus callosum is hypoplastic; this may result
not only from the disruption of early events in callosal
formation, but also from later dysregulation of axonal
pruning. Such mechanisms might also cause callosal
hypoplasia in other disorders such as schizophrenia
and autism25,31. Other environmental factors may also
influence postnatal and prenatal callosal development,
including musical training58–60, hypothyroidism61,62 and
enrichment or deprivation of experience63.
Behavioural impairment in AgCC
Consistent with the broad range of genetic factors
involved in AgCC, the cognitive, behavioural and neuro-
logical consequences of AgCC are wide-ranging. One
approach to defining clinical subsets of the AgCC patient
population is to categorize individuals according to spe-
cific neuroanatomical findings, and subsequently relate
these to the behavioural symptoms within these groups.
For example, a number of studies have suggested that
the presence of polymicrogyria, pachygyria (abnormally
broad gyri) and heterotopia, detected using MRI, corre-
lates with moderate to severe developmental delay64,65.
However, matching specific behaviours to anatomical
groups is difficult given the diversity of symptoms in
patients with similar antomical findings.
The comparison between complete and partial AgCC
has revealed conflicting data, with multiple studies show-
ing no difference in behavioural and medical outcomes
between the two conditions, whereas one earlier study
reported a worse outcome for individuals with complete
AgCC66. One hospital-based study reported that just
under a third of patients with AgCC were developmen-
tally ‘normal’ or only mildly delayed65. A longitudinal
study of 17 children prenatally diagnosed with isolated
AgCC showed that nearly all patients had at least mild
behavioural problems67. This suggests that isolated
AgCC, even when not ascertained clinically, still causes
behavioural and cognitive impairment. Parents often
report that when their child was diagnosed with AgCC,
they were told that the prognosis was unclear, ranging
from severely delayed to ‘perfectly normal’. However, as
more individuals with primary AgCC are identified and
assessed with sensitive standardized neuropsychological
measures, a pattern of deficits in higher-order cogni-
tion and social skills has become apparent even in the
so-called ‘normal’ individuals with AgCC.
Connectivity deficits. Historically, most research with
patients with AgCC focused on the consequences of
callosal absence, with the expectation that patients with
AgCC would exhibit a ‘disconnection syndrome’ similar
to that seen in commissurotomy patients4. The classic
disconnection syndrome (BOX 1) involves the complete
lack of IHT and interhemispheric integration of sensory
and motor information presented independently to each
of the hemispheres4, with surprisingly subtle behavioural
consequences in everyday life.
Studies using tachistoscopic presentation of visual
stimuli and studies of evoked potentials provide the most
compelling information about functional connections
and IHT in AgCC at various stages of sensory process-
ing. FIGURE 4 illustrates the visual cortical disconnection
Figure 2 | Examples of neuroanatomical findings in AgCC. Neuroanatomical features of
agenesis of the corpus callosum (AgCC) and callosal hypogenesis revealed by MRI and
diffusion tensor imaging (DTI). Structural T1-weighted MRI (top 3 rows) and directionally
encoded colour anisotropy dMRI (bottom row) are shown from a normal young adult male
volunteer (left column), a young adult male with AgCC (middle column), and a young adult
male with callosal hypogenesis (right column). The DTI images encode fibre orientation in
white matter tracts using a three-colour scheme such that fibre pathways with
predominantly left–right orientation are displayed as red, anteroposterior orientation as
green, and craniocaudal orientation as purple. AC, anterior commissure; ASB, anterior
sigmoid bundle; C, colpocephaly; CB, cingulum bundle; CC, corpus callosum; CM, cortical
malformation; PB, Probst bundle.
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The anatomical variant of
having more than the normal
number of digits on the hands
or feet. This is observed in
approximately 1:500 births
and is usually inherited as an
autosomal dominant trait with
A developmental disorder of
the enteric nervous system
resulting in absence of the
neuronal ganglion cells in the
distal colon, which in turn
results in a functional
obstruction of the colon. Can
present with a dramatically
distended colon (megacolon)
or with bowel perforation. It is
a cardinal feature of Mowat–
A clinically evident symptom
arising when one of the two
copies of a gene is mutated,
leaving a single functional copy
and a presumed reduction in
the level of the encoded
effects in AgCC, as well as the limits of these disconnec-
tion symptoms. As shown by visual evoked potentials,
there is a complete lack of IHT at the level of early visual
processing in AgCC68. The hemispheric disconnection
of the primary visual system in patients with AgCC
results in a unique pattern of deficits in laboratory tasks
that involve comparisons across the two visual fields:
intact comparisons of simple stimuli and impaired com-
parisons of complex stimuli. Despite the lack of trans-
fer of early visual information, individuals with AgCC
display a normal ability to make comparisons of simple
and easily encoded stimuli, indicating an intact inter-
hemispheric transfer of simple or familiar information.
For example, they exhibit an intact interhemispheric
Stroop interference effect69 and the typical bilateral field
advantage for comparison of familiar and easily encoded
visual information across hemifields68. These findings
confirm that information can be transferred between
the hemispheres in AgCC. One theory to explain the
preserved capacity for IHT of simple stimuli in patients
with AgCC is that simple information can be transferred
via other connecting pathways, such as the anterior com-
missure. Structural and functional exploration of these
alternate pathways for IHT is a crucial frontier in AgCC
By contrast, the capacity for IHT may be limited by
task complexity. For example, the performance of patients
with AgCC when comparing tachistoscopically presented
visual information is significantly less accurate for more
visually complex, less familiar and less easily verbalized
stimuli68 (FIG. 4). Similar limitations in IHT in patients
with AgCC were evident on other tasks that required
transfer or integration of the products of more complex
cognitive operations, required more rapid processing
and relied on less prior experience68,70–72. Taken together,
these studies indicate that there is a greater reliance
on the corpus callosum for rapid and effective inter-
hemispheric interactions as task requirements increase
(for example, when stimuli become more complex or
response criteria become more constrained).
The question remains, however, about what causes
the behavioural disturbances evident in primary AgCC.
Studies using dichotic listening73,74, positron emission tom-
ography75 and functional MRI (fMRI)76 have revealed that
language lateralization is normal in patients with primary
AgCC and is in some cases even exaggerated. Although
there is no published evidence for normal or abnormal
localization of other higher cognitive functions in this
population, we can suggest that, if localized functioning
in the cortex of patients with primary AgCC is normal, a
lack of information transfer between localized process-
ing regions in opposite hemispheres could contribute to
behavioural difficulties. This would leave callosal dis-
connection as a viable explanation for the behavioural
disturbances in patients with primary AgCC.
Neuropsychological impairment. The major anatomical
feature of primary AgCC is the absence of the corpus cal-
losum, and it is presumed to be the cause of the cognitive
and behavioural changes in these individuals. However,
colpocephaly (BOX 3) and Probst bundles are common in
people with primary AgCC (and never in those without
AgCC), and together with other more subtle anatomical
changes probably also affect behaviour. Functional and
anatomical imaging approaches, coupled with incisive
neuropsychological assessments, may in the future be
able to map the neural processes and neuropathology
associated with AgCC onto specific behavioural anoma-
lies. For now, we begin by describing the general symp-
tom profile found in primary AgCC.
Primary AgCC has a surprisingly limited impact
on general cognitive ability. Although the full-scale IQ
can be lower than expected based on family history,
scores frequently remain within the average range77.
Interestingly, in an unexpectedly large number of per-
sons with primary AgCC (as many as 60%), performance
IQ and verbal IQ are significantly different77,78. However,
there is no consistency with respect to which of the two
is more affected. Impairments in abstract reasoning79,80,
problem solving81–83, generalization (the ability to extrap-
olate from one case to others)84 and category fluency (the
ability to list multiple items that belong to a semantic
category, for example, names of animals)80 have all been
consistently observed in patients with primary AgCC.
Data from large sample sizes suggest that problem solv-
ing abilities become more impaired as task complexity
increases (W.S.B. and L.K.P., unpublished observations).
While neuropsychological research into domains such
as memory, attention and spatial skills is under way in
large samples of patients with primary AgCC, currently
published results in these domains are limited to a few
case studies that do not yet provide consistent findings.
The most comprehensively examined higher cogni-
tive domain in patients with AgCC is language. Overall,
Box 3 | Key diagnostic definitions in AgCC
Complete AgCC. A congenital condition characterized by total absence of the corpus
Hypogenesis of the corpus callosum. Also known as partial AgCC, this is a congenital
condition characterized by partial absence of the corpus callosum. The absence must
be evident from birth and not be representative of a degenerative condition.
Hypoplasia of the corpus callosum. Condition in which the corpus callosum is fully
formed, but is thinner than expected for age and sex of the individual.
Isolated AgCC. Neuroanatomical description which includes complete absence of the
corpus callosum, without other confounding brain abnormalities such as
polymicrogyria, heterotopia or schizencephaly. Individuals with isolated AgCC
frequently have colpocephaly and Probst bundles.
Primary AgCC. Primary AgCC refers to a symptom profile which includes isolated
AgCC and generally intact intellectual functioning, as indicated by full-scale IQ ≥ 80.
Anterior commissure. Small band of approximately 50,000 axons that connect the
cerebral hemispheres. The anterior commissure connects the temporal lobes and is
located at the base of the fornix.
Probst bundles. Misrouted callosal axons that run parallel to the interhemispheric
fissure and can be observed both in cases of complete and of partial AgCC.
Colpocephaly. Dilatation of the posterior aspect of the lateral ventricles, frequently
including the temporal horns. This does not represent hydrocephalus but may represent
the reduction of ipsilateral cortical association tracts.
Sigmoid bundle. A long heterotopic commissural tract found in some cases of partial
AgCC. It appears to connect the left frontal lobe with the right occipitoparietal cortex.
292 | APRIL 2007 | VOLUME 8
Sling glia or
Astrocytes Pioneer axons
A genetic syndrome defined as
a 47, XXY karyotype in a
phenotypic male. Patients
frequently have small testes,
minimal sperm production,
breast enlargement in puberty
and psychosocial problems.
Punched out lesions in the
pigmented layer of the retina
that cluster around the optic
disc that are pathognomonic
for Aicardi syndrome.
individuals with primary AgCC have intact general nam-
ing (for example, naming objects from line drawings85,86),
receptive language (for example, comprehension of
sentences78,86) and lexical reading skills87. However,
impairments have been reported in the comprehension
of syntax and linguistic pragmatics88,89, and in phonological
processing and rhyming86,88–90. With respect to linguistic
pragmatics, persons with primary AgCC are impaired
in the comprehension of idioms, proverbs, vocal pros-
ody91,92 and narrative humour93. Within humour, they
exhibit difficulty in overriding literal interpretation
bias and are poor at using context to infer meaning92–94.
Patients with primary AgCC also show marked difficul-
ties with expressive language, for example in the verbal
expression of emotional experience, which is consistent
with a diagnosis of alexithymia95. In a study of a large
sample of AgCC patients with adequate expressive lan-
guage skills, parents consistently described ‘meaningless’
or ‘out-of-place’ conversation as particularly common96.
Interestingly, recent studies of language support the
dynamic dual pathway model, according to which syntax
and semantics are lateralized to the left hemisphere and
prosody to the right hemisphere97–102. In this model, the
corpus callosum is the main path for coordination of this
lateralized information, particularly for coordinating
syntactic and prosodic information97–99, the very areas
of linguistic weakness evident in AgCC.
Parents of individuals with primary AgCC consist-
ently describe impaired social skills and poor personal
insight as the features that interfere most prominently
with the daily lives of their children96,103–105. Specific traits
include emotional immaturity, lack of introspection,
impaired social competence, general deficits in social
judgment and planning, and poor communication of
emotions (for example, individuals prefer much younger
friends, have a marked difficulty generating and sustain-
ing conversation, take all conversation literally, do not
take perspective of others, and are unable to effectively
plan and execute daily activities such as homework,
showering or paying bills96,105). Consequently, patients
with primary AgCC often have impoverished and
superficial relationships, suffer social isolation and have
interpersonal conflict both at home and at work due to
misinterpretation of social cues.
Responses of adults with primary AgCC on self-report
measures also suggest diminished self-awareness104. The
patients’ self-reports are often in direct conflict with
observations from friends and family. One potential
factor contributing to poor self-awareness may be a more
general impairment in comprehension and description
of social situations. For instance, when asked to gener-
ate stories about social pictures, adults with primary
AgCC provided stories lacking in logic, narrative con-
tent and social understanding106. It appeared that they
had difficulty recognizing the implications of pictures
depicting social scenes, imagining a sequence of events,
and organizing relevant ideas in order to present an
appropriate narrative. Similarly, when presented with
highly provocative social pictures (for example, photos
of mutilations), adults with AgCC tended to underesti-
mate the emotional valence and intensity of the pictures,
particularly for negatively valenced pictures107. Taken
together, the neuropsychological findings in primary
AgCC highlight a pattern of deficits in problem solving,
in social pragmatics of language and communication
and in processing emotion.
AgCC and neuropsychiatric disorders. The deficits in
social communication and social interaction in patients
with primary AgCC overlap with the diagnostic criteria
for autism (from the Diagnostic and Statistical Manual
of Mental Disorders, fourth edition; DSM-IV148).
Furthermore, people with primary AgCC may display
a variety of other social, attentional and behavioural
symptoms that can resemble those of certain psychiatric
disorders. Psychiatric diagnoses are based on clusters
of behaviours, which are very complex and probably
Figure 3 | Corpus callosum development. Midline structures support the
development of the corpus callosum in the human brain. Panels a–c depict coronal
sections of human fetal brains at 17 weeks gestation. Panel a is labelled with an anti-glial
fibrillary acidic protein antibody, panel b with an anti-neuropilin 1 (NPN1) antibody and
panel c with an anti-nuclear factor 1a (NFIA) antibody. Several midline glial structures are
present at the cortical midline, including the glial wedge (GW; a), the indusium griseum
glia (IGG; a) and the midline zipper glia (MZG; d,e). Pioneer axons, which form an
additional potential guidance mechanism, express the guidance receptor NPN1 (b,d,e)
and arise from the cingulate gyrus (b). In addition, the developing human brain contains
subcallosal sling neurons, stained here with an antibody to NFIA (c). Developing human
and mouse brains differ in two significant ways at the midline. First, in humans,
differentiating astrocytes are found across the entire width of the midline (a,d,e). These
cells can either be part of the subcallosal sling or an extension of the MZG. Second, a
population of NFIA/ neuronal-specific nuclear protein (NeuN)/calretinin positive cells is
present above the corpus callosum in humans (c), but not in mice. It is unclear whether
these cells are similar to the subcallosal sling neurons or whether they might form
neurons in the IGG (e). Scale bars: a and b, 3 mm; c, 400 mm. Panels a and b modified,
with permission, from REF. 9 © (2006) Wiley and Sons.
NATURE REVIEWS | NEUROSCIENCE
VOLUME 8 | APRIL 2007 | 293
(CGH). A method that
compares the quantity of DNA
across the whole genome
between two individuals. Two
DNA samples are labelled red
and green, respectively, and
are both hybridized to a slide
that has an array of many
thousands of spots containing
DNA from unique places in the
genome. The colour ratio at
each spot determines the
relative amount of DNA
present between the two
Also known as cerebral
gigantism, this is a genetic
disorder that results in early
physical overgrowth and
cognitive impairment. Most
cases are caused by
haploinsufficiency of the gene
NSD1, which is a coregulator
for steroid receptors.
In general, this term refers to
the displacement of neuronal
cell bodies into the white
Presentation of visual stimuli
more rapidly than the eyes can
presentation thus results in a
visual stimulus being perceived
in only one hemisphere;
representation of the image in
the opposite hemisphere will
transfer of information.
Stroop interference effect
A measure of reaction time
when identifying one feature of
a stimulus, while inhibiting a
dominant tendency to identify
it according to an interfering
feature (for example, the
normally increased reaction
time when naming the ink
colour of the word “red”
printed in green ink).
Bilateral field advantage
The normal decrease in
reaction time when comparing
two stimuli presented in
opposite visual hemifields,
compared with presentation of
both within one hemifield. The
reason for this advantage is
dual processing, that is, each
hemisphere only has to
process one stimulus. Without
transfer, there cannot be such
involve multiple neural mechanisms108. Examination
of symptom overlap between psychiatric disorders
and AgCC may help to isolate the symptoms that are
directly caused by a dysfunction in cortico-cortical
There are also structural similarities between AgCC
and some psychiatric disorders. Indeed, structural
correlates of abnormal brain connectivity are evident
in essentially every psychiatric disorder that has been
examined. For example, several studies have found
altered morphology of the corpus callosum in schizo-
phrenia patients, including changes in size and shape,
as well as microstructural changes in callosal regions
that are revealed by diffusion MRI (dMRI)31. There are
also a number of reports of complete AgCC in patients
with schizophrenia50,80,109, underscoring a direct con-
nection between AgCC and schizophrenia and coun-
tering claims that the smaller anatomical changes in
the corpora callosa in patients with schizophrenia are
not causally related to the condition. Corpus callosum
size, especially its anterior sectors, is also decreased in
some cases of autism110,111. Moreover, in one study, 8.5%
of individuals with AgCC had a diagnosis of autism,
compared to only 1% of their siblings112. Microstructural
Table 1 | Syndromes associated with AgCC*
With identified genes‡
Andermann syndrome (KCC3)
Mowat Wilson syndrome (ZFHX1B)
AgCC with fatal lactic acidosis (MRPS16)
HSAS/MASA syndromes (L1CAM)
AgCC seen consistently, no gene yet identified
AgCC seen occasionally (partial list)§
AgCC with spastic paraparesis (SPG11)
Microphthalmia with linear skin defects
Opitz G syndrome
Pyruvate decarboxylase deficiency
Septo-optic dysplasia (DeMorsier syndrome)
Warburg micro syndrome
AgCC, progressive neuropathy and dementia
Lissencephaly, AgCC, intractable epilepsy
Hirschsprung disease, AgCC
Complex I and IV deficiency, AgCC, brain malformations
Hydrocephalus, adducted thumbs, AgCC, MR
AgCC, polydactyly, craniofacial changes, MR
AgCC, chorioretinal lacunae, infantile spasms, MR
Hearing loss, hydrocephalus, AgCC, colpocephaly
Diaphragmatic hernia, exomphalos, AgCC, deafness
MR, AgCC, craniofacial changes, macrocephaly
Absent patellae, urogenital malformations, AgCC
AgCC, optic coloboma, craniofacial changes, MR
AgCC, craniofacial changes, cardiac defects, MR
AgCC, albinism, recurrent infections, MR
Progressive spasticity and neuropathy, thin corpus callosum
Coronal craniosynostosis, facial asymmetry, bifid nose
CDH, pulmonary hypoplasia, craniofacial changes
Blepharophimosis, micrognathia, contractures, AgCC
Encephalocele, polydactyly and polycystic kidneys
Microopthalmia, linear skin markings, seizures
Pharyngeal cleft, craniofacial changes, AgCC, MR
Tongue hamartoma, microretrognathia, clinodactyly
Lactic acidosis, seizures, severe MR and spasticity
Broad thumbs and great toes, MR, microcephaly
Hypoplasia of septum pellucidum and optic chiasm
Physical overgrowth, MR, craniofacial changes
Microcephaly, microopthalmia, microgenitalia, MR
Microcephaly, seizures, cardiac defects, 4p –
*Reliable incidence data are unavailable for these very rare syndromes. ‡Gene symbols in brackets. §Many of these may also
consistently have a thin or dysplastic corpus callosum, such as Sotos syndrome or agenesis of the corpus callosum (AgCC) with
spastic paraparesis (SPG11). The overlap between AgCC and these conditions is still under investigation. Other gene symbols are
omitted from this section. 4p –, deletion of the terminal region of the short arm of chromosome 4, defines the genotype for Wolf–
Hirschhorn patients; ARX, aristaless-related homeobox gene; CDH, congenital diaphragmatic hernia; KCC3, KCl co-transporter 3;
L1CAM, L1 cell adhesion molecule; MR, mental retardation; MRPS16, mitochondrial ribosomal protein S16; SPG11, spastic
paraplegia 11; ZFHX1B, zinc finger homeobox 1b.
294 | APRIL 2007 | VOLUME 8
EP: Left hemisphere EP: Right hemisphere
Evoked potentials recorded
from the left hemisphere
Match No match
A small band of approximately
50,000 axons that connects
the cerebral hemispheres. The
anterior commissure connects
the temporal lobes and is
located at the base of the
A research method testing
language lateralization by
different auditory input to each
ear. The degree to which
individuals preferentially recall
information from one ear or
the other is an indication of
which hemisphere is dominant
in language processing.
Grammatical arrangement of
words and phrases in a
sentence, which affects
relationships of meaning. For
example, changing the
placement of a word or phrase
can change the meaning.
The processes that allow one
to go beyond the literal
meaning of language and
actually interpret the speaker’s
intended meaning. This may
involve utilizing second-order
meanings, body language,
vocal inflection, context and
A continuous scale from
pleasant to aversive.
changes in the corpus callosum have also been found in
patients with Tourette’s syndrome113 and attention deficit
hyperactivity disorder (ADHD)114,115. One recent study
provides evidence linking genetic changes in KCC3 (the
gene mutated in Andermann syndrome) with bipolar
disorder116, even though these patients did not have
evident changes in callosal anatomy. As more causes of
AgCC are identified, we anticipate that further genetic
correlations between AgCC and neuropsychiatric
disorders will be found.
The functional consequences of structural changes
in brain connectivity, which might be revealed by
dMRI analysis117 including diffusion tensor imaging (DTI),
contribute to cognitive impairment. Functional con-
nectivity studies show that the strength of the correla-
tions between brain activation in different regions and
anatomical abnormalities is strikingly task-dependent.
For example, children with ADHD show a disturbed
transcallosally mediated motor inhibition118. Functional
connectivity studies in patients with AgCC might
reveal the means by which these highly atypical brains
attain such apparently ‘typical’ interhemispheric inter-
action. In turn, understanding the functional limits of
such connectivity may contribute to knowledge about
psychopathological conditions with apparent corpus
AgCC, like many psychiatric disorders but unlike
callosotomy, results from abnormal development of
connectivity. It may therefore be able to shed light
on the behavioural and cognitive consequences of
abnormal connectivity during development in gen-
eral, as well as on potential compensation due to early
intervention — a topic that is now receiving much
interest, especially in studies of autism119. Of course,
most people with autism, schizophrenia or ADHD do
not typically have gross absence of the corpus callo-
sum. Nonetheless, insofar as AgCC models one specific
component (namely, altered connectivity) of what is
likely to contribute to the cognitive symptoms of these
psychiatric diseases, it may allow us to isolate a subset of
symptoms that arise primarily from altered connectivity.
Because disorders as complex as autism are not likely to
Figure 4 | Interhemispheric transfer in AgCC. Illustration of interhemispheric transfer (IHT) limitations in individuals
with agenesis of the corpus callosum (AgCC). Panels a and b show an absence of interhemispheric conduction of the early
visual evoked potential components that index sensory activity in the extrastriate visual cortex (that is, P1 and N1
components), in both patients with commissurotomy and individuals with AgCC68. a | Visual evoked potential (EP)
recording paradigm. Right visual field (RVF) stimuli (top, solid lines) first result in evoked responses from locations within
the left hemisphere (bottom left), and then following IHT (middle, solid arrow) evoke responses in right hemisphere
locations (bottom right). The bottom panels show samples of typical evoked potentials from the left and right hemisphere
recording locations within a healthy brain in response to the RVF (solid lines) and left (dashed lines) visual field (LVF).
b | Comparison of left hemisphere evoked responses to stimuli in the right (solid lines) and left (dashed lines) visual fields.
In the normal brain (top), the delay created by IHT is indicated by the later and smaller P1 and N1 components to the LVF
stimuli (dashed lines) compared with RVF responses (solid lines). P1 and N1 components for LVF stimulation are absent in
left hemisphere recordings of both the person with AgCC (middle) and the patient with commissurotomy (bottom),
indicating that the corpus callosum is necessary for the IHT of visual information. c,d | Experimental conditions that reveal
limitations in the ability to compare visual information from right and left visual fields68. Each square is an example of a
stimulus used in a letter- (c) and dot pattern- (d) matching task. While participants looked at a central fixation point (solid
diamond), two stimuli to be matched were flashed tachistoscopically in various configurations (bilateral or unilateral) in
each trial. Patients with AgCC could make bilateral letter matches as well as control participants (presumably by using
extra-callosal pathways). However, patients with AgCC could not successfully match bilaterally presented dot patterns,
which is a more complex task that cannot use semantic simplification, suggesting that there is a limit on information
transfer via non-callosal pathways. Panel a modified, with permission, from REF. 147 © (1993) Elsevier Science.
NATURE REVIEWS | NEUROSCIENCE
VOLUME 8 | APRIL 2007 | 295
Diffusion tensor imaging
(DTI). Anisotropic diffusion
within tissues is modelled as a
second-rank tensor, which can
be calculated from diffusion-
weighted MRI acquired in six
or more non-collinear
directions. The tensor at each
point in the image can be
visualized as an oriented and
scaled ellipsoid. More simply,
quantities such as the mean
diffusivity and fractional
anisotropy can be calculated
from the tensor and visualized
as conventional images. The
tensor contains information
about likely axonal fibre
direction and can be used to
create virtual fibre tracts
through the DTI, reflecting
structural connectivity in white
A characteristic that is a subset
of a particular condition and
may be shared by individuals
who do not have the full
have a single correct explanation108, finding clear genetic
and neuroanatomical models that can dissect particular
aspects of such disorders may be invaluable. Considering
all of the above, AgCC might be a powerful model for
studying behavioural and cognitive aspects of a number
of psychiatric disorders.
Integrating findings across disciplines
We have emphasized the genetic and developmental
nature of AgCC, and described its cognitive neuropsy-
chology. How can data from these different domains
best be synthesized? In linking genes and development
to behaviour and cognition, one approach is to postu-
late multiple intermediate traits or endophenotypes, a
compelling concept that has been developed to dissect
the causes of complex psychiatric disorders120. One
category of endophenotype is the anatomy. We propose
that the principal anatomical endophenotype in AgCC
is the absence of the corpus callosum. Regardless of the
diverse genetic and developmental factors that result in
AgCC, callosal absence in itself may directly lead to the
behavioural and cognitive symptoms we have described
in this review. However, additional neuroanatomical
factors such as Probst bundles, colpocephaly, abnormal
ipsilateral connections and abnormal cortical folding
may well contribute separately to the clinical outcome
of patients with AgCC by functioning as independent
endophenotypes within the AgCC clinical complex,
as well as contributing to other disorders such as
schizophrenia and autism.
Thus, the endophenotype concept as applied to
AgCC proposes that the abnormal neuroanatomy is
generated by genetic and environmental factors oper-
ating on development, and that the neuroanatomy
in turn generates the behavioural phenotype seen in
AgCC. Such a picture offers intriguing possibilities for
drawing parallels with psychiatric illness. Are there
sets of genetic mutations or environmental factors
that might contribute both to AgCC and to psychiatric
illnesses such as schizophrenia and autism? Are there
sets of cognitive and behavioural impairments that
are common to both AgCC and those psychiatric ill-
nesses? Commonalities at either the genetic, environ-
mental, anatomical or behavioural level would provide
preliminary support for hypotheses that callosal and
other cortico-cortical white matter tract impairments
are central to these disorders. We therefore suggest
that geneticists, anatomists, cognitive neuroscientists
and psychiatrists need to collaborate closely to take
full advantage of the insights that AgCC can offer into
understanding psychiatric illness.
Conclusions and future directions
Research on AgCC holds great promise for multiple
scientific disciplines. In the field of genetics, much
needs to be learned about the mode of inheritance. Most
current data point to sporadic and polygenic inheritance.
Identification of additional fully penetrant genetic causes
will provide important insight into callosal development
and function. As these genetic causes are illuminated,
understanding the range of behavioural phenotypes
that correlate with the genetics may be particularly
useful for informing family planning decisions, for
understanding related psychiatric conditions and
for developing early intervention strategies for chil-
dren whose developmental trajectories can be more
The biological basis of AgCC is complex; this is
reflected by the large number of human congenital
syndromes associated with AgCC. It is perhaps one of
the most complicated neurological birth defects simply
because so many developmental processes are involved
in the final readout of a fully formed corpus callosum. It
is this observation that makes AgCC a plausible model
for many other neurological and psychiatric illnesses with
neurodevelopmental components. Callosal development
can be affected by defects in cellular proliferation and
migration, axon growth and guidance, glial development
and patterning at the midline. Understanding the basis of
the many disorders associated with AgCC, such as schizo-
phrenia and autism, requires not only the identification
of genes that regulate each of these processes but also a
deep understanding of the function of each of the genes
and how they work together in separate and overlapping
molecular pathways to produce a corpus callosum.
AgCC is also particularly interesting to those studying
network plasticity and compensation, as it does not result
in the classic disconnection syndrome seen following
surgical disconnection in adulthood. Careful integration
of imaging and electrophysiology methods may provide
important information about the intra- and interhemi-
spheric connections in AgCC, similar to current work with
split-brain patients5. In turn, AgCC provides a powerful
test-bed for the integration of methods such as dMRI,
fMRI and electroencephalograms to examine effective
connectivity: given the demonstrable gross absence of
specific structural connectivity, how does this translate
into the functional deficits? Generating a functional map
of AgCC brains will inform crucial questions about corti-
cal and subcortical reorganization: where are particular
functional regions (for example, specific visual areas and
areas involved in language) located? To what extent do
their locations differ from those in healthy brains? Are
there some functional regions whose anatomical location
remains relatively invariant, and are there others that can
shift location more variably? Such questions have been
much investigated in studies of plasticity in animal brains;
next to nothing is known about this in the human brain.
Of course, the people most invested in AgCC research
are the individuals and families who deal with this condi-
tion. Neuropsychological and behavioural characteriza-
tion of AgCC may help clarify distinctions between it
and various behavioural diagnoses (for example, autism,
Tourette’s syndrome and ADHD). Methods from cogni-
tive neuroscience will be the most fruitful route to under-
standing the mechanisms underlying the cognitive and
psychosocial characteristics that are common in primary
AgCC. In turn, by using this information clinicians can
develop more nuanced interventions for key deficits in
AgCC, such as social skills, problem solving and plan-
ning, with the goal of enhancing the everyday lives of
individuals affected by this disorder.
296 | APRIL 2007 | VOLUME 8
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148. APA. Diagnostic and Statistical Manual of Mental
Disorders 4th edn (American Psychiatric Association,
Washington DC, 1994).
We would like to thank the Sherr, Brown and Adolphs labs for
helpful suggestions. We would also like to thank the Pfeiffer
Foundation for supporting the 2006 AgCC interdisciplinary
Competing interests statement
The authors declare no competing financial interests.
The following terms in this article are linked online to:
ARX | Disc1 | Fgfr1 | Gap43 | Gfap | KCC3 | L1CAM | Nfia | Nfib |
autism | schizophrenia
Corpus callosum research program: http://www.emotion.
UCSF Department of Neurology Brain Development
National Organization for Disorders of the Corpus Callosum:
Access to this links box is available online.
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