D143
Anatomical connectivity of the posterior fusiform gyrus
1
Devlin
Joseph. T.
1Department
and Cathy J.
2
Price
of Psychology & 2Wellcome Centre for Imaging Neuroscience
University College London, U. K.
Email: joe.devlin@ucl.ac.uk
Seed regions
Background
Jules Dejerine (1) first identified the phenomena of “pure alexia” in a patient
named Courrière with a lesion to the left occipital lobe. Courrière could not
read words, although his ability to write to dictation was intact – a finding
which Dejerine interpreted as evidence for a disconnection between visual
processing and visual word forms, thought to be stored in the angular gyrus of
the inferior parietal lobe. He hypothesized that by encroaching on the white
matter of the inferior longitudinal fasciculus (ILF), the lesion severed the direct
anatomical connections linking these regions. Anatomical studies of the ILF,
however, suggest that it does not link occipital and inferior parietal regions
(2,3). Moreover, inferring these connections based on comparison to macaques
is difficult due to pronounced differences in the ventral regions of the brain (see
Box). Finally, human studies of “functional connectivity” in reading
consistently demonstrate that the posterior fusiform region of the occipitotemporal junction is important for reading but is part of a much larger,
interactive system critical for skilled reading (4-9). As a result, it is becoming
increasingly important to understand the anatomical connectivity of the
posterior fusiform as a basis for understanding both its functional interactions
in normal reading and the consequences of damage in acquired dyslexias.
Discussion
L
R
Using DWI-based tractography we found relatively homogenous
patterns of connectivity along the rostro-caudal extent of the posterior
fusiform gyrus. All three regions showed long range paths via the inferior
longitudinal fasciculus linking them with the middle occipital gyrus,
caudally, and the ventral temporal pole, rostrally. In addition, there were
full local connections between fusiform regions via U-fibres that form part
of the occipito-temporal projection system (3, 13). Finally, there was
preliminary evidence for different connectivity between hemispheres with
seeds near Y= –54 showing another U-fibre path linking the more lateral
inferior temporal gyrus with the fusiform gyrus in the left, but not right,
hemisphere.
These results suggest a possible anatomical pathway underlying the
so-called “semantic” route to reading: namely, a poly-synaptic path first
linking middle occipital cortex to posterior fusiform regions and then to
anterior temporal cortex via the inferior longitudinal fasciculus before
continuing on to ventral regions of Broca’s complex via the uncinate
fasciculus. Presumably all of these connections occur in both directions,
providing not only bottom-up input but also top-down control modulated,
at least in part, by the ventrolateral prefrontal cortex (i.e. Broca’s
complex).
In contrast, we were unable to identify an anatomical path linking the
posterior fusiform with the “non-semantic” reading route through the
inferior parietal lobe. It is possible that we lacked sufficient sensitivity to
detect these paths despite using robust data, probablistic tractography, and
an explicit model of cross-fibres. Another possibility is that no such
anatomical pathway exists and that the functional connectivity is driven by
occipital regions projecting visual information in parallel to both the
ventral temporal lobes and inferior parietal regions. Additional data will
be needed to investigate these possibilities further.
Y= -44
ots
cs
cs
cs
ots
ots
ots
cs
cs
cs
ots
ots
Y= -54
S1
S2
S3
Y= -64
 All seed regions consisted of fusiform voxels with at least 20% likelihood of being grey matter. Tissue type segmentation was done automatically (16).
Comparative anatomy
Macaque
Human
Fusiform tracts common to all three seed regions
Tracts started from any of the three seed regions consistently identified:
1. Inferior longitudinal fasciculus (ilf) projections linking posterior fusiform areas with the ventral temporal poles
rostrally and the middle occipital gyrus caudally, &
2. Occipito-temporal projection fibres (U-fibres) linking adjacent fusiform regions (black arrows).
cs
ots
S1
phg
fg
S2
L
ots
X = –31
Z = –26
X = 36
L
Figure: Ventral surface of the brain with cerebellum removed.
The occipito-temporal sulcus (ots) is labelled with blue
arrows and the collateral sulcus (cs) with green. The
human brain has two gyri on its ventral surface while the
macaque has only one. The fusiform gyrus (fg; blue area)
lies between the ots and cs. The parahippocampal gyrus
(phg; yellow region) lies medial to the cs. It is unclear
whether the macaque gyrus best corresponds to the fg or
phg. The scale bar denotes 1cm.
The human and macaque ventral temporal lobes differ on multiple levels
including macro- and micro-structure, connectivity, and functionality:
1. Unlike human brains which have two gyri on the ventral surface, the
macaque has only one which is sometimes referred to as the fusiform gyrus
(10) and sometimes the parahippocampal gyrus (11).
ilf
ilf
X = –37
X = 35
ilf
Z = –26
ilf
X = –35
Z = –26
X = 31
R
References
mid-occ
Y = –80
mid-occ
mid-occ
Y = –80
Z=0
Y = –80
Z=0
Z = –2
Specific fusiform tracts
To determine the direct connections from each region, additional analyses were conducted using the grey matter seed regions as exclusion masks. This
allowed us to investigate whether individual paths connected pairs of regions directly (AC) or indirectly (A  B, B  C) via multiple U-fibre links.
Y = –64 (masked at –54 & –44 )
Y = –54 (masked at –64 & –44 )
Y = –44 (masked at –64 & –44 )
S1
1.
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Price, C. J., & Devlin, J. T. (2003). The myth of the visual word form area.
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itg
2. The cytoarchitectonic fields of the ventral macaque brain, OA, TF, and TH
(10) do not directly correspond to any of the human regions defined by
Brodman (12).
3. There is evidence to suggest that the connectivity of the inferior
longitudinal fasciculus is different between the two species (3,13).
R
ilf
ilf
S3
X = –32
X = –31
X = 35
Y = –54
X = 35
X = –32
X = 35
S2
itg
4. Although the fusiform gyrus plays a role in face processing in humans, it
is the lateral, but not ventral, temporal areas that are important for
processing faces in the macaque (14).
For all of these reasons, then, inferring human fusiform connectivity from
anatomical tracing studies in macaques is extremely difficult.
Current Study
 Probabilistic DTI tractography (15) was used to investigate cortico-cortical connectivity
of three posterior fusiform regions centred on Y= –64, –54, and –44.
 Three healthy volunteers (all male, right handed, aged 18, 19, 19) received a high
resolution T1 scan (3D Turbo FLASH, TR = 12msec, TE = 5.65msec, 1  1  1mm) and
a diffusion weighted imaging (60 directions, b-value=1000s/mm2, 40mT/m, 2.5  2.5 
2.5mm resolution).
X = –32
X = –35
X = 42
Y = –60
X = 30
X = –34
10. von Bonin, G., & Bailey, P. (1947). The Neocortex of Macaca Mulatta. Urbana, IL:
University of Illinois Press.
11. van Hoesen, G. (1982). The parahippocampal gyrus. Trends in Neuroscience, 345-350.
S3
itg
X = –38
X = –41
X = 37
Y = –49
X = 36
X = –35
Path schematic
middle occipital
gyrus
12. Brodmann, K. (1909). Localisation in the Cerebral Cortex (L. K. Garey, Trans.).
Leipzig: Verlag von Johann Ambrosias Barth.
13. Tusa, R. J., & Ungerleider, L. G. (1985). The inferior longitudinal fasciculus: a
reexamination in humans and monkeys. Ann Neurol, 18(5), 583-591.
14. Pinsk, M. A., DeSimone, K., Moore, T., Gross, C. G., & Kastner, S. (2005).
Representations of faces and body parts in macaque temporal cortex: a functional MRI
study. Proc Natl Acad Sci U S A, 102(19), 6996-7001.
15. Behrens, T. E., Berg, H. J., Jbabdi, S., Rushworth, M. F., & Woolrich, M. W. (2007).
Probabilistic diffusion tractography with multiple fibre orientations: What can we
gain? Neuroimage, 34(1), 144-155.
Inferior
Longitudinal
Fasciculus
16. Zhang, Y., Brady, M., & Smith, S. (2001). Segmentation of brain MR images through
a hidden Markov random field model and the expectation-maximization algorithm.
IEEE Trans Med Imaging, 20(1), 45-57.
 T1-weighted scans were translated and rotated into the MNI-152 orientation
fusiform gyrus
(Y= –64)
U-fibre
fusiform gyrus
(Y= –54)
U-fibre
fusiform gyrus
(Y= –44)
ventral temporal
pole
Email: joe.devlin@ucl.ac.uk
U-fibre
 In addition, an “exclusion mask” was manually defined per subject to prevent tracts from
“jumping” sulci (i.e. to prevent false positives due to differences in the resolution of the
T1 and DWI images).
X = 34
From each of the three seed regions, two types of paths can be seen: i) local paths linking adjacent fusiform regions and ii) long distance paths that run dorsally into the inferior longitudinal
fasciculus before going both rostrally and caudally to the ventral temporal poles and middle occipital gyrus, respectively. Finally, from the middle fusiform seed region, a local connection
appears to link the fusiform to the inferior temporal gyrus (itg), but only in the left hemisphere.
U-fibre
 Seed regions were manually defined on each participant’s T1 scan using the collateral
and occipito-temporal sulci as the medial and lateral borders of the fusiform gyrus,
respectively (see next panel)
X = 35
inferior temporal
gyrus
Although the connection between the fusiform gyrus and the
inferior temporal gyrus appeared to be left lateralized, this is a
preliminary finding in a small sample (n=3) and should be
considered as such.