SYLVIAN FISSURE ASYMMETRY IN NORMAL YOUNG ADULTS C'M' Leonard, S'D' Towler, S' Welcome, L' Halderman

1
SYLVIAN FISSURE ASYMMETRY IN NORMAL YOUNG ADULTS
C.M. Leonard, S.D. Towler, S. Welcome, L.
Halderman, R. Otto, C. ChiarelloDepartment of
Neuroscience, McKnight Brain Institute,
University of Florida, Gainesville,Department of
Psychology, University of California, Riverside
MRI Diagnostic Imaging Center, Riverside CA
159.4
Method
Background
During a study of visual field asymmetries
(Chiarello et al., 2004) one young man had
unusually strong asymmetries in the accuracy with
which he read words in the left and right visual
fields. His bottom up, automated skills were poor
in the left visual field/right hemisphere
(LVF/RH) but normal in the RVF/LH. This asymmetry
was not seen for slower top-down processing
skills such as verb generation (see charts below).
110 college students (with a projected sample of
200) were recruited for a study comparing
asymmetries in linguistic processing and brain
structure
Volumetric scans obtained on a 1.5T GE scanner
were reconstructed into 1mm isovoxel images in
the Talairach planes. Each hemisphere was blindly
rated twice by an experienced neuroanatomist and
a novice by paging through sagittal images.
After the first run, discordant ratings (20)
were resolved by viewing the images together.
Agreement did not improve during the second run.
Fissures were classified as 1 or 4 if they
received 2 or more ratings of 1 or 4. The
remaining fissures were classified as type 2/3
Chi square analyses tested if the ratings
differed by hemisphere or sex. Anova or t-tests
tested if ratings were associated with hand
preference or reading skill
Type 4
Type 4 fissures have two different types of
morphology. In the four hemispheres in the bottom
row, the vertical ramus is continuous with
Heschls sulcus, fitting the criterion for a
Witelson/Kigar type V (no planum temporale). In
the top row, there is a measurable planum
temporale that extends caudally to the vertical
ramus. The only scan from a man is marked M,
the only scan from a left hander is marked with a
dot. The scan from a woman with a history of
reading disability is marked RD and a scan from a
woman with poor nonsense word reading is marked
WA-. All images are from right hemispheres.
Hand index (from Bryden) varies from -1
(completely left to 1 (completely right). WJ
Woodcock-Johnson tests of achievement
Results
Type 5
Examination of his structural anatomy revealed
that his Sylvian fissures were also very
asymmetrical. He had a Type 4 fissure on the
right (see category system described below). He
also had the cognitive profile characteristic of
compensated dyslexics. Low level math skills and
spelling were poor but he has achieved success in
a profession that involves the visualization of
complex mathematics (Chiarello et al., 2006).
Type 4 fissures had also been seen in a very
severely dyslexic man who was a successful
builder and entrepreneur (Leonard, et al., 1993)
and bilaterally, in Einstein (Witelson, et al.,
1999). We speculate that these uneven cognitive
profiles are associated with a bias towards the
use of parietal rather than temporal lobe
processing mechanisms (Chiarello et al., 2006).
The present investigation was undertaken to 1)
determine the frequency of type 4 and other
fissure types in the normal population 2)
determine if fissure types differ with sex,
handedness and reading skill.
Question 1. Is there a reliable hemispheric
asymmetry in the distribution of the fissure
types? Yes. The table below compares the
distributions reported by Steinmetz with those in
the present sample. No left hemisphere received a
rating of 4.
In the second run, the novice rater noted when
sulci in the parietal operculum formed a tongue
(see arrows above), rendering the Sylvian fissure
difficult/impossible to classify. Although we
have not yet identified reliable criteria for
this fissure type, it is worth noting that the 11
individuals (6 men/5 women) whose right
hemispheres received this classification were
significantly more likely to be left handed and
had significantly lower passage comprehension
scores than the remainder of the sample. The 12
individuals (9 men/3 women, p .06) who received
a 5 for their left hemisphere did not differ from
the remainder of the sample on any variable
Conclusions and Future Work
We undertook this study in order to determine the
frequency of type 4 fissures in the left and
right hemispheres of a large sample of college
students. We confirmed Steinmetz report that
this conformation is a right hemisphere
specialization. In a series of blind ratings no
left hemisphere was ever labelled a 4 (0/440
ratings). Unlike the individuals with type 4
fissures we had seen in previous studies, the
students with type 4 fissures in this study did
not have an elevated incidence of reading
difficulties or uneven cognitive profiles. We are
currently analyzing the visual field data to
determine if they have an unusual incidence of
extreme visual field asymmetries. We are
continuing our attempt to develop criteria that
capture behaviorally significant features of
Sylvian fissure morphology. References Chiarell
o C, Kacinik N, Manowitz B, Otto R, Leonard C.
Cerebral asymmetries for language evidence for
structural-behavioral correlations.
Neuropsychology 2004 18 219-31 Chiarello C,
Lombardino LJ, Kacinik MA, Otto R, Leonard CM.
Neuroanatomical and behavioral asymmetry in an
adult compensated dyslexic. Brain Lang 2006 98
169-81. Eckert MA, Galaburda AM, Karchemskiy A,
Liang A, Thompson P, Dutton RA, et al. Anomalous
sylvian fissure morphology in Williams syndrome.
Neuroimage 2006 33 39-45. Hiemenz JR, Hynd GW.
Sulcal/gyral pattern morphology of the
perisylvian language region in developmental
dyslexia. Brain Lang 2000 74 113-33. Leonard
CM, Voeller KS, Lombardino LJ, Morris MK,
Alexander AW, Andersen HG, et al. Anomalous
cerebral structure in dyslexia revealed with
magnetic resonance imaging. Arch Neurol 1993 50
461-9. Steinmetz H, Ebeling U, Huang Y, Kahn T.
Sulcus topography of the parietal opercular
region An anatomic and MR study. Brain Lang
1990 38 515-33. Witelson SF, Kigar D. Sylvian
fissure morphology and asymmetry in men and
women bilateral differences in relation to
handedness in men. J Comp Neurol 1992 323
326-40. Witelson SF, Kigar D, Harvey T. The
exceptional brain of Albert Einstein. Lancet
1999 353 2149-53. This research was supported
by NIDCD grant no R01 006957.
Question 2. Is the distribution of fissure types
different in men and women? Maybe. Men have
slightly fewer type 1s on the left and more type
1s on the right (P nonsignificant).
Question 3. Does hand preference or reading skill
vary with fissure type? Hand, maybe. Reading
skill no. Individuals with type 3s on the left
show a trend towards being less strongly right
handed.
.