Transcript SYLVIAN FISSURE ASYMMETRY IN NORMAL YOUNG ADULTS …

SYLVIAN FISSURE ASYMMETRY IN NORMAL YOUNG ADULTS
C.M. Leonard, S.D. Towler, S. Welcome, L. Halderman, R. Otto, C. Chiarello
# 159.4
Department of Neuroscience, McKnight Brain Institute, University of Florida, Gainesville,
Department of Psychology, University of California, Riverside & MRI Diagnostic Imaging Center, Riverside CA
Background
Method
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.
Type 4
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
Vertical
ramus
M
Hand index (from Bryden) varies
from -1 (completely left to +1
(completely right). WJ: WoodcockJohnson tests of achievement
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.
Results
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).
WA-
Type 4 fissures have two different types of morphology. In the four hemispheres in the bottom row, the vertical ramus is
continuous with Heschl’s 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.
Chi square analyses tested if the ratings
differed by hemisphere or sex. Anova or ttests tested if ratings were associated with
hand preference or reading skill
Central sulcus
RD
Type “5”
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.
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).
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.
Three women with fissures that were consistently (4/4)
rated type 1 in each hemisphere.
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.
Right
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
The Steinmetz and Witelson/Kigar Categories
Left
•Type HV/1: Horizontal and vertical rami; Vertical ramus
enters supramarginal gyrus (SMG)
Central sulcus indicated in red. Arrowhead indicates
origin of ascending ramus in SMG.
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).
•Type H/2: No vertical ramus (planum parietale)
•Type 3: Vertical ramus rises posterior to SMG
•Type V: No horizontal ramus (planum temporale)
•Type 4: Vertical ramus joins postcentral sulcus or rises
in sensory strip
Figure from Chiarello et al., 2006; Types 1-4 from Steinmetz et al., 1990; Types HV, H & V from Witelson
and Kigar (1992).
Steinmetz et al. collapsed types 2 and 3 together because many 2’s are also 3’s. Few investigators have
adopted the Steinmetz and Witelson systems. Hiemenz and Hynd (2000) found no type 4 fissures and
an equal proportion of type 3 fissures on the left and right in a sample of children with and without
reading disability. This group and others (Leonard et al., 1993; Eckert et al., 2006) have reported that
type 2, 3 & 4 fissures can be associated with information processing impairments.
Conclusions and Future Work
Question 2. Is the distribution of fissure
types different in men and
women?
Maybe. Men have slightly fewer
type 1’s on the left and more type
1’s 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 3’s on the left
show a trend towards being less
strongly right handed.
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.
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This research was supported by NIDCD grant no R01 006957.