Discriminating left from right with

1
Discriminating left from right with a Likert
rating scale Sylvian fissure asymmetry in
healthy adults
343
Christiana M. Leonard, Ph.D.1, Stephen Towler1,
Dawn Joseph1, Suzanne Welcome2, Laura
Halderman2, Ron Otto3 and Christine Chiarello2
1 University of Florida, McKnight Brain
Institute, 2 University of California, Riverside,
Department of Psychology, 3Riverside Imaging,
Riverside CA
Introduction
Results
In 1968, Geschwind and Levitsky reported that
the post mortem analysis of 100 human brains of
unknown sex and hand preference demonstrated
unequivocal evidence of structural asymmetry in
the superior surface of the temporal lobe..
The planum temporale (PT) was six times as
likely to be larger on the left as the right.
They speculated that this structural asymmetry
provided a biological basis for the localization
of language function to the left hemisphere.
As this interpretation has not been confirmed
in two recent fMRI studies (Eckert et al., 2006
Doursaint-Pierre et al., 2006) the functional
significance of this highly reliable and
replicated population asymmetry remains
mysterious. In a small sample, we found that
PT asymmetry was associated with asymmetry in the
speed and accuracy of word processing presented
to the left and right visual field (Chiarello et
al., 2004). We are now attempting to confirm this
relationship in a sample of 200 healthy adults.
As part of this study we have developed a
reliable way to capture asymmetries in (1)
parietal operculum morphology (Steinmetz et al.
1990) and (2) the ratio between PT and the
parietal planum (PP) (Witelson Kigar (1991). 
1
V
1
4V
4NV
4NV
2_3
1
1
V
The Steinmetz and Witelson/Kigar Categories

• Type HV/1 Horizontal (PT) and vertical (PP)
  rami Vertical ramus enters supramarginal gyrus
  (SMG)
• Type H/2 No vertical ramus/PP
• Type 3 Vertical ramus/PP rises posterior to SMG
• Type V No horizontal ramus (PT)
• Type 4 Vertical ramus joins postcentral sulcus
  or rises in sensory strip

Fig. 3. Five point scale. Treating the upper and
lower banks of the Sylvian fissure as two
independent dimensions demonstrates a dramatic
degree of asymmetry between the two hemispheres.
The left and right parietal opercula were
characterized by highly significant differences
in PT/PP ratio (top) and gyral number (bottom). 
The left hemisphere distribution of ratios and
gyral number was skewed towards longer PT and
extra gyri. A discriminant analysis with these
two ratings successfully classified 72 of the
left hemispheres and 71 of the right hemispheres
(F2,397 64.4, p lt .0001).
Fig. 4. Combinations of characters collapsed to a
three point scale. Chi squares 20.0, p lt
.0001. Yellow columns show the proportions of
type 3 fissures. Most type 3 fissures have very
high PT/P ratios. Blue columns show the
proportion of hemispheres with type 4 fissures.
Most type 4 fissures are also type V. No V4
fissures were found in the left hemisphere. The
individuals with this feature combination did not
have unusual visual field asymmetries, behavioral
profiles or reading histories. We are currently
using Freesurfer http//surfer.nmr.mgh.harvard.edu
/ to compare the brain morphology of individuals
with and without unusual asymmetries.
Fig. 1 (Adapted 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 2s are also 3s. There have been relatively
few publications reporting use of the Steinmetz
and Witelson systems. We have found it difficult
to achieve reliability on the classifications due
to the existence of intermediate forms.
The table to the left compares the proportions of
fissure types in the Steinmetz, Witelson/Kigar
and present hybrid system. As can be seen, type 4
and V are much more common in the right
hemisphere, while type 2_3 and H are much more
common in the left hemisphere. There were no
effects of sex, hand preference, or IQ on the
distribution of fissure types. It is
intriguing to speculate on the genetic and
neurobiological mechanisms underlying these
robust structural differences.
Fig 2. Extreme examples of types 3 and 4 in a
severely affected dyslexic but successful
builder. A type 4 fissure was also seen in
another compensated dyslexic (Chiarello et al.,
2006). The second individual was highly
inaccurate and slow in naming words presented to
his left but not his right visual field/right
hemisphere. He failed English in secondary school
and never learned his times tables, but had a
superior ability to visualize complex
mathematical equations which led to professional
success. We speculated that superior
visualization ability is associated with a type 4
fissure because of the the enlargement of the
posterior parietal lobe. Einstein reportedly had
this formation bilaterally (Witelson, et al.,
1999).
References
Method
Chiarello 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. Dorsaint-Pierre, R., Penhune, V. B.,
Watkins, K. E., Neelin, P., Lerch, J. P.,
Bouffard, M., et al. (2006). Asymmetries of the
planum temporale and Heschl's gyrus relationship
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1164-1176.Eckert, M. A., Leonard, C. M.,
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leftward asymmetries for planum morphology and
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39-45. Steinmetz H, Ebeling U, Huang Y, Kahn T.
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by NIDCD R01 006957 and the McKnight Brain
Institute.

• PARTICIPANTS
• 100 male, 100 female native English speakers
• 18-34 years of age
• 28 (14) are not right-handed
• PROCEDURE
• Volumetric MRI scans (1.2 mm thick sagittal
  images) on 1.5 GE Scanner
• Brain tissue extracted, reoriented, and
  segmented into isometric 1 mm voxels with FSL
  software http//www.fmrib.ox.ac.uk
• After paging through sagittal images of each
  hemisphere, two raters, blind to hemisphere and
  individual characteristics, used a 5 pt Likert
  scale to rate (1) the probability that there was
  an extra or missing gyrus in the parietal
  operculum and (2) the ratio between the PT and
  the parietal operculum (PP) on an image 51 mm
  lateral to the midline.
• As at least 86 of the two ratings were within 1
  point of each other, the two ratings were
  averaged.
• For some analyses adjacent ratings were
  collapsed to produce a three point scale on each
  dimension.