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Neuroanatomical and Behavioral Asymmetry
in an Adult Compensated Dyslexic
Christine
1
Chiarello ,
Linda
2
Lombardino ,
Department of Psychology, University of California,
Introduction
Individual differences in cortical anatomy are readily observable, but
their functional significance is not well-understood (Chiarello, et al., 2004).
For example, 25-30% of individuals do not show the typical leftward
asymmetry of the planum temporale, and the degree of the leftward
asymmetry, when present, can vary substantially from person to person.
Here we report a case of an individual with unusually large asymmetries on
several divided visual field lexical tasks, who also evidenced an extreme
leftward asymmetry of the planum temporale and an unusual form of Sylvian
fissure morphology (Steinmetz type 4, Steinmetz, et al. 1990). We report
data from psychometric testing, several divided visual field tasks including
measures of basic word recognition (word naming, nonword naming, lexical
decision) and tasks requiring more controlled word retrieval (verb, category,
and rhyme generation), information about the individual’s unusual
educational background provided in a detailed interview, and a description of
his unusual brain structure.
Psychometric Tests
The data (Table 1) suggest that this individual may be a
compensated phonological dyslexic. His reading comprehension and
nonverbal IQ were well into the high normal range, yet, given his level of
educational attainment, his basic word decoding skills (word and nonword
pronunciation), grammar, syntax, rapid naming, and arithmetic scores were
all low. This pattern of skills and deficits is characteristic of high
functioning adult compensated dyslexics (Felton, et al., 1990).
Percentile
Skill (Subtest)
Reading
Untimed word reading
(WRMT-R Word Identification)
32
Timed word reading
(TOWRE Sight Words)
13
Untimed nonword reading
(WRMT-R Word Attack)
29
Method and Results
Rapid
Naming
Ronald
3
Otto &
2
Gainesville
Christiana
This individual took part in several divided visual field (DVF) experiments which
measured various aspects of visual word recognition and controlled retrieval. In each
experiment, 3-6 letter concrete nouns were presented for 120-150 ms in the right or left visual
field. This student’s scores for each task were compared to asymmetries observed from 1419 male student controls. VF accuracy for each task was converted to a standard laterality
index (RVF-LVF)/(RVF + LVF), and, to permit comparison across tasks, z-scores were
computed for the laterality index by task. A z-score of zero indicates the ‘typical’ VF
asymmetry for that task (e.g., a moderate RVF/LH advantage).
Table 2. Accuracy Asymmetry Z-score for Case and Z-score
Range for Control Participants
35
Word Comprehension
(WRMT-R)
72
Task
Passage Comprehension
(WRMT-R)
95
Word Recognition Measures
61
Grammaticality Judgment
(CASL)
30
Syntax Construction
(CASL)
50
Letter Naming
(CTOPP)
50
Digit Naming
(CTOPP)
50
Nonverbal
IQ
Raven’s Advanced Progressive
Matrices
86
Math
Timed Calculations and Solving
Equations (WJ COG III Calculation)
78
Timed Arithmetic
(WJ COG III Math Fluency)
39
Neuroanatomy
A volumetric MRI with 1.2 mm thick sagittal images
was acquired in a 1.5T GE scanner. Visual inspection of the
images indicated a relatively uncommon form of sylvian
fissure morphology (Steinmetz type 4) previously reported
in Einstein's brain (Witelson, et al., 1999). In a type 4
fissure, the planum parietale ascends directly posterior to
Heschl’s gyrus and enters the postcentral gyrus, rather than
the supramarginal, gyrus (Fig 1: bottom left). We have
previously seen Type 4 fissures coupled with extreme
planar asymmetry in a severely dyslexic individual with a
history similar to the present case (Leonard, Alexander,
unpublished data), and in one identical twin diagnosed with
a word finding difficulty (Lombardino, unpublished data).
In individuals with type 4 fissures, the coefficient of
asymmetry [(R-L)/((R+L)/2)] for the planum temporale is
very large. In this case, it was 1.67, more than 2 standard
deviations larger than the mean for control participants. His
other brain measurements were unremarkable.
Case
Control
Participants’
Range
Word Naming
+3.21
-1.10…..+1.64
Nonword Naming
+3.61
-0.90….+1.21
Lexical Decision
+3.27
-1.43….+1.64
Controlled Word Retrieval Measures
Verb Generation
+0.99
-1.12…..+2.93
Category Generation
-0.11
-1.83…...+1.55
Rhyme Generation
3
CA
Conclusions
Timed nonword reading
(TOWRE Phonemic Decoding)
Untimed Spelling
(WRAT3)
2
Leonard
& Diagnostic Imaging Center, Riverside,
Divided Visual Field Tests
Category
Grammar
At the time of testing, the participant was a 28-year-old male Ph.D.
candidate in a social science field. He was strongly right-handed (+1.00)
based on a five-item hand preference measure (Bryden, 1982). His GREs,
taken 4 years earlier, were 440 (verbal), 750 (quantitative), 690
(analytical). He had several first-authored publications, and successfully
completed his Ph.D. work the following year. His research involved 2dimensional (“geometrical”) modeling of human data. He is currently has a
University position as a tenure-track Assistant Professor.
The participant stated that he had never been diagnosed with any
reading or learning disability, although he reported long-standing problems
with letter reversals. His kindergarten teacher suspected mental
retardation, but a psychological evaluation at that time showed no unusual
features, except for color blindness. He was a very poor student, did not
read for pleasure, and barely graduated high school. He reported
difficulties performing simple calculations, and took the lowest levels of
math in high school. He also found writing and grammar to be difficult. He
attended a community college, and was initially attracted to his field
because he thought it involved little math. He became intrigued by his
field, transferred to a 4-year institution, and “taught himself” study skills,
trigonometry, and geometry, and earned straight As in his major. In
graduate school, he specialized in data analyses that depended on
abstract mathematical principles and the visualization of mathematic
relationships. Yet he reported embarrassment about his poor ability to
perform simple calculations when teaching statistics.
University of Florida,
Table 1. Percentile Scores for Standardized Tests.
Spelling
Biographical Data
1
Riverside ,
Natalie
1
Kacinik ,
+0.89
-2.00…. +2.70
As indicated in Table 2, his accuracy asymmetries were quite atypical for all of the
word recognition tasks, falling outside of the range of scores obtained from the control
participants. His data indicated an exaggerated RVF/LH advantage, due to extremely low
accuracies for stimuli presented to the LVF/RH (23-38% correct). In contrast, his
asymmetries for the tasks involving more controlled word retrieval were within the control
range, with typical accuracies for both visual fields.
Figure 1. Top: Typical right and left sylvian fissures. Bottom: Type 4 fissure in right hemisphere of
present case. Planum parietale (purple) rises posterior to the central sulcus in the postcentral gyrus
rather than the supramarginal gyrus. Normally, the planum parietale (purple) rises posterior to
postcentral sulcus (black). In a type 4 fissure, there is a large parietal lobe posterior to the sylvian
fissure.
Although the individual we studied had never been diagnosed with dyslexia, his
extremely poor word decoding skills, calculation and grammatical deficits, and his weak
academic performance prior to college, are consistent with the profile reported for
phonological dyslexics (Felton, et al., 1990). Nevertheless, his reading comprehension was
excellent and he eventually reached a high level of academic achievement, particularly in
more advanced levels of mathematics that involve visuospatial abilities. In both the verbal
and mathematics domains, he was able to successfully compensate for his deficiencies in
lower level skills.
Indices of brain lateralization in this person were notable. On DVF tasks of word
decoding and basic word recognition, his LVF/RH performance was extremely poor, producing
exaggerated LH advantages relative to controls. However, on more reflective lexical tasks
that require controlled word retrieval his overall performance and asymmetries were well
within the control range. This suggests that he can rely on top-down strategies to compensate
for poor bottom-up skills, and it is interesting that abnormal behavioral asymmetries were
observed only in tasks that require bottom-up word decoding skills.
The exaggerated planum temporale asymmetry evidenced in this case is similar to that
reported in a study investigating adult “recovered dyslexics” (Leonard, et al., 1993, 2001). We
suggest that right, as well as left, hemisphere language substrates may be important for
mastery of the word decoding skills needed to acquire reading normally. Interestingly, a
recent PET study of compensated dyslexics indicated reduced activation in several RH
regions, compared to controls, during a word reading task (Ingvar, et al., 2002). The sylvian
fissure morphology (type 4 fissure) in the RH in this case results in greater parietal cortex at
the expense of superior temporal cortex, and was previously reported in Einstein's brain
(Witelson, et al., 1999), as well and one individual with language and reading disorders
(Leonard, et al., 1993). One can speculate that this “parietal shift” may enhance some
visuospatial skills, as evidenced by our case’s well-developed spatial mathematical skills. The
link between anatomy and behavior requires more research, as Steinmetz originally reported
that this formation was present in 15% of 58 right hemispheres presumed to be normal.
In conclusion, we suggest that the particular profile of neuroanatomical and behavioral
asymmetry described here may characterize some high functioning dyslexics with special
talents, and may differ from the brain organization in poor readers who cannot compensate for
deficient word decoding skills.
References
Bryden, M.P. (1982). Laterality: Functional asymmetry in the normal brain. New York:
Academic Press.
Chiarello, C., Kacinik, N., Manowitz, B., Otto, R., & Leonard, C. (2004). Cerebral
asymmetries for language: Evidence for structural-behavioral correlations.
Neuropsychology, 18, 219-231.
Felton, R. H., Naylor, C. E., & Wood, F. B. (1990). Neuropsychological profile of adult
dyslexics. Brain and Language, 39, 485-497.
Ingvar, M., af Trampe, P., Greitz, T., Eriksson, L., Stone-Elander, S., & von Euler, C.
(2002). Residual differences in language processing in compensated dyslexics revealed
in simple word reading tasks. Brain and Language, 83, 249-267.
Leonard, C. M., Eckert, M. A., Lombardino, L. J., Oakland, T., Kranzler, J., Mohr, C. M., et
al. (2001). Anatomical risk factors for phonological dyslexia. Cerebral Cortex, 11, 148-157.
Arrowheads: Borders of planum
temporale
Purple: Planum parietale
Black: Postcentral sulcus
Blue: Central sulcus
Leonard, C.M., Voeller, K.K.S., Lombardino, L.J., Morris, M.K., Hynd, G.W., Alexander,
A.W., Andersen, H.G., Garofalakis, M., Honeyman, J.C., Mao, J., Agee, O.F., & Staab,
E.V. (1993). Anomalous cerebral structure in dyslexia revealed with magnetic resonance
imaging. Archives of Neurology, 50, 461-469.
Witelson, S. F., Kigar, D., & Harvey, T. (1999). The exceptional brain of Albert Einstein.
Lancet, 353, 2149-2153.
Acknowledgment
The individual described in this case was enthusiastic about participating in our study
and signed an informed consent form approved by both institutions The research was
supported by NSF grant BCS-0079456.