Atypical Reading/Laterality Profile Associated with Reversed Planum Temporale Asymmetry
QuickTime™ and a
TIFF (Uncompressed) decompressor
are needed to see this picture.
University of California,
Introduction
We are investigating the neuroanatomical correlates of one acquired cognitive
skill, reading. There is substantial individual variation in reading skill among
college students, but the relationship of this to variation in cortical anatomy and
lateralization is unclear. It has been speculated that poor reading skill may be
associated with reduced or reversed planum temporale asymmetry, but findings
have been contradictory (Leonard & Eckert, 2008). Most previous approaches to
this issue investigate differences between groups defined a priori (i.e., sex,
handedness, diagnostic group) (e.g., Chiarello, et al, in press). Here we explore
an alternate approach, in which groups are identified in a bottom-up fashion by
patterns in their behavioral data (reading subtest scores and asymmetries in
divided visual field word reading tasks, see Chiarello, et al., in press). A cluster
analysis of data from 200 college students revealed four groups with distinctive
reading/lateralization profiles. However, 17 individuals were unclassifiable by this
approach (“outliers”). In this report we focus on these latter individuals to
examine whether their atypical behavioral outcomes are associated with unusual
asymmetry of the planum temporale. If absent or reversed planar asymmetry is
attributable to less regulated neural development, then atypical, but not
necessarily deficient, behavioral profiles may occur.
Method
PARTICIPANTS:
• 100 male, 100 female native English speakers
• 18-34 years of age
BEHAVIORAL MEASURES:
• Verbal IQ estimated from Wechsler Abbreviated Scale of Intelligence (Wechsler, 1977)
• Reading subscales from Woodcock Reading Mastery Test - Revised (Woodcock, 1998)
• Word Identification (word reading)
• Word Attack (nonword reading)
• Passage Comprehension
• Asymmetry scores across the following divided visual field tasks calculated separately
for accuracy and reaction time:
• Lexical Decision
• Masked Word Recognition (2 AFC procedure)
• Word Naming
• Nonword Naming
• Semantic (manmade vs natural) Decision
• Verb Generation
• Category Generation
BRAIN MEASUREMENTS FROM MRI:
• Volumetric MRI scans (1.2 mm thick sagittal slices) on 1.5 GE scanner
• Surface area of the planum temporale was calculated between sagittal coordinates x =
47 and 56, a region for which asymmetry has correlated with behavior in many studies
(Leonard & Eckert, 2008).
Suzanne
1
Welcome ,
& Christiana M.
1
Riverside ,University
of Florida,
2
Leonard
2
Gainesville
Analyses and Results
The following measures, which were not strongly correlated, were
used in the cluster analysis (Ward’s method): word attack; accuracy
asymmetry for masked word recognition, lexical decision, and verb
generation; RT asymmetry for nonword naming, masked word
recognition, lexical decision and verb generation. A four-cluster
solution (eigenvalue = 0.97) successfully classified all but 17 of the
participants. As shown below, the clusters represented differing
reading/VF asymmetry profiles. All groups showed the expected
leftward planar asymmetry.
Outliers
0.5
Cluster 1
Cluster 3
0.4
Cluster 2
Cluster 4
0.3
0.2
0.1
0
-1.0 to
-0.6
Clusters
Poorer Reader
Average Reader
Good Reader
Good Reader
Low-to-Average
VF Asymmetry
(N = 61)
Large
VF Asymmetry
(N = 63)
Low VF
Asymmetry
(N = 26)
VF Asymmetries
Vary by Task
(N = 33)
47.3
50.3
63.7
0.6
% of group
Christine
1
Chiarello ,
Word Att%ile
Word Id%ile
P Comp%ile
Mean VF
Asym1
(sd)
31.9
40.7
56.1
66.2
57.0
73.7
60.8
58.6
73.4
-.212
(.206)
+.249
(.320)
-.510
(.274)
+.163
(.495)
PT Asym2
.32**
.30**
.40**
.41**
1 Asymmetry
expressed as z-score; 0 = average asymmetry; negative = smaller than average asymmetry
2 Coefficient of asymmetry; positive values = leftward asymmetry; ** p < .001
Outliers
The 17 “outliers” showed quite variable performance on the measures
used in the cluster analysis (i.e., they do not form a fifth cluster).
However, they did share some characteristics that differentiated them
from the rest of the sample:
• Very good performance on Passage Comprehension (77.2 %ile)
• Very slow responses on VF tasks (mean RT-zscore = .317), relative
to any cluster (mean RT-zscores < .09)
• Extreme variability across task asymmetries - much higher
average standard deviations (sdRT= 1.64, sdAcc= .51) than any cluster
(sdRT<.30; sdAcc< .26)
• Very high percentage of consistent-handers (70.6%), relative to any
cluster (38.5% - 54%)
• Unusual planum temporale asymmetries:
Mean coeffiecient of asymmetry = .12, ns
Large % of reversed (i.e., rightward) asymmetries (41%), relative to
any cluster (12%-27%)
The distribution of planar asymmetries for the entire sample is shown in
the figure.
-.5 to
-0.1
0 to
+0.4
+0.5 to
+0.9
+1.0 to
+1.4
+1.5 to
+1.9
Planum Temporale Asymmetry
Conclusions
• Even among college students, there is diversity in reading ability. We identified 4
reading subgroups with differing VF laterality profiles. Normal leftward planum
temporale asymmetry was observed in each of these “typical” profiles.
• A minority of the sample could not be classified by their reading and VF
lateralization performance, showing no discernable pattern of performance on
measures used in the cluster analysis.
• These “outliers” had very good reading comprehension, but demonstrated extreme
variability and slowed responses in divided visual field reading tasks.
• This group also failed to demonstrate typical leftward planar asymmetry, and many
had reversed (rightward) asymmetry.
• Hence, among normal readers, reversed or absent planar asymmetry is not
associated with poor reading skill, and may not be a predictor of dyslexia.
• However, atypical planum temporale asymmetry was associated with an atypical
behavioral profile. This may reflect a less regulated pattern of neural development, in
which random genetic and environmental factors influence cerebral lateralization and
behavioral outcomes.
References
Chiarello, C., Welcome, S.E., Halderman, L.K., Towler, S., Julagay, J., Otto, R., Leonard, C.M. (In press). A
large-scale investigation of lateralization in cortical anatomy and word reading: Are there sex
differences? Neuropsychology.
Leonard, C.M., & Eckert, M.A. (2008). Asymmetry and dyslexia. Developmental Neuropsychology, 33, 663681.
Acknowledgment
This research was supported by NIDCD grant 5R01DC6957.