A Cluster Analysis of Individual Differences in Reading

Report
A Cluster Analysis of Individual Differences in Reading Skill and Behavioral Lateralization:
Associations with Structural Asymmetries
Christine
1
Chiarello ,
Suzanne
1,2
Welcome ,
& Christiana M.
3
Leonard
University of California, Riverside1,University of Western Ontario2, University of Florida, Gainesville3
Three questions were of primary interest:
(1) Are there subgroups within the college population characterized by differing
profiles of reading and VF/hemisphere asymmetry? The answer can shed
light on the relationship between reading skill and degree of lateralization.
(2) Do groups identified in the cluster analysis differ in structural asymmetry? If
so, this could suggest relationships between behavioral and
neuroanatomical profiles that might not be apparent from approaches
relying on a priori subject groups.
(3) Are there some individuals whose reading behavior and VF asymmetry
depart from typically observed outcomes? Do they show unusual
neuroanatomical features? If such individuals have less well-regulated
trajectories of neural development, we would expect a higher likelihood of
unusual neuroanatomical features than for those with more typical profiles.
Method
PARTICIPANTS:
• 100 male, 100 female native English speakers
• 18-34 years of age
BEHAVIORAL MEASURES:
• Word Identification (word reading), Word Attack (nonword reading), and Passage
Comprehension subscales from Woodcock Reading Mastery Test - R
• 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 and 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, planum parietale, Heschl’s gyrus, and the
pars triangularis were measured as described in Chiarello, et al (2009). The volumes
of the anterior cingulate and the paracingulate sulci were measured as described in
Leonard, et al (2009). The planum temporale, Heschl’s gyrus and the paracingulate
sulcus are all characteristically larger on the left and some studies have related these
asymmetries to linguistic and cognitive skill.
0.5
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.
Multivariate outliers (N = 17) were identified by calculating robust Mahalanobis
distances. A four-cluster solution (eigenvalue = 0.97) successfully classified the
remaining 183 participants, as shown in table below:
 Poorer readers, somewhat reduced RVF advantages (N = 61)
 Average readers, large RVF advantages (N = 63)
 Good readers, reduced RVF advantages (N = 26)
 Good readers, task-dependent asymmetries (very large RVF advantage for
masked word recognition only) (N = 33)
 All groups showed the expected leftward asymmetries of the planum temporale
and Heschl’s gurus, and rightward asymmetry of the planum parietale
Clusters
Word Attack %ile
Word Ident %ile
P Comprehension %ile
Mean VF Asymmetry
(sd)
Pl.Temporale Asym2
Pl. Parietale Asym2
Heschl’s Gyrus Asym2
Paracingulate Asym2
Cingulate Asym2
Poorer Reader
Average Reader
Good Reader
Good Reader
Low-to-Average
VF Asymmetry
31.9
40.7
56.1
-.212
(.206)
Large
VF Asymmetry
47.3
50.3
63.7
+.249
(.320)
Low VF
Asymmetry
66.2
57.0
73.7
-.510
(.274)
VF Asymmetries
Vary by Task
60.8
58.6
73.4
+.163
(.495)
.32**
.30**
-.43**
.13**
.44**
-.26**
.40**
-.30**
.11**
.40**
-.13*
Clus te re d
Subje cts
0.4
0.3
0.2
0.1
0
-1.0 to
-0.6
-.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
0.4
0.35
0.3
0.25
0.2
0.15
0.1
0.05
0
Outlie r s
Clus te re d
Subje cts
-1.5 to
-0.1.1
-1.0 to
-0.6
-.5 to
-.1
0 to
+0.4
+.5 to
+.9
+1.0 to
+1.4
Anterior Cingulate Asymmetry
Conclusions
.41**
-.73**
.15**
.13 ns
-.15 ns
• Even among college students, there is diversity in reading ability. We identified 4
reading subgroups with differing VF laterality profiles. Normal leftward perisylvian
asymmetries were observed in each of these “typical” profiles.
-.37**
.15**
.40**
-.29**
• A minority of the sample had reading and VF asymmetry profiles that differed from
the rest of the sample. These “outliers” had very good reading comprehension, but
demonstrated extreme asymmetry variability in divided visual field reading tasks.
1 Asymmetry
expressed as z-score; 0 = average asymmetry; negative = smaller than average asymmetry
2 Coefficient of asymmetry; positive values = leftward asymmetry; * p < .05, ** p < .001
Outliers Compared to Clusters
• To determine whether the outliers differed
from subjects showing more typical profiles
(as captured by cluster analysis), t-tests
compared the combined clusters to the
outliers (see adjacent table)
• The outliers had significantly better
passage comprehension, somewhat larger
VF asymmetries, and much more variable
asymmetries across tasks
• The outliers had reduced asymmetry of
the planum temporale, with 41% of this
group having atypical rightward asymmetry
• Outliers also had reduced asymmetry of
the anterior cingulate sulcus, with 47%
having atypical leftward asymmetry
Outlie r s
% of group
There is substantial individual variation in reading skill, even among college
students, but the relationship of this to variation in cortical anatomy and
lateralization is unclear. Previous approaches have investigated
lateralization differences between groups defined a priori (e.g., sex,
handedness), with conflicting results. However, such group differences may
not reflect distinct behavioral subtypes that exist within the population of
normal readers. Here we identify groups in a bottom-up fashion by patterns
in their behavioral data using cluster analysis. Our objective is to identify
common variants in the relationship between reading skill and behavioral
lateralization, and to examine neuroanatomical correlates.
Analyses and Results
% of group
Objective
0.6
Clustered
Subjects
Word Attack% 47.3
Word ID%
49.6
P Compreh%
64.3
Mean VF Asym1 -.023
(sd)
(.851)
Outliers
Pl Temporale
Pl Parietale
Heschl’s G
Paracingulate
Cingulate
.126*
-.500
.137
.389
.083*
1 Asymmetry
.343
-.417
.129
.291
-.207
55.2
51.2
77.2*
+.249*
(1.43)**
expressed as z-score; 0 = average
asymmetry; negative = smaller than average asymmetry
*p < .05, ** p < .0001
• The outliers also failed to demonstrate typical leftward planar asymmetry, and
typical rightward anterior cingulate sulcus asymmetry, with many showing reversed
asymmetries.
• We observed an association between atypical structural asymmetry and atypical
behavioral profiles. This relationship may reflect a less regulated pattern of neural
development, in which random genetic and environmental factors influence cerebral
lateralization and behavioral outcomes.
• It is notable, however, that this was not associated with poor reading outcomes.
References
Chiarello, et al. (2009). Neuropsychology, 23, 210-222.
Leonard, C.M., et al. (2009). Brain Structure and Function, 213, 553-569.
Acknowledgment
This research was supported by NIDCD grant 5R01DC6957.

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