Centre for Clinical Research in Neuropsychiatry and School of Psychiatry and Clinical Neurosciences, University of Western Australia, Claremont, Western Australia
School of Psychology, University of Western Australia, Claremont, Western Australia
Centre for Clinical Research in Neuropsychiatry and School of Psychiatry and Clinical Neurosciences, University of Western Australia, Claremont, Western Australia
Correspondence: Dr Milan Dragovic, Centre for Clinical Research in Neuropsychiatry, Private Mail Bag No. 1, Claremont, WA 6910, Australia. E-mail: milan{at}ccrn.uwa.edu.au
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ABSTRACT |
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Aims To describe a novel approach, which simultaneously integrates various lateralityindices and delineates complex phenotypes.
Method Grade of membership analysis was used to describe latent, complex lateralisation phenotypes in patients with schizophrenia (n=157), theirsiblings (n=74) and controls (n=77). The indices used were asymmetries of eye, foot and hand; hand motor proficiency; and handedness of patients first-degree relatives.
Results Three distinct pure types of lateralisation (right, left and mixed) were evident in patients compared with two (right and left) in siblings and controls. The mixed type in patients featured absence of eye and foot lateralisation and presence of familial sinistrality, despite a right-hand dominance for writing. Patients with schizophrenia expressing the left phenotype had a more severe course of illness, significantly increased scores ontwo schizotypy factors and poorer neurocognitive performance. The pure types in the siblings were similar to those in healthy controls.
Conclusions The findings suggest that a leftward reversal, rather than a reduction in lateralisation, is associated with clinical severity and neurocognitive deficits in patients with schizophrenia.
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INTRODUCTION |
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METHOD |
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Clinical assessment
All patients, siblings and controls were interviewed by a psychiatrist or
other trained mental health professional using the Schedules for Clinical
Assessment in Neuropsychiatry (SCAN; Wing
et al, 1990). In addition, all participants filled in the
self-administered Schizotypal Personality Questionnaire (SPQ;
Raine, 1991). The World Health
Organization Psychiatric and Personal History Schedule (PPHS;
Jablensky et al,
1992) was used to collect information on the patients from key
family members. Case notes were consulted to extract data on number and
duration of hospitalisations, and dosage of antipsychotic medication
prescribed.
Laterality measures
The Edinburgh Handedness Inventory (EHI;
Oldfield, 1971) was used to
assess hand preference in the study participants and to record the reported
handedness of their parents. Eye and foot preferences were included as
independent items in the analysis. On the basis of the EHI, participants were
classified into handedness categories before statistical analysis: individuals
with laterality quotients ranging from 7100 to 771 were classified as
left-handers, those with laterality quotients from +70 to +100 as
right-handers and the remaining individuals (score from 770 to +70) as
mixed-handers. These cut-off scores for the EHI, determined on the basis of
statistical criteria, were found to be in good agreement with handedness
categorisation based on hand demonstration tasks
(Dragovic, 2004). Writing hand
and familial sinistrality variables were also extracted from the EHI
responses.
Motor proficiency of each hand was assessed by the finger-tapping task (Reitan, 1969) with tapping speed and inter-tap interval variability as measures yielding a tapping laterality quotient. On the basis of this quotient, all participants were assigned to one of three motor skill lateralisation categories: left dominant, mixed (no clear dominance) and right dominant.
Neurocognitive assessment
Each participant was assessed with a neurocognitive battery administered by
a trained research psychologist. Current intellectual functioning was assessed
by the Shipley Institute of Living Scale (SILS;
Zachary, 1986) which comprises
two sub-tests (a vocabulary and an abstraction sub-test) and yields a reliable
estimation of the Wechsler Adult Intelligence ScaleRevised Full-Scale
IQ (Zachary et al,
1985; Phay, 1990).
Premorbid intellectual functioning was estimated using the revised National
Adult Reading Test (Nelson & Willison,
1991). Sustained attention was assessed with two forms of the
Continuous Performance Test the identical pairs version
(Cornblatt et al,
1988), which measures the effects of an increased processing load
on working memory, and the degraded stimuli version
(Rosvold et al,
1956), which measures the effects of an increased processing load
on visual encoding. For each version of the Continuous Performance Test we
used the discrimination index, dL
(Snodgrass & Corwin,
1988), which indicates ability to discriminate signal from noise
as a measure of processing sensitivity. Verbal learning was assessed with the
Rey Auditory Verbal Learning Test (RAVLT;
Rey, 1964). Speed of
information processing was assessed by the Inspection Time task
(Stough et al, 1996;
White, 1996), which produces a
more accurate assessment of an individuals speed of processing than
traditional reaction time measures. Verbal Fluency, FAS version
(Benton et al, 1994)
was used as a measure of executive lexical retrieval.
Statistical analyses
To identify complex patterns of laterality and their distribution within
the study population, we used a version of latent class analysis known as
grade of membership (Woodbury et
al, 1978; Woodbury &
Manton, 1982; Manton et
al, 1994). It estimates multivariate regression relationships
between sets of discrete or continuous variables, and partitions the data into
several analytically derived latent classes or pure types,
identified by conditional maximum likelihood. The number of pure types
providing optimal partitioning of the data is determined by a formal
criterion, based on changes in the significance of the likelihood ratio
2 in successive iterations of the model with increasing (or
decreasing) numbers of pure types. Pure types estimate the probabilities of
joint occurrence of variables and are described by profiles of attributes
where the probability (
kj) is estimated of each
attribute being manifested by an individual (j) belonging entirely to
a given pure type (k). Simultaneously, grade of membership quantifies
the degree (grade of membership score, gik) to which an
individual belongs to any of the identified (k) pure types
(gik values are constrained to add up to 1, so that a
person may belong exclusively to one pure type or partially to several). Grade
of membership represents the set of attributes characterising an individual as
a weighted linear combination of pure type coefficients
(
kj) and grade of membership scores
(gik), where all individual heterogeneity of the data is
accounted for by the gik scores. The input data for grade
of membership include both internal variables, used to identify pure types,
and external variables, estimated conditional on the identified pure types and
placing the latter in a context, without affecting their definition. The level
of correspondence of each variable to the final pure type definition is
assessed by the information content statistic (H), which can be
interpreted as effect size indicating the level of contribution of each
variable to the model likelihood (values of H less than 0.10 suggest
a non-significant contribution). Grade of membership operates with both
continuous and categorical data, and requires no assumptions about the
distributions of the dependent variables. It has been used in both psychiatry
research (Manton et al,
1994; Jablensky &
Woodbury, 1995; Nurnberg
et al, 1999; Cassidy
et al, 2001;
Szádóczky et al,
2003) and genetic studies
(Corder & Woodbury, 1993;
Corder et al, 2001;
Hallmayer et al,
2003), including a new multivariate test for genetic linkage
(Kaabi & Elston,
2003).
The pure types in this analysis were derived from two sets of internal variables: behavioural lateralities (writing hand, handedness category, motor proficiency, and foot and eye dominance) and familial indices (parents and siblings handedness, and familial sinistrality). As each individual may approximate to varying degrees (quantified by grade of membership scores) to more than one pure type, grade of membership allows individuals to be uniquely assigned to discrete groups, based on the pure type for which they exhibited the highest gik. Further characterisation of such groups can then be obtained by conventional statistical analyses for relevant external variables, including, in this study, neurocognitive performance (scores on each neurocognitive task), personality traits (three schizotypy factors derived from the SPQ (Raine et al, 1994)), and several clinical measures (for the schizophrenia group only).
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RESULTS |
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In the second stage we investigated whether a comparable latent structure,
using the identical set of internal variables, was present in separate
samples. The results showed that the best grade of membership model for the
schizophrenia group consisted of three composite pure types, whereas two pure
types (left and right) provided the best
description of the data in siblings of patients with schizophrenia and in the
control group. After obtaining the three-pure-type solution in the
schizophrenia group and the two-pure-type solution in the groups of siblings
and controls, further modelling of the data failed to produce significant
improvement in goodness-of-fit. In the schizophrenia group, a change in
2 was significant for the two-type solution
(P<0.0001), and for the three-type solution (P=0.005),
but not for the four-type solution (P=0.920). In the samples of
siblings and controls a change in
2 was significant for the
two-pure-type solution (siblings, P=0.001; controls,
P=0.003), but not for the three-type solution (siblings,
P=0.119; controls, P=0.117).
Laterality pure types and their expression in patients, siblings and controls
Table 1 describes the
internal variables defining each laterality pure type in the three study
groups in terms of probabilities (kj).
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Pure type left
Pure type left is characterised by a leftward lateralisation
on the majority of behavioural and familial indices. Expressed in patients
with schizophrenia, siblings and controls, it describes individuals with high
probability of leftward (left or mixed)
lateralisation on all behavioural indices. In contrast to other pure types,
this type was characterised by preference of the left hand for writing, left-
or mixed-footedness, and greater motor proficiency of the left hand.
Individuals expressing fully this type are more likely to have at least one
left-handed first-degree relative, compared with the right pure
type in patients, siblings and controls.
Pure type mixed
Pure type mixed is identified in patients with schizophrenia
but not in siblings or controls. Its main feature is lack of a clear
preference in overall handedness, footedness, eye dominance and motor
proficiency, despite a right-hand dominance for writing. This type was
associated with positive familial sinistrality and parental left-handedness,
in which the father was more likely to be left-handed. Although the
distributions of handedness categories and foot dominance in this pure type
were in a rightward direction, individuals fully expressing this type showed
absence of clear eye and motor dominance.
Pure type right
Pure type right is expressed in the majority of participants
in all three samples. It is characterised by a rightward lateralisation in all
domains (hand, foot, eye and motor proficiency) and matching right-handedness
in the first-degree relatives.
External variables associated with pure types
The pure types described by kj probabilities
represent extreme profiles, expressed in their entirety
(gik=1.0) by only a minority of individuals in the sample,
whereas the majority approximate any such profile to a varying, quantifiable
degree. In order to compare the lateralisation patterns identified by grade of
membership pure types in terms of conventional descriptive statistics,
participants were grouped into discrete clusters, based on each
individuals highest grade of membership for any laterality pure type.
Within the schizophrenia group, 33 (21%) were classified as
left, 56 (36%) as mixed and 68 (43%) as
right. Within the two comparison groups, 27 (36%) of the
siblings group and 27 (35%) of the control group were classified as
left, whereas 47 (64%) of the sibling group and 50 (65%) of the
control group were assigned to the right phenotype category.
Further characterisation of the pure types was achieved by performing analysis of variance and KruskalWallis tests (for the schizophrenia group), using pure type assignments as the main factor, and Student t-tests (for the sibling and control group) for external variables that had not been used in the identification of laterality pure type (Table 2). Since the patients with schizophrenia assigned to the three pure types did not differ in age (F(2,154)=0.496, P=0.610) or education (F(2,153)=0.016, P=0.984), these variables were not used as covariates.
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The comparison of clinical measures, schizotypy scores (based on the SPQ factors; Raine et al, 1994) and neurocognitive performance revealed that members of pure types patients with schizophrenia in particular differ on these measures. Within the schizophrenia group, a consistent (non-significant) trend of differences between the three pure types emerged on several clinical measures. The left patients had the highest total number of hospitalisations, the longest total length of stay in psychiatric hospital, and the greatest duration of the single longest in-patient admission. There was a non-significant increase in the median number of hospitalisations in the mixed and left patients combined, relative to the group of right patients. No difference was found across the three groups with regard to the median daily dosage of antipsychotic medication (converted into chlorpromazine equivalents). All the participants with schizophrenia had higher scores on the SPQ schizotypy traits than their siblings and the controls (the latter two groups did not differ consistently from one another on these measures). Within the schizophrenia group, patients assigned to the left pure type displayed significantly higher scores (F(2,124)=3.66, P=0.029) on the cognitive-perceptual dysfunction factor (ideas of reference, magical thinking, unusual perceptual experiences and paranoid ideation) than patients assigned to the right and mixed pure types. There was a nearly significant (F(2,124)=2.89, P=0.059) increase on the SPQ interpersonal factor (social anxiety, no close friends, constricted affect). The three schizophrenia subgroups did not differ on the SPQ disorganisation factor (odd behaviour, odd speech).
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DISCUSSION |
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The lack of clear behavioural lateralisation in patients with schizophrenia is best illustrated by the mixed subtype, which represents a close approximation to the mixed-handedness construct. In contrast to previous studies, where reduced functional lateralisation has been restricted to hand preference (Cannon et al, 1995; Malesu et al, 1996; Orr et al, 1999; Collinson et al, 2004), our model broadens this construct by including multiple measures of behavioural asymmetries. Furthermore, our finding of two non-right patterns (left and mixed) of anomalous lateralisation in schizophrenia implicates both reductions of lateral preferences and increases in left-sidedness, rather than merely an increase of mixed-handedness (Satz & Green, 1999). This is consistent with a number of studies (Katsanis & Iacono, 1989; Clementz et al, 1994; Tyler et al, 1995) that have reported an excess of left-handedness in patients with schizophrenia compared with healthy controls.
Our finding that participants with schizophrenia assigned to the left phenotype were more likely to have a left-handed mother (but not a left-handed father), whereas those assigned to the mixed phenotype were more likely to have a left-handed father (but not a left-handed mother), suggests a familial effect that might be either genetic or environmental. However, the clear absence of an excess of atypical lateralisation in the unaffected siblings of patients with schizophrenia is in line with the findings of several other studies (Clementz et al, 1994; Toommey et al, 1998; Byrne et al, 2004) and suggests that atypical lateralisation is unlikely to be a robust phenotypic marker of the genetic vulnerability to schizophrenia.
Neurocognitive and clinical correlates
The associations between the laterality phenotypes identified in this study
and selected clinical and personality traits and neurocognitive variables
implicated in the vulnerability to schizophrenia do not provide unequivocal
support for the hypothesis (Crow et
al, 1998; Leask &
Crow, 2001; Nettle,
2003) that cognitive performance increases with increasing
lateralisation of hand preferences in either direction, and that cognitive
abilities are lowest around the point of equal hand skillspoint
of interhemispheric indecision
(Crow et al, 1998).
In our sample of patients with schizophrenia, greater clinical severity (as
reflected in more frequent and longer hospitalisations), earlier age at onset,
and higher scores on self-assessed cognitive-perceptual dysfunction and
interpersonal deficit, characterised the left subgroup of
patients showing a nearly complete reversal of lateralisation (assessed on
multiple indices) and not the mixed subgroup, which did not
differ on these measures from the patients with complete right
lateralisation. On the other hand, the left and
mixed subgroups combined showed a consistent trend of poorer
cognitive performance than the right subgroup, but did not
differ from one another on these measures. Thus, our data suggest that some
degree of cognitive deficit is associated with a leftward laterality shift,
rather than with a mere reduction of behavioural asymmetry. However, since the
effect size of this association is small (Cohens d for
neurocognitive measures in this data-set was in the range 0.080.16), it
is likely that behavioural asymmetries explain only a small proportion of the
variance in cognitive performance in the schizophrenia group.
Limitations
The study has several limitations. First, the sample of unaffected siblings
was relatively small and may represent a biased proportion of the sibling
population (it might have been the case that mainly healthy,
high-functioning siblings participated). Inclusion of all siblings might
reveal differences from the controls in both laterality subtypes and
neurocognitive profiles. Second, several of the laterality measures (including
parental handedness) are based on self-report, which may not be entirely
reliable. However, the EHI is the most widely used questionnaire and is widely
accepted as the standard in eliciting handedness data
(Ransil & Schachter,
1994).
Implications
Notwithstanding such caveats, our study demonstrates that the complexity of
behavioural lateralisation can be effectively partitioned into distinct latent
types using a multivariate analysis, such as grade of membership. An important
finding is that, within individuals, the probabilities of association between
different measures are not uniformly distributed; for example, left or right
writing hand can be associated with varying probabilities for other
behavioural preferences, suggesting that the writing hand is a poor predictor
of other laterality measures. We suggest that the multivariate integration of
laterality measures and other relevant co-factors into composite laterality
traits might provide a more refined tool for further research into the
genetic, developmental and environmental underpinnings of behavioural and
cerebral lateralisation.
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Clinical Implications and Limitations |
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LIMITATIONS
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ACKNOWLEDGMENTS |
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REFERENCES |
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---|
American Psychiatric Association (1994) Diagnostic and Statistical Manual of Mental Disorders (4th edn) (DSMIV). Washington, DC: APA.
Benton, A. L., Hamsher, K. de S. & Sivan, A. B. (1994) Multilingual Aphasia Examination: Manual of Instructions (3rd edn). Iowa City, IA: AJA Associates.
Byrne, M., Clafferty, R. A., Cosway, et al (2004) Measurement of lateral preferences and schizophrenia: results of the Edinburgh High-Risk Study and methodological issues. Psychiatry Research, 125, 205 -217.[CrossRef][Medline]
Cannon, M., Byrne, M., Cassidy, B., et al (1995) Prevalence and correlates of mixed-handedness in schizophrenics. Psychiatry Research, 59, 119 -125.[CrossRef][Medline]
Cassidy, F., Pieper, C. F. & Carroll, B. J. (2001) Subtypes of mania determined by grade of membership analysis. Neuropsychopharmacology, 25, 373 -383.[CrossRef][Medline]
Clementz, B. A., Iacono, W. G. & Beiser, M. (1994) Handedness in first-episode psychotic patients and their first-episode biological relatives. Journal of Abnormal Psychology, 103, 400 -403.[CrossRef][Medline]
Collinson, S. L., Phillips, T. J., James, A. C., et al (2004) Is lateral bias anomalous in early-onset schizophrenia? Selected comparisons with normal populations. Psychiatry Research, 125, 219 -224.[CrossRef][Medline]
Corder, E. H. & Woodbury, M. A. (1993) Genetic heterogeneity in Alzheimers disease: a Grade of Membership analysis. Genetic Epidemiology, 10, 495 -499.[CrossRef][Medline]
Corder, E. H., Woodbury, M. A., Manton, K. G., et al (2001) Grade-of-membership sibpair linkage analysis maps IDDM11 to chromosome 14q24.3q31. Annals of Human Genetics, 65, 387 -394.[CrossRef][Medline]
Cornblatt, B. A., Risch, N. J., Faris, G., et al (1988) The Continuous Performance Test, identical pairs version (CPTIP): I. New findings about sustained attention in normal families. Psychiatry Research, 26, 223 -238.[CrossRef][Medline]
Crow, T. J., Crow, L. R., Done, D. J., et al (1998) Relative hand skill predicts academic ability: global deficits at the point of hemispheric indecision. Neuropsychologia, 36, 1275 -1282.[CrossRef][Medline]
Dragovic, M. (2004) Categorisation and validation of handedness using Latent Class Analysis. Acta Neuropsychiatrica, 16, 212 -218.[CrossRef]
Geschwind, N. & Galaburda, M. A. (1985) Cerebral lateralisation: biological mechanisms, associations, and pathology: I. A hypothesis and a program for research. Archives of Neurology, 42, 428 -459.[CrossRef][Medline]
Hallmayer, J. F., Jablensky, A., Michie, P., et al (2003) Linkage analysis of candidate regions using a composite neurocognitive phenotype correlated with schizophrenia. Molecular Psychiatry, 8, 511 -523.[CrossRef][Medline]
Heinrichs, R. (2001) In Search of MadnessSchizophrenia and Neuroscience. Oxford: Oxford University Press.
Jablensky, A. (2004) Researching psychiatry in Western Australia. Australian and New Zealand Journal of Psychiatry, 38, 306 -315.[CrossRef][Medline]
Jablensky, A. & Woodbury, M. A. (1995) Dementia praecox and manic-depressive insanity in 1908: a Grade manic-depressive of Membership analysis of the Kraepelinian dichotomy. European Archive of Psychiatry and Clinical Neuroscience, 245, 202 -209.
Jablensky, A., Sartorius, N., Ernberg, G., et al (1992) Schizophrenia: Manifestations, Incidence and Course in Different Cultures. A World Health Organization Ten-Country Study. Psychological Medicine Monograph Supplement 20. Cambridge: Cambridge University Press.
Kaabi, B. & Elston, R. C. (2003) New multivariate test for linkage, with application to pleiotropy: fuzzy HasemanElston. Genetic Epidemiology, 24, 253 -264.[CrossRef][Medline]
Katsanis, J. M. A. & Iacono, W. G. (1989) Association of left-handedness with ventricle size and neuropsychological performance in schizophrenia. American Journal of Psychiatry, 146, 1056 -1058.[Abstract]
Leask, J. S. & Crow, J. T. (2001) Word acquisition reflects lateralisation of hand skill. Trends in Cognitive Sciences, 5, 513 -516.[CrossRef][Medline]
Malesu, R. R., Cannon, M., Jones, P. B., Malesu, R. et al (1996) Mixed-handedness in patients with functional psychosis. British Journal of Psychiatry, 168, 234 -236.[Abstract]
Manton, K. G., Woodbury, M. A. & Tolley, H. D. (1994) Statistical Applications Using Fuzzy Sets. New York: John Wiley.
McManus, I. C. (1985) Handedness, Language Dominance and Aphasia: A Genetic Model. Psychological Medicine Monograph Supplement 8. Cambridge: Cambridge University Press.
McManus, I. C. (1999) Eye-dominance, writing hand, and throwing hand. Laterality, 4, 173-192.[CrossRef][Medline]
Nelson, H. E. & Willison, J. (1991) National Adult Reading Test Manual (2nd edn) . Windsor: NFERNelson.
Nettle, D. (2003) Hand laterality and cognitive ability: a multiple regression approach. Brain and Cognition, 52, 390 -398.[CrossRef][Medline]
Nurnberg, H. G., Woodbury, M. A. & Bogenschutz, M. P. (1999) A mathematical typology analysis of DSMIIIR personality disorder classification: grade of membership technique. Comprehensive Psychiatry, 40, 61-71.[CrossRef][Medline]
Oldfield, R. C. (1971) The assessment and analysis of handedness: the Edinburgh Inventory. Neuropsychologia, 9, 97 -113.[CrossRef][Medline]
Orr, K. G. D., Cannon, M., Gilvarry, M., et al (1999) Schizophrenic patients and their first-degree relatives show an excess of mixed-handedness. Schizophrenia Research, 39, 167 -176.[CrossRef][Medline]
Phay, A. J. (1990) Shipley Institute of Living Scale: Part 2Assessment of intelligence and cognitive deterioration. Medical Psychotherapy, 3, 17-35.
Raine, A. (1991) The SPQ: a scale for the assessment of schizotypal personality based on DSMIIIR criteria. Schizophrenia Bulletin, 17, 555 -564.[Medline]
Raine, A., Reynolds, C., Lencz, T., et al (1994) Cogitive-perceptual, interpersonal, and disorganised features of schizotypal personality. Schizophrenia Bulletin, 20, 191 -201.[Medline]
Ransil, B. J. & Schachter, S. C. (1994) Test retest reliability of the Edinburgh Handedness Inventory and global handedness preference measurement, and their correlation. Perception and Motor Skills, 79, 1355 -1372.
Reitan, R. M. (1969) Manual for Administration of Neuropsychological Test Batteries for Adults and Children. Indianapolis, IN: Indianapolis University Medical Center.
Rey, A. (1964) Lexamen Clinique en Psychologie. Paris: Presses Universitaires.
Rosvold, H. E., Mirsky, A. F., Sarason, I., et al (1956) A continuous performance test of brain damage. Journal of Consulting Psychology, 20, 343 -350.[Medline]
Satz, P. & Green, M. F. (1999) Atypical handedness in schizophrenia: some methodological and theoretical issues. Schizophrenia Bulletin, 25, 63-78.[Medline]
Snodgrass, J. G. & Corwin, J. (1988) Pragmatics of measuring recognition memory: applications to dementia and amnesia. Journal of Experimental Psychology, 117, 34-50.
Stough, C., Brebner, J., Nettlebeck, T., et al (1996) The relationship between intelligence, personality and inspection time. British Journal of Psychology, 87, 255 -268.
Szádóczky, E., Rózsa, S., Patten, S.,et al (2003) Lifetime patterns of depressive symptoms in the community and among primary care attenders: an application of grade of membership analysis. Journal of Affective Disorders, 77, 31-39.[CrossRef][Medline]
Toommey, R., Faraone, S. V., Seidman, L. J., et al (1988) Association of neuropsychological vulnerability markers in relatives of schizophrenic patients. Schizophrenia Research, 31, 89 -98.[CrossRef]
Tyler, M., Diamond, J. & Lewis, S. (1995) Correlates of left-handedness in a large sample of schizophrenic patients. Schizophrenia Research, 18, 37-41.[CrossRef][Medline]
White, M. (1996) Interpreting inspection time as a measure of the speed of sensory processing. Personality and Individual Differences, 20, 351 -363.[CrossRef]
Wing, J. K., Babor, T., Brugha, T., et al (1990) SCAN. Schedules for Clinical Assessment in Neuropsychiatry. Archives of General Psychiatry, 47, 589 -593.[Abstract]
Woodbury, M. A., Clive, J. & Garson, A. (1978) Mathematical typology: a grade of membership technique for obtaining disease definition. Computers and Biomedical Research, 11, 277 -298.[CrossRef]
Woodbury, M. A. & Manton, K. G. (1982) A new procedure for analysis of medical classification. Methods of Information Medicine, 21, 210 -220.
World Health Organization (1992) International Statistical Classification of Diseases and Related Health Problems (ICD10). Geneva: WHO.
Zachary, R. A. (1986) Shipley Institute of Living Scale, Revised Manual. Los Angeles, CA: Western Psychological Services.
Zachary, R. A., Crumpton, E. & Spiegel, D. E. (1985) Estimating WAISR IQ from the Shipley Institute of Living Scale. Journal of Clinical Psychology, 41, 532 -540.
Received for publication June 23, 2004. Revision received November 9, 2004. Accepted for publication November 17, 2004.
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