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Correspondence |
panielPsychiatric Center Prague, Ústavní 91, 181 03 Prague 8, Czech Republic.
Institute of Clinical and Experimental Medicine (IKEM), Magnetic Resonance Unit, Prague
Psychiatric Center, Prague
Institute of Clinical and Experimental Medicine (IKEM), Magnetic Resonance Unit, Prague
Correspondence: E-mail: spaniel{at}pcp.lf3.cuni.cz
Dr Sommer and colleagues (2004) reported decreased language lateralisation measured with functional magnetic resonance imaging (fMRI) in 12 monozygotic twin pairs discordant for schizophrenia compared with 12 healthy monozygotic twin pairs. The authors did not find significant differences in language lateralisation between affected twins and their co-twins without schizophrenia. In the December 2003 issue of the Czech peer-reviewed psychiatric journal Psychiatrie, we published preliminary data from a study (supported by grant NF 6794-3/2001 from the Internal Grant Agency of the Czech Republic) that examined hemispheric dominance for language processing by means of fMRI in four monozygotic twin pairs discordant for schizophrenia. Although the activation paradigm (a verbal fluency task) differed from the one employed by Dr Sommer et al, the lateralisation index was calculated according to the same method within identical volumes of interest. The results indicated that language processing was significantly less lateralised in affected twins compared with their co-twins without schizophrenia (P<0.05, Wilcoxon signed ranks test, robustness assessed by analysis of 10 000 Monte Carlo permutations; mean laterality index 0.90 (s.d.=0.12) for unaffected twins and 0.73 (s.d.=17) for affected twins). There were no statistical differences in the laterality index during the verbal fluency paradigm between unaffected twins from the discordant monozygotic twin pairs and the four control monozygotic twin pairs (unpublished data). The explanation of the discrepancies could lie in the participants enrolled in our study. Since the aim of our work was to assess relative contribution of non-genetic factors in previously reported decreased language lateralisation in schizophrenia, the exclusion criterion was (in contrast to Dr Sommer’s study) any family history of schizophrenia or other major psychiatric disorder. This particular study strategy allowed selection of an extreme population presumably represented by sporadic forms of the disease. In addition, stringent diagnostic criteria were used in that only participants with schizophrenia were enrolled in the study. The occurrence of psychiatric disorders in co-twins without schizophrenia and the fact that the participants were not controlled for family history of psychosis suggest a substantial degree of genetic predisposition for schizophrenia in unaffected co-twins expressed as overall decrease in language lateralisation within the discordant twin group studied by Dr Sommer and her colleagues.
REFERENCES
Sommer, I. E. C., Ramsey, N. F., Mandl, R. C.W., et al
(2004) Language activation in monozygotic twins discordant
for schizophrenia. British Journal of Psychiatry,
184, 128
–135.
paniel, F., Tintera, J., Hájek, T., et
al (2003) Language lateralization in monozygotic twins
discordant for schizophrenia. Evidence from functional MRI.
Psychiatrie, 4, 301
–303.
Department of Neuroscience, University Medical Center Utrecht, Heidelberglaan 100, 3584 CX Utrecht, The Netherlands.
Correspondence: E-mail: I.Sommer{at}azu.nl
We read with interest the results of the study by Dr paniel et
al. In parallel to our findings, they reported decreased language
lateralisation in (twin) patients with schizophrenia compared with healthy
(twin) controls. However, they did not report whether the decreased
lateralisation in the patients resulted from increased activation of the right
hemisphere, or from decreased activation of the left hemisphere.
This is an essential point, since decreased activation of frontal, temporal and parietal language areas in the left hemisphere of schizophrenia patients is frequently associated with decreased task performance (as reported by Artiges et al, 2000). Increased language-related activation of right cerebral areas, in contrast, may reflect a failure to establish cerebral dominance, which may be a genetic predisposition to develop schizophrenia.
In our study (Sommer et al, 2004), the language tasks employed were selected to be very simple in order not to cause a difference in performance between patients and healthy subjects. Left hemispheric language activation was not lower in patients than in their co-twins, which, in our opinion, reflects equal task performance.
In the paniel et al study, a verbal fluency task was
employed, which is known to generate a difference in performance between
schizophrenia patients and controls, and generally yields decreased activation
of left frontal areas in patients (Curtis
et al, 1999). This may explain why paniel et
al found lower lateralisation in patients compared with their
co-twins.
paniel et al mentioned that selection of co-twins without
schizophrenia and of control pairs may have caused the difference between
their results and ours, since the control twin pairs in their sample were
selected not to have relatives with schizophrenia. This was, however, also the
case in our sample. The second point of difference raised by paniel
et al is that the co-twins in their study had no psychiatric
disorder. However, in our article we described an additional analysis
comparing twins with schizophrenia with their co-twins after exclusion of all
pairs from which the co-twins had psychiatric pathology, which yielded the
same results as the analysis including the entire sample.
In sum, we find Dr paniel et al’s study an
interesting contribution; in our opinion it is differences in the language
activation tasks, rather than differences in sample selection, that are the
cause of the differences in outcome between the studies.
REFERENCES
Artiges, E., Martinot, J. L., Verdys, M., et al
(2000) Altered hemispheric functional dominance during word
generation in negative schizophrenia. Schizophrenia
Bulletin, 26, 709
–721.
Curtis, V. A., Bullmore, E. T., Morris, R. G., et al (1999) Attenuated frontal activation in schizophrenia may be task dependent. Schizophrenia Research, 37, 35 –44.[CrossRef][Medline]
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