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Neural correlates of formal thought disorder
- Lena Palaniyappan (19 February 2009)
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Re: Neural correlates of formal thought disorder
- Helge Horn, Andrea Federspiel (17 April 2009)
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Lena Palaniyappan, Academic Clinical Fellow Division of Psychiatry, Newcastle University, NE1 4LP
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Lena.Palaniyappan{at}ncl.ac.uk Lena Palaniyappan
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Horn et al (2009) investigate a very important, somewhat underexplored area of neural correlates of schizophrenic speech disturbance. Given the probability of underlying deficits in contextual integration and theory of mind, formal thought disorder (FTD) yields a fertile ground for structural and functional connectivity analysis in schizophrenia. While the use of hitherto unused techniques such as resting perfusion scan to study FTD must be lauded, the results of this preliminary study must be treated with caution for various reasons. The composite score of Thought, Language and Communication scale(TLC) has been used as a measure of severity of FTD. The authors have administered the scale 45 minutes before the scanning procedure for each subject. It is widely perceived that uncontrolled generation of thought is required to reliably measure FTD in schizophrenia. TLC itself lacks a standardised practical method of eliciting such thought flow in contrast to some recently developed instruments (Liddle, Ngan, Caissie, et al,2002) . The cross-sectional use of TLC to measure severity of FTD must be treated with prudence. The authors extract components from Positive and Negative Syndrome Scale (PANSS) using factor analysis and demonstrate that none of these components correlate with severity of FTD as measured by total score of TLC. The validity of factor analysis in such a small sample is questionable and not in synchrony with available factorial structures of PANSS (Fitzgerald, de Castella, Brewer, et al, 2003). As a result, all principal components extracted were from negative symptoms in PANSS (except the conceptual disorganisation item, which was rightly excluded from further analysis). Consequently the results only show a lack of correlation between severity of FTD and negative symptoms as measured by PANSS. The MRI findings may still be explained by positive symptoms alone and not by FTD. Lastly, the pervasive issue of sample size in neuroimaging studies becomes more prominent when correlation analyses are attempted in whole brain analyses. Fitzgerald, P. B., de Castella, A. R., Brewer, K., et al (2003) A confirmatory factor analytic evaluation of the pentagonal PANSS model. Schizophrenia Research, 61, 97-104. Horn, H., Federspiel, A., Wirth, M., et al (2009)Structural and metabolic changes in language areas linked to formal thought disorder. The British Journal of Psychiatry, 194, 130-138. Liddle, P. F., Ngan, E. T. C., Caissie, S. L., et al (2002) Thought and Language Index: an instrument for assessing thought and language in schizophrenia. The British Journal of Psychiatry, 181, 326-330. Declaration of interest: None |
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Helge Horn, consultant psychiatrist University Hospital of Psychiatry, Bern , Switzerland, Andrea Federspiel
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horn{at}puk.unibe.ch Helge Horn, et al.
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We welcome interest in our study of structural and metabolic changes in language areas linked to formal thought disorder (FTD). Palaniyappan suggest to use the Thought and Language Index (TLI) instead of the Thought, Language and Communication scale (TLC) in order to quantify FTD. The TLI, she argues, would have the advantage over the TLC of a standardized method of eliciting thought flow. Unfortunately, the TLI was established after the start of our study (Liddle, P. F., Ngan, E. T., Caissie, S. L. et al, 2002) whereas the TLC was an established instrument that has been successfully used in numerous studies as a reliable instrument to quantify FTD (Andreasen, NC and Grove, W, 1986). We agree that future studies might benefit from the application of the more standardized TLI. However, the distribution of the severity of FTD in the patient group should not change substantially, just by changing from TLC to TLI. Therefore, we regard our results as reliable. Furthermore Palaniyappan criticizes the factor analysis of the PANSS in small patient samples used in neuroimaging studies. She is right that a factor analytic approach in such small samples is critical, if the patients group is randomly selected. In order to be able to study FTD with neuroimaging techniques we recruited a specifically selected patient sample, which manly differs in the severity of FTD and barely in other psychopathological categories. To obtain such selected sample the recruitment took years. Patients were matched for all other psychopathology beside the items of FTD of the PANSS as good as possible. The factor analysis and the correlation of the factors with the severity of FTD were only used to document this special patient selection. Therefore, it is no surprise, that the factors of our factor analysis do not match the factors of unselected sample of schizophrenic patients (Fitzgerald, P. B., de Castella, A. R., Brewer, K. et al, 2003). To study specific psychopathological phenomena like FTD by means of Neuroimaging, such a specific selection of patients is necessary to extract reliable results. In our patient group we did not observe any significant correlation between FTD and i) positive symptoms (without PANSS item P2): r=0.39; p=0.19, and ii) negative symptoms (without PANSS item N5): r= 0.04; p= 0.90. We therefore can exclude that our results are due to positive or negative symptoms in general. Concerning the issue of the sample size in neuroimaging studies our results were corrected for multiple comparisons. This approach is a common and accepted way to handle this problem in neuroimaging studies in general. Andreasen, NC and Grove, W (1986) Thought, language and communication in schizophrenia: diagnosis and prognosis. schizophr.Bull., 12, 348-359. Fitzgerald, P. B., de Castella, A. R., Brewer, K., et al (2003) A confirmatory factor analytic evaluation of the pentagonal PANSS model. Schizophr.Res., 61, 97-104. Liddle, P. F., Ngan, E. T., Caissie, S. L., et al (2002) Thought and Language Index: an instrument for assessing thought and language in schizophrenia. Br.J.Psychiatry, 181, 326-330. Declaration of interest: None |
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