J Psychiatry Neurosci 2010;35(1):7-17
W.J. Speechley, J.C. Whitman, T.S. Woodward
Speechley, Whitman, Woodward — Department of Psychiatry, University of British Columbia; Whitman, Woodward — BC Mental Health and Addictions Research Institute, Vancouver, BC
Background: Previous schizophrenia research involving the “beads task” has suggested an association between delusions and 2 reasoning biases: (1) “jumping to conclusions” (JTC), whereby early, resolute decisions are formed on the basis of little evidence and (2) over-adjustment of probability estimates following a single instance of disconfirmatory evidence. In the current study, we used a novel JTC-style paradigm to provide new information about a cognitive operation common to these 2 reasoning biases.
Methods: Using a task that required participants to rate the likelihood that a fisherman was catching a series of black or white fish from Lake A and not Lake B, and vice versa, we compared the responses of 4 groups (healthy, bipolar, nondelusional schizophrenia and delusional schizophrenia) when we manipulated 2 elements of the Bayesian formula: incoming data and prior odds.
Results: Regardless of our manipulations of the Bayesian formula, the delusional schizophrenia group gave significantly higher likelihood ratings for the lake that best matched the colour of the presented fish, but the ratings for the nonmatching lake did not differ from the other groups.
Limitations: The limitations of this study include a small sample size for the group of severely delusional patients and a preponderance of men in the schizophrenia sample.
Conclusion: Delusions in schizophrenia are associated with hypersalience of evidence–hypothesis matches but normal salience of nonmatches. When the colour of the incoming data is uniform (fish of only one colour), this manifests as JTC early in a series, and when the colour of incoming data varies (both black and white fish), this manifests as an overadjustment midseries. This account can provide a unifying explanation for delusion-associated performance patterns previously observed in the beads task in schizophrenia.
Acknowledgments: The authors thank Ligaya Allmer, Tonya Kragelj, David Mitenko, Manuel Munz, Izabella Patyk, Rachel Richards and Jennifer Scott for assistance with data collection, task design and data management.
Funding: This study was funded by a Canadian Institutes of Health Research Operating grant (MOP-64431; FRN 64431 to T.S.W.), a Canadian Institutes of Health Research New Investigator Award (T.S.W.), a Michael Smith Foundation for Health Research Scholar Award (T.S.W.), a NARSAD Young Investigator Award (T.S.W.), a Mind Foundation of BC Operating Grant (T.S.W.), a BC Mental Health and Addictions Services Seed Grant (T.S.W.), a Michael Smith Foundation for Health Research Senior Trainee Award (W.J.S.), a Canadian Institutes of Health Research and Mind Foundation of BC Doctoral Research Award in the area of Schizophrenia and Psychosis (W.J.S.), a Natural Sciences and Engineering Research Council of Canada Post-Graduate Scholarship Award (J.C.W.) and a Michael Smith Foundation for Health Research Senior Trainee Award (J.C.W.).
Competing interests: None declared.
Contributors: Dr. Woodward designed the study and acquired the data. All authors analyzed the data, wrote and reviewed the article and approved its publication.
Submitted Mar. 5, 2009; Revised Jul. 12, Aug. 24, 2009; Accepted Aug. 25, 2009.
Correspondence to: Dr. T.S. Woodward, BC Mental Health and Addictions Research Institute, Translational Research Building, 3rd Floor, Room A3-A116, 938 W. 28th Ave., Vancouver BC V5Z 4H4; fax 604 875-3871; Todd.S.Woodward@gmail.com; www3.telus.net/Todd_S_Woodward