Each regressor of interest was convolved with the canonical hemodynamic response function followed by a time derivative. Cognitive subtraction (Price and Friston 1997; Nichols
et al. 2005) was used to contrast brain activation to all DD task trials>SMC trials, hard>easy trials, and easy>hard trials to produce statistical parametric contrast images to be carried into second-level analyses. Inhibitors,research,lifescience,medical We also included analyses of hard trials>baseline and easy trials>baseline (baseline being the period a fixation cross appeared between each pair of choices) for between-group comparisons. Individual-participant general linear models were created to estimate parameters for the contrasts of task trials versus SMC trials and contrasts related to difficulty. One-way repeated-measures ANOVAs
were used for within-group analyses (consistent SZ and consistent HC). Inhibitors,research,lifescience,medical Subsequently, groups were compared using a two-way (group x trial category) ANOVA. Cluster size was defined as the number of contiguous voxels for which P < 0.05, uncorrected, except for within-group contrasts of task>SMC for which P < 0.001, uncorrected, for all voxels in a cluster. Cluster-size threshold was defined within SPM8 on the basis of Gaussian random-field theory to maintain the false discovery rate (FDR) = 0.05 (Genovese et al. 2002; Chumbley and Friston 2009). Results Behavioral and clinical Inhibitors,research,lifescience,medical assessments When data from the magnet DD task were analyzed, the resulting distribution of R2 values from SZ participants was approximately bimodal (Fig. (Fig.2),2), with a majority showing high Inhibitors,research,lifescience,medical values and the remainder showing very low values, suggesting an inability to make consistent choices. Three HC also had low R2 (Fig. (Fig.2).2). As we did in a previous study (R. E. Weller, K. B. Avsar, J. E. Cox, M. A. Reid, D. M. White, A. C. Lahti, unpubl. ms.), we set a criterion of R2 > 0.60 to define consistent performance in order to analyze fMRI data from HC and SZ matched on consistent performance on the DD magnet task. As described below, this also resulted in the groups being very similar on rate of Inhibitors,research,lifescience,medical discounting. Figure 2 Individual model PAK6 fit
(R2) values during estimation of k values for healthy controls (HC) and patients with schizophrenia (SZ). The line at 0.60 indicates the minimum R2 value that was used to define consistent performance. Overall, data from seven of 21 controls were not included in the imaging analyses: in addition to the three who were inconsistent on the DD task, two exceeded movement criteria, one did not respond to the SMC trials, and one was unable to tolerate the scanner (i.e., was claustrophobic). Data from 16 of 35 SZ were excluded based on performance on the magnet DD task: nine on the basis of R2 < 0.60 and seven because their responses were nearly all (>85%) click here choices of either immediate or DRs, suggesting lack of engagement in the decision making task.