Automatization and working memory capacity in schizophrenia

The STERN task. Each epoch starts with presentation of the target set and is followed by ten probes. Subjects have to press a button as quickly as possible if the probe letter belongs to the set of targets.

Graphs show performance on the STERN task for both groups, during fMRI scans. A: reaction times of correct target responses. B: number of correct responses as percent of all trials (considering both misses and false alarms). C: Variance of reaction time of all correct target responses. Errorbars denote standard error of the mean. Abbreviations: NT novel task, PT practised task, CT control task.

Activity map of 18 patients and 18 controls combined, of the STERN task (NT versus CT contrast), showing WM regions: 1. left fusiform gyrus LFG, 2. left prefrontal cortex LPFC, 3. left superior parietal cortex LSPC, 4. right superior parietal cortex RSPC, 5. anterior cingulate cortex ACC. The numbers in the slices correspond to MNI z-coordinates (Collins et al., 1994). Threshold for significance corresponded to p<0.05 Bonferroni-corrected, with a minimum clustersize of 10 voxels. Slices are in radiological orientation (left side is right hemisphere and vice versa).

Brain activity for the STERN tasks in WM regions depicted in Fig. 3. Results are displayed for each of the tasks (NT, PT and CT) for controls (A) and patients (B) separately. Activity levels (y-axis) are represented by b-values obtained from the GLM analyses of fMRI data. Groups were analysed together (see methods section). Activity was reduced with practise (NT versus PT) in all regions, for both groups. A significant group difference was found in regions 2 (left prefrontal cortex) and 3 (left superior parietal cortex), in the magnitude of activity decline following practise (NT versus PT). Asterisks denote a significant interaction between group and practise, which was due to elevated activity during NT in patients (see methods for details).

Effect of performing two tasks simultaneously (dual task) on performance for the STERN and SAT tasks (dual task performance cost, outside the scanner). On the y-axis the difference in accuracy is shown between single and dual-task performance of STERN and SAT; a high value corresponds to a greater loss of performance. Patients show higher performance cost than controls, especially on SAT. Asterisk denotes a significant difference between patients and controls, on the NT and PT during SAT combined (p<0.05).

Correlation between WM efficiency and capacity. On the y-axis, the difference in activity between NT and PT is shown as a measure of practise-induced neurophysiological efficiency of WM function. A larger value indicates a larger reduction in brain activity, and therefore a greater efficiency. On the x-axis performance cost is displayed, which is computed as the difference in errors between single task performance and dual-task performance, and represents the capacity of the WM system. A larger value corresponds with larger performance decrement in the dual-task, and therefore a smaller capacity. The fitted lines and the corresponding rho values reflect the correlation between efficiency and capacity, computed for patients and controls separately. There was a correlation only in controls. The difference between correlations for the groups was significant (see Methods and materials).