The relationship between working memory capacity and broad measures of cognitive ability in healthy adults and people with schizophrenia

Abstract

Objective: Working memory (WM) capacity, typically measured with cognitively complex span tasks, is correlated with higher order cognitive abilities in healthy adults. The goals of this study were to determine whether a more focused measure of visual WM storage capacity would show similar higher order ability correlations in healthy adults and in people with schizophrenia (PSZ), thereby demonstrating the importance of simple storage capacity; determine whether the illness alters the pattern of correlations across cognitive domains; and evaluate whether between-groups differences in WM capacity could account for the generalized cognitive impairment in PSZ.

Method: Ninety-nine PSZ and 77 healthy controls (HCs) completed a visual WM change-localization task, the Wechsler Abbreviated Scale of Intelligence (WASI), and the MATRICS Consensus Cognitive Battery (MCCB).

Results: PSZ performed more poorly than HCs on all cognitive measures. The between-groups effect size for WM capacity was large (d = 1.11). WM robustly correlated with WASI and MCCB performance, with no significant differences in the magnitude or pattern of correlations across groups. When the groups were pooled, WM capacity correlated at r = .68 with MCCB composite score and at r = .56 with WASI estimated Full Scale IQ. WM capacity accounted for approximately 40% of the between-groups variance across the WASI and MCCB.

Conclusions: A simple measure of WM storage capacity is robustly associated with the higher order cognitive abilities assessed by the WASI and MCCB in HCs and PSZ. WM capacity reduction may be a critical determinant of the general cognitive impairment in PSZ.

Figure 1

Top panel: Time-line of the change localization task. Bottom panel: the distribution of K scores in PSZ and HC. The value under each bar reflects the lower limit of the corresponding K bin, with a bin width of 0.5.

Figure 2

Age-adjusted and age plus K-adjusted MCCB and WASI marginal mean group differences. All differences are calculated from T-scores except for WASI Estimated IQ Score, which is the difference of standard scores, rescaled to have an SD of 10. Adjusted differences are estimated with analysis of covariance models. All p-values for adjusted between group differences were p <0.001.

Figure 3

MATRICS Consensus Cognitive Battery (MCCB) composite score versus K by group, with plotted linear regressions (not adjusted for age) for HCs (composite score = 9.93 + 14.55 × K) and PSZ (composite score = −0.05 + 13.16 × K). Differences between the slopes of the two regression lines are not statistically significant (t=0.42, df=172, p=0.68).