Reduced capacity but spared precision and maintenance of working memory representations in schizophrenia

Abstract

Context: Working memory deficits are considered a core feature of schizophrenia. Several recent integrative articles have offered mechanistic computational and neurobiological models of the origins of this cognitive deficit.

Objective: To test predictions of these models using a new experimental paradigm from the basic science literature that makes it possible to determine whether patients with schizophrenia show (1) deficits in working memory storage capacity, (2) deficits in the precision of working memory representations, and (3) an amplification of these deficits as the retention interval increases.

Design: Case-control design. All subjects performed a color working memory test in which they were asked to recall 3 or 4 items after a 1- or 4-second delay. All subjects also received a standard measure of intelligence and the Matrics Consensus Cognitive Battery.

Setting: A tertiary care research outpatient clinic. Patients A total of 31 clinically stable patients with a DSM-IV diagnosis of schizophrenia or schizoaffective disorder and 26 healthy volunteers participated. The 2 groups were similar in age, sex, and ethnicity distribution.

Main outcome measures: (1) The number of items stored in working memory and (2) the precision of the working memory representations.

Results: Patients showed a clear reduction in the number of items stored in working memory. Patients did not differ from controls in the precision of their working memory representations. There was no evidence of delay-related amplification of impairment in either capacity or precision.

Conclusions: Patients do not show the type of imprecision or delay-dependent amplification of impairment that is predicted on the basis of current models of the neurobiology of schizophrenia. The models need to be revised to account for a pure reduction in the number of items that patients are able to store in working memory.

Figure 1

A. Stimuli from the color recall task. B. Model of performance. When the probed item is present in memory, the reported color is most likely to be at the original value, and the probability declines with distance from the original value. When the probed item is absent from memory, subjects report a randomly selected color, which adds a constant offset to the distribution of responses. The precision of the memory, when a memory was present, is inversely related to the width of the bell-shaped portion of the distribution, which is quantified as the standard deviation (SD). The height of the tails reflects the probability that the probed item was absent from memory, and the probability that the probed item was in memory (Pm) is 1 minus the probability that the probed item was absent from memory.

Figure 2

Observed distribution of recall error (difference between original value and reported value; symbols) and model fits (lines) for the sensory control task, the 1-second delay condition, and the 4-second delay condition).

Figure 3

The number of task stimuli represented in short-term memory (K) and the precision of the representations (SD) in patients and control subjects in the sensory control task (Sensory), and the color recall task at 1 s delay (Short) and 4 s delay (Long). The graphs represent averages (± SEM) over 31 patients with schizophrenia and 26 control participants.

Figure 4

Scatterplots and regression lines showing the relationship between WM capacity from the color wheel task (K, averaged over delays and set sizes) and MATRICS battery Total T score (left column) and IQ scores from the WASI (right column). Controls are shown in the top row, patients in the bottom row.