Control of working memory content in schizophrenia

Abstract

People with schizophrenia (PSZ) exhibit signs of reduced working memory (WM) capacity. However, this may reflect an impairment in managing its content, e.g. preventing irrelevant information from taking up available storage space, rather than a true capacity reduction. We tested the ability to eliminate and update WM content in 38 PSZ and 30 healthy control subjects (HCS). Images of real-world objects were presented consecutively, and a tone cued the item most likely to be tested for memory. On half the trials, randomly intermixed, a second tone occurred. Participants were informed that the item cued by the second tone was now the most likely to be tested, and the item cued by the first tone now the least likely, providing incentive to eliminate the first cued item from WM. Both HCS and PSZ displayed a robust performance advantage for cued items. Unexpectedly, PSZ more efficiently removed the no-longer-essential item from WM than HCS. The magnitude of the WM clearance of this first cued item correlated with memory performance for the newly prioritized second cued item in PSZ, indicating that it was adaptive. However, WM clearance was not associated with WM capacity, ruling out the need to budget limited resources as an explanation for greater clearance in PSZ. A robust correlation between WM clearance and poverty of speech in PSZ instead suggests that the propensity to rapidly clear non-essential information and minimize the number of items in WM may be the reflection of a negative symptom trait. This finding may reflect a more general tendency of PSZ to focus processing more narrowly than HCS.

Figure 1

An example of a single task trial. The object stimuli are shown enlarged relative to the size of the screen to be discernible in the figure. The size of the stimuli in the probe array was identical to that in the encoding array.

Figure 2

Average response accuracy (±SEM) of 38 people with schizophrenia (PSZ) and 30 healthy control subjects (HCS) for each tested object position on 1-cue trials (A) and 2-cue trials (B). Chance performance would be 25%, given that participants had to choose the correct picture from among four choices. The arrows mark the cued objects. * P<0.01, ** P<0.001, paired t-tests comparing accuracy for the cued object to accuracy averaged over the immediately preceding and subsequent object.

Figure 3

Relationship between working memory clearance of the deprioritized position-4 item (quantified as the difference in position-4 accuracy between 1-cue and 2-cue trials) and response accuracy for the newly prioritized position-6 item in 2-cue trials. Working memory clearance was associated with position-6 accuracy in people with schizophrenia but not in healthy control subjects.

Figure 4

Relationship between the working memory clearance effect and the Poverty of Speech item of the Scale for the Assessment of Negative Symptoms (SANS) in 38 people with schizophrenia.