Decomposing Working Memory in Recurrent Major Depression: Impaired Encoding and Limited Maintenance Immune-to-Encoding Constraint

Abstract

It is generally believed that working memory (WM) is dysfunctional in depression. However, whether this impaired performance originates from impaired encoding, maintenance or both stages is still unclear. Here, we aimed to decompose the abnormal characteristics of encoding and maintenance in patients with recurrent major depressive disorder (MDD). Thirty patients and thirty-nine healthy controls completed a spatial working memory task where the encoding time and the retention time could vary under different load levels. Encoding performance was assessed by comparing accuracies between short and long encoding times, and maintenance performance was assessed by comparing accuracies between short and long retention times. The results show a lower performance in depression than the controls. However, while the decreased accuracy by long retention (vs. short retention) was increased by a short encoding time in the control group, the retention performance of the depression group did not further suffer from the short encoding time. The generally impaired encoding, together with limited maintenance of immunity against the constrained encoding time, suggests a common bias for fixed internal processing over external processing in recurrent MDD. The paradigm provided in this study can be a convenient and efficient clinical test for assessing the WM encoding and maintenance function.

Keywords: encoding; load; maintenance; recurrent major depression; working memory.

Figure 1

Stimuli (a) and the workflow of an example trial (b). Memory load was manipulated by varying the number of shape pairs. The circle and white arrow in the right–bottom cell is illustrated to show the correct response for the current trial but was not presented in the experiment.

Figure 2

Accuracies (a) and mean reaction times (RTs) (c) with standard errors shown as a function of encoding time and retention time for each group. The difference in accuracy between short and long retention (b), and the difference in RT between long retention and short retention (d) with standard errors shown as a function of the encoding time for each group. Encode−S: short time for stimulus encoding; Encode−L: long time for stimulus encoding; Interval−S: short retention interval; Interval−L: long retention interval.