Contralateral Delay Activity Tracks Fluctuations in Working Memory Performance

Abstract

Neural measures of working memory storage, such as the contralateral delay activity (CDA), are powerful tools in working memory research. CDA amplitude is sensitive to working memory load, reaches an asymptote at known behavioral limits, and predicts individual differences in capacity. An open question, however, is whether neural measures of load also track trial-by-trial fluctuations in performance. Here, we used a whole-report working memory task to test the relationship between CDA amplitude and working memory performance. If working memory failures are due to decision-based errors and retrieval failures, CDA amplitude would not differentiate good and poor performance trials when load is held constant. If failures arise during storage, then CDA amplitude should track both working memory load and trial-by-trial performance. As expected, CDA amplitude tracked load (Experiment 1), reaching an asymptote at three items. In Experiment 2, we tracked fluctuations in trial-by-trial performance. CDA amplitude was larger (more negative) for high-performance trials compared with low-performance trials, suggesting that fluctuations in performance were related to the successful storage of items. During working memory failures, participants oriented their attention to the correct side of the screen (lateralized P1) and maintained covert attention to the correct side during the delay period (lateralized alpha power suppression). Despite the preservation of attentional orienting, we found impairments consistent with an executive attention theory of individual differences in working memory capacity; fluctuations in executive control (indexed by pretrial frontal theta power) may be to blame for storage failures.

Figures

Figure 1

Trial events in Experiment 1 and Experiment 2. Trial events are depicted from left to right. In Experiment 1, the memory array could contain one, three, or six items. In Experiment 2, the memory array always contained six items.

Figure 2

Behavioral performance in Experiment 1 (A) and Experiment 2 (B). Error bars represent 1 SEM.

Figure 3

CDA (A) and lateralized alpha power (B) as a function of set size in Experiment 1. Shaded error bars represent 1 SEM. During the delay period (400–1500 msec), CDA and lateralized alpha power both tracked memory load.

Figure 4

CDA (A) and lateralized alpha power (B) as a function of accuracy in Experiment 2. Shaded error bars represent 1 SEM. During the delay period (400–1500 msec), CDA tracked variability in memory performance but lateralized alpha power did not.

Figure 5

Bar graph of the lateralized ERP components in Experiment 2. The lateralized P1 (A) and N2PC (B) components were significantly larger than 0 but did not differ as a function of trial accuracy, indicating that participants attended the correct side of the memory array even during poor performance trials. On the other hand, CDA amplitude (C) was smaller for poor performance trials, indicating that participants successfully maintained fewer items. Error bars represent 1 SEM.

Figure 6

Frontal theta power as a function of trial accuracy in Experiment 2. Frontal theta power tracked trial-by-trial fluctuations in working memory performance, both during the pretrial period (−500 to 0 msec) and during the retention interval (400–1500 msec). Shaded error bars represent 1 SEM.

Figure 7

Individual differences in set size 6 CDA amplitude indexed behavior. Set size 6 CDA amplitude correlated both with performance on set size 6 trials during the EEG recording (left) as well as for a separate change detection estimate of capacity (right). The correlation includes all participants from Experiments 1 and 2 with at least 100 set size 6 trials (n = 72).