Contralateral Delay Activity Indexes Working Memory Storage, Not the Current Focus of Spatial Attention

Abstract

Contralateral delay activity (CDA) has long been argued to track the number of items stored in visual working memory (WM). Recently, however, Berggren and Eimer [Berggren, N., & Eimer, M. Does contralateral delay activity reflect working memory storage or the current focus of spatial attention within visual working memory? Journal of Cognitive Neuroscience, 28, 2003-2020, 2016] proposed the alternative hypothesis that the CDA tracks the current focus of spatial attention instead of WM storage. This hypothesis was based on the finding that, when two successive arrays of memoranda were placed in opposite hemifields, CDA amplitude was primarily determined by the position and number of items in the second display, not the total memory load across both displays. Here, we considered the alternative interpretation that participants dropped the first array from WM when they encoded the second array because the format of the probe display was spatially incompatible with the initial sample display. In this case, even if the CDA indexes active storage rather than spatial attention, CDA activity would be determined by the second array. We tested this idea by directly manipulating the spatial compatibility of sample and probe displays. With spatially incompatible displays, we replicated Berggren and Eimer's findings. However, with spatially compatible displays, we found clear evidence that CDA activity tracked the full storage load across both arrays, in line with a WM storage account of CDA activity. We propose that expectations of display compatibility influenced whether participants viewed the arrays as parts of a single extended event or two independent episodes. Thus, these findings raise interesting new questions about how event boundaries may shape the interplay between passive and active representations of task-relevant information.

Figure 1

Illustration of congruent and incongruent probe displays in sequential encoding change detection tasks. (A) M1 and M2 show memory arrays from memory displays used in Berggren and Eimer (2016; note that distractors are not shown here to clarify the point about the arrangement of targets). (B) Probe displays can be spatially congruent, that is, they are a 1:1 combination of M1 and M2 without spatial translation, similar to the ones used in this study (left) or they can be arranged in a square or diamond pattern close to the fixation, as used by Berggren and Eimer (right). Note that the latter manifests a spatial translation from memory to probe displays.

Figure 2

(A) Stimuli from the six Load Change × Side Switch conditions. The total WM load was always four items in all conditions. Memory items were successively added in M1 and M2 (1 + 3, 2 + 2, 3 + 1), and M1 and M2 items were presented in the same hemifield (top row) or in opposite hemifields (bottom). (B) Trial procedure for Experiment 1 and 2. Participants had to remember colored circles from two displays, M1 and M2, and ignore gray circles. All displays were identical in Experiments 1 and 2, except for the probe display. The probe display showed the combined memory items from M1 and M2 in a spatially congruent manner (Experiment 1) or (C) with interspersed positions (Experiment 2). Participants had to indicate whether all colors did not change from M1/M2 to the probe display (no change trials, top row) or whether one item changed (change trials, bottom row). (D) Spatial arrangement of probe displays in Experiments 1 and 2 in relation to memory displays M1 and M2. In Experiment 1, the probe display had the same spatial layout as M1 and M2, that is, no item position changed (top). In Experiment 2, the probe display comprising interspersed half circles from M1 and M2. This means that probes in M1 or M2 that appeared on the left (dotted circles, top) reappeared in the center and equally likely in the left or right hemifield (dotted circles, bottom).

Figure 3

Grand-averaged difference waves (contra–ipsi) for electrode pool of PO7 and PO8. Results for Experiments 1 and 2 are shown in the left column and right column, respectively. M1 and M2 denote the onset of the first memory display (0 msec) and the second memory display (700 msec). In both Experiments 1 and 2, there were 2 × 3 conditions. Rows show the Side Change conditions separately: The top row shows trials in which both memory displays M1 and M2 appeared on the same side (M1 and M2 both in left or both in right hemifield), and the bottom row shows trials in which memory displays appeared on opposite sides (M1 in left and M2 in right hemifield or vice versa). The colors in each panel code the Load Change condition: Pink lines show trials in which M1 shows one item and M2 shows three items; blue lines show trials in which M1 and M2 show two items each; green lines show trials in which M1 shows three items and M2 shows one item. Waveforms are filtered with a 30-Hz low pass filter for display purposes.

Figure 4

Model for contributions of M1 and M2 to CDA amplitude after M2 (1000–1400 msec) for Experiment 1 (top) and Experiment 2 (bottom). The projected CDA amplitude (empty bars) was calculated as a weighted sum of the grand-averaged CDA during the first retention interval (300–700 msec), for example, the projected CDA for “3+1” in Experiment 2 is 0.54 × CDA amplitude for Load 3 during Retention Interval 1+ 0.98 × CDA amplitude for Load 1 during Retention Interval 1. The observed CDA amplitude (filled bars) is shown for comparison. The closer empty and filled bars, the better the model prediction for that condition. Error bars indicate standard errors of the mean. There was no significant difference between the projected and observed CDA amplitude for any condition.