Contralateral delay activity provides a neural measure of the number of representations in visual working memory

Abstract

Visual working memory (VWM) helps to temporarily represent information from the visual environment and is severely limited in capacity. Recent work has linked various forms of neural activity to the ongoing representations in VWM. One piece of evidence comes from human event-related potential studies, which find a sustained contralateral negativity during the retention period of VWM tasks. This contralateral delay activity (CDA) has previously been shown to increase in amplitude as the number of memory items increases, up to the individual's working memory capacity limit. However, significant alternative hypotheses remain regarding the true nature of this activity. Here we test whether the CDA is modulated by the perceptual requirements of the memory items as well as whether it is determined by the number of locations that are being attended within the display. Our results provide evidence against these two alternative accounts and instead strongly support the interpretation that this activity reflects the current number of objects that are being represented in VWM.

Fig. 1.

Experiment 1. A: trial schematics for high contrast (top) and low contrast (bottom) conditions, set size 4. Trials were intermixed. B: luminance of colored squares and background. C: accuracy for high contrast (◇) and low contrast (■) conditions. Average working memory capacities (K) across subjects were 2.5 and 1.71 for high and low contrast conditions, respectively, and difference was significant paired t-test, t(14) = 6.763, P < 0.001.

Fig. 2.

Event-related potential (ERP) data from experiment 1, time-locked to the onset of the memory array. Posterior lateral recording sites (OL/R, T5/6, PO3/4) are shown. Purple and black lines are data from contralateral and ipsilateral sites, respectively. Negative is plotted up.

Fig. 3.

Difference wave averaged across 3 channels shown in Fig. 2. A: averaged difference wave across time for high (left) and low (right) contrast conditions. Set size 2 and 4 are shown in black and blue, respectively. In both contrast conditions, N2pc followed by contralateral delay activity (CDA) are visible. B: N2pc amplitude in the time window between 200 and 280 ms after the memory array onset. Error bars = 95% confidence intervals. Notice there is a significant difference between contrasts (P < 0.001) but not between set sizes. C: CDA amplitude in the time window between 300 and 900 ms after the memory array onset. There is a significant difference between set sizes (P < 0.02) but not between contrasts.

Fig. 4.

experiment 2. A: trial schematics for “same location” trial. B: ERP data from trials in which 4 items were remembered sequentially, either at the same location (red) or different location (blue) as the 1st memory array. Time-locked to the onset of memory array 1. C: CDA amplitudes 300–500 ms following the 1st and 2nd memory array. Mean CDA amplitudes from simultaneous 2- and 4-item conditions are shown in dashed lines. Error bars = 95% confidence intervals. Significant main effect of time window was found (1st delay vs. 2nd delay, P < 0.001), but no effect of locations was found. Regardless of the location, CDA amplitude after the second delay was statistically not different from simultaneous presentation of 4 items.