Working-memory signal changes sides as remembered objects cross the midline

A human electroencephalography study found that a brain signal associated with visual working memory changed its hemispheric bias as people mentally moved remembered objects across the vertical midline. The signal, called contralateral delay activity, or CDA, shifted from one side to the other when two colored discs were relocated into the opposite visual hemifield and shifted back when they returned. The pattern was consistent with a transfer of working-memory-related activity between cerebral hemispheres, although the scalp recordings did not directly establish that the remembered information itself crossed hemispheres.

Participants viewed four colored discs, two on each side of a central fixation point, while a cue identified the two colors they should retain. The discs then became colorless outlines and rotated along an invisible circle through unpredictable angles. Because the direction and distance of motion were not known in advance, accurate performance required participants to update the locations of the two items continuously rather than reconstruct their positions only when a question appeared.

At the end, one outline was highlighted, and participants indicated its remembered color by clicking on a color wheel. Behavioral results confirmed that participants generally tracked the objects as they moved. Error relative to the probed disc increased with rotation angle, with a Pearson correlation of 0.88 and p < 0.001, while error relative to the other retained disc decreased, with r = −0.89 and p < 0.001.

Model fits indicated that this pattern was mainly associated with more frequent swaps between the two remembered items. Estimated swap rates increased with movement angle, with r = 0.92 and p < 0.001, whereas modeled memory precision did not change detectably, with r = −0.02 and p = 0.93. Even at the largest rotations, average estimated swap rates were around 10%, and the average memory error on non-swap trials was 16.8 degrees.

The CDA was derived from the difference in EEG activity between electrodes contralateral and ipsilateral to the side on which the relevant objects had originally appeared. It is an electrophysiological correlate of working-memory representations and is sensitive to the number of relevant, rather than irrelevant, visual objects. The researchers organized the trials into five post-movement configurations: both discs remained in the original hemifield; one crossed the midline; both crossed; one crossed and then returned; or both completed a full circuit and returned.

Their primary model predicted that representations moving with the objects would preserve a negative CDA when both ended in the original hemifield, eliminate the lateralized difference when they occupied opposite sides, and produce a positive CDA when both ended in the opposite hemifield. Two alternatives were tested alongside that prediction. One proposed that neural representations remained anchored to their original encoding locations, so the CDA would stay negative after any amount of mental movement, perhaps weakening as memory quality declined.

The other proposed that mental movement transformed the representations into a format no longer tracked by the CDA, so the signal would disappear after a cross-midline update and would not return. The measured CDA followed the primary model closely: it began negative, reversed when both objects crossed into the opposite hemifield, and became negative again when the objects returned. Bayesian analyses favored the hemispheric-transfer model in all five conditions, although evidence for the full-circuit condition was only anecdotal, with a Bayes factor of 2.55.

Quantitative model comparisons favored the transfer interpretation over the anchored-representation account in the three conditions in which their predictions differed. The corresponding Bayes factors were 3.57 when one object crossed, 471.7 when both crossed, and 3.84 when one crossed and returned. A Bayesian mixed-effects comparison assigned 70.9% of its model weight to the transfer account, 18.5% to anchored representations, and 10.6% to the account in which the CDA disappeared after updating.

Within the favored model, estimated CDA effects were −0.56 microvolts when both objects remained in the original hemifield, 0.60 microvolts when both crossed, and −0.33 microvolts after a full circuit; the 95% highest-density intervals were −1.03 to −0.06, 0.10 to 1.06, and −0.72 to −0.001 microvolts, respectively. Pooling the three conditions that involved a midline crossing produced further evidence that the reversal was not confined to a single configuration. A one-sided cluster-based permutation test found two periods in the working-memory experiment when the post-movement CDA was significantly above zero.

Because the pooled data contained approximately equal contributions from the three crossing conditions, this indicated that the reversal was present when both objects crossed, when one crossed and returned, and when both completed a circuit. An exploratory source-level analysis was also compatible with relocation of activity, producing an odds ratio of 14.0 for reversal in the both-crossed condition compared with the conditions in which objects ended in their original hemifield. The researchers then used a second experiment to test how much of the pattern could be attributed to spatial attention.

Participants saw the same rotating outlines but were told to ignore color and track only the positions of the two cued discs. At the probe, they reported whether the highlighted disc had been one of the tracked objects. Accuracy was 94.2% with a standard deviation of 5.1%.

A negative CDA was detectable before movement, demonstrating that attentional tracking alone generated a weaker CDA-like signal, but the post-movement pattern differed quantitatively from the working-memory result. Most notably, the condition in which both objects crossed the midline did not show the positive polarity reversal observed when participants had to retain the colors. Bayesian model weights in the attention experiment were nearly evenly divided between the transfer model, at 52.2%, and the anchored-representation model, at 47.8%, while the disappearing-CDA model received negligible weight.

A hierarchical comparison found that the reversal profile relative to conditions in which objects remained in or returned to their original hemifield averaged 1.03 microvolts in the working-memory experiment and 0.57 microvolts in the attention experiment. The between-experiment difference had a 95% highest-density interval of −0.38 to 1.21 microvolts, although posterior odds of 7.3 to 1 favored a stronger modulation in the working-memory task. The CDA in the both-crossed condition was also larger in the working-memory experiment, with posterior odds of 27.1 to 1 favoring that direction of difference.

Additional analyses suggested that alpha-band oscillations and systematic eye movements could not fully explain the task-dependent CDA effect. Lateralized alpha power shifted with object positions in both experiments, consistent with spatial attention and tracking, but condition-related modulation did not differ meaningfully between tasks: the mean difference was 0.92 microvolts squared, with a 95% highest-density interval of −2.32 to 4.37. Horizontal gaze bias pointed toward the objects’ post-movement locations, but it did not differ significantly between experiments, and gaze bias did not predict CDA amplitude in most tested conditions.

Together, these results support a distinct contribution from working memory, rather than a purely attentional or oculomotor explanation for the CDA reversal. The study has important limitations. Scalp EEG has a low signal-to-noise ratio, and the physiological meaning of the CDA remains debated; it may reflect a mixture of memory, attention, indexing, or other processes rather than stored content alone.

The attention task did not formally prevent participants from partially remembering colors or engaging spatial working memory, so some implicit memory demand could have reduced the difference between the experiments. Direct decoding of object identity from EEG could test whether the hemispheric pattern reflects the remembered content itself. The findings therefore show that lateralized EEG activity dynamically follows mentally updated locations and is stronger when active working memory is required, while remaining consistent with, rather than definitive proof of, interhemispheric transfer of remembered information.

Subject of Research: Human EEG evidence for dynamic hemispheric lateralization during spatial working-memory updates

Article Title: Contralateral delay activity during dynamic spatial updates in working memory

Article References: Chernik, C., van den Berg, R., & Lundqvist, M. (2026). Contralateral delay activity during dynamic spatial updates in working memory. iScience, 29(11), Article 117916. https://doi.org/10.1016/j.isci.2026.117916

Image Credits: AI Generated

DOI: 10.1016/j.isci.2026.117916

Keywords: working memory, contralateral delay activity, electroencephalography, hemispheric transfer, spatial cognition, mental rotation, spatial attention, neural dynamics, human neuroscience, Contralateral, delay, activity