## 2 Materials and Methods
### Stimuli, Experimental Procedure, and Task
This trial is an example of high-visual-working-memory (VWM) capacity load (i.e., four abstract shapes are encoded in VWM), with a Chinese character as a distractor singleton and spatially incompatible distractor-to-memory-probe positions (i.e., the distractor singleton appeared on the top-right quadrant, and the memory probe appeared on the bottom-left quadrant). This is an example of a match memory probe-to-sample task trial (i.e., the abstract shape shown as a memory probe matches one of the abstract shapes presented as part of the sample array). Stimuli are drawn to scale. The distractor singleton as a Chinese character was lateralized and could appear unpredictably in one of four possible quadrants on screen either in a compatible (25% of valid trials) (i.e., spatial ignored repetition) or incompatible (75% of invalid trials) spatial position compared to the incoming lateralized target location, which also appeared unpredictably in one of four quadrants. Thus, in only 25% of trials, the spatial position of the distractor singleton in the prime display became the target's spatial position in the subsequent probe display. These experimental manipulations could lead to either spatial negative priming (SNP) or spatial positive priming (SPP) effects [47; 48]. SNP/SPP represents an inhibitory/faciliatory effect that occurs when shifting attention toward a perceived distractor's spatial position on a prime display, with a subsequent re-orientation of spatial attention, and this results in slower/faster and less/more accurate target detection when it appears at the same distractor stimulus' spatial location compared to elsewhere due to residual inhibition/facilitation being deployed to that location [47; 48]. To keep the target as a single-onset stimulus but clearly demarcate the distractor as a task-irrelevant singleton as part of the distractor array, we presented this singleton distractor (i.e., prime display) in a fixed temporal order of 1000 ms prior to the target onset (i.e., probe display), which is sufficient time to disengage from the distractor [49]. Unlike many previous manipulations of perceptual capacity using the VWM load, which requires a dual-task design (e.g., [29; 50; 51]), the current design is a single-task design and therefore mitigates any additional cognitive loads from maintaining multiple task rules in mind. Before the main part of the experiment, participants completed 20 practice trials to familiarize themselves with the task and adjust to the requirements. These practice trials were repeated until the average accuracy was greater than 60%. Participants completed 512 trials in eight blocks of 64 trials and were allowed to pause in between blocks. The experiment had the following experimental conditions: (1) VWM capacity load, with two levels of randomly distributed trials: low (256 trials) vs. high (256 trials); (2) distractor-to-memory-probe spatial compatibility, where each level of VWM capacity loads had four levels of randomly distributed distractor-to-memory-probe horizontal and vertical spatial compatibilities: horizontally compatible and vertically compatible (64 trials) vs. horizontally compatible and vertically incompatible (64 trials); horizontally incompatible and vertically incompatible (64 trials). The distractor-to-memory-probe spatial compatibility effects indicate the extent to which participants' selective attention was biased by the singleton distractor's features relative to direct electrophysiological measures and indirect behavioral measures in performing the modified, delayed match-to-sample abstract shape task. Speed and accuracy were encouraged. The number of trials where the memory probe was matching or mismatching relative to any of the items presented as part of the sample array was balanced across each combination of the eight experimental conditions: VWM capacity load (low 1 and high 4), horizontal spatial compatibility (incompatible and compatible) and vertical spatial compatibility (incompatible and compatible).