Holding biological motion (BM), the movements of animate entities, in working memory (WM) is important to our daily social life. However, how BM is maintained in WM remains unknown. The current study investigated this issue and hypothesized that, analogous to BM perception, the human mirror neuron system (MNS) is involved in rehearsing BM in WM. To examine the MNS hypothesis of BM rehearsal, we used an EEG index of mu suppression (8–12 Hz), which has been linked to the MNS. Using a change detection task, we manipulated the BM memory load in three experiments. We predicted that mu suppression in the maintenance phase of WM would be modulated by the BM memory load; moreover, a negative correlation between the number of BM stimuli in WM and the degree of mu suppression may emerge. The results of Experiment 1 were in line with our predictions and revealed that mu suppression increased as the memory load increased from two to four BM stimuli; however, mu suppression then plateaued, as WM could only hold, at most, four BM stimuli. Moreover, the predicted negative correlation was observed. Corroborating the findings of Experiment 1, Experiment 2 further demonstrated that once participants used verbal codes to process the motion information, the mu suppression or modulation by memory load vanished. Finally, Experiment 3 demonstrated that the findings in Experiment 1 were not limited to one specific type of stimuli. Together, these results provide evidence that the MNS underlies the process of rehearsing BM in WM.

The nature of the building blocks of information in visual working memory (VWM) is a fundamental issue that has not been well resolved. Most researchers take objects as the building blocks, although this perspective has received criticism. The objects could be physically separated ones (strict object hypothesis) or hierarchical objects created from separated individuals (broad object hypothesis). Meanwhile, a newly proposed Boolean map theory for visual attention suggests that Boolean maps may be the building blocks of VWM (Boolean map hypothesis); this perspective could explain many critical findings of VWM. However, no previous study has examined these hypotheses. We explored this issue by focusing on a critical point on which they make distinct predictions. We asked participants to remember two distinct objects (2-object), three distinct objects (3-object), or three objects with repeated information (mixed-3-object, e.g., one red bar and two green bars, green bars could be represented as one hierarchical object) and adopted contralateral delay activity (CDA) to tap into the maintenance phase of VWM. The mixed-3-object condition could generate two Boolean maps, three objects, or three objects most of the time (hierarchical objects are created in certain trials, retaining two objects). Simple orientations (Experiment 1) and colors (Experiments 2 and 3) were used as stimuli. Although the CDA of the mixed-3-object condition was slightly lower than that of the 3-object condition, no significant difference was revealed between them. Both conditions displayed significantly higher CDAs than the 2-object condition. These findings support the broad object hypothesis. We further suggest that Boolean maps might be the unit for retrieval/comparison in VWM.