## 1 Introduction  
Mobile augmented reality (MAR) is a technology that employs handheld smart devices or head-mounted display modes [1]. Since the MAR interface consists of virtual objects and actual scenes, users can freely view 3D virtual objects by rotating and moving the phone. In recent years, as augmented reality (AR) technology has been increasingly used in mobile applications, the application scenarios for MAR have also been expanding [2; 3; 4]. Among the various types of information in the MAR interface, 3D virtual elements are widely used as targets for interactions and are viewed simultaneously with the real environment [5]. Since AR technology allows users to perform unlimited virtual-reality interactions with 3D virtual objects in real environments, it brings users an overall experience of highly vivid, immersive environments and rich sources of information [6; 7; 8]. Therefore, these virtual elements play a key role in users' cognitive performance within the MAR interface. For example, [PERSON] et al. [9] have suggested that the identifiability of virtual object information is a property that MAR applications and systems fulfill. However, it remains unclear how to design the presentation form of 3D virtual elements in a mobile AR interface,especially how to design a reasonable quantity and complexity to maximize the interactive richness and user experience. Moreover, AR interaction on mobile devices is a complex, goal-oriented task that combines real scenes and virtual objects to present information, which places high demands on the perceptual and cognitive processes of these virtual objects, and therefore the involvement of VWM capacity in this process is important. By studying the existing AR-related studies, we found that most previous studies have focused on its technical feasibility, such as the perceptual factors that influence MAR [10; 11; 12], application scenarios [13; 14; 15], theoretical models [16; 17], and technical principles [18]. However, with the continuous development of mobile AR applications, the number of 3D virtual elements in MAR is increasing, and the cognitive tasks and presentation forms are becoming more complex, which may impose heavy cognitive loads on users [19]. Previous research has shown that visual working memory (VWM) capacity is closely related to overall cognitive ability, as it determines the amount of information we can process simultaneously and limits the efficiency of higher cognitive activities [20]. But few studies have focused on how the presentation form of 3D virtual elements in MAR affects users' VWM capacity, especially the complexity of virtual object interaction. In interface design-related research, complexity has long been considered an important factor affecting users' perception of interface information because highly complex interface information usually requires more cognitive resources [21; 22]. Notably, in practice, the complexity of 3D virtual elements in MAR interactions includes multi-dimensional depth information, digital elements, live views and calibration issues [14; 23]; thus, a reasonable complexity level and display quantity of these 3D virtual elements are important and necessary in MAR interaction design. Therefore, it is crucial to understand the effects of the complexity and quantity of 3D virtual elements on VWM performance in MAR interfaces, especially the relationship between virtual object complexity and VWM capacity. In this article, we also explore the applicability of two dominant theoretical models: the slot model [24; 25; 26] and the resource model [21; 27; 28] in MAR interfaces to better explain the effects of the complexity and quantity of 3D virtual elements and their interaction on visual working memory behaviour. The slot-based theory assumes that there is a finite amount of memory capacity, i.e., the number of items that can be stored in working memory is strictly limited and cannot be increased by decreasing the quality of the representations [29; 30].