## 2 Materials and Methods
### Materials and Design  
This experiment used a 2 (complexity level) \(\times\) 2 (stimuli type) design such that all subjects were exposed to four stimuli groups: complex and ordered, complex and disordered, simple and ordered, and simple and disordered. The definition of visual order was similar to those adopted in previous studies, i.e., the degree of symmetry, balance, and regularity in the organizational form of the components [44; 47], while the definition of complexity in stimuli was the amount of the components [16]. To exclude the potential influence of other factors affecting graphical symbol complexity (e.g., familiarity, illustration style and color palette), the stimuli were novel, meaningless graphical symbols with simple geometric components. Three types of lines (straight, curved, and angular) and three types of shapes (circles, rectangles, and triangles) were selected and randomly combined into a single graphical symbol via Adobe Illustrator 2019. Two hundred and forty unique graphical symbols were randomly produced under the following constraints: (1) simple graphical symbol consisted of two lines and one shape; (2) complex graphical symbol consisted of two lines and five shapes; (3) ordered graphical symbol had a symmetrical, balanced, and regular organizational form; (4) disordered graphical symbol had an asymmetrical, unbalanced, and irregular organizational form (see Figure 2). Therefore, a simple and ordered graphical symbol consisted of two lines and one shape, and its organizational form was symmetrical, balanced, and regular, while a complex and ordered graphical symbol consisted of two lines and five shapes, and its combination form was asymmetrical, unbalanced, and irregular. To further reduce the disparity between the objective and subjective descriptions of these images, 50 volunteers were chosen to rate the four factors on a five-point Likert scale (familiarity, concreteness, complexity, and order) of all the graphical symbols before the formal experiments. Instructions were the same as in previous studies [16; 43; 45; 52; 55]. For complexity ratings, symbols should be considered as complex if they contained a large amount of elements (1 = definitely simple, 5 = definitely); for order ratings, symbols should be considered as ordered if the elements in them are well organized or structured (1 = definitely disordered, 5 = definitely ordered); for concreteness ratings, graphical symbols should be considered as concrete if they depict real-like items (1 = definitely abstract, 5 = definitely concrete); for familiarity ratings, symbols should be considered as familiar if they look like an object that frequently appears in daily life (1 = definitely unfamiliar, 5 = definitely familiar). Based on the rating results, 80 graphical symbols from four groups were selected for the formal experiment (20 complex and ordered symbols, 20 complex and disordered symbols, 20 simple and ordered symbols, and 20 simple and disordered symbols), see Figure 3. Subsequently, we analyzed the rating results of each group via ANOVAs analyze, followed by Newman-Keuls comparisons. The results indicated that there were significant differences between the complexity ratings of the complex group and the simple group, F (1, 78) = 437.0, \(p\) < 0.001, and between the ordered group and disordered group, F (1, 78) = 349.07, \(p\) < 0.001. However, there was no significant difference among the familiarity ratings of the 80 graphical symbols, F (3, 76) = 0.034, \(p\) = 0.854, and there was also no significant difference among the concreteness ratings of the 80 graphical symbols, F (3, 76) = 0.565, \(p\) = 0.64. Table 1 shows the ratings  
Figure 2: Complexity and order constraints in the design of graphical symbols in experiments. results for complexity, concreteness, order, and familiarity in each group. Subsequently, two symbols from each of the four stimulus groups randomly formed a bi-symbol pair. Since each complex graphical symbols consisted of two lines and five shapes, each complex bi-symbol pair was twice as complex.