## 1 Introduction
Because the iconicity of symbols is one of the key variables that affect symbol learning [19], researchers that have long identified these challenges and have sought solutions proposed an iconicity hypothesis which states that graphic symbols with high iconicity promote symbol learning and comprehension and favor learning outcomes [20]. For years, the iconicity hypothesis has continued to garner attention [18], and learners have been able to easily learn and comprehend graphic symbols with high iconicity. Essentially, graphic symbols that bear a greater resemblance to their referent are more easily learned by individuals [17]. Often, the key challenge of learning AAC applications is whether the learner can quickly associate the graphic symbol iconicity with language and communication functions. Therefore, when communication training or instructional intervention is involved, instructors should select graphic symbols with high transparency and translucency from a variety of symbols and symbol systems to facilitate effective learning outcomes [5]. Most studies on symbol iconicity have compared various graphic symbol systems and reported that symbol systems with high specificity or a variety of symbol elements (e.g., picture communication symbols (PCSs)) yield favorable learning outcomes [16]. Empirical analyses have also revealed that the characteristics of corresponding referents affect the learnability of symbols [21]. Moreover, some studies have verified the iconicity hypothesis and reported that, in addition to favorable learning outcomes yielded by graphic symbols with high iconicity, the complexity of symbols and components are major factors of symbol learning [22]. Although numerous studies have been published on the characteristics and learnability of AAC symbols, few of such studies have been published in Taiwan. In light of this, preliminary studies on AAC symbols and iconicity of color PCS systems commonly used in Taiwan have revealed that the translucency of color PCS systems tended to be high, and nouns had higher translucency than both verbs and descriptors did. In addition, the results confirmed the graphic symbol iconicity hypothesis and demonstrated that PCSs with high translucency favored the participants' learning outcomes [18]. [PERSON] [13] investigated graphic symbol learning;the effects of 27 sets of self-compiled verb graphic symbols on the learning outcomes of 40 children with developmental delays and 120 typically developing children were measured. The results revealed that a wide variety of symbol components favored symbol recognition and learning for both children with and without developmental delays. In addition, participants demonstrated significantly higher performance on verb graphic symbols with two, four, and five components than they did on those with a single component in terms of accuracy. Furthermore, children with developmental delays required more time to identify verb graphic symbols with more components. However, from the cognitive psychology perspective, children should first pay attention to certain content they read to trigger their learning; the cognitive process varies with the time spent on reading pictures or texts [23]. The dual-coding theory believes that multi-element graphic symbols provide more visual clues and stimuli, which can help readers form a coherent mental representation, which is conducive to the recognition and memory of graphic symbols [24]. Some studies support a positive relationship between the number of symbol elements and the degree of recognition [13; 16; 19]. However, cognitive load theory holds the opposite position [25]. It believes that human beings have a limited ability to process information. Excessive information volume can cause readers to overload information. Under the premise of limited working memory capacity, if readers need to encode and organize multiple elements at the same time, it is easy to cause cognitive resources to compete and distract, which is not conducive to the identification and learning of graphic symbols [25; 26].