## 1 Introduction  
During the last 20 years, a wide variety of evidence has indicated that augmentative and alternative communication (AAC) can improve the communication skills of children with disabilities [1; 2; 3]. [PERSON] & [PERSON], [4] stated that AAC exhibits potential advantages to improve language, literacy, and communication skills even for nonspeaking children with intellectual disabilities and is imperative to their learning and communicative competence. Most researchers in AAC studies have employed graphic symbols as vocabulary items to improve the communication skills of children with disabilities; these children have been taught how to express their needs and interact with others through graphic symbols [5; 6; 7; 8; 9]. Graphic symbols are considered as the techniques for enhancing efficiency to communicate and primary vocabulary for children with disabilities [10; 11]. [PERSON] and [PERSON] [6] argued that, for AAC users, graphic symbols are the most credible alternative vocabulary, and the development of a graphic symbol vocabulary is absolutely critical and fundamental for pediatric AAC users with disabilities. The critical role of graphic symbols in AAC highlights the importance of symbol learning topics in AAC research. Applications of graphic symbols are extensive in both research and teaching practice. Numerous researchers have used graphic symbols to improve the communication skills of students with disabilities or mitigate their challenging behavior [1; 10]. Numerous studies have reported that picture book reading found effective for laying the foundation for children's future literacy capabilities [9; 12; 13; 14]. It is closely related to language development, print awareness, and emergent literacy [9; 12; 13; 14]. In addition, graphic symbols are frequently used as a teaching medium and as learning materials in both preschool and special education. [PERSON] and [PERSON] [15] encourage teachers and parents to use pictures and symbols to (a) improve oral instruction comprehension for children with disabilities and (b) promote social interaction and participation between all children with and without disabilities. Therefore, the use and learning of graphic symbols are imperative for young children with and without disabilities. Most studies have argued that graphic symbols can assist children with disabilities in functional competence (e.g., communication, writing, and learning) or improve their communication and literacy skills [6; 7; 8; 9; 10; 11]. However, graphic symbols are merely a part of a symbol system, and the so-called symbols refer to objects that stand for or represent referents. [PERSON] and [PERSON] [5] contended that if symbols characterize the referents, they exhibit a problem of recognizability or specificity. Researchers have also pointed out that many AAC studies have explored the learnability of various graphic symbol systems through the intrinsic characteristics of the systems, and iconicity of symbols has often drawn attention from researchers and practitioners [16; 17; 18]. Symbol iconicity refers to any association between a symbol and a referent. The association can be either based on a visual concept or any feature association by the viewer. Published studies have primarily measured symbol iconicity through their transparency and translucency [16]. Symbol transparency refers to the degree to which viewers can directly understand the referent or meaning represented by an individual graphic symbol. Essentially, transparency means the guessability of symbols for viewers without prior training or learning. By contrast, symbol translucency refers to the degree to which viewers can understand or infer the referent or meaning represented by a graphic symbol when some limited descriptive explanation or a contrast object is present. Symbol translucency is also known as the learnability of symbols.