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The advancements in communication and information technologies led to increased attention among the educational community on the importance of multimedia learning materials. The use of dynamic Visual-Auditory learning Materials (multimedia) in the educational context has increased significantly in the past decade. Visual-Auditory materials include several forms such as videos, presentations with audio, and interactive materials.

The cognitive theory of multimedia learning (CTML)
The superiority of multimedia or dynamic Visual-Auditory materials in learning is rooted in the cognitive theory of multimedia learning (CTML). The fundamental principle of the CTML is how does the student processes and understands the learning materials, develop meaningful associations, and assimilate new knowledge and information with existing knowledge and information schemes. The CTML is based on three key concepts: dual channels, finite capacity, and active processing. According to the concept of dual channels, verbal and visual materials are processed by two different, separated channels. The learner will select and process relevant visual images in visual working memory, while relevant verbal materials are processed in the verbal working memory. The concept of finite capacity assumes that each channel can only process a finite amount of information (visual or verbal). The concept of active processing assumes that deeper, stronger, and more meaningful learning strongly depends on the cognitive processing of the learner to be able to choose, process, organise, comprehend, and integrate the new visual or verbal information with previously-acquired knowledge. The interaction between the three concepts and a schematic representation of how the working memory works in instructional multimedia is shown in figure 1. There are five columns of boxes and two rows with arrows that link them (Mayer, 2014). The two horizontal rows include the information-processing channels. The auditory/verbal channel is followed by the visual/pictorial channel. First, the learning multimedia materials (words or pictures) are watched by learners. Second, the eyes and ears are used to access the sensory presented materials using sensory memory. Then, the learners decide what images and auditory/text will be kept in their long-term memory. However, the visual or written materials need to be kept in the working memory to allow the learner to integrate and process the visualisations and written text. Then, the learned integrates these elements (images or words) with the existing information and knowledge.

Strengths
On the one hand, a wide range of empirical research found that the use of dynamic audio-visual materials such as videos and interactive materials is not only preferred by students over text and static materials but is also more likely to lead to increased and better understanding and learning experience compared to the use of words along. Also, several researchers demonstrated that students prefer video-based learning materials compared to text-based learning materials. For example, a recent study compared the effectiveness of two forms of e-learning including static text/picture or video-based e-learning. The researchers found that the video-based group had better performance compared to the text/picture-based group one month later. Also, the CTML is learner-centred. In addition, the theory offers a simple guide and suggests five key principles for designing multimedia instructions that lead to effective learning materials.

Cognitive overload
On the other hand, the key limitation is that multimedia learning tends to overwhelm the brain, and therefore, it should be designed more effectively. It can overwhelm the brain because the CTML is heavily dependent on the working memory, which can be easily overloaded. Therefore, this may limit the efficacy of using the CTML as a basis for designing learning materials.

Solutions
However, several solutions to the problem of cognitive overload were proposed. In this regard, professors Mayer and Moreno referred to an overload scenario in which one or two channels are overloaded by incidental and essential processing (extraneous materials). An example of this scenario is inserting video clips with examples and/or the use of narrated animation with music. In this case, they recommended that designers can use weeding or simply eliminate extraneous materials. Also, signalling can be used. In other words, easy cues for processing materials can be used.

Another scenario is when the auditory-based or visual-based channels are overloaded with basic processing demands. For example, this can be manifested in a case in which an animation with concurrent explanatory subtitles is presented. In this case, it is recommended to use off-loading. In other words, text can be replaced by narration.

Another scenario is when the channels are overloaded by representational holding and essential processing. For example, this can be manifested in a case in which animation follows a narration. In this case, it is recommended to adopt "synchronising". In other words, animation and narration can be presented simultaneously. "Individualising" can also be used. In this case, skills are imparted to students in order to hold mental representations.

The expertise reversal effect of segmentation
Also, a key limitation is that most experiments on multimedia learning used learning materials segmented into smaller pieces and were less than 20 minutes. Instructional multimedia segmented pieces need a large extent of cognitive processing to understand, process, analyse, and synthesize the audio and visual-based information and assimilate the newly acquired information in the working memory. Based on the expertise reversal effect, the design measures that can positively affect learning and cognitive load for learners with limited prior knowledge might have an adverse impact on learning and cognitive load for learners with a high extent of prior knowledge. This is consistent with an empirical experiment on the impact of segmentation on students with varying levels of previous knowledge. For example, an experiment with animation-based learning stimuli found that segmented animations were more effectively processed compared to continuous animation for learners with a limited level of previous knowledge compared to their counterparts with a high level of previous knowledge. This provides empirical evidence for the expertise reversal effect of segmentation. Therefore, because the majority of studies on multimedia learning used segmented learning materials, this questions the validity of those studies and the theory itself. Limited studies on the impacts of cuing in animation are available and the limited existing ones provided mixed results. Also, the effective use of multimedia learning in designing instructional materials can be limited by several technical issues such as technical support, copyright material, time constraints, access to a server, and skilled designers.

The CTML is reductionist as other modes of learning are not taken into account
Another limitation of the CTML is that it only emphasises the importance of two modes of learning (visual- and auditory paths), but it does not take other modes and mechanisms into account. For example, the CTML is based on the concept that learners are passive, rather than active learners. On the contrary, there is a large body of evidence supporting the efficacy of active modes of learning such as kinaesthetic-based learning (KBL). In addition, students prefer kinaesthetic learning compared to auditory and visual learning. A kinaesthetic learner is an active participant that prefers to learn through physical activity, rather than passively watching an educational video. Therefore, the CTML does not take other effective modes of learning into account.

Conclusion
The cognitive theory of multimedia learning provided a valuable framework for designing effective learning materials. Although it is supported by a large body of empirical evidence, several critics questioned its efficacy. Cognitive overload is one of the key issues. However, this problem can be addressed by adopting several solutions. Extraneous materials can be addressed by using weeding and signalling. If the channels are overloaded by representational holding and essential processing, synchronising and Individualising can be used.

Also, the expertise reversal effect of segmentation is another issue. The theory is reductionist because it only focuses on the auditory and visual modes of learning, while other modes of learning (e.g., kinaesthetic learning) are not taken into account. Although the theory has some limitations, as shown above, it deepened my understanding of how learning materials can be designed. However, designers should take its limitations into account when using the theory in designing learning materials. They are also recommended to consider the solutions to the cognitive load issue.