Discussion
The present study examined the effects of cognitive load and emotional faces on WM performance using modified Aospan. The primary hypothesis of the current study was to examine how the distractors with different levels of cognitive load may affect the WM performance of emotional facial processing. The second objective was to examine the effect of emotional faces on WM performance. Overall the results confirmed proper manipulation of different distraction tasks as participants performed better in the low level of distraction than the high level of distraction. This also confirms the design that cognitive load was lower in the low than the high level of distraction and participants consumed less WM resources when performing the mathematical equations with fewer operands than larger operands. Utilizing varying levels of mathematical equations is effective in manipulating different amount of cognitive load, and these different levels of cognitive load affect performance of WM span.
Regarding the impact of cognitive load on WM performance, it was found that when participants were presented with distractor of low cognitive load, WM performance was not affected. However, in exposure to distractor of high cognitive load WM was significantly impeded. The cognitive load of distracting tasks in the modified Aospan are divided into three levels, with the difference in performance of WM span between difficult and control condition being the most prominent. In other words, the difficult distracting task taxes the highest cognitive resources thereby diminishing the WM span performance to the greatest degree. This result is consistent with the previous studies (Allen et al., 2017; Lavie, 2005; O’toole et al., 2011) contradictory that increasing the cognitive load of the distractor has a vast impact in WM performance. When there are two or more incoming stimuli that need to be processed, priority must be assigned to one or the other at a given time. To allocate cognitive resources appropriately between the two competing resources, the WM model (Baddeley & Hitch, 1974) explains that a critical component named the central executive plays a pivotal role in this moment (Baddeley, 2012). The central executive acts as a guide for the delegation of resources and attention allocation of resources to manage and maintain information in mind during the task. Under low cognitive load, this component is working adequately but when cognitive load increases, the WM is affected reflecting the limited capacity of attentional resources and WM span.
The present findings are also consistent with the assumption of the TBRS model which suggests that WM functions to temporarily store items and process information, and these function are fueled by limited attentional resources (Geday, Kupers, & Gjedde, 2007; Vergauwe, Barrouillet, & Camos, 2009). The TBRS model also states that the cognitive load of a given event corresponds to the proportion of time (Geday et al., 2007). In terms of the complex span paradigm, the TBRS model explains that the recall performance of storage component should be a function of the cognitive load of the processing component of the span task. This trade-off is due to the limited-capacity mechanism of WM suggested by Baddeley (2012). The present findings are in line with their suggestion in terms of the limited-capacity nature of WM. It is suggested that both the processing component (mathematical equations solving) and storage component (serial recall of emotional faces) require visuospatial WM, another component proposed by the WM model. This component provides a virtual environment for physical simulation, calculation, visualization and optical memory recall. It is a part of the WM that holds information during the initial processing to produce the recollection of an image such as a place or a face. Therefore, the current results revealed a trade-off between the two processes (emotional facial processing and calculation). This is consistent with the proposal of the limited-capacity mechanism of WM. As a result, the higher the cognitive load is, the lower the levels of performance are.
Regarding the effect of emotional faces on WM performance, the results showed no significant effect of emotional face on WM performance. Although previous studies such as Craig et al. (2014) lend support to the happiness superiority effect, wherein positive facial stimuli were found to be processed more quickly than negative and neutral ones in their visual search task. However, such effect was not apparent in the current study as the type of emotional stimuli being processed did not affect the WM performance. In an explanatory manner, participants were not influenced by the emotions of facial stimuli (bottom-up attention) when preforming the task. In fact, a study has investigated the strategies used in complex span paradigm and found using strategies such as imagery (Bailey, Dunlosky, & Kane, 2011). They found that participants used encoding strategies such as chunking of verbal letters. In this study, although the non-verbal emotional faces can prevent participants from rehearsing or chunking, they plausibly employ alternative methods to facilitate WM performance. For example, they have associated a name for each emotional face based on its features to help with their memory performance.
A key limitation of the current study is that participants were not asked to report the emotionality of the facial stimuli they were exposed to, it remains inconclusive as to whether emotions have no impact on WM or that the participants did not perceive the stimuli as emotionally arousing. Future studies should address this by asking participants to rate the emotionality of the stimuli in the experiment. As the study has only used three different types of emotional faces. The non-significant results might have been attributed to this. In support of this assumption, previous studies (e.g., Craig et al., 2014; O’toole et al., 2011) have used other emotional stimuli such as threat related stimuli. It was found that threatening or angry faces elicited a bigger effect (also known as Anger Superiority Effect) on capturing attention compared to general happy or sad faces. In future studies, more salient emotional facial stimuli are recommended. If the emotionality of faces is more prominent, participants may elicit stronger emotional responses toward them. As the current results are not consistent with the TBRS model (Barrouillet et al., 2004), the modified version of Aospan in the present study with varying cognitive load can be further verified by the TBRS model in the future to address the following questions. First, it has been found that affective state of individuals may either improve or diminish the WM performance, however, few studies (e.g. Yang, Yang, & Isen, 2013) have examined the effects of current mood state on WM using complex span paradigm. Second, the current modified Aospan used emotional facial stimuli as to-be-remembered items, future studies may employ emotional words, aversive or appetitive stimuli to investigate whether these non-verbal but meaningful stimuli have effects on WM performace because these stimuli are of evoluationarily importance.
In conclusion, the current study modified the Aospan in which changed the to-be-remembered items to emotional faces and varied the cognitive load by means of different levels of mathematical equations. In consideration of the non-verbal nature of emotional facial stimuli, the time limit plus 3 SD was applied for differences in ability of individuals. The effect of emotional faces on WM performance was non-significant. Instead, the results reveal a main effect of cognitive load on WM performance. These findings add evidence to the TBRS model.
Acknowledgements
We thank Aospan for providing us the software for data collection.
Compliance with ethical standards
All procedures performed in our experiments involving human participants were in accordance with the ethical standards of the institutional and/or national research committee and with the 1964 Declaration of Helsinki and its later amendments or comparable ethical standards. The experiment reported here was approved by the Local Ethical Committee of the University of Salford.

Informed consent

Informed consent was obtained from all individual participants included in the study.
Conflict of interest
The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.