“…The literature review for this work indicates more candidate metrics than those tested here. In the present study, RR and RR SD, were demonstrated to be successful as online indicators of working memory load, which is an encouragement for testing the remaining metrics, in particular frequency analyses such as: low-frequency (LF), highfrequency (HF), and LF/HF frequency-power [12], [13], [16], [27], [28]. In addition, the results of the candidate identification indicated other metrics that may be of interest that have not appeared in the literature yet, namely the derivative of the RR SD stream and the first and second derivatives of the RR stream.…”