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• We investigated how differing durations of sleep loss influence working memory. • Working memory assessed using tasks/EEG data after 24 h and 30 h sleep deprivation. • 30 h of sleep deprivation had a greater negative impact on working memory than 24 h. • Effects were linked to decreased effective connection from right insula to left ACC. Total sleep deprivation (TSD) causes a decline in almost all cognitive domains, especially working memory. However, we do not have a clear understanding of the degree working memory is impaired under prolonged TSD, nor do we know the underlying neurophysiological mechanism. In this study, we recorded EEG data from 64 subjects while they performed a working memory task during resting wakefulness, after 24 h TSD, and after 30 h TSD. ANOVA was used to verify performance differences between 24 h and 30 h TSD in working memory tasks: (1) reaction time and accuracy hit rates, (2) P200, N200, and P300 amplitude and latency in measurements of event-related potential, as well as (3) effective connectivity strength between brain areas associated with working memory. Compared to 24 h TSD, 30 h TSD significantly decreased accuracy hit rates and induced a larger N200 difference waveform. The effective connectivity analysis showed that 30 h TSD also decreased beta frequency in effective connection strength from the right insular lobe to the left anterior cingulate cortex (ACC). Effective connection from the left ventrolateral prefrontal cortex to the left dorsolateral prefrontal cortex increased in the match condition of the 2-back task. In conclusion, 30 h TSD had a greater negative impact on working memory than 24 h TSD. This impairment of working memory is associated with decreased strength in the effective connection from the right insula to the left ACC. [ABSTRACT FROM AUTHOR]

• We studied the activity of the PFC in the Sternberg WM task (ST) with three levels of difficulty using fNIRS. • Significant differences in HbO, HbT and Oxy changes depending on WM task load were found. • The PFC is more activated if the WM load is increased. Both ventrolateral and dorsolateral PFC seem to play a central role in WM processes. The Prefrontal cortex (PFC) has been highly related to executive functions such as working memory (WM). This study assesses the activity of the PFC in performing the Sternberg WM task (ST) with three levels of difficulty (easy, medium and hard) using the near-infrared spectroscopy (fNIRS) technique. Participants were 43 young and healthy right-handed women. Nine WM task blocks were pseudo randomly presented, three for each difficulty task. The results showed that the participant's performance was better in the easy trials than in the medium and hard trials. Performance in the medium trials was also better than in the hard ones. Bonferroni-corrected paired post-hoc t-tests indicated higher oxygenation in medium and hard tasks than in the easy ones for times between 13 and 42 s in the left lateral PFC and in both, medial and lateral, right PFC. Significant differences in Oxygenated hemoglobin (HbO), Total hemoglobin (HbT) and oxygenation (Oxy) changes depending on the Sternberg WM task were found. Unlike previous studies with fNIRS and WM, the current study uses a highly controlled WM task that differentiates between encoding, retention and retrieval phases, comparing different levels of task load. [ABSTRACT FROM AUTHOR]