Sustained attention under load: Neurophysiological mechanisms and behavioural consequences

Sustained attention research demonstrates the negative consequences of declining task focus with time-on-task. Separate research demonstrates the critical role of perceptual load in task focus: increased perceptual load is shown to draw neural energy into task-relevant, rather than irrelevant, processing (Bruckmaier et al., 2020) thus improving task focus (Lavie, 2005). However, how perceptual load affects the neurophysiological mechanisms underlying sustained attention decline with time-on-task remains unknown. Moreover, research on the aperiodic component of EEG has linked the 1/f slope with excitation-inhibition (E/I) balance: steeper slopes reflecting reduced E/I ratio (Gao et al., 2017), but whether sustained attention decline can be attributed to a change in the aperiodic slope is an open question. Here we recorded EEG during gradual continuous-performance task (detecting infrequent mountain-among-city scenes), under low or high perceptual load (with or without overlaid salt-and-pepper noise, respectively), and analysed periodic and aperiodic components. Time-on-task resulted in a widespread increase in alpha power and a steeper 1/f slope in a left temporoparietal cluster, accompanied by reduced detection sensitivity, increased response variability, and increased mind wandering, with reduced thought detail. Perceptual load improved task focus, indexed by reduced mind wandering, but exacerbated the effect of time-on-task on detection sensitivity, and the 1/f slope was steeper with time-on-task in a right parieto-occipital cluster under increased load. Overall, these findings suggest that sustained attention decline with time-on-task can be attributed to depletion of neural energy needed for excitatory signalling, which is further drained with increased processing demands in tasks of high perceptual load.