Effects of alertness on perceptual detection and discrimination

The level of alertness fluctuates throughout the day, exerting modulatory effects on human cognitive processes at any moment. However, our knowledge of how alertness level interacts with specific cognitive demands and perceptual rules of a task is still limited. Here we used perceptual decision-making paradigms to explore this issue. We analysed data from four different experiments involving a total of 113 participants: 1) auditory masking detection, 2) sensorimotor detection, 3) auditory spatial discrimination, and 4) auditory phoneme discrimination. We examined participant performance during the natural transition from awake (high alertness) to drowsy (low alertness). First, we fitted psychometric functions to the performance in EEG-defined high and low alertness metastable states. Second, we modelled slope and threshold from the fitted sigmoidal curves as well as signal detection theory measures, including perceptual sensitivity (d') and response bias (criterion). We found lower detection and discrimination sensitivity to stimuli as alertness level decreases, signalled by a shallower slope and a lower d', while the threshold increases slightly and equivalently across experiments. We observed no change in criterion during the transition. Zooming in, we observed that the decrease in sensitivity measured by slope was stronger for discrimination than for detection decisions, indicating that lower alertness impairs the precision of decisions in discriminating alternatives more than in identifying the presence of a stimulus around the threshold. Taken together, these results suggest that alertness has a common effect on perceptual decision-making and differentially modulates detection and discrimination decisions.