Sustained attention, or vigilance, is the ability to maintain alertness and responsiveness to infrequent target events over prolonged periods. First systematically studied during World War II in the context of radar monitoring, the vigilance decrement — a decline in detection performance over time on watch — has proven remarkably robust across tasks and settings, posing both theoretical challenges and practical problems.
The Vigilance Decrement
Norman Mackworth's (1948) classic clock test required observers to watch a clock hand making regular movements and detect occasional double-jumps. Performance declined significantly over a 2-hour watch period, with the sharpest decline in the first 15-30 minutes. This vigilance decrement has been replicated in hundreds of studies across diverse tasks, making it one of the most reliable findings in attention research.
The decrement is worse when signals are faint, infrequent, or difficult to discriminate from non-signals; when the monitoring period is long; when the task provides little feedback; and when the observer is sleep-deprived, fatigued, or poorly motivated. Using signal detection theory, the decrement often reflects a shift in response criterion (becoming more conservative) rather than a decline in perceptual sensitivity, though both can contribute.
Multiple theories attempt to explain why vigilance declines over time. Resource depletion theories propose that sustained attention consumes limited cognitive resources that are gradually exhausted. Mindlessness theories suggest that understimulating tasks promote task-unrelated thought (mind wandering). Overload theories propose that continuous demands produce information overload and cognitive fatigue. Recent neuroscience research points to declining activity in right-hemisphere attention networks and increasing activation of the default mode network during vigilance tasks, consistent with a shift from externally directed attention to internally directed thought.
Individual Differences
Sustained attention varies substantially across individuals, with differences related to age, personality, arousal level, and clinical status. Children show shorter vigilance periods than adults, with sustained attention capacity increasing through childhood and adolescence. Adults with ADHD show disproportionate vigilance decrements, and impaired sustained attention is a core feature of the disorder. Trait differences in extraversion, conscientiousness, and mindfulness also predict vigilance performance.
Practical Applications
The vigilance decrement has significant implications for occupations requiring prolonged monitoring: air traffic control, security screening, medical image reading, quality inspection, and long-haul driving. Countermeasures include scheduled rest breaks, task rotation, signal enhancement, knowledge of results (feedback), and environmental stimulation. Automation can help but introduces its own problems: when humans monitor automated systems, the monitoring task itself is a vigilance task subject to the same decrement.
Mind Wandering
Recent research has reframed the vigilance problem partly as a mind-wandering problem. When task demands are low, the mind spontaneously generates task-unrelated thoughts, drawing resources away from external monitoring. Jonathan Smallwood and Jonathan Schooler have shown that mind wandering occupies 30-50% of waking thought, and its frequency increases during monotonous tasks. Mind-wandering episodes are associated with activation of the default mode network and deactivation of task-positive networks.