A critical examination of the evidence for sensitivity loss in modern vigilance tasks.

It is well known that when human observers must monitor for rare but critical events, probability of detection tends to wane over time, a phenomenon known as the "vigilance decrement." Over 60 years of empirical study on this topic has culminated in the general consensus that performance suffers due to a loss in observers' ability to distinguish signal from noise (a loss in sensitivity) provided that the task loads memory and stimuli are presented at a relatively high rate. We challenge this assertion on 2 fronts: First, we contend on a theoretical level that the metrics employed to measure observer sensitivity in modern vigilance tasks (derived from signal detection theory) are inappropriate and largely uninterpretable. This contention is supported by an evaluation of recent empirical work in the vigilance domain. Second, we present the results of an experiment that demonstrates that shifts in response bias (the observer's "willingness to respond") over time can masquerade as a loss in sensitivity. Consequently, the basic underlying cause of the vigilance decrement is actually unclear, and may simply reflect a shift in response criterion rather than sensitivity. The theoretical, as well as practical implications of these conclusions are discussed with respect to sustained attention in general, and vigilance in particular.