Vigilance decrement during the on-the-road driving tests: the importance of time-on-task in psychopharmacological research.

Time dependent decrements in performance are characteristic of activities that are monotonous and require focused attention for an extended period of time. A vigilance task is a task that participants can perform without difficulty for a short period of time, but with time their performance becomes impaired. A real world example of such a vigilance task is prolonged highway driving. The on-the-road driving test in normal traffic was specifically designed to measure the effects of vigilance decrement associated with driving. The primary parameter of this test is the Standard Deviation of Lateral Position (SDLP), i.e. the weaving of the car. This methodological paper explains the typical vigilance decrement seen in the on-the-road driving test and discusses the importance of sufficient time-on-task to elucidate potential adverse drug effects on driving. Performance decrements (SDLP increment) as a function of time are seen after both drug and placebo treatment, following a similar pattern over distance/time traveled. However, whereas for some drugs SDLP differences between drug and placebo are constant, other drugs produce additional performance decrement that increases over distance traveled. It is concluded that driving tests of short duration (e.g. less than half an hour) may fail to detect drug-related impairment, because participants are capable of, at least in part, counteracting the impairment by increased effort and motivation to perform the test.