Differential effects of prolonged work on performance measures in self-paced speed tests.

Time-related changes in the speeded performance of complex cognitive tasks are considered to arise from the combined effects of practice and mental fatigue. Here we explored the differential contributions of practice and fatigue to performance changes in a self-paced speeded mental addition and comparison task of about 50 min duration, administered twice within one week's time. Performance measures included average response speed, accuracy, and response speed variability. The results revealed differential effects of prolonged work on different performance indices: Practice effects, being more pronounced in the first session, were reflected in an improvement of average response speed, whereas mental fatigue, occurring in both sessions, was reflected in an increase of response speed variability. This demonstrates that effects of mental fatigue on average speed of performance may be masked by practice effects but still be detectable in the variability of performance. Therefore, besides experimental factors such as the length and complexity of tasks, indices of response speed variability should be taken into consideration when interpreting different aspects of performance in self-paced speed tests.