Rhythmic oscillations in the performance of a sustained attention task.

Attempts to sustain a narrow focus of attention over a long period of time are effortful and are punctuated by lapses. Most studies of sustained attention performance obscure the presence and pattern oflapses by reporting measures that are summed across the entire period of an individual's performance, or that are average scores for blocks of trials across many participants. In the present study we attempted to explore fluctuation in the attention of individual participants over the course of a vigilance task and to quantify its periodicity, if any exists. Normal university students listened to letters of the alphabet, arranged randomly and presented at a rate of 2 per second for 20 min. They were instructed to press a hand-held button when they detected a target two consecutive identical letters). Continuous estimates of performance accuracy (correctly detected targets) at regularly spaced time intervals were created for each participant using a moving time window. The resulting functions were analyzed in order to detect and quantify periodicity using a Fast Fourier Transform (FF). The most often observed rhythms for those participants with adequate FFT power congregated at 1-2 min, 4-7 min and greater than 10 min. Performance functions from 36 of the 40 subjects displayed at least two of these frequencies. Other studies have identified cycles in performance during similar vigilance challenges, but without particular rhythms or with no particular shared frequencies amongst participants. The possible sources of these fluctuations and the differences in the findings of these studies and the present study are discussed.