The relationship between behavioural microsleeps, visuomotor performance and EEG theta.

Visuomotor performance and responsiveness deteriorates with time-on-task due to drowsiness and increased propensity to sleep. Frequent episodes of behavioural microsleep (BM) are also common during extended and monotonous tasks. In this study, simultaneous recording of EEG, eye-video, and continuous visuomotor response is used to investigate visuomotor performance and EEG activity during tonic drowsiness and phasic BMs. The data were collected from 20 healthy volunteers while they performed a continuous 2-D pursuit tracking task for 50 min. We identified episodes of BMs by expert visual rating of eye-video and visuomotor response using a set of pre-defined criteria. Visuomotor performance and EEG activity were correlated with and without BM events. A moderate correlation was observed between visuomotor error and theta activity in EEG at a posterior channel (Pz) before the removal of BMs. However, when BMs were removed from the data, the correlation dropped in most subjects. Furthermore, most of the large fluctuations in performance observed during the visuomotor task disappeared after the removal of BMs. This suggests that episodic behaviours such as BMs contribute substantially to fluctuations in performance and to EEG theta activity during an extended task, and that they should be taken into account when studying tonic drowsiness.