Inter-modal and intra-subject consistency in EEG correlates of vigilance

Abstract

Occipital EEG was monitored continuously for 20 subjects who took part in an auditory vigilance task and for 20 in a visual vigilance task. Ten subjects were common to both experiments. The two tasks were virtually identical. EEG samples were taken for each of 250 signal presentations. Automatic low frequency analysis was performed on 4 frequency bands between 2.0–19.5 cps. There were 5 types of signal in the task and each signal was followed by a rest period. The wanted signal was 4 consecutive odd and different digits. The other signals contained either 3 odd and 1 even digit, 2 odd and 2 even digits, 1 odd and 3 even digits or 4 even digits. Since the 5 signal types varied in their approximation to the wanted signal they were ranked on that criterion for their arousal value. There were a number of statistically significant findings as follows. A decreasing monotonic trend is demonstrated for EEG abundance across signal types, that is, EEG abundance is inversely proportional to the arousal value of the signal for both the auditory and the visual tasks. Subjects common to the 2 tasks show consistency of individual differences in the 2 EEG trends across the tasks. EEG alpha and beta abundance provide strong evidence for individual differences in the magnitude of abundance under different stimulus conditions. Subjects with faster median reaction times demonstrate a stronger EEG trend than those with slow reaction times. However, reaction time is not related to either pre-, during or post-signal presentation EEG. A decreasing monotonic trend is demonstrated for post-test subjective ratings of alertness for each signal type and also for the associated rest periods in both the auditory and the visual task. The trend in the subjective ratings for signals agrees with the trend in EEG abundance. Subjects are consistent in their ratings across the 2 tasks.