Effects of single and repeated cognitive tasks on autonomic balance as observed by an analysis of R-R intervals

The effects on sympathic-parasympathetic nervous system balance of single exposure and repeated exposures to a cognitively demanding task were examined by power spectrum analysis of cardiac R-R intervals. In the single task experiment, 32 healthy subjects engaged in a 90-min English language transcription task and in 90-min reading as a nontask control. In the repeated task experiment, 14 subjects experienced either intermittent exposure to the transcription task until late at night, or daytime exposure to the transcription task followed by relax wakefulness until sleep onset. The single task exposure significantly increased the normalized low frequency component (%LF; 0.05-0.15 Hz) of the R-R interval spectrum compared with the nontask control, but there was no significant task difference in the normalized high frequency component (0.15-0.50 Hz). The increased %LF values immediately returned to control levels after cessation of the task. Repeated exposure to the cognitive task until late at night produced a significantly greater %LF during the first nonrapid eye movement sleep period when compared to relaxed wakefulness. These results would suggest that a single exposure to a cognitive task may produce a shift to sympathetic nervous system dominance only during the period of the task, whereas repeated exposures to a cognitive task until late at night may produce a shift to sympathetic nervous system dominance which lasts into the subsequent sleep period.