Effects of Stroop color-word conflict test on the autonomic nervous system responses

The Stroop color-word test (CWT) is a mental stress test involving sensory rejection and has been used as a model of the defense reaction in humans. The present study was designed to investigate effects of CWT on resting cardiac autonomic nervous system activity evaluated by analyses of heart rate (HR) variability (HRV). Eight healthy subjects performed 21 min of CWT after 14 min of resting control followed by 14 min of recovery (RCV). Beat-to-beat R-R intervals (RRI, i.e., HRV), systolic blood pressure (SBP), and instantaneous lung volume were continuously monitored throughout these periods. Steady-state HRV data (10-12 min) for resting control and recovery (12 min) and for 1-11 min (CWT1) and 11-21 min (CWT2) of CWT were analyzed by coarse-graining spectral analysis to break down their total power into harmonic and nonharmonic (fractal) components. The harmonic component was further divided into low (0.0-0.15 Hz, LF)- and high (>0.15 Hz, HF)-frequency components. Cardiac sympathetic (SNS) and parasympathetic (PNS) nervous system activity indicators were evaluated as LF/HF and HF/TP, respectively [LF, HF, and total spectral power (TP) of HRV]. The fractal component had the spectrum of 1/f(beta), and the spectral exponent beta was calculated. RRI decreased and SBP, plasma norepinephrine, and epinephrine concentrations increased significantly (P < 0.05) in response to CWT (CWT1 and CWT2). PNS and SNS indicators did not show any significant (P > 0.05) difference between resting controls and CWT. Whereas the percentage of fractal components in total HRV remained unchanged, beta decreased significantly (P < 0.05) in response to CWT. Transfer magnitudes from SBP to RRI, calculated by cross-spectral analysis, decreased at CWT2. The transfer magnitudes from instantaneous lung volume to RRI were also decreased significantly (P < 0.05) by CWT. It was concluded that although CWT did not affect the SNS indicator of HRV, despite altered HR and vasomotor responses, tonic sympathetic nervous system influence was observed for norepinephrine and epinephrine. In addition, reflex blood pressure (CWT2) and respiratory modulation of HR (CWT1 and CWT2) decreased during CWT. Mental stress altered the fractal nature of HRV (as judged by decreased beta), but mechanism(s) responsible for this change remained unknown.