Application of wavelet analysis to the plethysmogram for the evaluation of mental stress

The present study proposes a new stress evaluation technique using the photoplethysmogram (PTG). Heart rate variability (HRV) is often used to evaluate mental stress. HRV can be measured from an electrocardiogram (ECG). The frequency variability of HRV and mental stress response are related. PTG also indicates changes in emotional response. PTG can easily be measured without body surface electrodes. This method is less invasive than ECG measurement. We attempt herein to evaluate mental stress by wavelet analysis of the PTG. PTG was measured in the resting and mental stress states, and wavelet transformed PTGs were compared. In nine out of ten subjects, the wavelet result for PTG revealed a decrease in the frequency band.