Table 3.

Time and frequency domain features.

Features	Description
HR	The rate of change associated with R-R intervals from HR represents HRV. Increases due to stress
SDNN	The standard deviation of interval between two normal heartbeats (NN). NN measures the total power. Decreases in response to stress. $SDNN=\sqrt{\frac{1}{N-1}\sum _{j=1}^N{(R{R}_j-\overline{RR})}^2}$
RMSSD	The root mean square of successive differences between normal heartbeats. Primarily manipulated by PNS activity. $RMSSD=\sqrt{\frac{1}{N-1}\sum _{j=1}^N{(R{R}_{j+1}-\overline{R{R}_j})}^2}$
pNN50	Represents the percentage of the difference associated with NN interval which differ more than 50 ms.It shares a strong correlation with PNS activity, RMSSD, HF
SD1	Non-linear variables derived from the Poincaré plot. Shares a high correlation with HF, RMSSD. Decreases due to stress
SD2	Non-linear variables derived from the Poincaré plot. Shares a high correlation with LF. Increases in response to stress
ApEN	Represents the ratio between SD2 and SD1. Shares a high correlation with LF/HF. Increases due to stress
GSR std	Standard deviation associated with electrodermal activity. Increases during stress
GSR mean	Mean value obtained from measuring the rate of change associated with EDA activity. Increases during stress
Resp Rate	Represents breathing rate, increase in Resp rate leads to increased PNS activity, HF and decreased LF, SNS activity. Increases in response to stress
VLF	Represented within the VLF band (0.0033–0.04 Hz) and it is mediated by SNS activity
LF	Represented through 0.04–0.15 Hz within the PSD, it is mostly used to indicate SNS activity but can specify PNS activity
HF	Represented by the frequency range of 0.15–0.40 Hz and solely indicates PNS activity
LF/HF	Represents ANS activity, increases in response to increased stress and decreased HRV