Photoplethysmographic Waveform Versus Heart Rate Variability to Identify Low-Stress States: Attention Test

Our long-term goal is the development of an automatic identifier of attentional states. In order to accomplish it, we should first be able to identify different states based on physiological signals. So, the first aim of this paper is to identify the most appropriate features to detect a subject's high performance state. For that, a database of electrocardiographic (ECG) and photoplethysmographic (PPG) signals is recorded in two unequivocally defined states (rest and attention task) from up to 50 subjects as a sample of the population. Time and frequency parameters of heart/pulse rate variability have been computed from the ECG/PPG signals, respectively. Additionally, the respiratory rate has been estimated from both signals and also six morphological parameters from PPG. In total, 26 features are obtained for each subject. They provide information about the autonomic nervous system and the physiological response of the subject to an attention demand task. Results show an increase of sympathetic activation when the subjects perform the attention test. The amplitude and width of the PPG pulse were more sensitive than the classical sympathetic markers (<inline-formula><tex-math notation="LaTeX">$P_{\text{LFn}}$</tex-math></inline-formula> and <inline-formula><tex-math notation="LaTeX">$R_{\text{LF/HF}}$</tex-math></inline-formula>) for identifying this attentional state. State classification accuracy reaches a mean of <inline-formula><tex-math notation="LaTeX">$\text{89} \pm \text{2}\%$</tex-math></inline-formula>, a maximum of <inline-formula><tex-math notation="LaTeX">$\text{93}\%$</tex-math></inline-formula>, and a minimum of <inline-formula><tex-math notation="LaTeX">$85\%$</tex-math></inline-formula>, in the 100 classifications made by only selecting four parameters extracted from the PPG signal (pulse amplitude, pulsewidth, pulse downward slope, and mean pulse rate). These results suggest that attentional states could be identified by PPG.