Table 2.

Definitions and ergonomic interpretations of multimodal eye–brain–heart features.

Modality	Feature	Calculation/definition	Ergonomic significance & physical meaning
Eye-tracking	Pupil Diameter	Mean diameter of the pupil (mm) over the task block	Reflects cognitive arousal and mental workload. Dilation typically indicates active information processing or high cognitive demand.
	Blink Rate	Number of blinks per minute (blinks/min)	Associated with visual fatigue and task monotony. A significant increase points to passive task disengagement and drowsiness.
EEG	Engagement Index (EI)	$P_{\beta }/(P_{\alpha }+P_{\theta })$	Used as an EEG spectral proxy for task engagement and sustained attentional allocation. Higher values indicate a relative dominance of beta-band activity over theta/alpha activity, which is commonly associated with active cognitive processing and increased attentional demand.
	Fatigue Index (FI)	$(P_{\alpha }+P_{\theta })/P_{\beta }$	Used as an EEG spectral proxy for fatigue-related cortical slowing and reduced task engagement. Higher values indicate a relative increase in theta/alpha activity compared with beta activity, which is typically associated with decreased vigilance, mental fatigue, or drowsiness-related changes.
ECG (HRV)	LF/HF Ratio	Ratio of low-frequency (0.04-0.15 Hz) to high-frequency (0.15-0.4 Hz) power	Represents sympathovagal balance. An elevated ratio indicates sympathetic dominance, often triggered by acute stress or active tension.
	RMSSD	Root mean square of successive R-R interval differences (ms)	Reflects parasympathetic nervous system activity. An increase in RMSSD correlates with physiological relaxation or passive underload.

$P_{\theta }$, $P_{\alpha }$, and $P_{\beta }$ denote the EEG spectral power in the theta (4–8 Hz), alpha (8–13 Hz), and beta (13–30 Hz) bands, respectively. EI and FI were used as operational EEG spectral indices rather than direct measurements of latent neurocognitive constructs.