Table 1.

Overview of the main implicit techniques used in human behaviour research.

Implicit Technique	Biometric Signal Measured	Sensor	Features	Psychological or Behavioural Construct Inferred
EDA (electro dermal activity)	Changes in skin conductance	Electrodes attached to fingers, palms or soles	Skin conductance response, tonic activity and phasic activity	Attention and arousal [80]
HRV (heart rate variability)	Variability in heart contraction intervals	Electrodes attached to chest or limbs or optical sensor attached to finger, toe or earlobe	Time domain, frequency domain, non-linear domain	Stress, anxiety, arousal and valence [81,82]
EEG (electroencephalogram)	Changes in electrical activity of the brain	Electrodes placed on scalp	Frequency band power, functional connectivity, event-related potentials	Attention, mental workload, drowsiness, fatigue, arousal and valence [83,84]
fMRI (functional magnetic resonance imaging)	Concentrations of oxygenated vs. deoxygenated haemoglobin in the blood vessels of the brain	Magnetic resonance signal	blood-oxygen-level dependent	Motor execution, attention, memory, pain, anxiety, hunger, fear, arousal and valence [85]
fNIRS (functional near-infrared spectroscopy)	Concentrations of oxygenated vs. deoxygenated haemoglobin in the blood	Near-infrared light placed on scalp	blood-oxygen-level dependent	Motor execution, cognitive task (mental arithmetic), decision-making and valence [86]
ET (eye-tracking)	Corneal reflection and pupil dilation	Infrared cameras point towards eyes	Eye movements (gaze, fixation, saccades), blinks, pupil dilation	Visual attention, engagement, drowsiness and fatigue [87]
FEA (facial expression analysis)	Activity of facial muscles	Camera points towards face	Position and orientation of head. Activation of action units	Basic emotions, engagement, arousal and valence [88]
SER (speech emotion recognition)	Voice	Microphone	Prosodic and spectral features	Stress, basic emotions, arousal and valence [89]