Table 5.

Some biological-based measures.

Biological Signals	Description
Electroencephalography (EEG) [73]	An EEG signal is a monitoring method that records the brain’s electrical activity from the scalp. It represents the microscopic activity of the brain’s surface layer underneath the scalp. Based on the frequency ranges (0.1 Hz–100 Hz), these signals are categorized as delta, theta, alpha, beta, and gamma.
Electrocardiography (ECG) [74]	ECG signals represent the electrical activity of the heart, which are acquired using electrodes placed on the skin. ECG monitors heart functionality, including heart rhythm and rate.
Photoplethysmography (PPG) [75]	PPG signals are used to detect blood volume changes. These signals are measured at the skin’s surface using a pulse oximeter. It is often used for heart rate monitoring.
Heart rate variability (HRV) [76]	HRV signals are used to monitor the changes in the cardiac cycle, including the heartbeats.
Electrooculography (EOG) [77]	EOG signals are used to measure the corneo-retinal standing potential between the front and back of the human eye and record the eye movements.
Electromyography (EMG) [78]	EMG signals are the collective electric signals produced from muscles movement.