**1.1 Heart-rate variability as a measure of hypnotic depth**

Ensuring sufficient depth to induce genuine hypnotic responsiveness is one of the major challenges in hypnotic research [7]. Hypnotic susceptibility, a stationary characteristic of a subject, is distinct from hypnotic depth, which is a dynamic property that represents the subject's momentary capacity for response to hypnotic suggestions [8].

Electro-encephalogram (EEG) is a candidate for monitoring neural activity. Higher power over the alpha, beta, and theta parietal region associated with sustained-attentional processing and greater EEG asymmetry was seen in the highly hypnotizable subjects [9].

In an investigation, heart rate, frequency of changes from a time-frequency analysis of heart-rate variability (HRV), and amplitude were compared with the continuous self-rated hypnotic depth (SRHD) of 10 volunteers. The investigation recorded significant linear relationships between the high-frequency (HF) component of the heart-rate variability (HRV) and the continuous self-rated hypnotic depth (SRHD), the continuous self-rated hypnotic depth (SRHD) correlated positively with the amplitude of the HF component and negatively with the frequency of the HF [10].
