Abstract
Hypnotic susceptibility is a major factor influencing the study of the neural correlates of hypnosis using EEG. In this context, while its effects on the response to hypnotic suggestions are undisputed, less attention has been paid to “neutral hypnosis” (i.e., the hypnotic condition in absence of suggestions). Furthermore, although an influence of opened and closed eye condition onto hypnotizability has been reported, a systematic investigation is still missing. Here, we analyzed EEG signals from 34 healthy subjects with low (LS), medium (MS), and (HS) hypnotic susceptibility using power spectral measures (i.e., TPSD, PSD) and Lempel-Ziv-Complexity (i.e., LZC, fLZC). Indeed, LZC was found to be more suitable than other complexity measures for EEG analysis, while it has been never used in the study of hypnosis. Accordingly, for each measure, we investigated within-group differences between rest and neutral hypnosis, and between opened-eye/closed-eye conditions under both rest and neutral hypnosis. Then, we evaluated between-group differences for each experimental condition. We observed that, while power estimates did not reveal notable differences between groups, LZC and fLZC were able to distinguish between HS, MS, and LS. In particular, we found a left frontal difference between HS and LS during closed-eye rest. Moreover, we observed a symmetric pattern distinguishing HS and LS during closed-eye hypnosis. Our results suggest that LZC is better capable of discriminating subjects with different hypnotic susceptibility, as compared to standard power analysis.
Highlights
Hypnosis refers to a variety of practices capable of inducing a highly focused, absorbed attentional state that minimizes competing thoughts and sensations [1,2]
Between-group analysis of total power spectral density (TPSD) did not show any significant difference among HS, MS, LS in any experimental condition; the results are not reported
Given the abovereported interpretation of Lempel-Ziv complexity (LZC) findings and given previous observations of enhanced fast-α amplitude at rest in the posterior brain of LS [76], we suggest that the significant increase in complexity of the α activity could indicate the tendency of LS to deeply relax after the hypnotic induction, rather than experience a mental dimension linked to neutral hypnosis
Summary
Hypnosis refers to a variety of practices capable of inducing a highly focused, absorbed attentional state that minimizes competing thoughts and sensations [1,2]. This is achieved through a two stage procedure involving induction and suggestion. Suggestions are, instead, communicable representations, typically conveyed through metaphors and visual imagery, intended to alter emotions, perceptions, cognitions, or ideomotor processes [3] Under this induction-suggestion paradigm, hypnosis has been widely used as an investigative tool for cognitive, emotional, and social neuroscience research through behavioral and neuroimaging studies [2,4], as well as an applicative tool in clinical settings [5].
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