Individual Variability in Muller-Lyer Illusion Difference Thresholds: Moderating Influences of Hypnotic Susceptibility in Waking and Hypnosis

Abstract

This study investigated the differential influences of hypnotic susceptibility on Muller-Lyer illusion difference thresholds in waking and hypnosis using the psychophysical method of constant stimuli. As assessed by the Harvard Group Scale of Hypnotic Susceptibility, Form A (HGSHS:A) and the Group Stanford Hypnotic Susceptibility Scale, Form C (GSHSS:C), sixteen high (scores = 9–12) and sixteen low (scores = 0–3) hypnotizables participated. In counterbalanced sessions of waking and hypnosis, each subject was sequentially exposed to 110 computer-generated Muller-Lyer illusion pairs of comparison stimuli and a standard stimulus. Difference thresholds for high and low hypnotizables did not differ significantly in waking, but were significantly lower for high than low hypnotizables in hypnosis. Difference thresholds were also significantly lower for high hypnotizables in hypnosis than in waking. Low hypnotizables performed similarly in waking and hypnosis.