When Flames Feel Real in Augmented Reality: Effects of Plausibility and Placement of Virtual Flames on the Burning Hand Illusion and Physiological Responses

In the Burning Hand Illusion (BHI), participants perceive illusory heat as they observe virtual flames on their hands in Augmented Reality. In this study, we investigate the role of stimulus location within peri-hand space and its resemblance to a realistic fire in the emergence of the illusion. Additionally, we examine the associated autonomic nervous responses, quantified through skin conductance and pupil dilation responses. We used the following four conditions: (A) Realistic flames on the right hand paired with the sound of a burning torch; (B) Green, flame-like visuals on the hand paired with a sound that does not evoke fire but is spectrally matched to the sound of the burning torch; (C) Realistic flames between the hands; and (D) Green, flame-like visuals between the hands. Sixteen out of twenty-five participants reported feeling heat in condition (A), while seven reported a sensation of warmth in condition (C). Condition (B) introduced ambiguity, as participants reported warmth as well as cold sensations, but overall, fewer participants reported thermal illusions. No participants reported thermal illusions in (D). These findings suggest that both the location and resemblance of the flames significantly influence the emergence of the BHI, with the response diminishing as the flames become less plausible or not placed on the actual hand. Skin conductance responses were highest in condition (A) and lower in condition (C), with the lowest responses observed in conditions (B) and (D). Pupil dilation responses were largest in conditions where the flames were directly on the hand. The observed autonomic responses reinforce the notion that the BHI elicits physiological effects that parallel those of real thermal stimuli. This provides evidence that the virtual fire in the BHI is sufficiently plausible to be perceived as affecting the hand, and is consequently processed by the brain’s homeostatic emotional systems.