Pupil responses objectively index pharmacologically altered tactile sensitivity

Tactile perception is a subjective experience, yet it can be physiologically quantified. This offers new avenues for studying sensory processing in contexts where verbal feedback is limited or unreliable. A growing body of research uses changes in pupil size, showing that stronger tactile stimuli lead to greater pupil dilation. Building on this, we investigated whether pupil responses could serve as an objective measure of tactile sensitivity. To explore this, we pharmacologically manipulated tactile sensitivity in healthy participants (n = 32). In separate sessions, an anaesthetic cream or a placebo cream was applied to one forearm. At the beginning and/or end of each session, Von Frey assessments and a vibrotactile detection task were conducted to confirm the efficacy of the anaesthetic cream in reducing tactile sensitivity. During each session, pupil responses to vibrotactile stimuli applied to both the cream and non-cream arms were recorded. Our results confirmed that the anaesthetic cream significantly reduced the perceived intensity of tactile stimulation, an effect that persisted throughout the session. Crucially, we observed weaker pupil dilation responses to vibrotactile stimuli applied to the anaesthetised arm compared to the placebo or non-cream arm. Exploratory analyses showed that participants for whom the anaesthetic cream was most effective in reducing tactile sensitivity also showed the weakest pupil responses when the anaesthetised arm was stimulated. Overall, these findings demonstrate that the pupil response is a reliable and objective index of tactile sensitivity, highlighting its potential for studying sensory processing in populations where verbal feedback is limited or unreliable.