Observing social stimuli influences detection of subtle somatic sensations differently for pain synaesthetes and controls

Introduction: There is preliminary evidence that viewing touch or pain can modulate the experience of tactile stimulation. The aim of this study was to develop an experimental paradigm to investigate whether the observation of needle pricks to another person’s hand facilitates the detection of subtle somatic sensations. Furthermore, differences between control persons and persons reporting synaesthesia for pain (i.e., experiencing observed pain as if it is their own pain) will be examined. Method: Synaesthetes ( n = 15 ) and controls ( n = 20 ) were presented a series of videos showing left or right hands being pricked and control videos (e.g., a sponge being pricked), whilst receiving occasionally subtle threshold sensations themselves on the hand in the same spatial location (congruent trials) or in the opposite location (incongruent trials) as the visual stimuli. Participants were asked to detect the sensory stimulus. Signal detection theory was used to compare whether sensitivity was different for both groups and both categories of visual stimuli. Results: Overall, perceptual sensitivity (d′) was significantly higher when the visual stimuli involved a painful situation (e.g., needle pricking another’s hand) compared to the control videos, and was significantly lower in synaesthetes compared to control participants. When no sensory stimulus was administered, participants reported significantly more illusory sensations when a painful situation was depicted compared to a non-painful situation. Discussion: This study suggests that the detection of somatic sensations can be facilitated or inhibited by observing visual stimuli. Synaesthetes were generally less sensitive, suggesting that they experience more difficulties in disentangling somatic and visual stimuli.