Fig 1.
To examine the role of tactile expectations on the magnetic touch illusion and on limb ownership in general, four experimental conditions were included: Visual stimulation in the form of a brush slowly approaching the rubber hand in a continuous ‘tapping’ motion, while synchronous taps were delivered to the hidden real hand using an identical brush (visuo-tactile). This condition should elicit the magnetic touch illusion [12]. The visual only—brush approaching condition featured identical visual stimulation as in visuo-tactile, but no tactile stimulation of the real hand. The visual only—hand approaching condition consisted of the experimenter’s hand slowly approach the rubber hand, as in [4]. Theoretically, both of these conditions should induce tactile expectations, because the trajectory of the visual stimulus is along a path on “collision course” with both the real and rubber hands [13]. In the visual only—hand static condition, the experimenter’s hand was held in a fixed position 30cm above the rubber hand. This condition should not be associated with tactile expectations and served as a baseline condition to control for the effect of simply looking at a rubber hand. *EDA = electrodermal activity. Illustration credit: Mattias Karlén.
Table 1.
Questionnaire statements.
Fig 2.
(A) Questionnaire results. Ratings of the questionnaire statements (Table 1) for each of the four experimental conditions. The error bars denote the SEM. (B) Mean threat-evoked skin conductance response (SCR), which was defined as the difference in conductance between the first moment the participant observed the knife threat and the peak in conductance occurring within 5 s. The error bars denote the SEM. (C) The temporal profiles of the threat-evoked SCRs, normalized relative to the conductance at t = 0s. The duration of the threat event was 2 s.
Fig 3.
(A) Motion tracking results. All motion tracking data points (x-, y- and z-coordinates), color-coded according to the real-time rating of illusory magnetic touch, are shown for all participants and each condition. The surface of the rubber hand was also mapped and is shown in black. (B) Real-time illusion vividness ratings. The strength of the magnetic touch illusion, averaged for each 1-cm-interval between 60 and 3cm, is shown for each condition separately. In conjunction with visual inspection of the graphs in panel A, these results show that the illusion was present in the visuo-tactile condition, but in neither of the conditions involving tactile expectations (brush and hand approaching), nor in the baseline condition consisting of simply looking at a rubber hand (hand static). (C) Skin conductance as a function of distance. None of the conditions were associated with a change in skin conductance level as a function of the distance between the rubber hand and the approaching object. (D) Average skin conductance for every 10-cm-interval. Error bars denote the SEM. *In the visual only—hand static condition, during which no approaching movement was performed, the ratings (panel B) and skin conductance (panel C) are plotted against time (average over each 0.5-s interval between 0-29s) instead of distance.