Finger Proprioception During Synchronous and Asynchronous Stroking

Participants were divided into 2 groups of 10 each. The first group was tested in the synchronous, the second in the asynchronous stroking condition. Participants were exposed to stroking with measurements of finger proprioception every 10 s for 7 min (timeline: Fig. 1, bottom). For the measurement the light was switched off under the semi-silvered mirror (no vision of the rubber hand or the frame of the monitor or the mirror) and the visual probe dot was projected into the field of view at roughly the same height as the participant's unseen hand and the visible rubber hand. Participants had to respond (forced choice) whether the dot was to the left or to the right of the perceived position of the invisible left index finger of their own hand. The location of the dot changed at every trial according to two alternating simple up-down staircase algorithms (staircase steps: 4 cm and 1 cm) [9] (link). The staircases move the dot at each step into the opposite direction that the participants report, so that, over time, the dot moves closer to the participant's perceived lateral location of the index finger. Both staircases started at the participant's real index finger position at 0 cm. Before stroking, participants' proprioceptively felt finger position was tested in two pre-tests (one in the dark, one when seeing the rubber hand). At the end of the experiment, participants had to fill out the RHI questionnaire [1] (link), supplemented with a German translation.