Pupillometric assessment
As auditory stimulation causes pupil dilation (Strauch et al., 2020), we ensured that participants could not hear the sound of the tapper. Participants had to wear earplugs and noise-cancelling headphones (Sony WH-1000x M3) playing brown noise. The volume was set to the highest level that the participant found comfortable. To check whether the sound of the tappers was sufficiently masked, a sound detection task was performed. Participants were presented with a grey screen for 1 s, during which a tapper could be fired. Participants then had to indicate whether or not they had heard a tapper by pressing a key, with written instructions presented on the screen after each trial. A tapper would fire in 20 out of 30 trials, in random order. Most participants (n = 11) could not discriminate between tapper-present and tapper-absent trials (d’ = 0), one showed limited sensitivity (d’ = 0.08), and two showed reversed sensitivity (d’ = -0.25 and d’ = -1.60).
Adhesive tape was then used to secure the tappers to the tip of the little finger, forearm (positioned in a downward orientation), and calf, ensuring that they were hidden from view (Figure 1A). A foam ring was placed at the base of the little finger to prevent contact with the adjacent ring finger. A fourth tapper was placed on a cushion, to be used in a control condition. The experiment consisted of four blocks, one for each of the three body locations and a control block, which were counterbalanced using a Latin square design. To ensure that the tactile stimulation was similar across locations, the same tapper was always used for the given stimulus location within a block and thus the tapper was swapped between blocks. Each block consisted of 25 trials for the given stimulus location. In addition, every sixth trial, a randomly selected different tapper was fired, to avoid habituation or expectation effects. These trials were not included in the analysis.
The trial sequence is shown in Figure 1B. Throughout the trial, participants had to look at a central fixation cross (white, grey background). Trials started with a variable baseline period of 0.5 to 2.5 s, which was set to 1.5 to 2.5 s after the first 14 participants to ensure a stable baseline pupil measurement. Tactile stimulation was then applied and the pupil response was recorded for a further 1.5 s. Trials were considered invalid if participants blinked or looked >3 visual degrees from the centre for >200 ms. The participant received feedback via a red (invalid) or grey (valid) cross of 0.5 s. After feedback, an intertrial interval of 1.5 s occurred. Invalid trials were repeated at the end of each block. If blinks or gaze deviations from the centre occurred during the baseline, the baseline timer was simply reset to zero and no feedback was given.