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.