Introduction
Touch perception is central to many aspects of our daily lives. We use
it to detect a mosquito on our arm, to comfort a friend, or to control
the amount of pressure we apply when grasping an object. Our perception
of a touch has many different characteristics, and it’s intensity is one
of them. Perceived touch intensity is influenced by a number of factors.
It depends on stimulus characteristics, such as the amount of pressure
applied to the skin, but also on non-stimulus characteristics, such as
the sensitivity of the part of the body being touched (Weinstein, 1968).
The ability to perceive touch, and differences in perceived touch
intensity are typically assessed using psychophysical methods involving
overt motor or verbal responses (e.g. Fritz & Zimmermann, 2023; Kusnir
et al., 2023). While this has many advantages, it does not allow for the
assessment of more implicit representations and prevents the testing of
touch perception in situations where overt verbal and motor responses
are not possible.
Changes in pupil size are a promising candidate for providing an
objective psychophysiological index. The eye’s pupil does not only
respond to changes in low-level vision, but also reflects attentional
processing (see Strauch et al., 2022 for a review). More specifically,
pupil size is arguably the most sensitive psychophysiological indicator
of the mental effort involved in any given physical or cognitive
process. This is likely due to the close link between pupil size and
activity in the norepinephrinergic locus coeruleus in the brainstem
(Alnaes et al., 2014; Aston-Jones & Cohen, 2005; Joshi et al., 2016;
Murphy et al., 2014; Schwarz et al., 2015; Strauch et al., 2022). The
locus coeruleus has widespread projections throughout the brain and is
thought to be involved in the coordination and collaboration of neural
populations, including flexibly switching between circuits and
synchronizing activity (Dahl et al., 2022; Poe et al., 2020; Wainstein
et al., 2022). As more intense tactile perception should go hand in hand
with more intense processing thereof, we predicted that pupils would
dilate in response to tactile stimulation, and that the more intense the
stimulation is perceived the more dilation would occur.
The effects of pain on pupil size were described already more than a
century ago (Bumke, 1911) and have been reported in a variety of
populations (Drummond & Clark, 2023; Ji et al., 2022; Macchini et al.,
2022; Sillevis et al., 2021; Yılmaz, 2022). Only a few modern-day
studies have directly investigated the effects of non-painful
tactile stimulation on pupil size, mostly in animals (Gusso et al.,
2021). The studies in humans have shown that pupils dilate in response
to tactile stimulation, with some indications that the magnitude of this
dilation is modulated by whether the stimulus is consciously perceived
(Gusso et al., 2022), the stroke speed (van Hooijdonk et al., 2019), the
frequency of vibrotactile stimulation (Mückschel et al., 2020), and the
type of the material that participants actively touched (Bertheaux et
al., 2020). While these findings suggest that pupil size changes scale
with stimulus intensity, previous studies suffer from serious
methodological limitations, such as the non-automated delivery of
tactile stimulation (Bertheaux et al., 2020; van Hooijdonk et al.,
2019). These shortcomings make it impossible to draw substantive
conclusions, for example about the temporal course of tactile
processing. However, if pupil size can indeed serve as a reliable
indicator of tactile perception, this would allow for the objective
investigation of a variety of questions: For example, how intensely is
tactile stimulation processed with differing degrees of attention paid
to a particular part of the body, or as a function of conscious
perception (Gusso et al., 2022)? How strong is the processing of tactile
stimulation as a function of stimulation intensity and frequency,
receptor density, or skin and receptor type in healthy subjects and in
pathology? Does pupil size show residual processing of tactile
stimulation intensity in patients with somatosensory impairments after
brain lesions?
In two experiments, for which the overarching hypotheses were
pre-registered, we investigated whether and how well changes in pupil
size can indicate the objective intensity of tactile processing – and
thus the basis for how intensely tactile stimulation is perceived. In
Experiment 1, we measured pupil size in response to stimulation by a
tapper on body parts which differ in tactile sensitivity (Weinstein,
1968) in addition to a non-stimulation baseline. We expected more
dilation with stimulation than without stimulation, and a greater
increase in pupil size for more subjectively sensitive body parts than
for less sensitive body parts. In Experiment 2, we stimulated only the
little finger, but at different vibration intensities and against a
non-stimulation baseline. Again, we expected more dilation with
stimulation than without stimulation, and more pronounced effects with
more intense stimulation than with less intense stimulation. Finally, we
expected differences in pupil size to reflect differences in subjective
tactile sensitivity as assessed by von Frey filaments (Experiment 1) and
differences in tactile discrimination performance (Experiment 2).