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).