Abstract
Touch is important for many aspects of our daily activities. One of the
most important tactile characteristics is its perceived intensity.
However, quantifying the intensity of perceived tactile stimulation
beyond subjective self-reports remains challenging. Here, we show that
pupil responses can objectively index the intensity of tactile
stimulation in the absence of overt participant responses. In Experiment
1 (n=32), we stimulated three reportedly differentially sensitive body
locations (finger, forearm, calf) with a single tap of a tactor while
tracking pupil responses. Tactile stimulation resulted in greater pupil
dilation than a baseline without stimulation. Furthermore, pupils
dilated more for the more sensitive location (finger) than for the less
sensitive locations (forearm, calf). In Experiment 2 (n=20) we extended
these findings by manipulating the intensity of the stimulation with
three different intensities, here a short vibration, always at the
little finger. Again, pupils dilated more when being stimulated at
higher intensities as compared to lower intensities. In summary, pupils
dilated more for more sensitive parts of the body at constant
stimulation intensity and for more intense stimulation at constant
location. Taken together, the results show that the intensity of
perceived tactile stimulation can be objectively measured with pupil
responses – and that such responses are a versatile marker for touch
research. Our findings may pave the way for previously impossible
objective tests of tactile sensitivity, for example in minimally
conscious state patients.