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. 2025 Aug 13;20(8):e0329625.
doi: 10.1371/journal.pone.0329625. eCollection 2025.

Modulation of the post-auricular reflex in response to social and CT-optimal touch

Birgit Hasenack  1   2 , Anouk Keizer  1 , Olga Wódecka  1 , Leonard B Schaafsma  1 , H Chris Dijkerman  1 , David Terburg  1   3
Affiliations

Modulation of the post-auricular reflex in response to social and CT-optimal touch

Birgit Hasenack et al. PLoS One. .

Abstract

The pleasantness perception of CT-optimal touch is usually assessed with subjective and explicit measures. As these can be prone to biases, it is important to develop implicit measures as well. The vestigial post-auricular muscle reflex (PAR) might be a good candidate, given its sensitivity to pleasant visual and auditory stimuli. As such, we investigated if the PAR can also be modulated by CT-optimal touch. We additionally compared how the PAR responds to social and robotic touch and conducted control experiments to replicate the reflex's specific sensitivity to primary rewards. The sample consisted of 43 non-clinical participants. PAR responses were recorded while participants were touched by an experimenter and a robot, with a velocity of 3 cm/s (CT-optimal touch) and 18 cm/s (CT non-optimal touch). After each trial, participants also subjectively rated the pleasantness of the touch. Although the results revealed that CT-optimal touch was subjectively perceived to be more pleasant than CT non-optimal touch, it did not result in a potentiation of the PAR. Interestingly, social touch was subjectively perceived to be more pleasant than robotic touch and potentiated the PAR. The control experiments confirmed that the PAR is particularly modulated by primary (food, erotica), and not secondary (adventure, cuteness, monetary) rewards. While additional research is needed to further investigate the relation between the PAR and CT-optimal touch, the current results do already suggest that this reflex responds to the primary reward value of social touch.

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Conflict of interest statement

The authors have declared that no competing interests exist.

Figures

Fig 1
Fig 1. Robotic device.
The robotic device consisted of two foam probes attached to a rotating band (A). A mark on the table (A) and a folded cloth (B) were used to correctly position the arm of the participant.
Fig 2
Fig 2. Visualization of the experimental procedure.
A visualization of two blocks is represented in the figure, with the experiment consisting of 24 blocks in total. The same touch stimulus was presented in trials of a corresponding block. X represents the fixation cross that was presented on a computer screen. The duration of the break varied slightly between participants.
Fig 3
Fig 3. Post-auricular muscle activity (averaged across ears).
Averaged post-auricular muscle activity time-locked with sound presentation (x = 0) and baseline-corrected for activity during the preceding 50ms. CT = CT-optimal touch. NCT = CT non-optimal touch.
Fig 4
Fig 4. Average baseline-corrected PAR for the four touch conditions.
CT = CT-optimal touch. NCT = CT non-optimal touch. Lines represent a 95% confidence interval.
Fig 5
Fig 5. Average subjective pleasantness for the four touch conditions.
CT = CT-optimal touch. NCT = CT non-optimal touch. Lines represent a 95% confidence interval.
See this image and copyright information in PMC

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