Social Responsiveness Correlates with Neural Response to Affective Touch: An fNIRS Study

Background: Functional magnetic resonance imaging (fMRI) studies from our lab have recently implicated the superior temporal sulcus (STS) and additional ‘social brain’ regions in processing slow, gentle touch targeting C-tactile (CT) afferents. These nerves are only present in the hairy skin and are hypothesized to support the role of the skin as a social organ because they respond especially well to stroking velocities characteristic of social, or affective, touch. Functional near-infrared spectroscopy (fNIRS) and fMRI studies indicate that the STS response to visual social stimuli (such as faces and biological motion) varies as a function of observers’ autistic traits. Using fMRI, we recently discovered a similar relationship between STS response to CT-targeted touch and individuals’ autistic traits. Autistic traits have been shown to vary in the general population; more autistic traits reflect less social responsiveness, as measured by the Social Responsiveness Scale (SRS).

Objectives: Using fNIRS, we sought to examine whether STS response to CT-targeted affective touch varies as a function of social responsiveness.  

Methods: To date, 14 typical adults participated in this study. We monitored regional cerebral blood volume changes using a 52-channel NIRS apparatus over the prefrontal cortex and STS to measure brain activity while participants received continuous brushing to the arm and palm in a block design procedure. Brushing was administered by a trained experimenter using a soft watercolor paintbrush. Participants also completed the SRS.

Results: To implement a region of interest analysis, we coregistered participants’ NIRS optode placement to their own structural MRI image to identify the four optodes measuring activity in the right STS. Preliminary results indicate that, as predicted, activation in the STS was greater during arm touch (CT-targeted) trials compared to palm touch (non-CT) trials during the 6-12 second post-stimulus onset period (p < .01). We identified a trend towards a negative correlation between SRS scores and each participant’s average difference in STS activation between arm touch and palm touch (p = .065) and peak difference in STS activation between the two types of touch (p = .116) within the 6-12 second post-stimulus onset period.

Conclusions: Preliminary results from the current fNIRS study are consistent with our fMRI findings of STS involvement in processing CT-targeted touch. Importantly, we identified a negative correlation between participants' social responsiveness and their STS response to CT-targeted affective touch. Individuals with less social responsiveness exhibited a diminished STS response to social touch to the arm relative to the palm. This is the first fNIRS study to examine the neural response to social touch and lays the foundation for follow-up studies with individuals with and without autism.