Rapid Processing of Observed Touch through Social Perceptual Brain Regions: An EEG-fMRI Fusion Study

Abstract

Seeing social touch triggers a strong social-affective response that involves multiple brain networks, including visual, social perceptual, and somatosensory systems. Previous studies have identified the specific functional role of each system, but little is known about the speed and directionality of the information flow. Is this information extracted via the social perceptual system or from simulation from somatosensory cortex? To address this, we examined the spatiotemporal neural processing of observed touch. Twenty-one human participants (7 males) watched 500 ms video clips showing social and non-social touch during EEG recording. Visual and social-affective features were rapidly extracted in the brain, beginning at 90 and 150 ms after video onset, respectively. Combining the EEG data with fMRI data from our prior study with the same stimuli reveals that neural information first arises in early visual cortex (EVC), then in the temporoparietal junction and posterior superior temporal sulcus (TPJ/pSTS), and finally in the somatosensory cortex. EVC and TPJ/pSTS uniquely explain EEG neural patterns, while somatosensory cortex does not contribute to EEG patterns alone, suggesting that social-affective information may flow from TPJ/pSTS to somatosensory cortex. Together, these findings show that social touch is processed quickly, within the timeframe of feedforward visual processes, and that the social-affective meaning of touch is first extracted by a social perceptual pathway. Such rapid processing of social touch may be vital to its effective use during social interaction.Significance StatementSeeing physical contact between people evokes a strong social-emotional response. Previous research has identified the brain systems responsible for this response, but little is known about how quickly and in what direction the information flows. We demonstrated that the brain processes the social-emotional meaning of observed touch quickly, starting as early as 150 milliseconds after the stimulus onset. By combining EEG data with fMRI data, we show for the first time that the social-affective meaning of touch is first extracted by a social perceptual pathway and followed by the later involvement of somatosensory simulation. This rapid processing of touch through the social perceptual route may play a pivotal role in effective usage of touch in social communication and interaction.

Characteristics of vicarious touch reports in a general population

“…For instance, previous self-report studies found that observing a threatening touch-such as a knife tip touching a face-evokes stronger and more frequent mirrored sensations than benign touches like those from a fingertip or feather (Holle et al, 2011;Ward et al, 2018). While functional Magnetic Resonance Imaging (fMRI) studies have investigated the various neural mechanisms involved in observing social, affective, pleasant, and unpleasant touches, (Bolognini et al, 2013;Ebisch et al, 2011;Gazzola et al, 2012;Lamm et al, 2015;Lee Masson et al, 2018;Lee Masson and Isik, 2023;Morrison et al, 2011;Riva et al, 2018), it remains largely unclear if these varying observed touches evoke differing reportable sensations in the observer.…”