Somatosensory cortical representations of assimilation effect by vibrotactile stimulation

This study investigated neural activity related to the assimilation effect in the perception of vibrotactile stimuli. The assimilation effect refers to a tactile perceptual bias in which the perception of the vibrotactile frequency on one finger is biased towards a distracting vibrotactile stimulus targeting other types of mechanoreceptors on the different finger. The assimilation effect occurs not only between fingers on the same hand (in-hand) but also between fingers on different hands (across-hand). These behavioral aspects of the assimilation effect lead to an assumption that neural processes related to the assimilation effect would involve the integration of different tactile information mediated by the somatosensory cortex. We addressed this hypothesis by investigating brain responses using functional magnetic resonance imaging (fMRI) to vibrotactile stimuli that induced assimilation effect under in-hand and across-hand conditions. We first observed that the vibrotactile stimuli presented in this study activated primary (S1) and secondary (S2) somatosensory cortices. Yet, neural responses in these regions did not show correlations with individual assimilation effects, indicating that neural processing of vibrotactile signals in S1 and S2 would not be directly linked to the assimilation effect. Instead, we found that connectivity between S1 and medial prefrontal cortex (mPFC) was correlated with individual in-hand assimilation effects and that connectivity between S2 and posterior parietal cortex (PPC) was correlated with individual across-hand assimilation effects. These results suggest that the assimilation effect may be related to tactile information integration via functional connections between the somatosensory cortex and higher-order brain regions.