Neural correlates of tactile localization in external and somatotopic frames of reference

Abstract

Previous behavioral work has shown that tactile stimuli are represented in both somatotopic and external frames of reference. Our knowledge of the neural correlates of external representations of tactile stimuli is limited. For example, multi-voxel pattern analysis (MVPA) studies of tactile localization have not disambiguated between somatotopic and external reference frames. To examine this, we used MVPA in two fMRI experiments to identify brain regions involved in somatotopic and external tactile processing. Vibrotactile stimuli were presented to two possible somatotopic locations on the hand in two different postures. To examine hand-centered external processing, vibrotactile stimuli were presented to either the index or ring finger of the right hand while the palm was facing up or down (Experiment 1). In Experiment 2, we presented vibrotactile stimuli to either the left or the right index finger while the hands were crossed or uncrossed to examine trunk-centered external processing. Whole-brain MVPA analyses were used to decode activation patterns for somatotopic and external spatial representations. For both experiments, we found typical regions associated with somatotopic processing (contralateral S1, S2, and premotor cortex), with additional somatotopic regions in Experiment 2 (contralateral supramarginal gyrus, SMA). In Experiment 1, we found significant decoding of hand-centered space in contralateral anterior cingulate and surprisingly, in contralateral secondary visual cortex. However, we found no evidence for external body-centered processing in Experiment 2. Altogether, our results suggest a visual component relevant to tactile spatial representation in an external, hand-centered representation. We also demonstrate the presence of somatotopic representations of touch in areas beyond primary somatosensory cortex. ☐ Keywords: neural correlates, fMRI, multi-voxel pattern analysis, touch localization, frames of reference, somatotopic, external space