Palm-to-finger cortical functional interactions in primary somatosensory cortex: a 7T fMRI study

Many studies focused on the cortical representations of fingers, while the palm is relatively neglected despite its importance for hand function. Here, we investigated palm representation (PR) and its interactions with finger representations (FRs) in primary somatosensory cortex (S1). Few studies in humans suggested that PR is located medially with respect to FRs in S1, yet to date, no study directly quantified the somatotopic organization of PR and the five FRs. Importantly, the relationship between the somatotopic organization and the cortical functional interactions between PR and FRs remains largely unexplored. Using 7T fMRI, we mapped PR and the five FRs at the single subject level. First, we analyzed the cortical distance between PR and FRs to determine their somatotopic organization. Results show that the PR was located medially with respect to D5. Second, we tested whether the observed cortical distances would predict palm-finger functional interactions. Using three complementary measures of functional interactions (co-activations, pattern similarity and resting-state connectivity), we show that palm-finger functional interactions were not determined by their somatotopic organization, that is, there was no gradient moving from D5 to D1, except for resting-state connectivity, which was predicted by the somatotopy. Instead, we show that the representational geometry of palm-finger functional interactions reflected the physical structure of the hand. Collectively, our findings suggest that the spatial proximity between topographically organized neuronal populations do not necessarily predicts their functional interactions, rather the structure of the sensory space (e.g. the hand shape) better predicts the observed functional interactions.