Tactile force evokes a biphasic BOLD response in ipsilateral primary somatosensory cortex

Tactile perception is fundamental to how we engage with and interpret our surroundings. While the contralateral primary somatosensory cortex (S1) is well known to play a role in processing tactile information, lesser understood responses in the ipsilateral hemisphere have been observed. We aim to characterize the blood-oxygen-level-dependent (BOLD) response pattern of ipsilateral S1 to unilateral tactile stimuli. Three stimulus force levels were applied in an event-related design. As expected, we observed an apparent difference in BOLD responses between the contralateral and ipsilateral hemispheres. We found differing response patterns between the Brodmann's areas (BAs) within ipsilateral S1. Ipsilateral BA2 had a positive BOLD response similar to that of the contralateral hemisphere. In contrast, ipsilateral BA1 and BA3b appeared to have a biphasic response. That is, those regions had an initial negative response, followed by a secondary positive one. Using multi-echo analysis, we evaluated whether this biphasic response pattern is BOLD related. We hypothesize that this biphasic response is driven by two periods of neural activity. In addition, the secondary (positive) phase was more sensitive to tactile stimulus force level than the initial "negative" BOLD phase. The ipsilateral BA1 and BA3b responses support a previous hypothesis of bilateral sensory gating. This study extends previous research reporting negative responses in ipsilateral S1, suggesting that the response is consistent with a BOLD signal, biphasic, and that the previously overlooked secondary positive phase is actually more sensitive to stimulus intensity.