Studies of Somatosensory and Pain Neural Circuits with High Field Functional MRI

Abstract

The primary somatosensory cortex (SI) plays a principal role in the processing of both somatosensory and pain information. The organization and function of this cortical region have been studied for over a century using invasive anatomical and physiological methods that give us insight into the architecture of the human cortex and are practical for studies in most mammalian species, but are unfeasible approaches to studying this region in most people. With the advent of ultra-high (7-Tesla) non-invasive magnetic resonance imaging (MRI) we are now able to study the cortex with the resolution and precision once available by invasive techniques. In this work we use 7-T MRI to explore how human SI responds to and processes somatosensory and noxious stimuli. We aimed to evaluate the ability of BOLD signals at high MRI field (7 T) to map fine-scale activations in subdivisions of SI in individual subjects. We acquired BOLD fMRI data from cortical areas around the central suclus in healthy human subjects while stimulating individual finger pads with innocuous tactile or noxious thermal stimuli. In both areas 3b and 1 of contralateral SI, the responses to tactile stimulation of individual digits organized somatotopically. The spatial and temporal profiles of the BOLD signal showed greater overlap of digit responses in area 1 than area 3b, suggesting that these areas play different roles in touch perception. Within individual subjects, the cortical responses to tactile stimulations and the magnitude of the BOLD signals were reproducible across imaging runs and were comparable across subjects. We also identified cortical activation in bilateral areas of SI (3a, 3b, 1, and 2) that correlated to intensity estimation of pain. Functional connectivity between these areas was significant in the resting state, suggesting an intrinsic pain network. Moreover, it appears that resting state correlations may vary with time before and after a painful episode and thus show dynamic properties that merit further study. Our findings demonstrate that BOLD fMRI at 7T is capable of revealing the functional organization of areas 3b and 1 of SI with good within-subject reliability and reproducibility, and activation and correlation maps can be acquired within a reasonably short time window, which are essential characteristics for several neurological and psychiatric applications within patient populations.