Intra- and inter-hemispheric interactions in somatosensory processing of pain : dynamical causal modeling analysis of fMRI data
Pain is a perceptual experience comprising many dimensions. These pain dimensions interrelate with each other and recruit neuronal networks that process the corresponding information. Elucidating the functional architecture that supports different perceptual aspects of the experience is thus, a fundamental step to our understanding of the functional role of different regions in the cerebral pain matrix that are involved in the cortical circuitry underlying the subjective experience of pain. Among various brain regions involved in the processing of nociceptive information, primary and secondary somatosensory cortices (S1 and S2) are the main areas generally associated with the processing of sensory-discriminative aspect of pain. However the functional organization in these somatosensory areas is not completely clear and relatively few studies have directly examined the integration of information among somatic sensory regions. Thus, several questions remain regarding the hierarchical relationship between S1 and S2, as well as the functional role of the inter-hemispheric connections of the homologous somatic sensory areas. Likewise, the question of serial or parallel processing within the somatosensory system is another questionable issue that requires further investigation. The purpose of the present study was to test a number of causal hypotheses regarding the functional interactions between S1 and S2, while subjects were receiving painful electric shocks. We implemented a connectivity modeling approach, which utilizes a causal description of system dynamics, in order to study the interactions among activation sites defined by a data set derived from a functional imaging study. Our paradigm consists of 3 experimental scans using electric shock stimuli, with the stimulus intensity changing from moderately painful (level 3), to slightly painful (level 2), and to completely non-painful (level 1) during the final scan. Therefore our paradigm allowed us to investigate how stimulus intensity is encoded within our network of interest, and how the connectivity of the different regions is modulated across the different stimulus conditions.   Our result is in favor of serial mode of somatosensory processing with thalamocortical input to S1 contralateral to stimulation site. Thus our results implicates that pain information is propogated from S1 contralateral through our network of interest comprising of bilateral S1 and S2. Our analysis indicates that S1→S2 connection is modulated by pain, which suggests that S2 is higher on the hierarchy of pain processing than S1, in accordance with previous neurophysiological and MEG findings. Lastly, our analysis provides evidence for the entrance of somatosensory information into the hemisphere contralateral to the stimulation side, with inter-hemispheric connections responsible for the transfer of information to the ipsilateral hemisphere.
Pain Somatosensory cortex FMRI Connectivity Causality IRMf