Serial processing in primary and secondary somatosensory cortex: A DCM analysis of human fMRI data in response to innocuous and noxious electrical stimulation

• We explored parallel versus serial mode of processing in human somatosensory cortex.
• DCM was applied to fMRI brain responses to cutaneous electrical stimulation.
• Results support serial processing from S1 to S2 at innocuous and noxious intensities.
• Connectivity patterns change as a function of pain at the more intense level.

The anatomy of the somatosensory system allows both serial and parallel information flow but the conditions involving each mode of processing is a matter of debate. In this functional magnetic resonance imaging (fMRI) study, cutaneous electrical stimulation was applied to human volunteers at three intensities (low-innocuous, moderate-noxious and high-noxious) to investigate interactions between contralateral primary and secondary somatosensory cortices (S1c and S2c), and between contralateral and ipsilateral S2 (S2c and S2i), using dynamic causal modeling (DCM). Our results are consistent with serial processing with a key role of the direct input to S1c for all three intensity levels. The more intense stimulus also induced significantly more interactions between S2i and S2c, consistent with an increase in inter-hemispheric integration associated with the additional recruitment of nociceptive inputs. However, stronger pain reports were also associated with reduced information flow from S1c to S2c at both the moderate (r = −0.81, p = 0.004) and the high stimulation level (r = −0.63, p = 0.037). These findings suggest that the connectivity pattern driven by innocuous inputs is modified by the additional activation of nociceptive afferents.