Comparison between SI and SII responses as a function of stimulus intensity.

In this MEG study we investigated the differences in responses to somatosensory electrical stimuli between primary (SI) and secondary (SII) sensory cortices using 10 different levels of stimulus intensity, starting from below the sensory threshold up to a weak painful level. SI dipole source linearly increased in amplitude as the stimulus intensity raised up to a strong motor level and then saturated at higher stimulation levels. The contralateral and ipsilateral SII dipole source strengths followed the stimulus intensity growing up to the motor threshold, but showed a decrease at the strong motor level, followed by an increase as the stimulus intensity raised towards the weak painful threshold. These results suggest different responses of SI and SII cortices as the intensity of stimulation rises from non-painful to painful values.