During the early part of non-nociceptive/nociceptive processing, in addition to the decreased activity in SI area, the decreased contralateral frontal and left precuneus activation in the second and third phase was also found respectively. During phase 4, only significant decreased brain activity in the contralateral SI area was found for nociceptive condition compared to non-nociceptive condition. Component in this phase is consistent with the well-established ERP component of N2. The locations of N2 were consistently reported to be mainly focused on the bilateral SII areas by dipolar source analysis [8, 9]. During phase 4, only significant decreased brain activity in the contralateral SI area was found for nociceptive condition compared to non-nociceptive condition. The role of late SI activity in pain processing is still unknown. In this phase, the nociceptive/non-nociceptive stimuli have been transferred to pain/somatosensory perception, and subjects would detect the spatial of stimulation sites so that they could make decision about whether the stimulus is target or not in next step.

The promising result suggests the different processing and pathway between pain and tactile inputs. Further study with time-varying source connectivity would provide a useful tool to identify pain and tactile sensations.