Abstract
We reviewed the recent progress in the ability of electroencephalography (EEG) and magnetoencephalography (MEG) to detect pain perception in humans. Positron emission tomography (PET) and functional magnetic resonance imaging (fMRI), which reflect metabolic or blood flow changes by painful stimulation, have excellent spatial resolution in the order of mm, but their temporal resolution is not very high. In contrast, EEG and MEG, which reflect physiological changes, have an excellent temporal resolution in the order of msec, however, it is difficult for EEG and MEG to detect activities in deep areas such as the thalamus. Since the spatial resolution of EEG is not very high in the order of cm, MEG is useful for detecting activated areas following painful stimulation.
For recording activities following Aδ fiber stimulation relating to the first pain, MEG were usually recorded following painful CO2 laser stimulation, but our new method, epidermal stimulation (ES), is also very useful. The primary small activity was recorded from the primary somatosensory cortex (SI), probably in area 1, in the hemisphere contralateral to the stimulation. Then, the secondary somatosensory cortex (SII) and insula were activated with the second activity in SI. These three regions were activated in parallel with almost the same time period. This is a very characteristic finding in pain perception. Then, the cingulate cortex and medial temporal area (MT) around the amygdala and hippocampus were activated. In the hemisphere ipsilateral to the stimulation as well, the above regions were activated, except for SI. Therefore, SI is considered to play a main role in localization of the stimulus point, the SII and insula are important sites for pain perception, and the cingulate and MT are mainly responsible for cognitive or emotional aspects of pain perception.
For recording EEG and MEG activities following C fiber stimulation relating to the second pain, we recently developed a new method of applying weaker CO2 laser stimuli to tiny areas of the skin. MEG findings following C fiber stimulation were also similar to those following Aδ fiber stimulation. However, the effect of sleep and attention on MEG following C fiber stimulation was much larger than that following Aδ fiber stimulation. This findings may suggest the greater effects of cognitive or emotional functions on the second pain than on the first pain.
