Physiological investigation of pain perception in humans

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We investigate pain-related somatosensory evoked potentials follwing CO_2 laser stimulation (pain SEPs) to elucidate the mechanisms of pain perception in humans. We caluculated conduction velocity (CV) of small myelinated fibers (Adelta) and the spinothalamic tract which pain sensation ascends through by using pain SEPs.CV of both of them were about 10 m/sec.We investigated machanisms of change of pain perception induced by various modalities of sensation in normal subjects. Concurrently applied vibratory stimuli to and active movements of fingers significantly reduced and prolonged pain SEPs, but continuous cooling did not. The findings were consistent with gate control theory (Melzack & Wall). Pain SEPs were also significantly attenuated by voluntary and passive movements of remote areas such as the foot. This finding was not produced by vibration or movement imagery of the limbs without active movement. Therefore, interactions between pain perception and movement-related cortical activities must take place in some areas of the brain without relieving pain.We studied pain SEPs in patients with various neurological diseases such as peripheral neuropathies, syringomyelia, multiple sclerosis (MS) and HTLV-I-associated myelopathy (HAM). Results of pain SEps showed a significant correlation with a degree of pain impairment. In addition, subclnical lesions could be detected by pain SEPs in MS and HAM.Pain SEPs were very useful for clinical testing, particularly when we recorded and analyzed both pain SEPs and conventional electrically-stimulated SEPs.