Effects of stimulus intensity on signals from human somatosensory cortices

WE recorded somatosensory evoked magnetic fields (SEFs) to left median nerv e electric stimulation from seven healthy subjects. The stimulus intensity was varied in three sessions: sensory stimuli evoked a clear tactile sensat ion without any movement, weak motor stimuli exceeded the motor threshold, and strong motor stimuli caused a vigorous movement. Responses were modelle d with sources in the contralateral primary somatosensory cortex (SI), the contralateral and ipsilateral secondary somatosensory cortices (SIIs) and t he contralateral posterior parietal cortex (PPC). The amplitude of the 20 m s response from the SI cortex and the subjective magnitude estimations foll owed the stimulus intensity whereas signals from the three other areas satu rated already at the level of the motor threshold. The results implicate di fferential roles for various somatosensory cortices in intensity coding. (C ) 1998 Lippincott Williams & Wilkins.