Two evoked responses with different recovery functions in the primary somatosensory cortex in humans

Objectives: We investigated the recovery function of somatosensory evoked magnetic cortical fields (SEFs) to confirm the temporal aspects of the somatosensory process in humans. Methods: SEFs were recorded following median nerve electrical stimulation in 6 healthy subjects. Double stimulation, with interstimulus intervals (ISIs) from 3 to 100 ms, was applied, and the SEF components for the second stimulation were analyzed. In a supplementary experiment, responses to single stimulations of various intensities from the sensory threshold to the motor threshold were studied. Results: The first SEF component (1M) diminished when the ISI was less than 10 ms, while the second component (2M) remained even when the ISI was 3 ms. The two components showed a very similar attenuation with decrease of stimulus intensity. There was no significant difference in dipole location between 1M and 2M in the primary somatosensory cortex (SI). Conclusions: The results suggested that at least two independent pathways with different recovery functions exist in a similar area in the SI.