Somatosensory information processing in area 3b for thumb and middle finger in humans

Abstract

It is widely accepted that the human thumb plays an important role in precision grasping, and that its morphology has achieved prominent evolution compared with those of the monkeys. Thus, it is possible that the somatosensory information from the thumb is processed differently for a fine motor control. Therefore, we examined the relationship between somatosensory N20m primary responses and high-frequency magnetic oscillations (HFOs) after thumb and middle finger stimulation. Somatosensory evoked magnetic fields (SEFs) from 12 normal subjects were measured following electrical stimulation of the thumb and middle finger, respectively. SEFs were recorded with a wide bandpass (3–2000 Hz), and then N20m and HFOs were separated by subsequent 3–300 and 300–900 Hz bandpass filtering. The N20m peak-to-peak amplitude did not differ significantly between thumb and middle finger SEFs. In contrast, HFOs were significantly larger in the number of peaks and higher in the maximum amplitude, and the total amplitude after thumb stimulation than after middle finger stimulation. We speculate that the enhanced HFOs after thumb stimulation might facilitate primary somatosensory processing, which in turn provides motor cortex finely tuned somatosensory information for fine motor control of the thumb.