Human somatosensory evoked magnetic fields to vibratory stimulation of the index finger: is there frequency organization in SI?

Objective: Frequency organization in the human somatosensory cortex was studied.

Design and methods: Somatosensory evoked magnetic fields (SEFs) from 12 subjects were measured following vibratory stimulation of the index finger by using a 122 channel whole head SQUID system. Sensory stimuli comprising a 40 ms vibration at frequencies of 50, 100, 200 and 400 Hz were delivered to the volar surface of the tip of the right index finger. Using a single dipole model, the sources of the magnetic fields were estimated and mapped onto magnetic resonance images of each subject. The analysis of variance test (ANOVA) was used for statistics.

Results: Source localization was determined on the main two peaks (M60 and M110) of the SEFs. All of the sources were located in the area 3b of somatosensory cortex (SI). There were no statistically significant differences between the locations of the dipoles evoked by different frequency stimulations.

Conclusions: These results demonstrate the absence of systematic frequency organization at the hand representation area of the SI cortex. We speculate that high frequency vibration above 100 Hz are coded by the fast-spiking interneurons which synapse with Pacinian pyramidal neurons in SI.