Differential effects of stimulus intensity on peripheral and neuromagnetic cortical responses to median nerve stimulation.
To study the differential effects of tactile stimulus intensity on cortical and peripheral responses, we measured neuromagnetic cortical responses, compound muscle action potentials (CMAP), sensory nerve action potentials (SNAP), and the subjective estimation of tactile magnitude to electric median nerve stimulation at the wrist in 13 male healthy adults. The sensory perception threshold (ST) for electric pulses at wrist skin was determined and then various levels of stimulus intensity (1 approximately 6 ST) were given to each subject. At 1 ST, only the P50m components of the primary somatosensory (SI) cortical responses were recorded. The second somatosensory (SII) cortical responses were saturated at 2 ST, while the SI responses reached maximum at 3 ST equivalent to the subjective threshold intensity for "strong" tactility. The CMAP and SNAP were maximum at 4-5 ST. At 2 ST, >70% of maximum SI responses were produced, whereas only <40% of maximum CMAP or SNAP responses were obtained. We concluded that the stimulus intensities for activating or saturating somatosensory cortical responses were lower than those for CMAP and SNAP. The differential intensity effects on cortical and peripheral responses suggest a polysynaptic organization underlying the central amplification for somatosensory cortical activation. The optimal intensity levels for producing maximum SI and SII responses were 3 and 2 ST, respectively. Compared with the SII, the SI plays a crucial role in the coding of the tactile stimulus intensity.
Lin
Yung-Yang
YY
Integrated Brain Research Unit, Department of Medical Research and Education, Taipei Veterans General Hospital, Taipei, Taiwan. yylin@vghtpe.gov.tw
Shih
Yang-Hsin
YH
Chen
Jen-Tse
JT
Hsieh
Jen-Chuen
JC
Yeh
Tzu-Chen
TC
Liao
Kwong-Kum
KK
Kao
Chuen-Der
CD
Lin
Kon-Ping
KP
Wu
Zin-An
ZA
Ho
Low-Tone
LT
D000200:Action Potentials / Q000502:physiology; D000328:Adult; D001931:Brain Mapping; D002540:Cerebral Cortex / Q000502:physiology*; D004558:Electric Stimulation; D004574:Electromagnetic Fields; D005073:Evoked Potentials, Somatosensory / Q000502:physiology; D006801:Humans; D015225:Magnetoencephalography; D008297:Male; D008475:Median Nerve / Q000502:physiology*; D008959:Models, Neurological; D009475:Neurons, Afferent / Q000502:physiology; D010525:Peripheral Nerves / Q000502:physiology*