EFFECTS OF STIMULUS RATE ON REGIONAL CEREBRAL BLOOD-FLOW AFTER MEDIANNERVE-STIMULATION

The primary motor cortex and supplementary motor area (SMA) are purpor tedly involved in the generation of the P22 and N30 components of soma tosensory evoked potentials (SEPs) evoked by electrical stimulation of the median nerve at the wrist. We used regional cerebral blood flow ( rCBF) measurements and PET in 10 normal subjects to study the cerebral areas activated by median nerve electrical stimulation. PET scans wer e performed with the subjects at rest and during stimulation of the ri ght median nerve at frequencies of up to 20 Hz. Stimulation evoked a s ingle focus of activation in the primary somatosensory area (SI). An i ncrease of rCBF in this area was linearly correlated with stimulus fre quencies of up to 4 Hz and then reached a plateau. The SMA was not sig nificantly activated by stimulation at arty of the frequencies tested. In contrast to the SI, the SMA showed no trend toward a correlation b etween the rCBF changes and the stimulus repetition rate. In order to achieve maximal resolution in the sensorimotor cortex, regions of inte rest were placed in individual co-registered MRI-PET images on both si des of the central sulcus. There was no significant increase of rCBF i n the crown of the precentral gyrus. These results suggest that a cont ribution of the primary motor cortex and the SMA to the generation of the P22 and N30 components of SEPs is unlikely. Consequently, function al clinical interpretations derived from P22 or N30 abnormalities must be reconsidered.