Somatosensory-evoked potentials and vibration.

Vibratory stimuli (250 Hz) with amplitudes between 50 and 200 microns were delivered to the middle finger knuckle of 15 human subjects. The cortical-evoked potentials and psychophysical magnitude estimations were simultaneously recorded. Only the large, late components of the evoked cortical potentials showed significant correlation to the stimulus intensity. While the psychophysical response showed a slight positive acceleration, the evoked potential stimulus-response relations were slightly nonlinear with negative acceleration. The early components of the evoked potential were well localized over the contralateral, postcentral hand area; the late components showed a wide distribution over both hemispheres. The late, bilateral components resembled the alpha rhythm in frequency and distribution. It is concluded that late components resembling alpha rhythm may be due to stimulus-synchronized oscillations of alpha generators. The underlying neuronal events are probably of less importance for perception. By contrast, the important neuronal activity in the first 100 ms after arrival of the impulses at the cortex does not appear in the scalp-recorded evoked potential.