Somatosensory brain responses evoked by vibrotactile stimulation of the distal phalanx in normal subjects

Abstract

The sensory response upon vibrotactile stimuli is still not completely understood. Previously, the responses of single units from mechanoreceptive a erents and cortical neurons have been studied. On the other hand, there is a thorough psychophysical literature on judgements of the presence, magnitude, and frequency of vibrotactile stimuli. However, non-invasive recording of evoked somatosensory brain responses on the scalp was not explored adequately due to technical di culties. Dependency of the evoked somatosensory response to vibrotactile stimuli, the frequency following response, is a phenomenon in which the human brain has a tendency to change its dominant EEG frequency towards the frequency of the external stimulus. This may arise from the synchronous ring of many cortical neurons. In this study, 40 and 230 Hz vibrotactile stimuli were applied to 10 adult subjects (5 females and 5 males) and psychophysical detection thresholds were measured at various frequencies using the two-interval forced choice paradigm. The psychophysical results were consistent with the literature. To measure the frequency following brain responses, stimulus amplitudes were determined based on the psychophysical sensation levels. Stimuli amplitudes were set at 10, 20 and 30 dB higher than the thresholds (SL=10, SL=20 and SL=30 dB respectively). EEG recordings were obtained over the primary somatosensory cortex with gold surface electrodes placed on the scalp at CPi-CPc. The results were examined by fourier transforms and wavelet transforms.Wavelet transform plots showed that, as the mechanical stimulus amplitude was increased, the background activity was suppressed and the frequency-following activity during the stimulus period increased. This nding was statistically signi cant. The origin of the frequency-following responses were discussed based on current results.|Keywords: taction, somatosensory cortex, mechanoreceptor, wavelet transform.