The effect of psychophysical forward masking in the tactile Pacinian channel on somatosensory evoked responses

Tactile perception originates in the four psychophysical tactile channels. These channels are mediated by four corresponding receptor systems. Particularly, for the Pacinian tactile channel, the lowest tactile threshold of humans is approximately at 250 Hz. In this study, we measured frequency-following brain responses to 250-Hz vibratory stimuli with intensities at varying sensation levels. Initially, the absolute threshold of the Pacinian channel was measured using the two-interval forced-choice paradigm. The experiments were repeated with a masking stimulus (frequency: 250 Hz; intensity: SL+10, SL+20, SL+30 dB) which preceded the test-stimulus interval. As a result, the masked thresholds were elevated as a function of the masking-stimulus level, consistent with previous studies. Then, EEG signals were recorded over the somatosensory cortex (CPc-CPi) during vibrotactile stimulation, using the same protocol as described in psychophysical part of the experimets. The EEG signals were averaged 500 times, and analyzed by using Fourier and wavelet transforms. The difference between the total area of power spectrum during the masked test-stimulus and during the unmasked test stimulus interval was significant for SL+30 masking level (Wilcoxon signrank test p=0.013). In addition, the difference of power spectrum area at driving frequency between two cases was significant for SL+20 and SL+30 masking level p=0.013 and p=0.019 respectively. This is the first time that a neural correlate of psychophysical forward masking is demonstrated in the tactile system.