Somatosensory evoked potentials associated with tactile stimulation at detection threshold in man

Somatosensory evoked potentials elicited by sub- and supraliminal tactile pulses were studied during continuous threshold measurement. Two threshold methods were used: a modified tracking method and a detection method. With both methods threshold estimates of the same order of magnitude were obtained. Stimuli just above the threshold elicited a distinct somatosensory evoked potential with components P50, P100, N190 and P400. No such SEP was associated with the subliminal stimuli. However, in some cases some potential oscillations were obtained, time-locked to the subliminal stimuli; these are suggested to be due to errors in responding. In control experiments detected stimuli elicited a distinct SEP, but no SEP was associated with undetected stimuli of identical amplitude. The results indicate that in the psychophysical threshold determination the neural processing reflected in the SEP is associated only with supraliminal tactile stimulation. The lack of evoked brain activity associated with subliminal tactile stimuli supports the hypothesis derived from human microneurographic studies, stating that the tactile detection threshold may be based on an extremely small peripheral input, perhaps only on a single impulse in a single peripheral fiber.