Time course and magnitude of movement-related gating of tactile detection in humans. II. Effects of stimulus intensity

This study examined the effect of systematically varying stimulus intensity on the time course and magnitude of movement-related gating of tactile det ection and scaling in 17 human subjects trained to perform a rapid abductio n of the right index finger (D2) in response to a visual cue. Electrical st imulation was delivered to D2 at five different intensities. At the lowest intensity, approximately 90% of stimuli were detected at rest (1 X P-90); f our multiples of this intensity were also tested (1.25, 1.5, 1.75, and 2.0 X P-90). At all intensities of stimulation, detection of stimuli applied to the moving digit was diminished significantly and in a time-dependent mann er, with peak decreases occurring within +/-12 ms of the onset of electromy ographic activity in the first dorsal interosseous (25-45 ms before movemen t onset). Reductions in the proportion of stimuli detected were greatest at the lowest stimulus intensity and progressively smaller at higher intensit ies. No shift in the timing of the decreases in performance was seen with i ncreasing intensity. Once the weakest intensity at which most stimuli were perceived during movement had been established (2 X P-90), magnitude estima tion experiments were performed using two stimulus intensities, 2 X P-90 (5 subjects) and 3 X P-90 (3 subjects). Significant movement-related decrease s in estimated stimulus magnitude were observed at both intensities, the ti me course of which was similar to the time course of reductions in detectio n performance. As stimulus intensity increased, the magnitude of the moveme nt-related decrease in scaling diminished. A model of detection performance that accurately described the effect of stimulus intensity and timing on m ovement- related reductions in detection was created. This model was then c ombined with a previous model that described the effects of stimulus locali zation and timing to predict detection performance at a given stimulation s ite, intensity, and time during movement. Movement-related gating of tactil e perception represents the end result of movement-related effects on the t ransmission and subsequent processing of the stimulus. The combined model c learly defines many of the requirements that proposed physiological mechani sms of movement-related gating will have to fulfill.