When a relatively weak stimulus, such as a mild tone, precedes a reflex-eliciting stimulus by an appropriate interval, the amplitude of the elicited reaction is reduced. Three experiments examined the relative impact of tonal frequency and intensity modulations, occurring prior to a glabella tap, on the amplitude of the elicited glabella reflex. Experiment 1 indicated that a frequency shift in an otherwise continuous pure tone yielded an amount of inhibition equivalent to that produced by a shift following an intensity decrement (i.e., gap) in the tone; further, this amount of inhibition was greater than that produced by a gap with no shift. Experiment 2 replicated this pattern of findings at 2 different tonal intensity levels, and showed that the lower tonal intensity led to less inhibition. Experiment 3 indicated that a 24-ms constant tonal frequency burst and a 24-ms shifting tonal frequency chirp presented prior to the glabella tap led to equivalent amounts of reflex inhibition; further, this amount of inhibition was not significantly different from that produced by a frequency shift, a gap or a frequency shift coincidental with a gap.
