Numerosity and rhythmicity in stimulus-response compatibility

When people must respond discriminatively to 1 or 2 stimuli by making 1 or 2 taps of a response key, they initiate the response more rapidly when the correct number of taps matches the number of stimuli (compatible condition) than when it mismatches (incompatible condition; J. O. Miller, S. G. Atkins, & F. Van Nes, 2005). Miller et al. sometimes found an effect of compatibility on response execution time, as reflected in the interresponse intervals between successive taps. The authors report 2 further experiments (N = 8 participants) in which they generalized the numerosity compatibility effects on response-initiation time and interresponse intervals to 2- versus 3-stimulus sequences. In addition, they varied gap length between stimuli to see whether the rhythm of the stimulus would influence that of the response. Weak rhythmicity effects were repeatedly found, but those were too small to suggest a plausible alternative explanation for the numerosity compatibility effect on response-initiation time.