However when subjects were required to respond when the SOA was changed, then delay in responding to the second signal rose to 50msec. Experimenters like Pashler have sought to find the locus of the effect. Most widely accepted view is that the PRP effect is one of a bottleneck in response selection. Selection of the second response cannot begin until the first response is selected. This is not universally held though.
Subjects have to time taps with both hands.
Easy to coordinate taps if timing cycles are identical, but when timing cycles were different interference occurs. Difficult for subjects to separate continuous stimuli of regular, but independent periodicity.