The bottleneck of the psychological refractory period effect involves timing of response initiation rather than response selection.

The Psychological Refractory Period (PRP) effect is a delay in responding that is assumed to be caused by a bottleneck that prevents preparation of a second action until preparation of the previous action has been completed. The bottleneck is usually attributed to a limitation that prevents concurrent selection of two responses. However, evidence reviewed here challenges this selection interpretation. We propose instead that the bottleneck is due to a process that programs the timing of response initiation, and which must be completed immediately prior to responding. This hypothesis is based on two conclusions from recent developments in research, which includes findings from paradigms that do not involve the PRP. The first conclusion is from studies involving the startle response and single-task simple (precued) reaction time; these studies indicate that programming the timing of response onsets is needed to enable response initiation. This programing takes longer for more complicated timing, and must be delayed until just prior to responding. The second conclusion is from studies of concurrent rhythmic movements demonstrating that the representation of timing is restricted to one temporal frame unless very rapid performance enables parallel timing. These findings reveal limitations that can combine to produce the PRP bottleneck. This interpretation clarifies otherwise puzzling aspects of the PRP effect and indicates that a fundamental restriction concerning response timing may underlie both limitations and the PRP effect that they produce. This restriction might arise because timing is controlled by subcortical neural structures with limited working memory.