When the first response isn't finished, processing of the next stimulus is postponed

What it is

When two stimuli, each requiring its own response, appear one after another with a short gap between them, the second response is noticeably delayed compared to when it appears on its own. This is the psychological refractory period (PRP): the delay is not because the second stimulus is intrinsically harder to process, but because the first response hasn't finished processing yet, forcing the second stimulus's processing to wait in line.

Why it happens

Somewhere in the central processing system — usually taken to be the decision / response-selection step — there is a stage that can only handle one task at a time, forming a central bottleneck. While the first stimulus occupies that stage, the second stimulus has to queue at the bottleneck even if its sensory information has already fully arrived and is ready to go, and it cannot begin being processed until the processing tied to the first response releases that stage — which is directly tied to how quickly the first response itself is completed.

Studying it

The typical paradigm presents two stimuli (S1, S2) that each require a distinct response (R1, R2), and systematically varies the interval between them (stimulus onset asynchrony, SOA), comparing the second response's RT across different SOAs. If RT2 grows systematically as SOA shrinks, and the growth roughly fills the "gap" completely (RT2 ends up close to RT1 plus a fixed amount minus SOA), that is the classic signature of a central bottleneck.

Where it stops holding

This phenomenon requires both responses to genuinely need their own independent central processing (both require a decision); if the second task is extremely simple and doesn't involve a real decision step (a pure reflexive action), the bottleneck effect may not be noticeable. Whether the two tasks share the same sensory channel or responding limb also affects the observed delay, independently of the sheer number of tasks involved.

Applying it

When an interface triggers two prompts in quick succession that both require the user to judge and respond independently (two dialogs popping up back-to-back, each requiring a choice), recognize that the user's response to the second prompt will inevitably be delayed because the first hasn't finished being processed — this is not a lapse in the user's attention but a queuing limit built into processing capacity itself. Avoid having an important second prompt follow immediately after another decision-requiring prompt; consider having the second prompt wait until the first response is complete, or space the two out further in time. How to check: record the actual interval between two consecutive prompts and the user's reaction time to the second one, and confirm whether the interval and the delay follow the "shorter interval, bigger delay" pattern — that tells you whether the problem lies in the prompt-timing design.

Related

• Same group: A8.04.2 The shorter the interval between two stimuli, the greater the delay in the second response · A8.04.3 Rapid successive prompts queue in the central system rather than being processed in parallel · A8.04.4 This limit cannot be trained away
• Nearby: A8.01 The structure of reaction time
• Search terms: psychological refractory period · PRP · central bottleneck