This limit cannot be trained away

What it is

The queuing delay produced by the psychological refractory period reflects a limitation built into the structure of central processing itself, not a lack of skill or practice — so it cannot simply be trained away. Even after extensive repeated practice on the same dual-task pairing, a participant's delay can shrink somewhat, but the queuing phenomenon itself, and its basic relationship with the interval between stimuli, continues to hold — practice does not make it disappear.

Why it happens

What practice can shorten is the processing time each individual task requires on its own (classifying the stimulus faster, selecting the response more fluently), which indirectly shortens the total time the bottleneck stays occupied and so makes the observed delay smaller. But that amounts to "making the queue shorter," not "getting rid of the queue" — as long as both tasks still need to pass through that shared decision stage, the serial-processing structure is still in effect. This is the same kind of limitation seen in motor-skill training: practice can make a single movement's execution more accurate, but it cannot turn the movement into one that needs no execution time at all.

Studying it

Confirming "practice shortens but doesn't eliminate" involves having participants undergo long, extensive practice on the same dual-task pairing (on the order of thousands of trials), continuously tracking the size of the PRP effect (the delay, or the slope of the delay-versus-interval curve) as practice accumulates. The typical result shows the effect size shrinking with practice but converging on a level above zero, rather than trending toward zero.

Where it stops holding

A handful of studies report that, under specific, tightly constrained task pairings (where both tasks are highly automatized and barely draw on the shared decision resource), long-term training can shrink the effect down to something close to measurement error. But such results usually come with strict task-design conditions attached, and cannot be generalized into "with enough practice, any dual-task queuing delay can be eliminated."

Applying it

Don't treat the second-response delay caused by two consecutive prompts as a problem that can be solved by users simply getting more practiced with the product — even the most experienced user will still respond more slowly to the second of two prompts that both require an immediate judgment, compared to when that prompt appears alone. Product design should address this by cutting the number of prompts requiring an independent decision within the same window and spacing prompts further apart, rather than assuming the delay will naturally disappear once users get used to the product and speed up. How to check: test the second-response delay for the same sequence of prompts with both novice and experienced users; if the experienced users' delay, though smaller than the novices', is still clearly higher than their reaction time to that prompt in isolation, the delay is a structural limit rather than a matter of skill, and should not be expected to resolve itself through accumulated user experience.

Related

• Same group: A8.04.1 When the first response isn't finished, processing of the next stimulus is postponed · 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
• Nearby: A8.01 The structure of reaction time
• Search terms: PRP practice effect · practice-resistant bottleneck · dual-task training