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psychological refractory period

Your brain cannot process two independent tasks at once.

The psychological refractory period causes a delay in reaction to a second stimulus if it arrives during the processing of the first.

Even if you are skilled at multitasking, the central bottleneck in your brain must process one task before it can fully allocate resources to the next. The delay is usually only a few hundred milliseconds, but it is unavoidable. You are not multitasking; you are switching channels and incurring a cost each time.

Your brain is a single-threaded processor with an effective switching mechanism.

Where this comes from

Study
Pashler, 1994, Psychological Bulletin
What the check found
The definition and identification of the phenomenon are accurate. Pashler's 1994 paper is the foundational work for the modern response-selection bottleneck theory describing this effect.
Confidence
Well replicated
psychological refractory periodmultitaskingattention

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One of these every day, and every one names the study it came from. When a finding is contested, it says so.