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A third limitation is that choosing a response to a stimulus - braking when you see a child
in the road, for instance, or replying when your mother tells you over the phone that she’
s thinking of leaving your dad - also takes brainpower. Selecting a response to one of
these things will delay by some tenths of a second your ability to respond to the other.
This is called the “response selection bottleneck” theory, first proposed in 1952.
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But David Meyer, a psychologist at the University of Michigan, Ann Arbor, doesn't buy the
bottleneck idea. He thinks dual-task interference is just evidence of a strategy used by
the brain to prioritise multiple activities. Meyer is known as something of an optimist by
his peers. He has written papers with titles like "Virtually perfect time-sharing in dual-task
performance: Uncorking the central cognitive bottleneck". His experiments have shown
that with enough practice - at least 2000 tries - some people can execute two tasks
simultaneously as competently as if they were doing them one after the other. He
suggests that there is a cen
tral cognitive processor that coordinates all this and, what’s
more, he thinks it uses discretion sometimes it chooses to delay one task while completing
another.
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Marois agrees that practice can sometimes erase interference effects. He has found that
with just 1 hour of practice each day for two weeks, volunteers show a huge improvement
at managing both his tasks at once. Where he disagrees with Meyer is in what the brain
is doing to achieve this. Marois speculates that practice might give us the chance to find
less congested circuits to execute a task - rather like finding trusty back streets to avoid
heavy traffic on main roads - effectively making our response to the task subconscious.
After all, there are plenty of examples of subconscious multitasking that most of us
routinely manage: walking and talking, eating and reading, watching TV and folding the
laundry.
I
It probably comes as no surprise that, generally speaking, we get worse at multitasking