[extropy-chat] Neural bottleneck found that thwarts multi-tasking

Keith Henson hkhenson at rogers.com
Sun Jan 21 18:27:01 UTC 2007

[It's been interesting lately finding things on one mailing list that 
should be shared on others. KH]

Neural bottleneck found that thwarts multi-tasking

René Marois and Paul Dux

Many people think they can safely drive while talking on their cell phones. 
Vanderbilt neuroscientists Paul E. Dux and René Marois have found that when 
it comes to handling two things at once, your brain, while fast, isn't that 

"Why is it that with our incredibly complex and sophisticated brain, with 
100 billion neurons processing information at rates of up to a thousand 
times a second, we still have such a crippling inability to do two tasks at 
once?" Marois, associate professor of Psychology, asked. "For example, what 
is it about our brain that gives us such a hard time at being able to drive 
and talk on a cell phone simultaneously?"

Researchers have long thought that a central "bottleneck" exists in the 
brain that prevents us from doing two things at once. Dux and Marois are 
the first to identify the regions of the brain responsible for this 
bottleneck, by examining patterns of neural activity over time. Their 
results were published in the Dec. 21 issue of Neuron.

"In our everyday lives, we seem to complete so many cognitive tasks 
effortlessly. However, we experience severe limitations when we try to do 
even two simple tasks at once, such as pressing a button when a visual 
stimulus appears and saying a word when a sound is presented. This is known 
as dual-task interference," Dux, a postdoctoral research associate in the 
Department of Psychology, said. "We were interested in trying to understand 
these limitations and in finding where in the brain this bottleneck might 
be taking place."

The research is particularly timely, as additional states consider banning 
the use of cell phones while driving.

"While we are driving, we are bombarded with visual information. We might 
also be talking to passengers or talking on the phone," Marois said. "Our 
new research offers neurological evidence that the brain cannot effectively 
do two things at once. People think if they are using a headset with their 
cell phone while driving they are safe, but they're not because they are 
still doing two cognitively demanding tasks at once."

Identifying the information bottleneck responsible for this dual-task 
limitation required the use of functional magnetic resonance imaging, or 
fMRI, an imaging technology that reveals the brain areas active in a given 
mental task by registering changes in oxygenated blood concentration in 
these regions. While fMRI is an excellent tool for identifying a particular 
area in the brain involved in a given task, it generally provides limited 
information about how that area responds over time.

To overcome this limitation, Dux and Marois rapidly sampled brain activity 
using fMRI while subjects were performing two demanding tasks. Evaluation 
of the data produced by this rapid sampling method allowed them to 
characterize the temporal pattern of activity in specific brain areas.

The two tasks consisted of pressing the appropriate computer key in 
response to hearing one of eight possible sounds and uttering an 
appropriate syllable in response to seeing one of eight possible images. 
Different senses and motor responses were enlisted in order to ensure that 
any interference between the two tasks was not specific to a particular 
sensory or motor modality, but instead originated at a central 
information-processing bottleneck.

The results revealed that the central bottleneck was caused by the 
inability of the lateral frontal and prefrontal cortex, and also the 
superior frontal cortex, to process the two tasks at once. Both areas have 
been shown in previous experiments to play a critical role in cognitive 
"We determined these brain regions responded to tasks irrespective of the 
senses involved, they were engaged in selecting the appropriate response, 
and, most importantly, they showed 'queing' of neural activity--the neural 
response to the second task was postponed until the response to the first 
was completed," Dux said.

"Neural activity seemed to be delayed for the second task when the two 
tasks were presented nearly simultaneously - within 300 milliseconds of 
each other," Marois said. "If individuals have a second or more between 
tasks, we did not see this delay.

"This temporal delay is the essence of dual-task interference for tasks 
that require actions. By using time-resolved fMRI, we can see its signature 
in the brain," he continued. "These findings allow us to really now focus 
on this set of brain areas and to understand why these areas cannot process 
two tasks at once."

The researchers are interested in further exploring what is happening in 
the bottleneck to slow performance and believe the work may have future 
implications for people performing complex tasks.

"It may be possible to look to the sort of tasks people are going to have 
to do in a very complex environment, such as flying a plane, and find out 
under what circumstances these tasks may be less vulnerable to dual-task 
interference," Dux added.

For the record, neither Marois nor Dux use their cell phones while driving.

"I'm Australian, and it's illegal there, so I'm trained not to," Dux said. 
"Even so, I would never do it. Dual-task costs can be up to a second, and 
that's a long time when you're traveling at 60 miles per hour."

Source: Vanderbilt University

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