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Memory bottleneck limits intelligence
Single spot in brain determines size of visual scratch pad.
15 April 2004
TANGUY CHOUARD
Your working memory can be seen on a scan.
© Photodisc
The number of things you can hold in your mind at once has been traced to one penny-sized part of the brain.
The finding surprises researchers who assumed this aspect of our intelligence would be distributed over many parts of the brain. Instead, the area appears to form a bottleneck that might limit our cognitive abilities, researchers say.
"This is a striking discovery," says John Duncan, an intelligence researcher at the Cognition and Brain Sciences Unit in Cambridge, UK.
Most people can hold three or four things in their minds at once when given a quick glimpse of an image such as a collection of coloured dots, or lines in different orientations. If shown a similar image a second later, they will be able to recognise whether three or four of these spots and lines are identical to the first set or not.
But some people can only catch one or two things in a glance, while others can capture up to five.
This very short-term memory capacity is thought to be related to intelligence. In the same way that a computer with a larger working memory can crank through problems more quickly, people with a greater capacity for holding images in their heads are expected to have better reasoning and problem-solving skills.
A person's working memory capacity can be determined using simple psychological tests. But now two teams of researchers report in Nature that they can see it in brain scans too.
Keep it in mind
One of the teams, led by Edward Vogel of the University of Oregon in Eugene, found that the electrical activity in a single section of the brain, as detected through electrodes attached to the scalp, is directly related to short-term working memory1.
The team first tested subjects with an image of two coloured dots, waiting a second between flashes and asking the subjects if the image had changed. They then ramped up the test to four dots.
A large increase in the subject's brain activity on the four-dot test indicated that his or her memory capacity had not been pushed to its limit. No increase in electrical activity indicated that his or her working memory had topped out on the two-dot test. By graphing these responses, the team worked out the exact size of each subject's working memory.
A second team, led by René Marois of Vanderbilt University in Nashville, Tennessee, used functional magnetic resonance imaging during similar tasks to accurately locate the part of the brain being used for short-term visual memory2.
Both teams concluded that everything depended on the same tiny spot in the posterior parietal cortex.
"It is amazing that both groups should converge on the same area in the end," says Duncan. Since the task involves remembering many different aspects of each object, including spatial position, orientation and colour, most people thought that several parts of the brain would be involved, he says.
There are still many other aspects to human intelligence that are governed by other parts of the brain, the authors of both studies warn. But the capacity of one's working memory may form a bottleneck for certain kinds of intelligence, they say.
Tanguy Chouard is a senior biological sciences editor at Nature
References
Vogel, E. K. & Machizawa, M. G. . Nature, 428, 748 - 751, doi:10.1038/nature02447 (2004). |Article|
Todd, J. J. & Marois, R. . Nature, 428, 751 - 754, doi:10.1038/nature02466 (2004). |Article|
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