| Brain Signal Predicts Working Memory Prowess
Some people are better than others at remembering what they have
just seen holding mental pictures in mind from moment to moment.
An individuals capacity for such visual working memory can
be predicted by his or her brainwaves, researchers funded by the
NIHs National Institute of Mental Health have discovered.
A key brain electrical signal leveled off when the number of objects
held in mind exceeded a subjects capacity to accurately remember
them, while it continued to soar in those with higher capacity,
report University of Oregon psychologist Edward Vogel, Ph.D., and
graduate student Maro Machizawa, in the April 15, 2004, Nature.
Analogous to a computers RAM, working memory is the ever-changing
content of our consciousness. Its been known for years that
people have a limited capacity to hold things in mind that theyve
just seen, varying from 1.5 to 5 objects. Our study identifies
signals from brain areas that hold these visual representations
and allows us to coarsely decode them, revealing how many objects
are being held and their location in the visual field, explained
To find out if the amplitude of detectable signals reflects the
number of object representions held in visual memory, the researchers
presented 36 subjects with a series of trials containing an increasing
number of objects. Subjects briefly viewed a picture containing
colored squares, followed by a one-second delay, and then a test
picture. They pressed buttons to indicate whether the test picture
was identical to or differed by one color from the
one seen earlier. The more squares a subject could correctly identify
having just seen, the greater his/her visual working memory capacity.
Subjects averaged 2.8 squares.
Electrodes on the scalp recorded neural activity during the one-second
delay to pinpoint signals reflecting activity of brain areas involved
in holding the images in working memory. Asking subjects to remember
just one of two sets of colored squares that appeared on the left
and right sides of the screen revealed signals near the opposite
rear side of the head as emanating from the brain area involved.
The researchers found that the more squares a subject correctly
identified, the higher the spike of corresponding brain activity
up to a point. Amplitude of the signal for correct trials
was much higher than incorrect ones, suggesting that the delay activity
specifically reflects the maintenance of successful representations
in visual memory. Neural activity of subjects with poorer working
memory scores leveled off early, showing little or no increase when
the number of squares to remember increased from 2 to 4, while those
with high capacity, who correctly remembered more squares, showed
Using a similar task with functional magnetic resonance imaging
(fMRI), a research team at Vanderbilt University reports in the
same issue of Nature that the posterior parietal cortex,
an area at the top rear part of the brain, is the brain area responsible
for holding representations in visual working memory and
likely source of the signal in the Oregon study.
"Simply by measuring the amplitude increase across memory
array sizes, we can accurately predict an individual's memory capacity,
Since working memory capacity is strongly predictive of performance
on a broad array of cognitive abilities reasoning, language,
flexible problem solving Vogel foresees the physiological
measure as finding applications in assessing individuals who are
behaviorally or verbally impaired, such as in cases of stroke or
paralysis. The technique has also been used to study development
of cognitive abilities in pre-verbal children.
NIMH is part of the National Institutes of Health (NIH), the Federal
Government's primary agency for biomedical and behavioral research.
NIH is a component of the U.S. Department of Health and Human Services.
Signal Rises with Memory Capacity Graph (http://www.nimh.nih.gov/ImageGallery/Press/prwmprowress1.jpg)