A key brain electrical signal leveled off when the number of objects held in mind exceeded a subject's 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 computer's RAM, working memory is the ever-changing content of our consciousness. It's been known for years that people have a limited capacity to hold things in mind that they've 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 Vogel.
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
Contact: Jules Asher
NIH/National Institute of Mental Health