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Release: Immediate

UI researcher documents capacity of visual short-term memory

IOWA CITY -- You're cruising down an unfamiliar road at a good clip watching for your next turn. Along the roadside you see billboards advertising hotels, shopping, restaurants, museums, and then, finally, a sign saying your exit is two miles ahead. Unfortunately, you've missed the new speed limit sign posted in the same location and soon the state trooper is behind you flashing his lights and waving you over to the shoulder.

It may not get you out of the ticket and the fine, but Steven J. Luck, an assistant professor of psychology at the UI, has documented for the first time that the human mind can not record more than a few visual objects at the same time, even if they're very simple. Therefore, the driver could not have been expected to see the speed limit change immediately following the exit sign for which he had been looking.

Luck's findings are published in the Nov. 20 issue of the journal Nature, in a paper he wrote with Edward K. Vogel, a UI graduate student in psychology.

Luck says that visual working memory -- information you see and store in your memory only for a few seconds -- is limited to storing four objects at once. However, he says, the memory can record several features of those four objects including color, size, and orientation. This means it is possible to retain a good deal of information about a limited number of objects, Luck says. So in the highway example, for instance, the driver could remember the color, shape, and size of each sign he saw, but still would not have processed the fifth sign -- the all-important speed limit.

The research is significant because little work has been done in the area of visual working memory and no one has ever documented the capacity of this type of memory for objects defined by very simple features or for combinations of simple features.

By contrast, Luck says, it is well known that verbal working memory -- information you can store in verbal form for a few seconds -- is capable of holding about seven pieces of information. That's why it is possible to retain a telephone number by repeating it to yourself for the few seconds between looking it up in the telephone book and dialing the numbers.

Luck says earlier research had also shown that people could retain more verbal information by combining individual items into larger "chunks," and he wondered if the same would hold true for visual memory. His experiments showed that people could combine the separate features of an object into a single chunk, allowing them to retain as many as 16 features divided over four objects. But they could not retain visual memories of 16 separate objects -- in that case, they were limited to only four.

Luck says his experiments were unique because he presented his subjects with simple shapes and colors to remember and then asked them to notice only a change in color or in shape on second viewing.

"We're starting from a very simple level and then working up to more complex images," Luck says.

Other experiments have shown subjects pictures of natural scenes and then asked them to identify any changes in the picture on second viewing. In the more complex pictures, subjects have often failed to notice major differences between first and second viewing. Luck says his research explains why this occurs -- the subjects cannot record and retain all the visual elements of the more-complex pictures.

Luck's research projects are funded by the National Institute of Mental Health and by the McDonnell-Pew Program in Cognitive Neuroscience.

Luck says there will be many practical applications for his research since people are constantly faced with complex visual stimuli, and it is possible to overload the limited working memory system and therefore miss something more vital.

"This has tremendous implications for things like driving where noticing changes is very important," Luck says.

For more information, Luck is available by phone at (319) 335-2422 or by email at