Anyone who's ever crammed just before an exam knows that you can't fit many facts into short-term memory. In fact, the space limit seems to be about seven words or digits--the length of a telephone number. But what about images? Psychologists report in today's Nature that humans can instantly recall the details of only four different objects. A better understanding of this memory limit could have practical applications, the researchers say, such as improving the design of dashboard displays or street signs.
To gauge the capacity of visual short-term memory, Steven Luck and Edward Vogel of the University of Iowa in Iowa City, set up a simple test. They flashed several squares, each a single block of color, onto a screen for 1/10 of a second. After 1 second in the dark, they showed the squares again for 2 seconds, sometimes with one color altered. Ten college students were repeatedly tested to see if they could recall if there had been a change. With three or fewer squares, they answered almost perfectly, but the success rate began to drop with four or more squares.
Next, the researchers complicated the task by asking the students to remember multiple features such as color, orientation, size, or the presence or absence of a gap in rectangles. Again, they noted a drop in accuracy when more than four rectangles were on the screen. "They could remember a total of 16 features, as long as it was only four objects," says Luck. This shows that working memory seems to store the entire object--the whole square--rather than a collection of individual features, just like remembering a list of words, rather than a collection of letters.
"It's an interesting study and certainly the topic is one of great importance," says Jonathan Cohen, a psychologist who studies memory at Carnegie Mellon University in Pittsburgh and the University of Pittsburgh. For example, Nissan Research and Development, Inc. in Cambridge, Massachusetts, studies how drivers, faced by countless images of street signs and traffic signals, store only some of the images in their working memory. "If you designed the dashboard of the car properly or you designed the layout of street signs, you might be able to minimize accidents," says Luck.