For many years, scientists have held that short-term memories don't suddenly disappear, but grow gradually more imprecise over the course of several seconds. However, a new study has found just the opposite.
In the study, researchers at the University of California, Davis, found that their subjects retained temporary memories of an object's colour or shape for at least four seconds.
After that, the memories began to wink out like streetlights at daybreak, remaining quite accurate until they suddenly disappeared.
To test the accuracy of short-term visual memory, Weiwei Zhang, a postdoctoral scholar, and Steve Luck, a professor of psychology, both at the UC Davis Center for Mind and Brain, devised a pair of tests, both of which could separately measure two things: the accuracy of a short-term memory and the probability that the memory still existed. Each test was given to 12 adults.
In the first test, three squares - each with a different colour fill - flashed for a tenth of a second on a computer screen. After an interval of one, four or 10 seconds a wheel showing the entire spectrum of colours appeared on the screen.
The three squares also reappeared, only now they were colourless and one of them was highlighted. Subjects were asked to recall the colour of the highlighted square and click on the area of the wheel that most closely matched it. Each subject repeated this test 150 times for each of the three memory retention intervals.
When subjects retained a memory of the colour, they clicked very close to it on the wheel - the distance between the click and the actual colour indicating the accuracy of the memory. When colour had disappeared from memory, however, subjects clicked at random on the wheel.
The second test was similar to the first, but used shapes instead of colours.
Luck said that the study revealed that subjects 'either had the memory or didn't have the memory and the probability of having it decreased between four and ten seconds. The memories did not gradually fade away.'
The finding provides insight into the underlying mechanisms behind memory formation and retention.
"The memories are not like flashlights that get progressively weaker as the battery runs low. They are more like a laptop computer that continues working at the same speed until it suddenly shuts down," Luck said.
The study has been published in the April issue of the journal Psychological Science.