Memory only counts to four
What about our short-term memory's ability to store visual objects? Scientists found that the limiting factor was not how many details, such as color, etc., the subject had to remember. The decisive factor was the number of complete objects. Anyone who's ever tried to cram knowledge into their head right before an exam knows that short-term memory just doesn't absorb some facts. In fact, the limit appears to be seven words or numbers - no more than, say, a phone number. But what about pictures? In a report in Nature (November 20, 1997), psychologists state that most people are only able to remember details of four different objects.
Steven Luck and Edward Vogel of the University of Iowa used a simple test to check visual short-term memory capacity: they flashed a few solid-colored squares on a screen for a tenth of a second. After a second of darkness, they showed the squares again for two seconds. But the second time around, sometimes one of the colors had been swapped out. Ten students had to determine whether a change had taken place. With three or fewer squares, this was not a problem for them, but after four squares the success rate began to drop.
Then the task was made more difficult. Various properties such as color, orientation, size or the presence of gaps were queried. Again, more errors occurred as soon as there were more than four objects on the screen.
The subjects were able to remember up to 16 characteristics, says Luck. But only as long as there were no more than four objects. He says these results show that short-term memory stores whole objects - like the complete square - rather than a collection of individual properties. This would be like remembering a list of words instead of a string of letters.
The researchers hope that these findings will find practical application in the design of dashboards or street signage, for example.