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Memory is a curious thing, and visual memory is even more curious. In some ways, we don’t remember much about the scene that’s right in front of us. As countless change blindness studies have shown, we often don’t notice even obvious changes taking place in a scene. Other studies have concluded that visual short term memory has a capacity of just three or four objects.
Yet I have vivid visual memories of scenes I have only glimpsed for a few seconds: A deer below the rim of the Grand Canyon; Michael Jordan draining a three-pointer to win the NBA championships; the standing ovation our daughter received at the school play. If I could only retain three or four of the items in those scenes in short-term memory, why are my long-term memories so vivid? How could they have ever become long-term memories if my short-term memory couldn’t even contain them?
Some researchers have suggested that any additional details in those memories are filled in by verbal descriptions — the “gist” of the scenes, instead of the actual visual imagery. But this explanation doesn’t match up with the rich visual memories I have of some places. Could some other process be responsible for the richness of visual memories?
There is some evidence that visual memory for scenes increases with longer viewing time. But can we remember more than the four items supposedly retainable in short-term visual memory?
David Melcher showed photos of scenes to volunteers for periods ranging from 5 to 20 seconds. But he also added a twist:
Sometimes viewers saw the images uninterrupted, for 5, 10, or 20 seconds. Then they were quizzed on the contents of the images (“What color were the flowers in the pots?” “What was in the view in the background?”). Other times, after either a 5- or 10-second interval, there was no quiz, and the next image was presented, followed by its quiz. Then later in the test, the unquizzed image reappeared for another 5 or 10 seconds, followed by the quiz for that image. So this image had been viewed for either 10 or 15 seconds in total, but in two separate viewings. Here are the results:
Accuracy seems to be directly related to the total viewing time. For each increase in viewing time, accuracy increased significantly. And look at the 10-second point, where some images were seen in a single viewing, and some were seen in two sessions. There was no significant difference in the results!
On the basis of this experiment, Melcher could calculate the average number of objects retained in memory. After 5 seconds, about 5.1 objects were recalled, compared to 7.0 objects after 20 seconds. And while central objects in a scene were recalled more frequently than peripheral objects, the same pattern held for both types of objects — the longer the scene was observed, the more likely viewers were to remember objects.
But how much time can elapse between the first and second viewings of a picture while preserving the effect? In his second experiment, Melcher systematically varied the time between the first and second viewing. This time, instead of having viewers rate other pictures during the break, they either performed a reading or a visual short-term memory task. There was either a 0-, 10-, or 60-second delay between the first (10-second) and second (1-second) presentation of the picture. Here are those results:
As you can see, there was no significant difference in the results, regardless of the interval between the first and second presentation of the picture or what viewers were doing during the interval — even when they completed a task designed to occupy visual short-term memory. As a control, sometimes pictures were rated after just a single 1-second viewing, and accuracy on the test was significantly lower.
So far from finding a fixed limit to short-term visual memory for scenes, Melcher found that the memory improves after longer exposure to a scene. In previous experiments, Melcher had found that scene memories diminish after a 24-hour period, so it doesn’t appear that they have simply moved into long-term memory.
Melcher suggests that there might be something like a “proto-long-term memory,” not as robust as true long-term memories, but with a larger capacity than short-term visual memory.
Melcher, D. (2006). Accumulation and persistence of memory for natural scenes. Journal of Vision, 6, 8-17.
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