But this, it turns out, is not true of all memories. In a recent paper, Researchers at MIT and Harvard found that, if a memory can survive long enough to make it into what is called “visual long-term memory,” then it doesn’t have to be wiped out at all. Talia Konkle and colleagues showed participants a stream of three thousand images of different scenes, such as ocean waves, golf courses or amusement parks. Then, participants were shown two hundred pairs of images—an old one they had seen in the first task, and a completely new one—and asked to indicate which was the old one.
Participants were remarkably accurate at spotting differences between the new and old images—96 percent. In other words, despite needing to remember nearly 3,000 images, they still performed almost perfectly.
However, it turns out that they were only this accurate when the new and old images came from different types of scenes (e.g., a golf course and an amusement park). In order to test just how detailed these memories really were, the psychologists also analyzed how participants performed when the images were from the same types of scenes (e.g., two different amusement parks). Since images from the same scene type differ from each other in fewer ways than do images from different scene types, the only way participants would’ve been able to succeed at pointing out differences between these similar images is if they had remembered them with a truly vast amount of detail.
As you might expect, participants were worse at discriminating between same-category images, but not by much, scoring as high as 84 percent. In fact, even when the experimenters increased the number of images that participants initially needed to remember for a given type of scene, participants were still good at distinguishing the old image from the new—with only slight decreases in performance. That said, the fact that memory performance decreased at all shows that, although our memories are very detailed, they are not photographic.
These two separate experiments present a paradox: why are we capable of remembering such a massive number of images with great detail in some instances, and not even a few images after a couple of seconds in others? What determines whether an image is stored in long-term vs. short-term memory?
In a recent review, researchers at Harvard and MIT argue that the critical factor is how meaningful the remembered images are—whether the content of the images you see connects to pre-existing knowledge about them. In the Zhang & Luck experiment, you try to remember meaningless, unrelated colors, and so no connection with stored knowledge is made; it’s as if the white board is scrubbed clean before you get a chance to copy the scribbles into your notebook. But in the Konkle et al. experiment, you see images of recognizable scenes that you already have meaningful knowledge about—such as where the roller coaster is likely to be located relative to the ground. This prior knowledge changes how these images are processed, allowing thousands of them to be transferred from the whiteboard of short-term memory into the bank vault of long-term memory, where they are stored with remarkable detail.
Together, these experiments suggest why memories are not eliminated equally— indeed, some don’t seem to be eliminated at all. This might also explain why we’re so hopeless at remembering some things, and yet so awesome at remembering others.
Are you a scientist who specializes in neuroscience, cognitive science, or psychology? And have you read a recent peer-reviewed paper that you would like to write about? Please send suggestions to Mind Matters editor Gareth Cook, a Pulitzer prize-winning journalist at the Boston Globe. He can be reached at garethideas AT gmail.com or Twitter @garethideas.