The human brain is a marvel of power and flexibility, and a pair of new studies out Monday demonstrates that when it runs up against the limits of its capacity to take in and store information, the brain often relies on its agility to fill the gap. In the process, however, information can be lost.
Two unrelated research studies published in the journal Proceedings of the National Academy of Sciences explore how people visually process and remember information about a panorama of similar and dissimilar objects. The new research supports the idea that our brains use different sites in the visual cortex to interpret different categories of visual stimuli -- suggesting that, when confronted with too much of the same thing, those circuits can be overloaded. And it tells us that when we upload such groupings for long-term storage, we leave by the wayside objects that our experience tells us don't really fit into the picture. Although efficient, such selective memory can be wrong.
In one of the studies, 55 participants did a visual perception exercise in which they were shown, for less than a second, a grouping of four things. Sometimes the four pictures were all faces; sometimes they were all scenes; sometimes all objects; and sometimes all bodies. And sometimes, researchers included two images of one things (say, faces) and two images of another (say, scenes).
Immediately after seeing the group of pictures, the researchers found, subjects were more accurate at recalling all the images when the four images had been a mix of two types of things than when all four images came from the same category. This study was led by Michael A. Cohen of Harvard University's psychology department.
Subsequent brain scans on a smaller group of subjects suggested why: The Harvard team found evidence that viewing faces activated neurons in one part of the occipitotemporal cortex, while viewing scenes activated neurons in a wholly separate part of the region. The sight of bodies or objects stimulated still different clusters of brain cells.
But there are only so many routes into those places in the brain where faces or scenes are represented, and trying to take in too many different images in the same category appeared to cause a sort of traffic jam of neural traffic, preventing some of the visual information from imprinting itself on a person's conscious awareness. When the four images sent visual information for processing to two different sites, traffic jams were averted and all the pictures got processed.
In the other study, researchers at the Princeton Neuroscience Institute and the University of Texas at Austin set out to explore the relationship between subjects' expectation of a given visual stimulus and their ability later to remember what they actually saw.
Presenting participants with a repeating sequence of images (scenes, faces and objects), the researchers first set up expectations on the participants' part that the third image he or she would see would be, say, a human face.
Then, in certain instances, they foiled that expectation: Instead of showing a face as the third image, researchers showed, say, a scene instead.
The subjects were told they were performing a classification task -- was the face male or female; was the scene indoor or outdoor? But 10 minutes after a subject had looked at a group of sequences, the researchers administered a surprise test of memory: They showed the subject 144 of the images he or she had just seen in groups of three, and threw in 48 previously unseen images. They asked subjects to identify which images they remembered having seen, and which were not in the initial group of images, and to rate how confident they were of their judgment.
Images that had been presented "out of context" -- not when a subject expected to see them -- were much less often remembered (or remembered with lower confidence) than were images that were shown in the order that subjects anticipated. Researchers suggested this was the brain's parsimonious use of storage space at work: When an image didn't match the predicted order, the subject appeared likely to give it a low priority for commitment to memory.
"Forgetting seems disadvantageous, but plays an essential role in maintaining the efficiency of memory operations," the researchers wrote. Participants reported to researchers that they had not even noticed that images were being presented in a predictable order -- or that their predictions had ever been upended. And yet, when an image appeared where it was not expected to be, it was automatically deemed less suitable for memory storage and often dropped by the wayside.
"Automatic retrieval occurs constantly in the background, pruning invalid memories without burdening our conscious mind" with a yes-or-no decision about allotting precious memory space to them, the study's authors wrote.