On Perception (And Pancakes): How The Brain Keeps Vision Stable

You probably didn't think Julia Roberts could teach you much about subtle, yet critical, brain functions.

But, it turns out, she can. Recall Roberts in her iconic film “Pretty Woman.” In one scene, she is eating a croissant. But as the camera pans back to her, the croissant turned into a pancake.

It's likely that many of us missed that blooper, and now we know why. Scientists have discovered a brain mechanism that smooths our field of vision so that we don't notice certain subtle visual changes — such as a croissant becoming a pancake in an otherwise identical scene.

In a paper published last month in Nature Neuroscience, researchers from the University of California, Berkeley have identified a brain mechanism that helps to stabilize our field of vision. They call it, a “continuity field” — a process the brain uses to merge similar objects seen within a 15-second timeframe.

“It seems like a very odd thing the brain is doing that could make us less accurate,” said the study's lead author, Jason Fischer, who is now a postdoctoral fellow in the Department of Brain and Cognitive Sciences at MIT. “But in fact there is this huge benefit to it — and that is stabilizing perception over time.”

To measure this process, researchers showed study participants an image with alternating light and dark bars, or “gratings,” at a random angle every five seconds. The participants were then asked to move a white bar to match the tilt of the grating that had been shown.

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Researchers found that while the white bars generally aligned with the image, there were subtle differences that were biased toward the previous three or so images. These differences could be attributed to the continuity field.

Imagine, now, for example, you are driving down a highway in the pouring rain and you’re trying to read a road sign. The windshield wipers are moving; the raindrops are hitting your windshield. As you’re looking at the sign, you’re experiencing constant interruptions in your visual stream. In that case, the changes that the continuity field is causing us to miss are the raindrops and windshield wipers — you may even fail to notice them after a while. The continuity field, for the most part, is beneficial — it blocks the stuff we don’t want to see.

“Continuity fields are an advantageous development in the visual system, otherwise we wouldn’t have them,” Fischer said. “But I think there are going to be those rare cases where it does cause us to miss something important.”

Here's another example: say you’re at the airport going through the security line, and you place your computer in a bin on the conveyer belt. After you pass through the checkpoint, you walk over to pick up your computer. But instead of grabbing your own computer, you pick up the computer of the person in front of you who is busy putting their shoes back on. You missed the subtle changes between the computers because the continuity field had stabilized the images.

Without a continuity field, we would be hypersensitive to every single change in our field of vision. It would be similar to being on hallucinogenic drugs, according to the researchers. We might even perceive a change happened when there was no real change, said Fischer. The continuity field enables us to discard the irrelevant changes.

In effect, it helps us to stay sane.


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