Neuroscientists explore size constancy in the brain

Mel Goodale is relentlessly interested in how our brain enables us to understand the world.

“One of the most amazing things about the way we see the world is that it seems very stable, very sensible and we can understand things,” said the Canada Research Chair in Visual Neuroscience and director of Western’s Brain and Mind Institute.

Recent research released by Goodale goes even deeper, explaining how vision and the brain co-exist, allowing doctors to better regulate the necessary treatments for individual patients.

“To maintain a stable visual world, we need to know how far away objects are from us so that our brain can make the appropriate adjustments to the size of the image coming from the retina,” Goodale said. “Findings from this research are relevant not only for understanding how our brain represents the size of an object but also to understand medical conditions, where patients experience visual distortions in which the world gets oddly smaller or bigger.”

For example, when we see a car driving away from us down the road, Goodale said, it’s not actually shrinking in size, even though clearly in our eyes it looks as if it is. That’s called size constancy.

Goodale said if the human brain didn’t invoke size constancy, the world would appear to be a very strange place, with objects expanding as we moved closer to them and shrinking as we moved away, comparable to the ‘Alice in Wonderland Syndrome.’

While size constancy has been known for centuries, Goodale wants to understand how the brain enables us to see objects at different distances.

A key to understanding this is afterimage, a phenomena Goodale said we’ve all experienced. For instance, you’re at a wedding, a camera flash goes off and when you look way at a wall, you suddenly see a dark spot corresponding to where that flash excited your eye.

“When you have an afterimage and you project it on different surfaces, it appears to be the size of an object at that distance,” Goodale said. “So when you have the same afterimage and you project it on a piece of paper in your hand it looks very small, but if you look up at the wall, suddenly that after image looks much larger.”

So how, and why, does the brain do this? Goodale and his colleagues, Irene Sperandio and Philippe Chouinard, employed the use of Western’s 3T fMRI (functional magnetic resonance imaging) scanner to tackle this question.

Subjects were asked to stare at a light long enough to create an afterimage on the retina. As they projected the afterimage onto surfaces at different viewing distances, their brains were scanned. As expected, people reported the farther the surface on which they saw the afterimage, the larger it appeared.

The brain scans revealed this difference in the perceived size was playing out very early in the visual pathway – in a brain area typically thought to reflect only what is happening on the retina.

“When visual signals from the eye reach the brain, they enter what we call the cerebral cortex,” Goodale said. “We know there is a map of the retina on that visual cortex, kind of a map of the world out there. The question we addressed was is that map reflecting what’s on the back of your eye, or is reflecting what we really see out there in the world. The answer we found was that it reflects what’s out there in the world, and not what’s going on in the retina.”

The findings, revealed this week by Nature Neuroscience, have some real-world implications for patient treatment.

“We must also recognize that when people have damage to their brain from a stroke, it isn’t just their speech that’s affected, or their ability to move arms or legs, but they also have fundamental problems in seeing the world,” Goodale said. “As we begin to understand how the brain converts those bits of information into meaningful images that we can understand, the more we will be able to diagnose people correctly as to what sort of therapy or rehabilitation is needed.”

He added the principles illuminated by this work can also be usefully applied to computer-based recognition devices and artificial visual systems that use brain implants.