How brain fills gaps

August 20, 2007
A vase from China´s Song dynasty demonstrates the use of very faint contrast borders to create the illusion of shading on a one-color background. The phenomenon is known as edge induction. The image of the vase is overlaid over the Cornsweet illusion, in which the left half of a rectangle divided in two looks lighter and the right area darker. Holding one´s hand over the center of the image reveals that the left and the right are in fact the same color. The brain "fills in" the color on the left and the right in response to information from the middle border. Courtesy of Anna Roe

When in doubt about what we see, our brains fill in the gaps for us by first drawing the borders and then "coloring" in the surface area, new research has found. The research is the first to pinpoint the areas in the brain, and the timing of their activity, that are responsible for how we see borders and surfaces.

The research was published online by Nature Neuroscience on Aug. 19.

"When you look at objects, they can be defined as either the contour of the object or surface features, like color and brightness. There's been a debate in neuroscience about how this occurs: Do you first see the contour and then fill it in like a coloring book, or do you see the surface and from there grow it out to build the contour?" says Anna Roe, Vanderbilt University associate professor of psychology and one of the study's authors. "Our examination of individual neurons in the visual cortex revealed that the former is true — our brains process the border information first and fill in the surface information second, causing us to perceive something that is in fact not really there."

The authors open the paper with the example of vases from China's Song dynasty on which faintly contrasting carved lines create the illusion of shading on a one-color background. The phenomenon is known as edge induction, and it is believed to help us distinguish objects in dim light or through fog, or when we see objects through dappled light, such as would be found in a forest. In these conditions, the authors hypothesized that our brain seizes upon the edge and then fills in the rest of the object. In the case of the vase, we see the contrasting border and perceive that the areas within the border also are of that contrasting color, even though in fact they are the same color as the rest of the background.

The authors set out to understand what is happening at the neural level in these situations by examining activity in individual neurons in the visual cortex of cats while the cats were looking at an illusion much like the one presented by the vase. The illusion, called the Craik-O'Brien-Cornsweet illusion, is a rectangular field of gray divided in half by a shaded middle border. The area to the left of the border appears brighter than that to the right. In reality, the brighter and darker areas exist only at the border; the surrounding areas to the left and the right are the exact same brightness. The illusion causes the brain to apply the brightness and darkness it sees at the border to the areas to the left and the right.

"The Cornsweet illusion is a very good example of edge induction — taking information from the edge of an object and applying it to the rest of the object," Roe said. "It demonstrates that a lot of what you perceive is actually a construction in your brain of border information plus surface information. In other words, a lot of what you see is not accurate. We were interested in understanding how the border and surface information combine to achieve what you end up seeing."

Roe and her colleagues found that when presented with the illusion, the neurons that respond to edges fired first and the neurons that respond to texture fired second. This firing delay was only seen when the subjects perceived a brightness difference within an image; when presented with an image that did not appear different in brightness, the neurons fired at the same time.

"We found that the timing of neuronal firings is not a fixed thing in the brain, it depends on what you are looking at," Roe said. "This is a great example of neuronal activity being dependent on a stimulus that is directly correlated to how we perceive objects. It is not hardwired — neural activity and relationships between neurons change depending upon the stimulus."

The authors also discovered that the neuronal response to the illusion took place by neurons residing in two separate areas of the visual cortex.

"It seems like this kind of border-to-surface delay was really prevalent in cell pairs in the two different areas of the visual cortex," Roe said. "This is the first example of interaction between two areas underlying border-surface perception. It emphasizes in a way that hasn't been emphasized before how important inter-area relations are in visual perception.

An important implication of this study is that it emphasizes the key role of neuronal interactions in the brain, rather than simply neuronal activity level, in visual perception," Roe said. "Thus, methods that are good at detecting activity levels, such as fMRI, may miss some of these basic mechanisms. So, it's important to have different tools to assess different aspects of brain response."

Roe's co-authors were Chou P. Hung, National Yang Ming University, Taipei, Taiwan, and Benjamin M. Ramsden, West Virginia University School of Medicine.

Source: Vanderbilt University

Explore further: Opponent activity of a new type of neuron is responsible for selective motion vision

Related Stories

Opponent activity of a new type of neuron is responsible for selective motion vision

July 17, 2015
Motion despite immobility. The illusion of self-motion is created, for example, in an IMAX cinema with the help of large-format movies. This is possible, because the brain calculates self-motion from the visual surround moving ...

Discovering Autism: An unsettling boom

December 19, 2011
Amber Dias couldn't be sure what was wrong with her little boy.

Recommended for you

Scientists produce human intestinal lining that re-creates living tissue inside organ-chip

February 16, 2018
Investigators have demonstrated how cells of a human intestinal lining created outside an individual's body mirror living tissue when placed inside microengineered Intestine-Chips, opening the door to personalized testing ...

Data wave hits health care

February 16, 2018
Technology used by Facebook, Google and Amazon to turn spoken language into text, recognize faces and target advertising could help doctors fight one of the deadliest infections in American hospitals.

Researcher explains how statistics, neuroscience improve anesthesiology

February 16, 2018
It's intuitive that anesthesia operates in the brain, but the standard protocol among anesthesiologists when monitoring and dosing patients during surgery is to rely on indirect signs of arousal like movement, and changes ...

Team reports progress in pursuit of sickle cell cure

February 16, 2018
Scientists have successfully used gene editing to repair 20 to 40 percent of stem and progenitor cells taken from the peripheral blood of patients with sickle cell disease, according to Rice University bioengineer Gang Bao.

Appetite-controlling molecule could prevent 'rebound' weight gain after dieting

February 15, 2018
Scientists have revealed how mice control their appetite when under stress such as cold temperatures and starvation, according to a new study by Monash University and St Vincent's Institute in Melbourne. The results shed ...

First study of radiation exposure in human gut Organ Chip device offers hope for better radioprotective drugs

February 14, 2018
Chernobyl. Three Mile Island. Fukushima. Accidents at nuclear power plants can potentially cause massive destruction and expose workers and civilians to dangerous levels of radiation that lead to cancerous genetic mutations ...


Please sign in to add a comment. Registration is free, and takes less than a minute. Read more

Click here to reset your password.
Sign in to get notified via email when new comments are made.