Neurological basis for perceptual illusions discovered

Credit: Courtesy of Milton Leite via Flickr Creative Commons Credit: Courtesy of Milton Leite via Flickr Creative Commons

Optical illusions, such as the Penrose staircase or the Fraser spiral, shift our perception of reality.

The idea of a person climbing a staircase forever, but never getting any higher, or of overlapping black and white arc segments that seem to form a spiral, but are actually concentric circles, is enough to make anyone dizzy. But how can we see things that aren’t even there?

New research from Carnegie Mellon University Assistant Professor of Biological Sciences Sandra J. Kuhlman reveals that optical illusions may be the product of your brain reacting to feedback between neurons in different parts of the brain’s visual system. Understanding this feedback has the potential to help scientists understand how the brain interprets sensory stimuli.

The Kanizsa triangle is another classic optical illusion. Through a series of black wedges and lines, you think you see a clear outline of a white triangle in the middle.

“We see with both our brain and our eyes,” said Kuhlman in a university press release.

“Your brain is making inferences that allow you to see the triangle. It’s connecting the dots between the corners of the wedges. Optical illusions illustrate some of the amazing things our visual system can do.”

This visual system is the result of complex interactions between light, our eyes, and our brains.

When the light from an object reaches our eyes, it travels through circuits of neurons in the retina, through the thalamus, and into the visual cortex of the brain.

The information is then processed in multiple stages and sent to the prefrontal cortex, where the brain decides how to respond to the given stimulus. But not all neurons make it to the prefrontal cortex. In the second stage of processing, some neurons send information back to the first stage of processing in a feedback loop, potentially altering the signal from being sent to the prefrontal cortex. Kuhlman’s team of colleagues decided to explore how the information moving backwards from the second stage to the first stage impacts how it is encoded in the visual system.

In order to do so, they needed a way to quantify the magnitude of information being sent back to the first stage. They recorded normal neuronal firing in the first stage as a mouse looked at moving patterns and then silenced the neurons in the second stage using modified optogenetic technology, halting the feedback to the second stage and allowing them to determine how much activity in the first stage was the result of feedback.

The researchers found that around 20 percent of neuronal activity in the visual cortex was the result of feedback, indicating that some information we perceive is not a direct response to visual stimuli, but to how the stimuli are perceived by higher cortical areas of the brain. This is a concept Kuhlman calls reciprocal connectivity, and may be the reason why our brain completes the triangle in the Kanizsa triangle optical illusion. “This represents a new way to study visual perception and neural computation.

If we want to truly understand the visual pathway, and cortical function in general, we have to understand these reciprocal connection[s],” Kuhlman said in a university press release.

Kuhlman is also a member of Carnegie Mellon’s BrainHub neuroscience initiative and the joint Carnegie Mellon and University of Pittsburgh Center for the Neural Basis of Cognition (CNBC). This study, recently published in The Journal of Neuroscience, received funding from the Knights Templar Eye Foundation, the Howard Hughes Medical Institute Undergraduate Program, the Fight-For-Sight Foundation, and the National Institutes of Health’s National Eye Institute.