Study helps explain ancient phenomenon

Dr Nicholas Price.
Dr Nicholas Price.

Our visual systems receive a barrage of stimulation due to continually changing lighting, shadows, and shifts in the position and orientation of objects. Processing these dynamic stimuli is a challenging task for the brain.

One way that the individual neurons that make up the visual system cope with ever-changing scenes is to optimise the way they process information by taking into account the prevailing status of the world. For example, when you walk from sunshine into a darkened cinema the eye and the brain adjust to the lower levels of light. This ‘adaptation’ allows neurons to maximise their sensitivity to the light levels that dominate the current environment.

But exactly how they adjust to changes in stimuli has been a mystery.

Monash Biomedicine Discovery Institute (BDI) scientist Dr Nicholas Price and his team have produced research that sheds light on this puzzle.

To conduct his study, Dr Price and his multidisciplinary team used a ‘movie’ that was updated 120 times a second to include all possible orientations, contrasts and luminances. These factors critically affect all neurons in the early visual parts of the brain. The researchers measured how neurons in the visual cortex encoded orientation information following changes in contrast or luminance.

They used computational techniques to extract information about how different features in the stimulus activity cause changes in the responses of individual neurons on a millisecond by millisecond basis.

Their study, published in Nature Communications, demonstrates how the coding and perception of orientation can remain relatively stable and reliable, despite continual changes in luminance and contrast, and revealed the dynamic neural coding mechanisms responsible.

“We think we’ve found a mechanism in the early visual processing parts of the brain that is able to dynamically adapt the way the brain represents scenes in a way that conserves information in the brain and optimises the way the brain works in the context of the current scene,” Dr Price said.

In doing so, the researchers have contributed to understanding a phenomenon observed by the Greek philosopher Aristotle more than 2000 years ago.

“It’s been known since then that perceptually we adjust to changing scenes,” Dr Price said.

“Aristotle sat next to a stream and watched the water flowing past. After a while he looked at the bank and it seemed to be moving. It’s called the ‘motion after-effect’,” he said.

“Although not a particularly evolutionarily adaptive example, it highlights how malleable the visual system can be.”

Dr Price says the findings may in the future help inform research into the bionic eye, on which he works.

“If we understand how the brain encodes complex scenes we can optimise the way we’re stimulating the brain, which means that we can produce better, more effective, more reliable stimulation in the bionic eye,” he said.

First author was PhD student Masoud Ghodrati.

The study was supported by the Australian National Health & Medical Research Council (NHMRC), the Human Frontier Science Program (HSFP) and the Australian Research Council (ARC).

Read the full paper in Nature Communications titled Contrast and luminance adaptation alter neuronal coding and perception of stimulus orientation.

About the Monash Biomedicine Discovery Institute 

Committed to making the discoveries that will relieve the future burden of disease, the newly established Monash Biomedicine Discovery Institute at Monash University brings together more than 120 internationally-renowned research teams. Our researchers are supported by world-class technology and infrastructure, and partner with industry, clinicians and researchers internationally to enhance lives through discovery.