Visual snow causes being revealed
Neuroscientists have recently characterised how visual processing is disrupted in the condition known as Visual Snow Syndrome (VSS).
The hallmark symptom of VSS is constant and dynamic “snow” across the entire field of vision.
For many sufferers, one minute you can see everything clearly, and in an instant you’re viewing the world through a veil of static, like an old-fashioned noir film or badly tuned analogue TV. Add little floating black specks, darting across your eyes, to this image. Or shadowy outlines of images persisting that you were previously looking at, despite looking elsewhere or closing your eyes.
What about living and experiencing these symptoms all the time, every day?
This is Visual Snow Syndrome, and its impact on those who experience the condition is immense, affecting their capacity to work, socialise and have effective relationships with others.
Eye tests of individuals with VSS don’t uncover anything, as it is not an eye disorder but rather a brain malfunction. It is believed to be a neurological problem whereby the brain incorrectly processes visual information sent from the eye.
Yet the cause of VSS is a mystery. Not knowing its cause presents a challenge in how clinicians are able to treat the condition. The typical symptoms of VSS experienced by people suggest that there may be problems in visual processing in the brain. Visual processing is responsible for converting visual information from the eyes into a cohesive and perceptible image within the brain. This image can then be used to inform a response, to allow us to interact with our environment.
For the first time, research has been able to display objective functional changes to the ocular motor networks in the brains of people with VSS. A recent study published in the journal Neurology has uncovered evidence of processing speed and error differences which can be measured with objective testing approaches.
Associate Professor Joanne Fielding from the Department of Neuroscience (Central Clinical School) led the study which used three different ocular motor tasks which placed different demands on the ocular motor network in the brain which controls eye movement. This study also allowed the team to determine at which stage visual processing was affected. Ms Emma Solly, PhD student, and Dr Meaghan Clough, a postdoctoral fellow in the Fielding / White research group worked with a group of patients diagnosed with VSS and compared their eye movements with another group of people with no health problems.
Senior author A/Prof Fielding said, “The ocular motor network and its processing of visual information is complex, involving a number of distinct steps that allow us to make sense of how we see the world.”
She said that the network includes areas of the brain involved in sending visual information from the eyes to the brain. It then puts together this visual information using cognitive processes to make a cohesive image, resulting in an eye movement.
“Our study provides the first evidence of objective and quantifiable behavioural changes in patients with visual snow syndrome.”
The team discovered that patients with VSS moved their eyes faster than healthy patients towards a suddenly appearing stimulus. In addition, when asked instead to stop that action and move their eyes in the opposite direction, VSS patients were more likely to erroneously move their eyes towards the stimuli.
Interestingly, when the difficulty of each of these tasks increased, therefore requiring increased demands on an individual's higher order visual processing in the brain, the results were no different. The patients with VSS still responded with faster eye movements and the proportion of erroneous eye movements did not change.
A/Prof Fielding said, “These results suggest that in people with VSS the visual processing changes are not a consequence of disruption to decision-making centres of the brain. Rather, patients with VSS appear to be processing visual stimuli faster than healthy people, leading to hyper-accelerated eye movements.
“This analysis has provided an essential first step into defining a behavioural signature of VSS, and identifying the brain areas and processes affected.
“Once we know what's causing the issue, that information can be used to develop targeted treatment and better management of this debilitating syndrome.”
Reference: Emma J Solly, Meaghan Clough, Allison M McKendrick, Paige Foletta, Owen B White, Joanne Fielding. Ocular motor measures of visual processing changes in visual snow syndrome. Neurology. First published July 16, 2020, DOI: https://doi.org/10.1212/WNL.0000000000010372