“Visual Snow” and its Possible Origins

Post by Anastasia Sares

The takeaway

Visual snow syndrome is a condition where people continually see “static” or “snow” interfering with their vision, which may be accompanied by other visual problems. Visual neurons fire randomly all the time, but for people with visual snow, it seems that the brain is amplifying this random firing, bringing it to conscious awareness.

What's the science?

Visual snow is a newly-described condition that is currently estimated to affect around 2% of the population, and there are a few hypotheses as to why it occurs. One is that the neurons in the visual system could be producing an excessive amount of noise – random fluctuations that have nothing to do with the outside world – that are then interpreted as light signals. The second, slightly more nuanced theory is that the total amount of noise in visual neurons is the same for people with visual snow and people without it, but that the gain, or amplification of this noise is especially high for people with visual snow (think of someone turning up the volume on a bad-quality radio station). This week in Brain, Brooks and colleagues pitted these two possible causes of visual snow against each other and found a victor.

How did they do it?

The authors recruited people with and without visual snow to participate in some tasks. They also recruited people in both groups with and without migraines, because these people can also have visual snow as a symptom, though it may have a different cause. One task was designed to measure the total amount of noise (random activity) in the visual system. Participants were shown two squares and had to decide which one contained a lighter-colored circle inside it. The images were made increasingly “noisier” by adding a bunch of lighter and darker pixels (the authors called this “external noise” as opposed to the “internal noise” generated by the visual system itself). By doing the experiment three times with the same stimuli, they could get an idea of a person’s consistency in their responses, which should be related to the amount of internal noise in the visual system (low internal noise allows people to make more consistent responses).

The second task was designed to measure the gain of the visual system. Participants were shown four squares and asked which one’s brightness had been different from the others. The higher the contrast between the squares and the background, the more contrast is needed to identify the odd one out—this is called contrast gain, and the researchers suspected it would be especially strong in people with visual snow.

What did they find?

When comparing the different groups of participants, the authors found no difference in the total amount of noise in the visual system. On the other hand, contrast gain was increased only in people with visual snow, regardless of migraine status. Variations in the contrast gain experiment showed that the difference was specific to neurons in the parvocellular pathway—a pathway with slower-response neurons responsible for high-resolution color vision.

What's the impact?

By comparing different clinical groups (with/without migraine and with/without visual snow), the authors showed that abnormal contrast gain is specific to visual snow. Of course, these tasks are indirect measures of what’s actually going on in the brain; other studies are needed to examine the actual neural activity involved. The better we understand this syndrome, the more likely we will be to find a treatment.

Brooks et al. Visual contrast perception in visual snow syndrome reveals abnormal neural gain but not neural noise. Brain (2021). Access the original scientific publication here