Symptomology and mechanisms of visual snow syndrome

Abstract

Visual snow syndrome (VSS) is a condition in which innumerable tiny flickering dots cover the entire visual field. VSS can interfere with daily activities like driving and reading and is estimated to affect between 2 and 7% of the population. The precise details of visual snow’s symptomology and its underlying mechanisms are poorly understood, limiting the development of effective treatments. We address this lack of understanding by providing the first detailed measurements of the appearance of visual snow and demonstrating that it responds to illusions that affect neural activity in visual cortex. In the first experiment, participants with VSS matched the settings of a simulation to their visual snow. On average, visual snow was very small, fast, low density, and low contrast, and participants’ responses were highly reliable across trials. The logic of our second experiment is based on the phenomena that viewing an intense stimulus reduces both neural activity and perceptibility of similar stimuli, an effect called adaptation. Participants with VSS adapted to a high contrast snow-like stimulus, then judged the effect on their visual snow, pressing a button as soon as the adapted region of the screen matched an unadapted region. Adaptation reduced (and in many cases, eliminated) the snow temporarily. The duration of the effect was related to adapter duration by a power function, which is typical of classic adaptation. In a third experiment, participants adapted to stripe patterns drifting towards or away from the center of the screen, which is known to produce illusory motion when the stripes stop moving (called the motion aftereffect). People with VSS judged whether there was a motion aftereffect in their visual snow. Participants reported a motion aftereffect in their snow and again, increasing exposure to the adapter resulted in longer illusions. Together, these results suggest that visual snow arises from spontaneous (non-stimulus driven) activity in the visual pathways, which reaches motion-selective regions (e.g., V5/MT). This work establishes new and powerful tools for investigating the origins of VSS and may improve assessment of treatment efficacy and diagnostic testing.