eye (phosphenes), or spontaneous photopsia. iii. Photophobia. iv. Nyctalopia; impaired night vision. Symptoms are not consistent with typical migraine aura. Symptoms are not better explained by another disorder (ophthalmological, drug abuse). Normal ophthalmology tests (best-corrected visual acuity, dilated fundus examination, visual field, and electroretinogram); not caused by previous intake of psychotropic drugs. Additional and non visual symptoms like tinnitus, ear pressure or brain fog and more might be present. It can also be diagnosed by PET scan. Comorbidities Migraine and migraine with aura are common comorbidities. However, comorbid migraine worsens some of the additional visual symptoms and tinnitus seen in "visual snow" syndrome. This might bias research studies by patients with migraine being more likely to offer study participation than those without migraine due to having more severe symptoms. In contrast to migraine, comorbidity of typical migraine aura does not appear to worsen symptoms. Psychological side effects of visual snow can include depersonalization, derealization, depression, photophobia and heliophobia in the individual affected. Patients with visual "snow" have normal equivalent input noise levels and contrast sensitivity. In a 2010 study, Raghaven et al. hypothesize that what the patients see as "snow" is eigengrau. This would also explain why many report more visual snow in low light conditions: "The intrinsic dark noise of primate cones is equivalent to ~4000 absorbed photons per second at mean light levels; below this the cone signals are dominated by intrinsic noise". Causes The causes are unclear. The underlying mechanism is believed to involve excessive excitability of neurons within the cortex of the brain, specifically the right lingual gyrus and left cerebellar anterior lobe of the brain. Persisting visual snow can feature as a leading addition to a migraine complication called persistent aura without infarction, commonly referred to as persistent migraine aura (PMA). In other clinical sub-forms of migraine headache may be absent and the migraine aura may not take the typical form of the zigzagged fortification spectrum (scintillating scotoma), but manifests with a large variety of focal neurological symptoms. Visual snow does not depend on the effect of psychotropic substances on the brain. Hallucinogen persisting perception disorder (HPPD), a condition caused by hallucinogenic drug use, is sometimes linked to visual snow, but both the connection of visual snow to HPPD and the cause and prevalence of HPPD is disputed. Most of the evidence for both is generally anecdotal, and subject to spotlight fallacy. Another proposed mechanism