Ocular disorders

silverwolf0 [subject #111], Still from the Visual Snow anime video clip illustrating his experience of visual snow, 2006. Click here to see the full video.

Silverwolf0

A respondent to Sofia Greene's internet survey, a 19-year-old Chinese male living in Texas, USA (subject #111), reported a diagnosis of cone dystrophy with persistent visual snow in addition to the distinctive triad of loss of color vision, photophobia, and reduced central vision. "I have had these issues, and also VS, since birth. It is 24/7 static in my vision. One facet, although not exactly true to how it looks, can be viewed from this anime clip [see here for a lower resolution version]. It is constantly moving grains or dots that is similar to what you see on TV when the reception is bad. It is also similar to when one uses a cheap digital camera (or phone camera) to take a picture at night and all the noise can be seen on the screen as well as on the final picture. An analogy of the movement would be of an electron cloud where you cannot determine the exact position of any particle as the speed is too fast. It is also similar to the haze/fog of an electron cloud. It also takes on the general color of whatever it is hovering over, as can be seen on the video clip. VS occurs also when I dream and when my eyes are closed so it is a true 24/7 ordeal."

(silverwolf0 [subject #111], Email to Sofia Greene, May 25, 2006; additions in square brackets by Klaus Podoll)

nibart [subject #24], My vision flickers, 2004. Visual snow in the area of a ring scotoma resulting from rod-cone dystrophies in a patient with retinitis pigmentosa. © 2004 nibart (larger image see here) [more]

nibart

Another participant in Sofia Greene's internet survey, a 32-year-old Caucasian female living in Israel (subject #24), reported a diagnosis of retinitis pigmentosa as cause of her progressive visual disturbances affecting night vision and peripheral vision. "It must have been around 1997 when I first became aware that there was something like tv static twinkling at the far edges of my visual field. I am sending you this sketch that I made almost two years ago. This is roughly how my vision was at that time; in life-size, sitting approximately 40 cm from the computer screen. Everything seems to oscillate against whatever is next to it, like under heavy rain or electric current. I didn't draw dots in all the puddles, but they all look the same. I tried different backgrounds for the transparency effect. I also added the bright white flashes that swirl occasionally when I begin an activity or, sometimes, even just blink. Notice the shadows inside, they were bouncing back and forth like little flames and some dots were popping randomly inside. I had a visual field test where you can clearly see a ring of blurred vision exactly where I claim my puddles are. Today the entire structure has narrowed even more, but there is still an almost clear distinction between heavily snowed areas and mildly snowed areas in the centre. If it's snowy outside the circle, I cannot tell."

(nibart [subject #24], Email to Sofia Greene, March 2, 2006)

Sylvia12

A third participant in Sofia Greene's internet survey, a 25-year-old Finnish female (subject #109) living in the UK, wrote: "My symptoms started in the autumn of 2005, with floaters, then sparklies, then afterimages, then visual snow; all of these visual phenomena lasting 24 hours a day, 7 days a week. Usually the negative or (to a lesser extent) positive afterimages occur right after seeing the stimulus and are very brief. This year I lost my vivid colours and got a single instance of palinopsia (i.e., I saw an afterimage of a character in a movie about 2 hours after I had stopped watching the film). This year also my night blindness started. Sometimes I see a bright lattice when I open my eyes at night to go to the bathroom. At the onset of my symptoms there was nothing else wrong with me except a sore neck and I had used a muscle relaxant called Sirdalud [tizanidine] for 3 weeks. No one in my family has had migraines. I have only had headaches related to neck tension, never a migraine. I haven't used drugs. A MRI of the brain, VEP and ERG were normal. A multifocal ERG showed overall depressed signal everywhere else except in fovea, hence my doctors suspect a retinal disease."

(Sylvia12 [subject #109], Email to Klaus Podoll, May 15, 2006; additions in square brackets by Klaus Podoll)

Except for her single episode of palinopsia, Sylvia12's visual symptoms and her pathological multifocal electroretinogram (mfERG) suggest the presence of a retinal disease of unknown origin. Palinopsia is most commonly associated with posterior cerebral disturbances, but has also been described in patients with diseases apparently confined to the eye (Pomeranz and Lessell, 2000), so the aforementioned single episode of visual perseveration does not exclude a diagnosis of retinal disease.

Lars Frisén MD, PhD, Retina's matrix of non-overlapping receptive fields (top image) and disturbance of retina's matrix due to randomly distributed loss of one-third of the receptive fields (lower image), 2007. © 2007 Lars Frisén MD, PhD

Neuro-ophthalmologist Lars Frisén MD, PhD, professor at the Department of Ophthalmology at Göteborg University in Sweden, suggested a retinal disturbance with randomly distributed loss of its receptive fields as cause of the visual snow experience. "It is well known", he wrote on his website, "that the retina samples the image of the outside world discretely, by means of a seamless matrix of non-overlapping receptive fields (top figure). It is rational to expect that a loss or disconnection of receptive fields should result in a splotchy neural image. The second figure from the top mimics a randomly distributed loss of one-third of the receptive fields. Intuitively, such a severe deficit should be easy to detect and describe. Yet, patients often fail to detect even more severe damage, and those who do detect a deficit, don't provide descriptions that are even remotely reminiscent of a splotchily depleted neural matrix. A possible exception is formed by patients who see numerous small dots wherever they look. The phenomenon is usually called 'visual snow'. There are several websites dedicated to this subject." (Frisén, 2007)

However, this model only accounts for white visual snow, whereas in many cases it is black or coloured. It would also predict a very specific size of snow particles, equal to the size of a single receptive field, which doesn't match with the variability of the particle size of visual snow as described by the experients. Also it does not account for its movement dimension, i.e. the random movements of the visual snow particles. Action patterns like the transition from visual snow to geometric forms, which are described by a small number of subjects, cannot be explained by Frisén's model, too. The incomplete fit of this model to the phenomenology of visual snow results from the fact that it is based on the assumption of a retinal disturbance, whereas visual snow is, in the majority of cases, a symptom of cerebral disorders such as persistent aura without infarction or HPPD.

Left and middle panel: PULSE TYPE SNOWY VISION according to Fulton (2004a). Left panel: BLACK noise pulses. No perceived noise in black window frame. Middle panel: WHITE noise pulses. No perceived noise in white window frame. Right panel: BROADBAND (analog) SNOWY VISION according to Fulton (2004a). White noise perceived in black areas and black noise perceived in white areas. According to Fulton (2004a), it is difficult to display snowy vision in simple pictures because of the continual motion of the noise elements in the actual case relative to the background scene. (Reproduced from Fulton's webpage on SNOWY VISION ITS PERCEPTION AND ITS CAUSE, last update August 25, 2004) © 2004 James T. Fulton (larger image see here)

References

Frisén L. On seeing with defective vision - Interactive simulations. Website, Last update November 1, 2007.
Fulton JT. SNOWY VISION ITS PERCEPTION AND ITS CAUSE. Website, Last update August 25, 2004a.
Fulton JT. Biological Vision: A 21st Century Tutorial. No Publisher's House, 2004b.
Pomeranz HD, Lessell S. Palinopsia and polyopia in the absence of drugs or cerebral disease. Neurology 2000; 54: 855-859.

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