Several visual symptoms have been reported that introduce excess noise into the normal visual system. They fall into two distinct classes. The following discussion is based on a close study of the literature and about two dozen case studies that have arisen in response to this site since its inception.

In the first class, labeled broadband snowy vision, the noise appears as a fine texture (similar to that seen on a TV turned to a blank channel) of low amplitude obscuring the scene in object space. The noise may appear both lighter and darker than the background.

The second type,labeled pulse type snowy vison, involves noise that appears as widely spaced fine dots appearing randomly over the field of view. It occurs in two forms where the individual dots are either darker than the local background or lighter than the local background. All three of these situations are shown below.

It is difficult to display the symptoms of the above diseases in simple pictures because of the continual motion of the noise elements in the actual case relative to the background scene.

The broadband and pulse type of snowy vision have distinctly separate underlying causes (etiologies) and will be discussed separately.

Liu, et. al. have presented a paper describing a wide variety of persistent positive visual phenomena (labeled Aura) that they relate to migraine headaches. They did not offer any treatment for their phenomena. This webpage will only address a specific group of their reported symptoms. It will also explore the possibility that some sources of the symptoms are in the eyes as well as their assumption that the source was in the brain. The symptoms discussed here last for more than an hour.

Alison Hale has provided a web site devoted to her medical condition. It includes a series of four images simulating the disease of Snowy Vision as she observes it personally. The last three images illustrate the character of the noise more clearly than the first. Her observations have been compared with two other unrelated sufferer, O. S. and S. S. The symptoms are also comparable to those reported for subjects 6, 7, 8 & 9 in the paper by Liu, et. al. In the process of correlation, additional details about the disease have become evident, along with its potential appearance within other syndromes (groups of symptoms).

Most of the subjects have reported the noise appears to be a veil in front of, but near, the actual image. A few subjects have described it as a veil that appears to be in a different plane than the scene. Sometimes this plane is discontinuous, giving a stereographic effect. Only a very few subjects have defined any color aspect to the snow.

A more complete discussion of these diseases appears in Chapter 18, Section 18.8.2 of PROCESSES IN BIOLOGICAL VISION. Download Chapter 18 from Table of Contents.

• Major symptoms of pulse type of SNOWY VISION


• Major symptoms of broadband type of SNOWY VISION


• Test tablet to quantify a subjects symptoms of SNOWY VISION



• First Order Analyses of the Cause of the Disease


• Detailed Description of the failures leading to SNOWY VISION


• Most likely source of the extraneous signal pulses causing SNOWY VISION

A rare disease has been reported that causes the visual field to be obscured by high frequency noise (very fine dots appearing widely spaced and at random positions over the field of view) under all conditions of illumination (including with the eyes closed in the dark). The symptoms appear to include:

• The noise is seen as very fine asynchronous dots scattered randomly across the field of view. They do not clump in order to form larger dots.
• The dots take on the color of their background.
• The dots are not intrinsically black and white nor do they exhibit a color shift relative to the background.
• O. S. reports the contrast associated with the noise spikes is less than that displayed in the simulations of Hale. He used the term 80% transmissibility to describe the mask associated with the noise. Alternately, the dots appear to have a peak amplitude of about 20% of the scene contrast.
• The signal associated with the noise is monopolar. It can occur in either of two polarities. In the case of the "BLACK" pulse noise, the dots are always darker than the scene they obscure as illustrated on the left above. In the case of the "WHITE" pulse noise, the dots are always lighter than the scene they obscure as illustrated in the middle frame above. The "WHITE" pulse noise is most prominent under low light conditions (mesotopic and scotopic conditions).

Major symptoms of BROADBAND SNOWY VISION

Hale has summarized the broadband form of the disease within her larger syndrome quite carefully. The symptoms of the disease include the following:

• The perpetual presence of excess noise in the perceived imagery of the visual system largely independent of the illumination level but directly linked to the contrast of the scene.
• The noise appears to be spread evenly across the visual field with one possible (and if verified, important exception). Hale has reported in one instance that the noise is absent from the area of her field of view represented by her foveola.
• The signal associated with the dots is bipolar. Dark spots are observed in light areas and light spots are observed in dark areas of the scene.
• The dots are variable in size as illustrated on the right above.

Additional discourse is under way to determine the amplitude of the noise spikes. This will involve observing black to white step patterns of various step heights. Additional observations will be made to determine if the noise shows any spatial structure near high contrast edges associated with the observed external world.

Hale has provided observations that suggest the mechanism causing Snowy Vision may also cause several associated spatial perception problems that form the Hale Syndrome.

• She notes the basically two-dimensional perception of the world within her field of view. However, at one point, She notes that the perception of parallel lines in object space at times can become quite distorted (sometimes violently) and take on a three-dimensional character.

As will be developed below, the broadband type of noise she describes under the title "scotopic sensitivity syndrome" appears to be closely linked to the same mechanism, but possibly not the same source as the noise in scotopic vision.

First Order Description of the Visual System

The Overall Block Diagram of the visual process in Humans and other Chordates, [Chapter 15, Part II] can be used to localize the failures resulting in these diseases. In the following figure, the lateral geniculate nuclei (LGN) and perigeniculate nuclei (PGN) are shown along with the superior colliculus. These are all major elements of the thalamus, a portion of what is commonly called the midbrain or upper brain stem as opposed to the cerebral hemispheres. These elements receive information over the optic nerve from the eyes shown on the left and mounted on the ocular platform. The LGN is divided into two major element called the magnocellular and parvocellular regions. The magnocellular region deals primarily with luminance information while the parvocellular region deals primarily with chrominance information. The magnocellular region also contributes significantly to the operation of the stereoptical subsystem of vision by sending signals to the superior colliculus and the motor neuron controllers labeled LTN, MTN and DTN. These elements, along with the muscles of the eyes form the oculomotor servomechanism.

POTENTIAL FAILURE LOCATIONS IN THE OVERALL BLOCK DIAGRAM OF VISION

First Order Description of the Perceived Images in BROADBAND SNOWY VISION

• A critical observation concerns the uneven distribution of the noise across the visual field. Hale has described a "hole" in the noise pattern centered on her line of fixation. If this feature is confirmed, it would be significant. This hole relates to the foveola of the retina. The photoreceptors of the foveola are processed separately (channel labeled [Y] in the figure)from the remainder of the retina.
• Subject S. S. has also described a nonuniformity in the noise distribution. However, he did not indicate it was centered on the line of fixation. Further discussions are attempting to quantify the distribution.
• In addition, S. S. has described a stereoscopic effect related to the noise pattern. He states the noise appears to be associated with a semi-transparent screen that oscillates in distance at a rate of 3-5 Hz.
• N. B. has reported from first hand experience, her snowy vision only occurs in one eye (visual field of view).

Since the disease is known to arise in midlife, the cause of the problem is not likely to be genetic or of prenatal origin. Alternately, the appearance of the disease during early development suggests it is not an electrical failure (such as the creation of a unique electrical leakage path) brought on by aging. At the moment, one or more electrical bias errors, due to either chemical or hydraulic imbalance appear to be the most likely cause of these symptoms. Further perceptual tests can further isolate the location of these potential errors.

The causes described here are too subtle to be detected using modern clinical procedures based on nuclear imaging techniques. Liu, et. al. did speculate in their Discussion that the symptoms may be due to spontaneous neural discharges.

Details related to the PULSE TYPE of SNOWY VISION

NOTE: The following discussion does not refer to scotopic noise that originates primarily in the stellate cells of the midbrain and is perceived under very low incident illumination of a scene in object space. Nor does it refer to photon noise which originates in the conversion of photon energy into electrical charges within the chromophores and adaptation amplifiers of the photoreceptors under mesotopic illumnation conditions. The disease is observed under all illumination conditions and is not related to the sensitivity of the eyes under scotopic conditions.

• The observation that the color of the noise does not differ significantly from the background largely eliminates the parvocellular portion of the LGN and the channels marked [P] and [Q] as a source of the failure.
• The observation that the noise pattern takes on the color of the background suggests the failure is localized to the magnocellular portion of the LGN, the channel marked [R] in the figure and the circuits of the luminance processing matrix.
• The absence of snow of a different color than the background in the perceived image of object space eliminates the photoreceptors of the retina (shown in the upper left as four parallel bars labeled L, M, S & UV) as a source of the disease.

As a result of the above observations, the source of the failure leading to PULSE TYPE SNOWY VISION can be isolated to the shaded portion of the preceding figure (including the magnocellular portion of the LGN but excluding the vertical shading leading to the three elements labeled DTN, LTN and MTN and the superior colliculus. The latter path provides the signal path that can lead to the disease of Snowy Vision contributing to other conditions found in both Hale's Syndrome and Irlen's Syndrome.

The following figure describes the signal processing associated with the eyes of the visual system. The major individual circuits found in each of photoreceptors used in human vision are shown on the left. The major circuit elements of the luminance processing matrix are shown within the dotted box. Other circuit elements are only shown to demonstrate that the problem is isolated to the elements associated with the luminance or [R-channel] of vision. The elements within the dotted box are only shown in skeleton form to emphasize their function. Not shown within this box is the synapse between the bipolar cell and the parasol ganglion cell. This is a passive circuit that is not believed able to cause the failure under study. The bipolar cells of the retina sum the signals from the three spectral channels used in the majority of human vision to create an analog luminance channel signal. This signal is then processed by the parasol type ganglion cells of the retina to create a pulse type version of the luminance signal more effectively transmitted over the optic nerve to the thalamus and other elements of the brain.

While little known to the lay public and much of academia, the human visual system operates as a change detector and relies upon the fine motion of the eyes, known as tremor, to generate the signals used in vision. As a result of tremor, all of the pulse signals, known as action potentials, traveling over the optic nerve between the eyes and the brain are normally synchronous with the motion of the eyes. Snowy Vision is causes by the introduction of non-synchronous action potentials into the signals carried by the individual neurons of the optic nerve. The non-synchronous nature of these action potentials makes them appear at random locations within the field of view.

Locating the source of these non-synchronous action potentials is the goal of this discussion.

SPECIFIC FAILURE LOCATION causing PULSE TYPE SNOWY VISION (Within dotted box)

There appear to be three candidate errors leading to pulse type Snowy Vision. First, extraneous noise can be introduced by a noise source related to the parasol ganglion cell. These potential sources are shown as i_ne and i_np in the figure. i_ne stands for a noise current source in the emitter Or dendritic) lead of the Activa (the biological transistor) shown and labeled AG (Activa of the Ganglion type). i_np stands for a noise current source in the poditic (or base) lead of the Activa. Second, a bias error associated with the emitter to base potential of the Activa could cause the generation of extraneous action potentials. These could be caused by an imbalance in the electrostenolytic mechanisms associated with the two rectangular boxes shown leading to the ground connections in this circuit. Third, bipolar cells are known to oscillate uncontrollably when excessive capacitance is introduced into their circuits by an investigators test probe. Such capacitance could be introduced by natural means and would lead to the observed extraneous noise.

Details related to the BROADBAND TYPE of SNOWY VISION

NOTE: The following discussion suggests broadband type snowy vision is closely related to the mechanisms associated with scotopic vision. HOwever, the source of the mechanism may be different. Normally, these mechanisms are associated with the stellate cells of the midbrain and particularly the lateral geniculate and perigeniculate nucleus (LGN & PGN). This symptom is observed under all illumination conditions and not just the low light levels normally described as scotopic.

Every time a neural signal is transmitted more than one or two millimeters, the analog information is encoded using action potentials. While encoded, the signal is largely immune to broadband noise. The recovery of this encoded signal is accomplished by the stellate cells who are always susceptable to broadband noise. However, subsequent electrotonic circuits are also susceptible to broadband noise. The fact the noise is visuotopic does suggest it occurs early in the circuitry of the LGN or PGN and is therefore likely to be due to excess noise in the stellate circuits.

A Stereoscopic hallucination closely linked to snowy vision

The following test tablet is designed to further localize the source of the disease. Copy the following questions before attempting to view the target and record your answers.

1. The goal of the first test is to determine if you see light spots on a dark background. Do you see light spots inside the rectangle marked 100? Question 1: Yes or No


2. The goal of the second test is to determine if you see dark spots on a light background. Do you see dark spots inside the rectangle marked 0? Question 2: Yes or No


3. The goal of the third test is to determine if the apparent noise spikes are higher or lower in contrast than each of the transitions in the step target. Look along the top edges of the test tablets. Compare the darkest noise spots with the blackness of the adjacent tablet. What test tablet number describes the darkest spot you see? Question 3: Give a number


4. The goal of the fourth test is to determine if the density of the noise spikes is higher near the transitions in the step tablet. Is the density of spots different on the two sides of the contrast edges formed by the tablets? Question 4a: Yes or No Is the density of spots higher on the dark side or the light side of each step? Question 4b: Darker or Lighter

• The goal of the fifth test is to move back from the screen until the smaller BLACK circle is equal in diameter to 0.5 inches at 30 inches (or 0.25 inches at 15 inches) Then, fixate on the center of the smaller circle. Is the noise spike density higher or lower inside the small circle compared to the outside? Question 5a: Is the density higher, lower or same At the same distance, do you see any difference between the spot density inside the big circle compared to outside of it? Question 5b: Yes or No
  


• The goal of the sixth test is to determine the predominant size of the noise dots that you see from the same distance as used above. Note the number next to the dots on the left that most closely approximate the size of your BLACK dots. Question 6: Give a number.
  
  

• The goal of the seventh test is to move back from the screen until the smaller WHITE circle is equal in diameter to 0.5 inches at 30 inches (or 0.25 inches at 15 inches) Then, fixate on the center of the smaller circle. Is the noise spike density higher or lower inside the small circle compared to the outside? Question 7a: Is the density higher, lower or same At the same distance, do you see any difference between the spot density inside the big circle compared to outside of it? Question 7b: Yes or No


• The goal of the eighth test is to determine the predominant size of the noise dots that you see from the same distance as used above. Note the number next to the dots on the left that most closely approximate the size of your WHITE dots. Question 8: Give a number.

If you suffer from Snowy Vision, please e-mail me the answers to these questions along with your name, age, address (at least the country so I know the time zone) and any other comments you may have about your condition (such as do you wear glasses). I will tailor the test chart further based on the comments I receive.

At this time, the most likely cause of the disease known as Snowy Vision is a chemical or hydraulic imbalance associated with the interneural matrix (INM) of the retina located near the vitreous humor of the eye. The specific nature of this imbalance is yet to be determined. Until then, no treatment or cure for the disease can be offered.

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