Palinopsia

Palinopsia (Greek: palin for "again" and opsia for "seeing") is a visual disturbance that causes images to persist to some extent even after their corresponding stimulus has left. These images are known as afterimages and occur in persons with normal vision; however someone with palinopsia experiences them to a significantly greater degree, to the point where they become difficult or impossible to ignore, and often result in mild to severe anxiety and/or depression.

Description
There are two kinds of afterimages. The first kind is of a relatively short duration and is positive, meaning that the color of the afterimage is the same as that of the original image. The other type is a negative afterimage, meaning that the colors of the original image are inverted, which tends to last comparatively longer. The duration of the positive afterimage generally does not depend upon the length of exposure to the original image, while the intensity and duration of the negative afterimage depend on that of the original image. In other words, viewing an image for a longer period of time, or being exposed to a brighter image, can lead to either a longer or more intense afterimage, or both.

Both positive and negative afterimages are experienced by those with normal vision. For example, a very fast-moving object will often be perceived as having a trail, as might a point of light, such as a moving hand-held flashlight, in an otherwise unlit room. These are both examples of positive afterimages, or "trails." Lasting negative afterimages can occur after prolonged exposure to an unchanging visual stimulus, due to the "tiring" of cone cells. The canonical example of this uses the inverted American flag (see afterimage).

For palinopsia sufferers, the effects are largely the same, however the stimulus required to produce an afterimage is much less. Virtually any moving object will often be accompanied by trails. Negative afterimages can be formed after viewing an object for seconds or less, and as such have the propensity to "stack up". For example if a person with palinopsia glances at a lit TV screen in a dark room, and then shifts his or her gaze to the left of the TV, he or she will see a negative afterimage in the center of his or her vision. If he or she shifts their gaze downwards, another afterimage of the TV will appear, in the place where it had just been; in this case, to the right, and the first afterimage might still be visible alongside the second, and so forth. Furthermore, as the television is a bright light source (particularly to a palinopsia sufferer), each shift in gaze will be accompanied by a fleeting image of the lit TV screen. Thus, palinopsia is a condition which mimics normal phenomena, but with far greater intensity.

Pathology
The pathology which leads to palinopsia is unknown. Though normal negative afterimages are generally understood to be a retinal phenomenon, palinopsia is thought to be a brain-related disorder, and not an eye-related disorder. This is likely because palinopsia is most commonly encountered in connection with diseases, drugs or injuries which affect the brain. Palinopsia is a common symptom of hallucinogen persisting perception disorder, or HPPD, and can be the only symptom. It can also be caused by vascular disease, brain tumors, certain prescription medications or head trauma. It is hypothesized by some that palinopsia results from a lack of inhibitory neural signals in the visual center of the brain.

Some people suffering from ocular migraine or, rarely, occipital lobe epilepsy, can also have bouts of palinopsia, or persistent palinopsia. A rare condition known as persistent migraine aura without infarction, in which one experiences a migraine aura constantly, can accompanied by or described by palinopsia.

Research
Currently research into the causes and treatment palinopsia is all but nonexistent. Research articles concerning palinopsia, which are relatively rare, most often only document its occurrence, typically as either a reversible or irreversible effect of a prescribed medication or injury. Due to the comparatively few sufferers of the disorder, awareness is largely restricted to those affected and their confidants. As such, funding is difficult to come by.

From a practical point of view, studying the causes and treatment of palinopsia presents many challenges to the researcher. Since the condition does not produce effects apparent to an outside observer, animal testing, an invaluable tool for the study of many diseases, is impossible unless more is known about the pathology. If, as is likely, the problem stems from differences at the neuronal level, cell culture experiments or animal model experiments could lead to an understanding of how to induce, and possibly correct, the malfunction. However, characterizing that pathology would be a difficult process, as the option of isolating known dysfunctional neurons from a sufferer would be very difficult, as palinopsia is not fatal (by way of contrast, much of our early knowledge of the pathology in Alzheimer's Disease was learned by examining the brains of recently deceased Alzheimer's sufferers). Future research, such as advances in MRI technology, or immunofluorescence experiments, might allow insight into the physiological differences between the visual systems of persons with normal vision and those with palinopsia, without requiring invasive procedures. In addition, advances in knowledge of the brain, particularly in how the brain recycles visual stimuli, might provide an explanation for palinopsia.

Treatment
Current treatments typically seek to diminish the anxiety which accompanies the condition, such as with benzodiazapenes. But as the neurological basis for palinopsia is unknown, treatments which target the condition itself are as yet non-existent. In some instances, cases resolve themselves after a period of time, though the length of time varies considerably and can be lifelong; in fact what little evidence there is suggests that the condition is lifelong more often than not. Most sufferers seek to adapt to their condition by accepting it as part of normal experience. However, this adaptation is not as easy as it might seem and can take many years.