Cortical visual impairment

Symptoms of CVI usually include several (but not necessarily all) of the following:

• Variable vision. Visual ability can change from one day to the next but it can also fluctuate from minute to minute, especially when the person is tired.  When undertaking critical activities, people with CVI should be prepared for their vision to fluctuate, by taking precautions such as always carrying a white cane even if they don't always use it to the full, or always having giant print available even if they don't always need it (for example, consider the consequences of losing vision while giving a public speech).  Managing fatigue can reduce fluctuations but does not eliminate them.
• One eye may perform significantly worse than the other, and depth perception can be very limited (although not necessarily zero).
• The field of view may be severely limited.  The best vision might be in the centre (like tunnel vision) but more often it is at some other point and it is difficult to tell what the person is really looking at.  Note that if the person also has a common ocular visual impairment such as nystagmus then this can also affect which part(s) of the visual field are best (sometimes there exists a certain gaze direction which minimises the nystagmus, called a "null point") and both conditions come into play when restricting the field of view.
• Even though the field of view may be very narrow indeed, it is often possible for the person to detect and track movement outside this area.  (Movement is handled by the 'V5' part of the visual cortex, which may have escaped the damage.)  Sometimes a moving object can be seen better than a stationary one; at other times the person can sense movement but cannot identify what is moving (this can be annoying if the movement is prolonged, and to escape the annoyance the person may have to either gaze right at the movement or else obscure it).  Sometimes it is possible for a person with CVI to see things while moving their gaze around that they didn't detect when stationary.  However, movement that is too fast can be hard to track; some people find that fast-moving objects "disappear".
• Some objects may be easier to see than others.  For example, the person may have difficulty recognising faces or facial expressions but have fewer problems with written materials.  This is presumably due to the different way that the brain processes different things.
• Importance of colour and contrast.  The brain's colour processing is distributed in such a way that it is more difficult to damage, so people with CVI usually retain full perception of colour.  This can be used to advantage by colour-coding objects that might be hard to identify otherwise.  Sometimes yellow and red is easier to see, as long as this does not give poor contrast between the object and the background.
• Strong preference for a simplified view.  When dealing with text, the person might prefer to see only a small amount of it at once.  This allows for more magnification, which may be needed due to ocular visual impairments or in order to ensure that important things such as letters are not completely hidden behind scotomas (small defects in parts of the functioning visual field), but more importantly it simplifies the view and reduces the chances of getting lost in it.  However, the simplification of the view should not be done in such a way that it requires too rapid a movement to navigate around a larger view, since too much motion can cause problems (see above) although some is acceptable.
• For the same reason (simplified view), the person may also dislike crowded rooms and other situations where their functioning is dependent on making sense of a lot of visual 'clutter'.
• Visual processing can take a lot of effort.  Often the person has to make a conscious choice about how to divide mental effort between making sense of visual data and performing other tasks.  For some people, maintaining eye contact is difficult, which creates problems in Western culture.  (Note however that difficulty in eye contact can also be due to nystagmus.)
• People with CVI can sometimes benefit from a form of blindsight, which manifests itself as a kind of awareness of one's surroundings that cannot consciously be explained (for example, the person correctly guesses what they should do in order to avoid an obstacle but does not actually see that obstacle).  However, this cannot be relied on to work all the time.
• CVI can also cause varying degrees of photophobia in some cases.  It can take longer than usual to adjust to large changes in light level, and flash photography can be painful.  On the other hand, CVI can also in some cases cause a desire to compulsively gaze at light sources (including such things as candle flames).  The use of good task lighting (especially low-temperature lamps which can be placed at very close range) is often beneficial.
• Although people (with or without CVI) generally assume that they see things as they really are, in reality the brain is doing a certain amount of guessing and "filling in", which is why people sometimes think they see things that turn out on closer inspection not to be what they seemed.  This occurs much more frequently when a person has CVI.  Hence, a person with CVI can look at an optical illusion or abstract picture and perceive something that is significantly different from what a person without CVI will perceive.

The presence of CVI does not necessarily mean that the person's brain is damaged in any other way, but it can often be accompanied by other neurological problems, the most common being epilepsy (possibly in a mild form, and it may or may not have an associated increased susceptibility to emotional changes).  Some people with CVI have limited language skills, but this is not necessarily the case.

Diagnosing CVI is difficult.  A diagnosis is usually made when visual performance is poor but it is not possible to explain this from an eye examination.  Before CVI was widely known among professionals, some would conclude that the patient is faking their problems or has for some reason engagaed in self-deception.  However, there are now testing techniques that do not depend on the patient's words and actions, such as FMRI scanning, or the use of electrodes to detect responses to stimuli in both the retina and the brain.  These can be used to verify that the problem is indeed due to a visual cortex malfunction.

IMPORTANT: Just because I added something to Wikipedia, does not mean I agree with everything else on Wikipedia.  In 2005 a local student ran a hate campaign (I was one of the targets) and he used out-of-context questionable Wikipedia extracts as "facts" to support his views.  Despite its efforts, Wikipedia can temporarily contain highly misleading statements, and you shouldn't just accept everything you read.  Always do your research responsibly.

(The campaign also involved a fake scholarly paper by a hate group and the ``take it up with them'' fallacy I've written about separately, but certain Wikipedia editors didn't help.)