My most recent article titled Visual Hallucinations Clinical Occurrence and Use in Differential Diagnosis which can be found (http://www.ncbi.nlm.nih.gov/pmc/articles/PMC1307180/pdf/westjmed00149-0042.pdf). The article defines visual hallucinations as a visual sensory perception without external stimulation, or operationally, as a behavioral syndrome in which a patient claims to see something or behaves as if he or she sees something that an observer cannot see. One common form of hallucinations are from optic nerve disease which can be characterized by bright light in conditions where bright light is absent such as low light scenarios and closed eyelids. This is usually stimulated by horizontal movement of the eye. For whatever reason, the optic nerve is enflamed which causes the hallucinations. Narcolepsy has also been found to cause hallucinations in cases of hypnagogic (falling asleep) of which 15-50% of subjects report experiencing hallucinations often occurring during sleep paralysis in which REM is interrupted. Hypnopompic (waking up) subjects did not have a set % of occurrences. Toxic or metabolic disorders often manifest different hallucinations due to low or high levels of chemicals affecting activity of optical nerves.
The article also touches on area 17 or the primary occipital striate cortex. This area of the brain is more likely to produce more unformed hallucinations or hallucinations that are noticeable but not clear. In addition, area 18 or the area of the brain known as the peristriate visual association area has been found to produce patterned hallucinations such as swirls or checkered boxes. Higher areas of visual association (area 19) produce complex hallucinations such as people and animals or larger more detailed hallucinations.
I also found it very interesting that children are much more likely than adults to have hallucinations such as imaginary friends or play objects as well as the ability to recall previously seen images as hallucinations. Finally if anyone knows a great spot for more information on how LSD effects the brain let me know!