This morning I cracked my back. Upon re-straightening, I noticed that every visible surface had been coated with glitter. While less back pain certainly improves my worldview, this seemed a bit extreme. I've taken plenty of LSD over the years, do you think there's a connection here?
Dear Spinal Tapped,
You may be right. We are guessing that you might be experiencing what people often call "flashbacks." The scientific term is "hallucinogen persisting perceptual disorder," or HPPD. HPPD is the second most common problem associated with repeated use of LSD, following only "bad trips" in frequency.
HPPD takes many forms, but the most common are visual disturbances like you experienced. These range from a sense of movement in the visual field, to visual afterimages or trails, or dots or patterns in your visual field (which is most likely what you experienced).
Sometimes people also feel strange and disassociated -- similar to what they once experienced while tripping. The frequency and severity of HPPD varies from lasting only a few seconds to its most intense, when the person constantly experiences this altered perception. Some LSD users aren't bothered at all, because they have these experiences very rarely, or they actually enjoy the "mini-trip." Other users experience the feelings so constantly that they are incapacitated and can't even work. While they usually diminish in intensity over months or years, sometimes, as you experienced, they appear abruptly a long time after the last LSD use. They can last for years in some people.
The first reports of HPDD emerged in 1954 in patients who received LSD for therapeutic purposes and have continued to the present day. Unofficial estimates range from 10 to 40 percent of regular users. This jibes with a survey conducted by some of our students that found that of a group of 100 students who had used LSD at least 30 to 50 times, about one-third had HPPD. The likelihood of developing HPPD is related to the number of LSD experiences. Although there are reports of HPPD in a person who only took LSD once, it occurs more often with those who have experimented on a more regular basis. HPPD is most common when you are tired, experiencing strong emotional arousal or smoking pot. It also can happen as you fall asleep.
We don't know what causes HPPD, although there are plenty of theories. Some believe there is a heightened sensitivity of neurochemical sensors in the brain, while others hypothesize that there is damage to the brain cells. We think it occurs as a result of adaptive changes in the brain. But, nobody has ever shown cell damage after LSD use, so that's just speculation. (There is also absolutely no evidence that LSD "lurks" in the spinal cord, releasing the drug slowly over time.)
The best scientific studies indicate that LSD acts upon a neural circuit that makes your brain keep paying attention to sensory (especially visual) stimuli that are no longer there. Chemical receptors can become "sensitized" when they are stimulated repeatedly by LSD. In fact, some scientific studies of visual function showed just that: The brains of regular LSD users kept responding to visual images that were no longer there. So maybe your cracked back and the intense stimulation had you "seeing stars," a common sensation reported by many people who have an intense knock on the head. And the LSD experience maintained this feeling for long enough that you got the glitter effect. We can reassure you that nothing terrible is happening to your brain because of the LSD, but you might expect occasional strange visuals from time to time. If it bothers you, consult your doctor, but if not, relax and enjoy the show.