Hallucinogen Persisting Perception Disorder
Hallucinogen Persisting Perception Disorder, HPPD, is a long lived problem arising from past use of strong hallucinogenic drugs. The majority of patients with HPPD report a prior use of lysergic acid diethylamide (LSD). Life LSD dose does not appear to increase the risk to developing HPPD. That is, I have seen some patients who have tripped on acid multiple times and then developed HPPD, and others develop the same disorder after one or two trips. I liken tripping on acid to playing Russian roulette, but using chemicals instead of bullets. Developing HPPD without ever tripping on acid can also happen, but in my experience this is quite rare, and suggestive of another disorder in the nervous system that needs medical attention.
The usual HPPD patient knows better than his doctors what is going on. The patients I see in consultation often have seen an average of six other medical specialists before they found their way to my office. This is probably because HPPD is a rare disorder, and not something that neurologists, psychiatrists, psychologists, and ophthalmologists usually encounter in training. Ironically these are the specialists most often consulted by the HPPD patient. Too often the first (and mistaken) clinical impression is that the patient has a psychosis. This can set treatment on a wrong path.
The majority of HPPD patients do not suffer from psychosis, or other signs of psychotic illness, such as auditory hallucinations and delusions. The disorder is for the most part a perceptual disorder in which visual information from the perceived world enters the brain but then cannot shut itself off. The result is lingering visual information, or a disinhibition of visual information processing, in the form of after-images, the trailing of images as they move through the visual field, flashes of light, and the formation of complex imagery on otherwise blank surfaces. Typical drawings from HPPD patients of what they see are shown on the right.
The first scientific description of the persisting visual phenomena described by LSD users may be found here.
Research in my laboratory later documented quite clearly that in the HPPD patient, when a visual signal from an image enters the brain, the signal stays around in consciousness longer than it does in the control subject who does not suffer HPPD. This finding has been found in three different studies of visual psychophysics.
One finding, shown in Figure Five, was that LSD users see a flickering light as fused more often than non-users, because the eyes of the LSD user continues to see the light after it’s gone. For details click here.
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Fig. 5 |
A similar event occurred if a subject was exposed to bright light, and then tested for the ability to adapt to darkness. In this experiment, Dr. Ernst Wolf and I found that the dark adaptation of LSD users was reduced compared to non-drug controls because the LSD group could not mentally shut off the original light enough to see a tiny light when in the dark. See Figure 6.
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Fig. 6 |
This impairment could be important, for example, to persons needing night vision such as airplane pilots.
My colleague, Frank Duffy, and I also found characteristic abnormalities in the brain’s electrical activity in HPPD subjects, documenting that HPPD is a disorder which clearly takes place in the brain, and not simply in the imagination. Figures 7 and 8 below are examples of brain electrical activity maps (BEAMs) of an HPPD patient side by side with a group of normal individuals for comparison.
Figure 7 is a BEAM study of a single HPPD patient whose brain electrical activity is significantly increased in both temporal regions during visual function when compared to controls. This is signified by the large white areas on each side of the head, with the viewpoint looking down on the patient’s head, nose on top of the image, and ears to each side.
Figure 8 shows the results of a group of HPPD subjects compared to non-drug users. The measure in this study is called electrical coherence. It is a way of assessing the degree to which parts of the brain show connectivity with one another. In HPPD there appears to be excessive connectivity in regions of the brain that process visual information: the posterior lobes of the brain, and the right temporal region. This excess coherence in actively hallucinating patients is noted by the white, yellow and red color bands. They may represent an overreaction of these regions to normal visual stimulation.
Click on each thumbnail below for a larger image.
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Figure 7 |
Figure 8 |
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