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Van Toi V, Abraham H, Bursac N. 
“Post-LSD Hallucinosis Is Associated With Decrease In Flicker-Fusion Sensitivities ”. 
Investigative Ophthalmology and Visual Science (supp). 1996;37(3):3300.

PURPOSE: LSD and similar agents may alter visual perceptions continuously and permanently in certain users resulting in hallucinogen persisting perceptual disorder (HPPD)1. In the present study, the psychophysical De Lange curves (TMTF curves) were established to determine how HPPD affected former LSD abusers.

METHODS: Sinusoidal waveform stimulation was used and generated by a visual stimulator2. The stimuli were achromatic, with 200 troland luminance, 500 field size, and were seen under a Maxwellian view. Nine flicker frequencies were tested. At each frequency, the measurement was triplicated using the 'fusion to flicker with preview' method3 and the results were averaged. The examiner was blinded to the diagnostic status of the subjects. Eighteen subjects participated in the study. They formed three equal number groups: LSD naive controls (age: 25 2.9 years), post-LSD subjects without HPPD (age: 27.2 8.5) and post-LSD subjects with HPPD (age: 22.4 4.2). The LSD groups had a history of LSD use 2-3 years prior to the study. At the time of testing, subjects were drug-free verified by toxicological screening. Binocular corrected visual acuity was normal in all subjects.

RESULTS: The De Lange curves were established in 10-15 minutes for each subject. Among the three groups: 1) The difference in high frequency sensitivities (> 30 Hz) was not statistically significant; 2) By contrast, the sensitivities at lower frequencies differed markedly: the lower the frequencies, the greater the difference, e.g., at 5 Hz the sensitivities of the control group were more than 3 times those of the LSD subjects without HPPD, and 5 times those of LSD with HPPD; and 3) The differences of the CFF values were statistically significant using an ANOVA (p< 0.001).

CONCLUSIONS: 1) LSD reduces sensitivities to the flicker at frequencies less than 30 Hz; 2) Patients with post-LSD hallucinations have the least sensitivity to the flicker perception, followed by asymptomatic LSD users, followed by LSD naive controls; 3) The De Lange curve is a sensitive, specific and reliable method to determine this; 4) Decreased sensitivity to flicker is consistent with the hypothesis that HPPD is associated with disinhibition of visual information processing.

1 Abraham et al., J. Ab. Psycho., 97, 1988
2 Vo Van Toi and Grounauer, Rev. Sci. Instrum, 49, 1978;
3 Vo Van Toi et al., App. Optics, 30, 1991.

Tufts University, Electrical Engineering and Computer Science Department, Biomedical Engineering Lab, Medford, MA 02155 and *Brown University, Department of Psychiatry and Human Behavior, Providence, RI 02906

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