EEG coherence in post-LSD visual hallucinations

LSD use in certain individuals may result in chronic visual hallucinations, a DSM-IV syndrome known as hallucinogen persisting perception disorder (HP PD). We studied 38 HPPD subjects with a mean of 9.7 years of persistent vis ual hallucinations and 33 control subjects. Measures of local and medium di stance EEG spectral coherence were calculated from all subjects. Coherence, a measure of spectral similarity over time, may estimate cortical coupling . In the eyes-open state in HPPD subjects, widespread reduction of coherenc e was noted. However, upon eye closure, the occipital region demonstrated a ugmented regional coherence over many frequencies but with reduced coherenc e of the occipital region to more distant regions. This occipital coherence increase correlated with previously reported shortened occipital visual ev oked potential latency for HPPD subjects. We speculate from coherence and k nown clinical and psychophysical data that, in HPPD, there is widespread co rtical inhibition in the eyes-opened state, but localized and isolated occi pital disinhibition upon eye closure, a state known to facilitate hallucina tory experiences. An analogy is drawn to findings in the interictal and ict al epileptic focus. In HPPD, we speculate that occipital EEG hypersynchrony resulting from increased regional coherence, when coupled with relative is olation of visual cortex, especially upon eye closure, facilitates hallucin ations and illusions. (C) 2001 Elsevier Science Ireland Ltd. All rights res erved.