Ayahuasca Research

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Study 1.

Effects of the Amazonian Psychoactive Beverage Ayahuasca on Binocular Rivalry: Interhemispheric Switching or Interhemispheric Fusion?

Ede Frecska, Keith D. White, and Luis E. Luna

Binocular rivalry belongs to a class of visual illusions in which an ambiguous but unchanging sensory input leads to sudden perceptual switches. Rivalry phenomena occur when dissimilar images stimulate corresponding retinal areas of the two eyes. During rivalry, the image present in one eye disappears from awareness (i.e., is suppressed), while the other eye’s image is seen (i.e., is dominant). Phases of suppression and dominance alternate between images: first, one eye’s view is suppressed and the other’s is dominant; a moment later, the second eye’s view is suppressed and the first eye’s view dominates. With prolonged viewing, such reversals can continue indefinitely in a quasi-regular manner, usually alternating every few seconds. This visual competition has a resemblance to spontaneous alternation of reversible figures and ambiguous pictures (e.g. Necker cube, Rubin’s vase, or Salvador Dali’s Slave Market and the Disappearing Bust of Voltaire).

Several theories have been proposed to account for binocular rivalry. One of the most recent theories of binocular rivalry is the interhemispheric switching model (http://www.uq.edu.au/nuq/jack/InterhemisphericSwitching.html). According to this theory each hemisphere adopts one of the rival stimuli, and that the perceptual alternation reflects switching of activity between the two hemispheres.

The real importance of binocular rivalry may be that it sheds light on the problem of visual awareness. The reason is simply that the stimulus is relatively constant yet the perception changes radically over time. If the interhemispheric switching model is right, then binocular rivalry can be a useful tool in studying variation of interhemispheric effects on conscious experience and it can give more insight into the nature of altered states of consciousness.

Changes in hemispheric integration were discussed as a possible basis for hallucinogenic effects (Mandell, A. 1985. Interhemispheric Fusion. In: Journal of Psychoactive Drugs 17(4):257-266.). It was suggested that hallucinogens diminish the usual hemispheric dominance and may lead to an increased amount of hemispheric coupling (i.e. bilateral coherence, interhemispheric fusion).

The purpose of present study was to use binocular rivalry as a tool for revealing the temporal characteristics of cerebral dominance in altered state of consciousness induced by ayahuasca. If Mandell’s theory on interhemispheric fusion is correct, then hemispheric dominance is relatively suspended by the hallucinogen and, according to the interhemispheric switching interpretation of binocular rivalry, longer nonrivalry responses (nondominance periods, or phenomenal fusion) are expected in the altered state of consciousness. Accordingly, the authors hypothesize that ayahuasca ingestion will increase the duration or the frequency of nondominance responses.

Subjects were tested both with and without ayahuasca ingestion, to serve as their own control. Ingestion of ayahuasca resulted in a decrease of rivalry alternation rates, increased length of one percept and there was evidence of perceptual fusion. Our findings – very indirectly though – indicate a shift in interhemispheric dominance and support the concept of interhemispheric fusion in altered states of consciousness.