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Increased Activation of Indirect Semantic Associations under Psilocybin

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The spread of activation in semantic networks can be measured using semantic and indirect semantic priming effects in lexical decision tasks (Spitzer et al 1993a and b). For example, in thought-disordered schizophrenic patients, activation spreads faster and farther than in non-thought-disordered patients and normal subjects, which results in an increased direct and indirect semantic priming effect (see below). This has been interpreted as the result of a decreased signal-to-noise ratio in cortical neural networks that process semantic information. Such a decreased signal-to-noise ratio has been related to a decreased dopaminergic modulation (Servan-Schreiber et al. 1990; Cohen and Servan- Schreiber 1992, 1993), which we recently were able to confirm directly in a study on the effects of L-dopa on semantic and indirect semantic priming (Kischka et al 1995).

As dopamine was found to have a focusing effect on the activity in semantic networks, i.e., it increases the signal-to-noise ratio and reduces the spread of activation (measured as reduced indirect semantic priming), we set out to investigate the effect of the hallucinogenic agent psilocybin on this task. Since psilocybin is known to act on the serotonin (5-HT) system and has effects of “broadening” conscious experiences, we hypothesized that it might exert a defocusing effect on semantic networks (i.e., decrease the signal-to-noise ratio), which should lead to an increased indirect semantic priming effect.

To test this hypothesis directly, we conducted a double-blind, placebo-controlled study on the effects of psilocybin on semantic and indirect semantic priming as part of a larger project that was designed to assess the behavioral effects and pharmacokinetic properties of this hallucinogenic agent (the results will be reported elsewhere; cf. Holzmann 1995).

Spitzer, M., Thimm, M., Hermle, L., Holzmann, P., Kovar, K., Heirnann, H., … Schneider, F. (1996). Increased Activation of Indirect Semantic Associations under Psilocybin. Biological Psychiatry, 39(12),  1055–1057.
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