This is the first study to investigate the direct effects of dopaminergic drugs on reasoning biases. The JTC bias and overconfidence in errors showed a differential pattern of dopaminergic modulation, suggesting that they represent different facets of reasoning abnormalities that interact with each other to produce delusions in susceptible individuals.
IntroductionThe semantic priming effect refers to the observation that reaction times (i.e., pronunciation or speeded lexical decision) to a target word, such as "dog" are faster when the word is preceded by a related prime (e.g., "cat") than when it is preceded by an unrelated word (e.g., "pen").1,2 This effect is attributed to an automatic spreading of activation, initiated by the prime word, to adjacent nodes within the semantic network, 3,4 but also to controlled processes not related to the semantic store, such as attention-based expectancy (i.e., generation of predictions) and postlexical matching (i.e., evaluation of the semantic association between prime and target). 4,5 These processes can be differentially tapped by manipulating various aspects of priming paradigms, such as the primetarget interval (stimulus onset asynchrony [SOA]), the proportion of related word pairs or the nonword ratio in lexical decision designs. It is generally accepted that the contribution of controlled processes to priming becomes more pronounced with the increase of any of the aforementioned variables and is greater in lexical decision tasks than word pronunciation tasks. 5,6 Semantic priming tasks have been widely used in the schizophrenia literature. Several studies have reported increased single-word priming in patients with schizophrenia, especially those with formal thought disorder. Background: Enhanced automatic spreading of activation in the semantic network has been suggested to underlie formal thought disorder in patients with schizophrenia, but it is not clear how this relates to the dopaminergic dysfunction implicated in the disorder. Previous studies on dopaminergic modulation of priming in healthy volunteers have focused on controlled rather than automatic processes. The present study aimed to examine the effects of both a dopaminergic agonist and a dopaminergic antagonist on semantic priming while minimizing the contribution of controlled processes. Methods: We investigated the effects of levodopa (L-Dopa; 100 mg), haloperidol (2 mg) and placebo on priming in healthy participants within a randomized, double-blind, crossover design. We used a pronunciation priming task with word triplets; the middle word was an ambiguous word, whereas the first word of the triplet served to provide either a congruent, incongruent or unbiased context for the target word. Two stimulus onset asynchronies (SOA) were used: 150 ms and 750 ms. Results:The study involved 34 participants. At an SOA of 150 ms, L-Dopa accelerated responses to incongruent targets and subordinate targets of ambiguous words, whereas haloperidol was associated with faster responses in congruent contexts and dominant targets. At an SOA of 750 ms, haloperidol accelerated responses to subordinate targets. Limitations: Modulations in the relative magnitude of priming according to substance and condition rather than absolute priming were assessed. Conclusion: Effects of L-Dopa on automatic priming processes appear to be different than those on controlled processes....
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