BACKGROUND. Reduced data-gathering and imbalanced sensory precision have been linked to psychotic-like experiences. These patterns arise in decision tasks that typically involve the posterior parietal cortex (PPC). However, a causal link between activity in PPC and the decision patterns associated with the psychotic phenotype has not been established yet. By the use of 1 Hz repetitive transcranial magnetic stimulation (TMS), we tested whether PPC excitability modulate data-gathering and precision encoding differently in high and low psychotic phenotypes. METHODS. We compared performance in both the random dot motion task (RDM; perceptual inference) and the beads task (probabilistic reasoning) in two groups of participants (total, N = 68) undergoing TMS or sham-TMS over the right PPC. Hierarchical drift-diffusion models estimated drift rates (sensory precision proxies) and decision thresholds. We evaluated differences between TMS and sham-TMS groups and tested for interactions of these TMS groups with delusional and hallucinatory phenotypes. RESULTS. In RDM, TMS increased decision thresholds compared to sham-TMS in the low psychotic phenotype group. This decision threshold effect was not present in participants with higher psychotic phenotypes. Drift rates, in contrast, were lowered in participants with higher delusional phenotype. We did not find any significant effect on beads task performance. CONCLUSIONS. Our findings suggest a causal role of PPC in decisions to end data-gathering during perceptual inference. The absence of this effect in the psychotic phenotype yields new hypotheses on the role of PPC excitability in neural mechanisms underlying decision-making patterns in the psychotic phenotype.

The authors have declared no competing interest.

This work was supported by a grant from the NOMIS Foundation. The authors declare that they have no financial or personal relationships that could influence the work reported in this article.

I confirm all relevant ethical guidelines have been followed, and any necessary IRB and/or ethics committee approvals have been obtained.

Yes

The details of the IRB/oversight body that provided approval or exemption for the research described are given below:

Our study received approval from the Ethics Committee of Royal Holloway, University of London.

I confirm that all necessary patient/participant consent has been obtained and the appropriate institutional forms have been archived, and that any patient/participant/sample identifiers included were not known to anyone (e.g., hospital staff, patients or participants themselves) outside the research group so cannot be used to identify individuals.

Yes

I understand that all clinical trials and any other prospective interventional studies must be registered with an ICMJE-approved registry, such as ClinicalTrials.gov. I confirm that any such study reported in the manuscript has been registered and the trial registration ID is provided (note: if posting a prospective study registered retrospectively, please provide a statement in the trial ID field explaining why the study was not registered in advance).

Yes

I have followed all appropriate research reporting guidelines, such as any relevant EQUATOR Network research reporting checklist(s) and other pertinent material, if applicable.

Yes