Posterior parietal cortex modulates perceptual decisions depending on psychotic phenotype

Reduced data-gathering and altered sensory precision are associated with psychotic phenotypes in tasks engaging the posterior parietal cortex (PPC). We investigated whether PPC excitability - modulated via 1 Hz repetitive transcranial magnetic stimulation (TMS) - differentially affects these behavioural patterns in high vs. low psychotic phenotypes. Based on prior work, we hypothesised that delusional and hallucinatory traits would moderate TMS effects on sensory precision (proxied by drift rates), while hallucinatory traits would additionally moderate effects on decision thresholds.

We compared performance in both the random dot motion task (RDM) and the beads task 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.

In RDM, TMS increased decision thresholds compared to sham-TMS in the low psychotic phenotype group. This effect was not present in participants with higher psychotic phenotypes. Drift rates, in contrast, were lowered in participants with higher delusional phenotype. No significant effect was found on beads task performance.

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.