Sampling Reality: Exploring Evidence Accumulation Mechanisms in the Psychotic Phenotype

Francesco Scaramozzino

Research output: ThesisDoctoral Thesis

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The way the brain samples evidence could be crucial in shaping our
experience of reality. Predictive coding accounts hypothesise that increased
sensory precision might bias sampling and impact inferences of reality. In a
predictive coding framework integrated with the use of sequential sampling
models, we investigated the relationship between evidence accumulation
mechanisms and psychotic phenotypes in the general population.
Study 1 explored the association of psychotic phenotypes with alterations of
evidence accumulation in perceptual inference. We fitted drift-diffusion
models (DDM) to behavioural data and evaluated the relation between
psychotic phenotype and DDM parameters using the drift rate as a proxy of
precision of sensory evidence. Additionally, we sought to replicate findings
linking reduced data-gathering in probabilistic reasoning with delusion-like
experiences, a bias known as the Jumping to Conclusions bias (JTC). Results
showed that both hallucination- and delusion-like experiences were associated
with increased sensory precision in perceptual inference. Only
hallucination-like experiences predicted lower decision thresholds, while we
did not find JTC in the psychotic phenotypes.
Study 2 aimed to modulate evidence accumulation by applying inhibitory
transcranial magnetic stimulation (TMS) to the posterior parietal cortex (PPC)
and examining its effects along the psychotic phenotype continuum. PPC
activity has been correlated with sampling behaviour and might be involved in
alterations of evidence accumulation associated with psychosis. We replicated
results from Study 1 with both psychotic phenotypes showing increased
sensory precision. Notably, participants with a higher delusional phenotype
undergoing TMS showed decreased sensory precision.
In conclusion, our findings indicate that increased precision of sensory
evidence characterises perceptual inference in both delusional and
hallucinatory phenotypes. Specifically for the delusional phenotype, PPC
activity might be implicated in alterations of precision encoding. Overall, our
studies point to evidence accumulation mechanisms potentially influencing
our experience of reality and contributing to the psychotic phenotype in the
general population
Original languageEnglish
Awarding Institution
  • Royal Holloway, University of London
  • Furl, Nicholas, Supervisor
  • McKay, Ryan, Supervisor
Award date1 Mar 2024
Publication statusPublished - 22 Sept 2023


  • psychosis
  • decision making
  • perception
  • Parietal cortex

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