We use sensory learning to process information about our surroundings and predict what is likely to happen. Recent theories propose that an impaired ability to make these predictions could be a cause of psychotic experiences. Instead of reliably predicting future events based on past experiences of their sensory environment, some people might assign too much importance to irrelevant sensory information. This could explain why people with schizophrenia, for example, can experience delusions and perceive the world differently than others.
Brain Function CoE researchers tested these theories by measuring people’s ability to judge simple sound patterns. The research team was led by Ilvana Dzafic and Marta Garrido at the University of Melbourne.
The team recruited 66 participants across the psychosis continuum – from healthy people who had experienced psychotic-like symptoms to people with schizophrenia experiencing severe psychosis. The participants completed an auditory oddball task, which tests how the brain responds to unexpected sounds. Their brain activity was recorded during the task using electroencephalography (EEG).
By analysing the EEG recordings, the researchers found that the brain responses to unexpected sounds were smaller in participants with schizophrenia than in those without the disorder.
The researchers also used EEG recordings to measure connectivity between different regions of the brain. In people with schizophrenia, altered connectivity is known to affect brain responses to unexpected sounds.
The researchers found that the participants who had experienced the most severe psychotic symptoms – regardless of a schizophrenia diagnosis – had weaker connectivity within the right inferior frontal gyrus (IFG). Decreased activity in this region was also linked to impaired sensory learning.
Moreover, participants who had experienced more psychotic symptoms showed stronger connectivity from the left IFG to a region called the superior temporal gyrus (STG). By contrast, participants with more severe hallucinations had weaker connectivity from the left STG to the IFG. This indicates that sensory information may be suppressed, relative to previous predictions about the environment, in people who experience psychosis.
These findings suggest that weaker connectivity in the IFG may underlie an impaired ability to make judgments about the sensory world in people experiencing psychosis.
The research team would like to study participants over longer time periods, including people who are at ultra-high risk of developing psychosis or have recently experienced their first episode of psychosis. This would help them to see when the relevant changes in the brain emerge.
Dzafic, I., Larsen, K. M., Darke, H., Pertile, H., Carter, O., Sundram, S., & Garrido, M.I. (2021). Stronger top-down and weaker bottom-up frontotemporal connections during sensory learning are associated with severity of psychotic phenomena. Schizophrenia Bulletin, sbaa188. doi: 10.1093/schbul/sbaa188