Hallucinations – percepts in the absence of an external stimulus – constitute an intriguing model of how percepts are generated and how perception can fail. They can occur in psychotic disorders, but also in the general population.
Healthy adults varying in their predisposition to hallucinations were asked to identify “speech” in ambiguous sounds. Listeners qualifying as more hallucination-prone in two established questionnaires perceptually down-weighted the speech-typical low frequencies (purple subgroup in the figure for illustration). Instead, the hallucination-prone individuals prioritised high frequencies in their “speechiness” judgements of ambiguous sounds.
At the same time, the higher one scored on hallucination-proneness, the more confident on a given (always ambiguous!) trial they were. Hallucination-proneness and actual sensory evidence had a comparable impact on confidence, consistent with the idea that the emergence of hallucinations is rooted in an altered perception of sounds.
This research may contribute to improving early diagnosis and prevention strategies in
individuals at risk for psychosis.
From the abstract:
“Hallucinations constitute an intriguing model of how percepts are generated and how perception can fail. Here, we investigate the hypothesis that an altered perceptual weighting of the spectro-temporal modulations that characterize speech contributes to the emergence of auditory verbal hallucinations. Healthy adults (N=168) varying in their predisposition for hallucinations had to choose the ‘more speech-like’ of two presented ambiguous sound textures and give a confidence judgement. Using psychophysical reverse correlation, we quantified the contribution of different acoustic features to a listener’s perceptual decisions. Higher hallucination proneness covaried with perceptual down-weighting of speech-typical, low-frequency acoustic energy while prioritising high frequencies. Remarkably, higher confidence judgements in single trials depended not only on acoustic evidence but also on an individual’s hallucination proneness and schizotypy score. In line with an account of altered perceptual priors and differential weighting of sensory evidence, these results show that hallucination-prone individuals exhibit qualitative and quantitative changes in their perception of the modulations typical for speech.”
The paper is available here.