Perceptual Gains and Losses in Synesthesia and Schizophrenia

Abstract

Individual differences in perception are widespread. Considering inter-individual variability, synesthetes experience stable additional sensations; schizophrenia patients suffer perceptual deficits in, eg, perceptual organization (alongside hallucinations and delusions). Is there a unifying principle explaining inter-individual variability in perception? There is good reason to believe perceptual experience results from inferential processes whereby sensory evidence is weighted by prior knowledge about the world. Perceptual variability may result from different precision weighting of sensory evidence and prior knowledge. We tested this hypothesis by comparing visibility thresholds in a perceptual hysteresis task across medicated schizophrenia patients (N = 20), synesthetes (N = 20), and controls (N = 26). Participants rated the subjective visibility of stimuli embedded in noise while we parametrically manipulated the availability of sensory evidence. Additionally, precise long-term priors in synesthetes were leveraged by presenting either synesthesia-inducing or neutral stimuli. Schizophrenia patients showed increased visibility thresholds, consistent with overreliance on sensory evidence. In contrast, synesthetes exhibited lowered thresholds exclusively for synesthesia-inducing stimuli suggesting high-precision long-term priors. Additionally, in both synesthetes and schizophrenia patients explicit, short-term priors—introduced during the hysteresis experiment—lowered thresholds but did not normalize perception. Our results imply that perceptual variability might result from differences in the precision afforded to prior beliefs and sensory evidence, respectively.

The gut microbiome is associated with brain structure and function in schizophrenia

“…We inferred that the sensory and cognitive impairments might be related to the alteration of ReHo indexes and microbial alpha diversity in SZ. For example, gut microbial alpha diversity in SZ may be associated with visual hallucinations 62 , 67 , and this relationship could be mediated by functional brain activity in the calcarine cortex.…”

From hallucinations to synaesthesia: A circular inference account of unimodal and multimodal erroneous percepts in clinical and drug-induced psychosis

“…Second, the current functional account of synaesthesia constitutes a model of druginduced synaesthesia and it was not designed to account for developmental synaesthesia (e.g., grapheme-colour synaesthesia, experienced by a small number of people without any drug consumption (Hubbard and Ramachandran, 2005)). That said, it is difficult to disregard the similarity between the descending loops (amplification of priors) and ideas such as the "disinhibited prior" proposed for developmental forms of synaesthesia (Neufeld et al, 2012;van Leeuwen et al, 2020).…”

The Gut Microbiome Was Associated With Brain Structure and Function in Schizophrenia

“…We inferred that the sensory and cognitive impairments may be related to the alteration of ReHo indexes and microbial alpha diversity in SZs. For example, gut microbial alpha diversity in SZs may be associated with visual hallucinations 54,59 , and this relationship could be mediated by functional brain activity in the calcarine cortex.…”