Abstract
Perceptions are inherently probabilistic; and can be potentially manipulated to induce illusory experience by the presentation of ambiguous or improbable evidence under selective (spatio-temporal) constraints. Accordingly, perception of the McGurk effect, by which individuals misperceive specific incongruent visual and auditory vocal cues, rests upon effective probabilistic inference. Here, we report findings from a behavioral investigation of illusory perception and related metacognitive evaluation during audiovisual integration, conducted in individuals with schizophrenia (n = 30) and control subjects (n = 24) matched in terms of age, sex, handedness and parental occupation. Controls additionally performed the task after an oral dose of amisulpride (400 mg). Individuals with schizophrenia were observed to exhibit illusory perception less frequently than controls, despite non-significant differences in perceptual performance during control conditions. Furthermore, older individuals with schizophrenia exhibited reduced rates of illusory perception. Subsequent analysis revealed a robust inverse relationship between illness chronicity and the illusory perception rate in this group. Controls demonstrated non-significant modulation of perception by amisulpride; amisulpride was, however, found to elicit increases in subjective confidence in perceptual performance. Overall, these findings are consistent with the idea that impairments in probabilistic inference are exhibited in schizophrenia and exacerbated by illness chronicity. The latter suggests that associated processes are a potentially worthwhile target for therapeutic intervention.
Highlights
Cross-modal cue integration is adaptively advantageous given the likelihood that real-world events perturb multiple sensory systems; and is sensible from a computational standpoint, as uncertainty in one modality can be reduced by combining with signals from others, increasing the overall precision of a sensory percept
Age was inversely associated with McGurkillusory rate in the schizophrenia group and accounted for 41.4% of related variance
Principal findings of this behavioral study of audio-visual integration were: that individuals with schizophrenia were less likely to perceive the McGurk illusion than healthy controls; and that, in these individuals but not control subjects, age significantly and inversely predicted the rate of illusory experience. The latter finding, in limiting associations with age to the patient group, implies a role for illness chronicity. This is substantiated by the observation that illness duration robustly predicted rates of illusory perception
Summary
Cross-modal cue integration is adaptively advantageous given the likelihood that real-world events perturb multiple sensory systems; and is sensible from a computational standpoint, as uncertainty in one modality can be reduced by combining with signals from others, increasing the overall precision of a sensory percept. As inter-sensory inconsistencies are likely abundant, perceptual decisions should take into account stimulus-specific uncertainty. Our experiences can be understood as the most likely explanation of the world (determined via posterior probability distributions), with these inferences biased by our own expectations (or priors reflecting past inferences) as well as data from available stimuli (the conditional likelihood of the data given a model). These computations lead to updated posterior probability distributions about the world which serve to adjust future expectations. The fundamental units are prediction error (on expected sensory input) and expected value (of actions executed in the environment), that can be modeled as free energy in a general optimization theory of perception and action (Feldman and Friston, 2010; Friston, 2010)
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