Neural basis of auditory expectation within temporal cortex.

Predictive coding frameworks of perception propose that neural networks form predictions of expected input and generate prediction errors when the external input does not match expectation. We therefore investigated the processing of unexpected sounds and silence in the auditory cortex using fMRI. Unexpected sounds, when compared to expected sounds, evoked greater activation in large areas of the left temporal and insular cortices. Additionally the left middle temporal gyrus exhibited greater activation to unexpected events in general, whether sounds or silence, when compared to the corresponding expected events. These findings support predictive coding models of perception, which suggest that regions of the temporal cortex function to integrate sensory information with predictive signals during auditory perception.

Auditory hallucinations: expectation-perception model.

In this paper, we aimed to present a hypothesis that would explain the mechanism of auditory hallucinations, one of the main symptoms of schizophrenia. We propose that auditory hallucinations arise from abnormalities in the predictive coding which underlies normal perception, specifically, from the absence or attenuation of prediction error. The suggested deficiencies in processing prediction error could arise from (1) abnormal modulation of thalamus by prefrontal cortex, (2) absence or impaired transmission of external input, (3) dysfunction of the auditory and association cortex, (4) neurotransmitter dysfunction and abnormal connectivity, and (5) hyperactivity activity in auditory cortex and broad prior probability. If there is no prediction error, the initially vague prior probability develops into an explicit percept in the absence of external input, as a result of a recursive pathological exchange between auditory and prefrontal cortex. Unlike existing explanations of auditory hallucinations, we propose concrete mechanisms which underlie the imbalance between perceptual expectation and external input. Impaired processing of prediction error is reflected in reduced mismatch negativity and increased tendency to report non-existing meaningful language stimuli in white noise, shown by those suffering from auditory hallucinations. We believe that the expectation-perception model of auditory hallucinations offers a comprehensive explanation of the underpinnings of auditory hallucinations in both patients and those not diagnosed with mental illness. Therefore, our hypothesis has the potential to fill the gaps in the existing knowledge about this distressing phenomenon and contribute to improved effectiveness of treatments, targeting specific mechanisms.