How our expectations can make us hallucinate: the neural mechanisms underlying perception

The way we perceive the world is strongly influenced by our expectations about what we are likely to see at any given moment. In certain situations – namely when sensory signals are very weak or noisy and expectations are very strong – expectations can even induce hallucinations: seeing an expected stimulus despite its absence. However, the neural mechanisms by which the brain integrates sensory inputs and expectations, and thereby generates the contents of perception, have yet to be established. Previous work, including my own, has demonstrated that processing in the visual cortex is strongly modulated by prior expectations. However, perhaps surprisingly, previous studies have not yet explained how these modulations relate to subjective perception, leading to a lacuna in our knowledge of how the brain supports perception. Here, I propose to study the neural mechanisms underlying subjective perception by using strong visual expectations to induce hallucinations in healthy human participants. I hypothesise that, upon presentation of a predictive cue (e.g., a siren), memory systems pre-activate templates of expected stimuli (an ambulance) in the deep layers of visual cortex, leading to biased processing of sensory inputs from the very moment they arrive. I will test this proposal by addressing three complimentary questions: 1) How do expectations filter perception? 2) What is the computational architecture underlying perceptual inference? 3) What is the neural source of expectations? I will combine psychophysical tasks probing participants’ perception with neuroimaging tools with exquisite spatial (high-field fMRI) and temporal (MEG) resolution to address these questions. The overarching aim of my research is to provide a mechanistic account of subjective perception. Ultimately, these insights may improve our understanding of clinical disorders characterised by aberrations in perception, such as psychosis.