Datensatz

Zusammenfassung

In the natural environment our senses are bombarded with many different signals. To form a coherent percept, the brain should integrate signals originating from a common source and segregate signals from different sources. This fMRI study investigated how humans combine bottom-up inputs (i.e. congruent VS incongruent signals) and top-down predictions (i.e. common source prior) to infer if sensory signals should be integrated. Sixteen participants were shown movies of congruent (e.g. visual Ti with auditory Ti), incongruent (e.g. visual Ti with auditory Pi) and McGurk syllables (e.g. visual Ki with auditory Pi, which can be fused into the illusionary percept Ti). We manipulated participants’ top-down predictions by presenting the McGurk stimuli in a series of congruent or incongruent syllables. Participants reported their syllable percept in forced choice procedure with 6 response options. At the behavioural level, participants were more likely to fuse auditory and visual signals of a McGurk trial into an illusionary percept in congruent relative to incongruent contexts. This indicates that participant’s top-down predictions influence whether or not they integrate sensory signals. At the neural level, incongruent relative to congruent bottom-up inputs increased activations in a left fronto-parietal network. The left prefrontal activations also increased for McGurk trials, when participants selectively reported their auditory percept and did not fuse auditory and visual signals. This effect was enhanced for incongruent contexts when participants expected that sensory signals needed to be segregated. Our results show that the left inferior frontal sulcus determines whether sensory signals should be integrated by combining top-down predictions generated from prior trials with bottom-up information about sensory conflict in the incoming signals. Furthermore, it exerts top-down control enabling independent sensory processing and report of only one sensory modality.