How the brain uses prior expectations to improve perception of degraded speech
In a new study published in BLOS Biology, Helen Blank and Matt Davis have shown that the brain uses predictive neural mechanisms to combine sensory signals and prior knowledge during the perception of degraded speech.
Perception inevitably depends on combining sensory input with prior expectations. This is apparent, for instance, when TV programmes are subtitled to help hearing-impaired individuals or to enhance comprehension of accented or degraded speech. Subtitles don’t just allow viewers to read instead of listen, they actually make the sound-track to the programme sound clearer. Although these effects have long been apparent the underlying neural mechanism by which prior expectations influence sensory processing has remained unclear. Recent theories suggest that the brain passes forward the unexpected part of the sensory input while expected properties are suppressed (Prediction Error). However, evidence to rule out the opposite and perhaps more intuitive mechanism, in which the expected part of the sensory input is enhanced or sharpened (Sharpening), has been lacking.
Combining multivariate fMRI analyses with computational simulations of both these mechanisms, however, allows us to determine the underlying neural mechanism. The key finding was an interaction between sensory input and prior expectations: for unexpected speech, increasing speech clarity increased the amount of information represented in sensory brain areas. In contrast, for speech that matches prior expectations (i.e., increasing speech clarity reduced the amount of this information.
The observations are uniquely simulated by a computational model of speech perception in which predictions for upcoming speech sounds are compared with the sensory input to generate neural signals of Prediction Errors. This model explains how subtitles enhance the perception of degraded speech and has implications for many other sensory domains in which prior knowledge has been shown to influence perception.