  Expectations aid and bias our perception. In speech, expected words are
easier to recognise than unexpected words, particularly in noisy environments,
and incorrect expectations can make us misunderstand our conversational
partner. Expectations are combined with the output from the sensory pathways to
form representations of speech in the cerebral cortex. However, it is unclear
whether expectations are propagated further down to subcortical structures to
aid the encoding of the basic dynamic constituent of speech: fast
frequency-modulation (FM). Fast FM-sweeps are the basic invariant constituent
of consonants, and their correct encoding is fundamental for speech
recognition. Here we tested the hypothesis that subjective expectations drive
the encoding of fast FM-sweeps characteristic of speech in the human
subcortical auditory pathway. We used fMRI to measure neural responses in the
human auditory midbrain (inferior colliculus) and thalamus (medial geniculate
body). Participants listened to sequences of FM-sweeps for which they held
different expectations based on the task instructions. We found robust evidence
that the responses in auditory midbrain and thalamus encode the difference
between the acoustic input and the subjective expectations of the listener. The
results indicate that FM-sweeps are already encoded at the level of the human
auditory midbrain and that encoding is mainly driven by subjective
expectations. We conclude that the subcortical auditory pathway is integrated
in the cortical network of predictive speech processing and that expectations
are used to optimise the encoding of even the most basic acoustic constituents
of speech.
