  Frequency modulation (FM) is a basic constituent of vocalisation in many
animals as well as in humans. In human speech, short rising and falling
FM-sweeps called formant transitions characterise individual speech sounds.
There are two representations of FM in the ascending auditory pathway: a
spectral representation, holding the instantaneous frequency of the stimuli;
and a sweep representation, consisting of neurons that respond selectively to
FM direction. To-date computational models use feedforward mechanisms to
explain FM encoding. However, from neuroanatomy we know that there are massive
feedback projections in the auditory pathway. Here, we found that a classical
FM-sweep perceptual effect, the sweep pitch shift, cannot be explained by
standard feedforward processing models. We hypothesised that the sweep pitch
shift is caused by a predictive interaction between the sweep and the spectral
representation. To test this hypothesis, we developed a novel model of FM
encoding incorporating a predictive feedback mechanism. The model fully
accounted for experimental data that we acquired in a perceptual experiment
with human participants as well as previously published experimental results.
We also designed a new family of stimuli for a second perceptual experiment to
further validate the model. Combined, our results indicate that predictive
interaction between different frequency encoding and direction encoding neural
representations plays an important role in the neural processing of FM. In the
brain, this mechanism is likely to occur at early stages of the processing
hierarchy.
