Frequency modulation (FM) is a basic constituent of vocalisation . Formant transitions in speech are characterised by short rising and falling FM-sweeps in the kilohertz frequency range. These sounds elicit a pitch percept that deviates from their average frequency. This study uses this perceptual effect, termed here the sweep pitch shift, to inform a model characterising the neural encoding of FM. First, a reexamination of the classical effect, consisting of two perceptual experiments, provides a quantitative characterisation of the dependence of the sweep pitch shift with the properties of the sweeps. Next, simulations carried on the new experimental data show that classical temporal and spectral models of pitch processing cannot explain the pitch shift. Conversely, a modified spectral model considering a predictive interaction between frequency and FM encoding fully reproduces our and previous experimental data . The model introduces a feedback mechanism that modulates the neurons that are expected to respond to future portions of the sweeps, accelarating their onset response. Combined, the experimental and modelling results suggest that predictive feedback modulation plays an important role in the neural encoding of FMeven at early stages of the processing hierarchy.
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.
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. Formant transitions in speech are characterised by
in many animals as well as in humans. In human speech,
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in
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,
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kilohertz frequency range. These sounds elicit a pitch percept that deviates from their average frequency. This study uses this perceptual effect, termed here the
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pitch shift, to inform a model characterising the neural encoding of FM. First, a reexamination of the classical effect, consisting of two perceptual experiments, provides a quantitative characterisation of the dependence of the sweep
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with the properties of the sweeps. Next, simulations carried on the new experimental data show that classical temporal and spectral models of pitch processing cannot explain the pitch shift. Conversely, a modified spectral model considering a
, cannot be explained by standard feedforward processing models. We hypothesised that the sweep pitch shift is caused by a
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frequency and FM encoding fully reproduces our and previous experimental data
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
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The model introduces a feedback mechanism that modulates the neurons that are expected to respond to future portions of the sweeps, accelarating their onset response.
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the experimental and modelling results suggest that predictive feedback modulation
our results indicate that predictive interaction between different frequency encoding and direction encoding neural representations
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encoding of FMeven
processing of FM. In the brain, this mechanism is likely to occur