Enhancement of speech perception in noise by periodicity processing: A neurobiological model and signal processing algorithm

• There is a fundamental relationship between pitch strength and loudness.
• This inspired the formulation of a new conceptual neurobiological model.
• A computer algorithm to enhance speech in noise was based on this model.
• Noise could be reduced by 14 dB without effecting speech intelligibility.
• Periodicity processing may have evolved to boost the salience of vocalizations.

The perceived loudness of sound increases with its tonality or periodicity, and the pitch strength of tones are linearly proportional to their sound pressure level. These observations suggest a fundamental relationship between pitch strength and loudness. This relationship may be explained by the superimposition of inputs to inferior colliculus neurons from cochlear nucleus chopper cells and phase locked spike trains from the lateral lemniscus. The regularity of chopper cell outputs increases for stimuli with periodicity at the same frequency as their intrinsic chopping rate. So inputs to inferior colliculus cells become synchronized for periodic stimuli, leading to increased likelihood that they will fire and increased salience of periodic signal components at the characteristic frequency of the inferior colliculus cell. A computer algorithm to enhance speech in noise was based on this model. The periodicity of the outputs of a Gammatone filter bank after each sound onset was determined by first sampling each filter channel at a range of typical chopper cell frequencies and then passing these amplitudes through a step function to simulate the firing of coincidence detecting neurons in the inferior colliculus. Filter channel amplification was based on the maximum accumulated spike count after each onset, resulting in increased amplitudes for filter channels with greater periodicity. The speech intelligibility of stimuli in noise was not changed when the algorithm was used to remove around 14 dB of noise from stimuli with signal–noise ratios of around 0 dB. This mechanism is a likely candidate for enhancing speech recognition in noise, and raises the proposition that pitch itself is an epiphenomenon that evolved from neural mechanisms that boost the hearing sensitivity of animals to vocalizations.