Vocoder Simulations Explain Complex Pitch Perception Limitations Experienced by Cochlear Implant Users

Abstract

Pitch plays a crucial role in speech and music, but is highly degraded for people with cochlear implants, leading to severe communication challenges in noisy environments. Pitch is determined primarily by the first few spectrally resolved harmonics of a tone. In implants, access to this pitch is limited by poor spectral resolution, due to the limited number of channels and interactions between adjacent channels. Here we used noise-vocoder simulations to explore how many channels, and how little channel interaction, are required to elicit pitch. Results suggest that two to four times the number of channels are needed, along with interactions reduced by an order of magnitude, than available in current devices. These new constraints not only provide insights into the basic mechanisms of pitch coding in normal hearing but also suggest that spectrally based complex pitch is unlikely to be generated in implant users without significant changes in the method or site of stimulation.

The role of cochlear place coding in the perception of frequency modulation

“…Results from the present study suggest that resolved harmonics, which are most important for human pitch perception, may be represented by their place of stimulation rather than by the temporal fine structure information encoded via the stimulus-driven spike timing (phase locking). This conclusion is consistent with other studies showing that pitch perception is possible even with spectrally resolved harmonics that are usually assumed to be too high in frequency to elicit phase locking (Oxenham et al, 2011;Lau et al, 2017;Carcagno et al, 2019) and with studies showing that steep filter slopes are required to represent harmonics from filtered noise in noise-vocoder simulations (Mehta and Oxenham, 2017). It is also supported by recent data from the inferior colliculus of the rabbit, showing that place coding of low-numbered harmonics from high F0s in the midbrain is robust over a relatively wide range of sound levels (Su and Delgutte, 2020), a finding that should generalize to low F0s in humans, given our superior frequency selectivity.…”

Solenoidal Micromagnetic Stimulation Enables Activation of Axons With Specific Orientation

“…In this work, we also expand this finding to our in vivo rodent model, showing for the first time, our ability to stimulate the brain stem of a rodent with a net sensitivity to the directionality of the magnetic flux. A similar μMS-orientation sensitivity was shown ( Lee and Fried, 2017 ) in layer V pyramidal neurons (PNs) and the asymmetric fields arising from such microcoils did not simultaneously activate horizontally oriented axon Furthermore, μMS was shown to stimulate in confined narrow regions (<60 μm) cortical pyramidal neurons in brain slices in vitro , which helped to avoid the simultaneous activation of passing axons ( Mehta and Oxenham, 2017 ). μMS coils were also surgically introduced 8–10 mm into the cochlea of anesthetized deafened felines ( Lee and Fried, 2017 ), thus unresponsive to acoustic stimuli, and auditory responses were then recorded during magnetic stimulation.…”

“…μMS coils were also surgically introduced 8–10 mm into the cochlea of anesthetized deafened felines ( Lee and Fried, 2017 ), thus unresponsive to acoustic stimuli, and auditory responses were then recorded during magnetic stimulation. These experiments were aimed at showing that magnetic field steerability of μMS may solve the low-resolution stimulation shortcomings of the state-of-the-art cochlear implants that are limited by their ability to reproduce accurately pitch in music and speech in the presence of background noise, which instead may require as much as four times the number of channels currently available ( Mehta and Oxenham, 2017 ). In the cochlea ( Macherey and Carlyon, 2014 ) as well as in cortex ( Matteucci et al, 2016 ) stimulation resolution is limited by the channel to channel cross-talk rather than electrode sub-millimeter size and spacing.…”

Auditory Stream Segregation and Selective Attention for Cochlear Implant Listeners: Evidence From Behavioral Measures and Event-Related Potentials

“…The spectral resolution of CIs is limited both by the number of electrodes (typically 22 or less) and the interaction of the electrical current across electrodes. Thus, current CIs may not provide enough spectral resolution to support melody recognition (e.g., Kong et al, 2004 ; Mehta and Oxenham, 2017 ). Therefore, the poor performance observed by Cooper and Roberts (2009) could reflect inherent limitations of CI listeners to recognize familiar melodies and may not be related to poor stream segregation.…”