Grant title: Sensitivity to envelope and pulse timing interaural time differences in prosthetic hearing
Grant scheme: General Research Fund
Funding amount: HK$ 526,038
Grant duration: 1-1-2021 - 31-12-2023
Abstract: The lives of many severely deaf individuals have been improved enormously with the advent of cochlear implants (CIs). These implantable prosthetic devices restore enough auditory sensation to allow patients to understand speech and hold conversations. However, the auditory performance afforded by these devices falls short of natural hearing, particularly in the areas of pitch discrimination and spatial hearing. Some researchers have blamed an inevitable period of deafness in early childhood for the insensitivity of CI users to so called interaural time difference (ITD) cues for sound source location, positing that the brain’s binaural processing system fails to develop properly in the absence of appropriate, early stimulation. However, another, to our mind more likely, explanation is that contemporary CIs do not faithfully encode ITD information present in the “fine structure” of auditory inputs. The sound processing devices used for CIs translate incoming sound waves into series of electrical pulses which are sent through electrode arrays to the auditory nerves of the patients. CIs vary the amplitude of these electrical pulses in a manner that encodes the so-called “sound envelope”, that is, the manner in which sound intensity varies over time, but the timing of the electrical pulses does not reflect the timing of peaks or troughs of the sound waves. In other words, in contemporary CIs, the “temporal fine structure of the electrical stimulation” is independent of the temporal fine structure of the incoming sounds, and any ITD information contained in the temporal fine structure of the sounds is not encoded by the timing of the electrical pulses. In normal hearing, both fine structure and envelope ITDs have been shown to be audible and to contribute to our ability to localise sound sources, but electric hearing with current prosthetic devices is sufficiently different from natural hearing that the hope that CI users might be able to make adequate use of envelope ITDs encoded by current processing strategies is probably ill founded. In this project we plan to use behavioural and physiological studies on bilaterally cochlear implanted animals in order to determine how sensitive the hearing inexperienced auditory system can be to envelope and pulse timing ITDs respectively. This will provide valuable data, guiding the development of improved sound processing strategies for future generations of binaural auditory prostheses.