Temporal Spatial Perception under Electric Hearing


Student thesis: Doctoral Thesis

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Award date23 Jun 2021


Spatial hearing in cochlear implant (Cl) patients remains a major challenge with many early deaf users reported to have no measurable sensitivity to interaural time differences (ITDs). This thesis aims to investigate influencing factors in ITD sensitivity under bilateral electric hearing.

Deprivation of binaural experience during an early critical period is often hypothesized to be one of the major causes of temporal spatial perceptual shortcomings in Cl users. Additional influencing factors include: 1) carrier rates, where typical clinical speech processor rates are thought to be too fast to encode ITDs; 2) the prevalence of onset dominance for these ITD cues, which is known to be a prominent feature under normal hearing circumstances but has not bee elucidated in electric hearing and 3) the ability to make use of ITDs carried on the extracted sound amplitude envelopes.

In order to address these ITD influencing factors in the work presented in this thesis I have investigated the following questions: does ITD sensitivity have a critical period?, what are the effects of hearing experience on ITD sensitivity?, does ITD sensitivity exist at clinically relevant pulse rates under appropriate stimulation?, does ITD sensitivity show onset dominance in the absence of early hearing experience? and is ITD sensitivity more reliably carried on the fixed rate pulses or the modulating amplitude?

These studies were all conducted in a newly developed animal model together with a novel behavioural setup and design in order to assess awake, behaving ITD sensitivity under bilateral Cl. This enabled the control of several confounding variables in particular: aetiology of deafening, age of onset, age of implantation and therefore duration of auditory deprivation together with stimulation strategy not limited to clinical speech processor technology.

My research conducted in pursuance of this thesis has demonstrated that, even in the absence of early hearing experience, and while using high carrier rates, Cl users are capable of developing good ITD sensitivity when stimulated appropriately. These results are of clinical importance as they prove that uncontrollable biological factors, such as age of deafness and length of period of deprivation, are most likely not the limiting factors for developing usable ITD sensitivity under Cl stimulation which they have hitherto been suspected to be. Rather, better control of the stimulation strategy to improve pulse time delivery is what is needed.