Abstract
Central auditory processing in humans was investigated by comparing the perceptual effects of temporal parameters of electrical stimulation in auditory midbrain implant (AMI) and cochlear implant (CI) users. Four experiments were conducted to measure the following: effect of interpulse intervals on detection thresholds and loudness; temporal modulation transfer functions (TMTFs); effect of duration on detection thresholds; and forward masking decay. The CI data were consistent with a phenomenological model that based detection or loudness decisions on the output of a sliding temporal integration window, the input to which was the hypothetical auditory nerve response to each stimulus pulse. To predict the AMI data, the model required changes to both the neural response input (i.e., midbrain activity to AMI stimuli, compared to auditory nerve activity to CI stimuli) and the shape of the integration window. AMI data were consistent with a neural response that decreased more steeply compared to CI stimulation as the pulse rate increased or interpulse interval decreased. For one AMI subject, the data were consistent with a significant adaptation of the neural response for rates above 200 Hz. The AMI model required an integration window that was significantly wider (i.e., decreased temporal resolution) than that for CI data, the latter being well fit using the same integration window shape as derived from normal-hearing data. These models provide a useful way to conceptualize how stimulation of central auditory structures differs from stimulation of the auditory nerve and to better understand why AMI users have difficulty processing temporal cues important for speech understanding.
Highlights
Auditory midbrain implants (AMIs) are designed for electrically stimulating the neurons of the inferior colliculus (IC) to elicit sound sensations in patients who are profoundly deaf and are not suitable for cochlear implantation (CI)
The current study investigated the temporal processing abilities of the AMI recipients related to these auditory features, with the aim of gaining knowledge that would underpin future development of the device
At the comfortably loud level, the results of all three AMI subjects showed a similar pattern in which the current adjustments required to make the pulse-pair stimuli the same loudness as the reference were close to zero for the smallest interpulse interval (IPI) and increased with increasing IPI up to around 2 ms
Summary
Auditory midbrain implants (AMIs) are designed for electrically stimulating the neurons of the inferior colliculus (IC) to elicit sound sensations in patients who are profoundly deaf and are not suitable for cochlear implantation (CI) (e.g., those without a functional auditory nerve [AN] or implantable cochlea). To predict the way that the stimulus level (or modulation depth) must be changed across experimental conditions to maintain a fixed criterion value (fixed maximum window output, fixed difference in window output, or fixed fluctuation of window output), a model of how neural excitation changes with current level was applied This is an unknown relationship that will vary across subjects and electrodes. The differences between subjects and stimulus levels in the shape of the current-for-equal-loudness versus IPI functions could be modeled by variation in the proportion of ‘available’ neurons that fired on the first of each pulse pair, and their refractory recovery time (see “TI model” section). The four balances were averaged to find the current reduction (compared to the current in the reference stimulus) required for the pulse-pair stimulus to be the same loudness as the reference stimulus
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
