How the modulation frequency affects the electrically evoked auditory steady-state response in CI users

Robin Gransier (1), Hanne Deprez (1,2), Michael Hofmann (1), Marc Moonen (2), Astrid van Wieringen (1), and Jan Wouters (1)
(1) ExpORL, Dept. of Neurosciences, KU Leuven, Herestraat 49 bus 721, 3000 Leuven, Belgium
(2) STADIUS, Dept. of Electrical Engineering (ESAT), KU Leuven Kasteelpark Arenberg 10, 3001 Leuven, Belgium

Currently, the fitting of threshold(T)- and comfort(C)-levels of cochlear implants (CI) in profoundly hearing-impaired infants and adults with multiple disabilities is challenging and time consuming. An objective method to fit T- and C-levels is therefore highly desirable. T-levels are pulse rate dependent, and therefore it is a prerequisite for an objective method to use the same pulse rate as during everyday stimulation. There lies potential in the electrically auditory steady-state response (EASSR) method as an objective fitting method because it can use pulse rates that are similar to everyday stimulation. ASSRs are stationary responses that originate from the auditory pathway and are elicited by periodically varying or modulated sounds, and can be derived from electroencephalography (EEG) recordings. One unique feature of ASSRs is that different modulation frequencies elicit responses from different regions of the auditory pathway. However, how the modulation frequency affects the EASSR in CI users, and whether EASSRs can be obtained reliably (i.e., free from stimulation artifacts) when stimulating in a monopolar configuration needs to be explored.

In the present study, we investigate how the modulation frequency affects the EASSR in four adult CI users with a 64-channel EEG-recording system. All subjects were stimulated at their C-level on a single electrode with an amplitude modulated pulse train, at a rate of 500 pulses per seconds, and in a monopolar configuration. The EASSR modulation frequency transfer function (MFTF) was obtained from 30 to 100 Hz with a 3 Hz step size. Preliminary results show that it is feasible, when appropriate methods are used, to measure responses on the contralateral side free of stimulation artifacts. We will present the MFTFs measured in CI users and compare them with the ASSR MFTFs of normal hearing subjects.

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