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Project

Neurale processen van de elektrisch gestimuleerde auditieve zenuw

Worldwide, Cochlear Implants (CIs) have already
partially restored hearing to
over 219000 profoundly hearing-impaired people. CIs attempt to
stimulate the survival Auditory Nerve (AN) fibers directly by means of
electrical pulses. Electrical stimulation is provided via an array of
electrodes implanted inside the cochlea, which directly activates the
AN fibers by means of biphasic Symmetric (SYM) pulses, i.e. an initial
phase followed by a second phase with the same amplitude but with
opposite polarity. Because electrical stimulation is delivered
extracellularly, AN fibers could become depolarized and hyperpolarized
at several neural sites and the excitation may be substantially
different depending on AN status, electrode-neuron interface, and
polarity. The necessity of charge balanced stimulation has precluded the
use of monophasic stimuli (pulses consisting only in one phase) to
study polarity effects in human CI users and most of the research
concerning
polarity effectiveness has been investigated in animal and
computational models. However, recently, asymmetric charge-balanced
pulse shapes and CI devices with recording capabilities have allowed
researchers to study polarity effectiveness on CI users either by means
of Electrically evoked Compound Action Potential (ECAP) or by means of
behavioral measurements. Contrary to the majority of the results
obtained with animals and computational models, those studies suggest
that the positive (anodic) current stimulates the AN more effectively
than the cathodic one in human CI users. The studies presented in this
document aim to examine how neurons at different sites are affected by
different stimulus shapes as well as the potential benefits of combining
non-standard pulse shapes by meansof objective ECAP and Electrically
evoked Auditory Brainstem Response (EABR) as well as behavioral
experiments.
In chapter 2 we study whether the high sensitivity to
the anodic current observed formerly for an electrode in the middle of
the array, can be generalized to stimulation at the base and apex of the
CI array. Three
experiments studied the effect of stimulus polarity
on the ECAP obtained with the masker-probe paradigm on different sites
along the cochlea inCI users. The results of all the experiments extend
those of a previous study, showing that the AN in humans is
preferentially activated by anodic stimulation, to different sites along
the cochlea by means of Monopolar (MP) stimulation.
In chapter 3 we
present a quantitative analysis of ECAP responses in order to determine
an objective detection criterion based on the statistical properties of
the noise. The study quantitatively investigates the properties of theĀ 
Background Noise (BN) and the remaining post average Residual Noise (RN)
in ECAP responses recorded from human CI users with the Across-sweeps
Noise Estimator (ASNE) method. The analysis of the statistical
properties of the noise indicated that a Signal-to-Noise Ratio (SNR)of
1.7 dB as a detection criterion corresponds to a false positive
detection rate of 1% with the used measurement setup.
In chapter4
we study whether the high sensitivity to the anodic current is also
observed in Bipolar (BP) stimulation mode. Spreads of Excitation (SOEs)
produced by different masking pulse shapes presented in MP and BP
mode are examined by means of the ECAP, obtained using the
forward-masked paradigm in human CI users. The SOEs obtained with the MP
masker showed a main excitation peak close to the masker electrode,
whereas SYM maskers produced bimodal excitation patterns showing two
peaks close to the electrodes of the masker channel. The SOEs obtained
using Symmetric with a long Inter-Phase Gap (SYM-IPG) (for which the
maskers second phase is responsible for most of the masking) and
Pseudomonophasic (PS) maskers were more pronounced close to the masker
electrode for which the effective phase was anodic. These results showed
that the anodic polarity is the most effective one in BPmode and that
the bimodal patterns produced by SYM maskers could bepartially reduced
by using asymmetric pulses.
Chapter 5 investigates polarity effects
at the level of the brainstem. Previous behavioral studies showing that
the human AN of CI users is mainly excitedby the anodic polarity have
only been obtained with asymmetric PS, and it was assumed that only the
short high-amplitude phase was responsible for the excitation.
Similarly, in our previous studies it has been shown that ECAPs could
only be obtained in response to the anodic phases of asymmetric pulses.
However, it is possible that ECAP responses to cathodic responses were
not detected due to the characteristics of the effective place of
excitation across and along the AN fibers. In this chapter we measured
EABRs to SYM and asymmetric pulse shapes presented for both polarities.
It is shown that responseswere time-locked to the short high-amplitude
phase of asymmetric pulses, and were smaller, but still measurable, when
that phase was cathodic
than when it was anodic. A behavioral
experiment studied the polarity sensitivity at different intensities by
means of a loudness balancing task between asymmetric stimuli of
opposite polarity. The results showed that cathodic stimuli required
higher amplitudes than anodic stimuli to reach the same loudness at
different stimuluslevels. This study provides the first evidence that
cathodic stimulation can excite the auditory system of human CI
listeners, and confirms that this stimulation is nevertheless less
effective than for the anodic polarity.
Datum:1 feb 2009 →  18 mrt 2013
Trefwoorden:cochlear implant, neuroscience, compound action potentials, brainstem respons, hearing
Disciplines:Neurowetenschappen, Biologische en fysiologische psychologie, Cognitieve wetenschappen en intelligente systemen, Ontwikkelingspsychologie en veroudering
Project type:PhD project