Detection of pulse trains in the electrically stimulated cochlea: Effects of cochlear health

Abstract

Perception of electrical stimuli varies widely across users of cochlear implants and across stimulation sites in individual users. It is commonly assumed that the ability of subjects to detect and discriminate electrical signals is dependent, in part, on conditions in the implanted cochlea, but evidence supporting that hypothesis is sparse. The objective of this study was to define specific relationships between the survival of tissues near the implanted electrodes and the functional responses to electrical stimulation of those electrodes. Psychophysical and neurophysiological procedures were used to assess stimulus detection as a function of pulse rate under the various degrees of cochlear pathology. Cochlear morphology, assessed post-mortem, ranged from near-normal numbers of hair cells, peripheral processes and spiral ganglion cells, to complete absence of hair cells and peripheral processes and small numbers of surviving spiral ganglion cells. The psychophysical and neurophysiological studies indicated that slopes and levels of the threshold versus pulse rate functions reflected multipulse integration throughout the 200 ms pulse train with an additional contribution of interactions between adjacent pulses at high pulse rates. The amount of multipulse integration was correlated with the health of the implanted cochlea with implications for perception of more complex prosthetic stimuli.

Relationship between multipulse integration and speech recognition with cochlear implants

“…For multipulse integration, the animal studies have demonstrated that a significant amount of the variance in the slopes of the functions could be accounted for by the density of the spiral ganglion cell bodies of the auditory neurons in Rosenthal's canal near the stimulation sites (Kang et al, 2010;Pfingst et al, 2011b). The mechanism for the effects of auditory fiber density on the ear's ability to integrate multiple electrical pulses is not fully understood.…”

“…The pulse duration was 25 ls/phase with an interphase interval of 8 ls, and the pulse-train duration was 500 ms. The pulse rates used were chosen to be below 1000 pps because threshold change at pulse rates above 1000 pps is believed to involve a mechanism of residual cell charge and the animal studies showed that spiral ganglion density was more strongly correlated with the slopes of the multipulse-integration function below 1000 pps than above (Pfingst et al, 2011b). For each ear, detection thresholds were measured three times for each of the two pulse rates for all functioning electrodes in a monopolar configuration (MP 1 þ 2).…”

“…Several psychophysical and electrophysiological measures have been developed to probe sensory and neural health in implanted animals. These measures include psychophysical threshold functions such as temporal integration of electrical pulse trains (threshold-versus-stimulus-duration functions) as well as electrophysiological measures such as electrically evoked compound action potentials and auditory brainstem responses (Hall, 1990;Prado-Guitierrez et al, 2006;Smith and Simmons, 1983;Kang et al, 2010;Pfingst et al, 2011a;Pfingst et al, 2011b;Ramekers et al, 2014). Advantages of an animal model include that histological analysis can be conducted a short time after these functional measurements are made and that the conditions of the inner ear of the animals can be systematically manipulated.…”

“…One of the psychophysical measures that showed significant correlation with spiral ganglion cell counts is a multipulse-integration measure (Kang et al, 2010;Pfingst et al, 2011b). The multipulse-integration measure is based on psychophysical detection thresholds as a function of pulse rate for pulse trains fixed in duration.…”

Low-frequency fine-structure cues allow for the online use of lexical stress during spoken-word recognition in spectrally degraded speech

“…The current signal processing strategies for CIs are designed to preserve temporal envelope cues (i.e., amplitude changes over time) for speech recognition, while discarding the spectral and temporal fine-structure cues [i.e., the carrier defined by the Hilbert transform (Hilbert, 1912)] that are important for pitch perception (Smith et al, 2002;Zeng et al, 2005). The reduced frequency selectivity in CI listening is also due to other factors, including limited neural survival, current spread, and channel interaction (e.g., Bierer et al, 2011;Fu and Nogaki, 2005;Hughes, 2008;Jones et al, 2013;Pfingst et al, 2011). The spectral details provided by CIs resemble that of 4-8 channels of vocoded speech to normal-hearing listeners (Fishman et al, 1997;Friesen et al, 2001;Stickney et al, 2004).…”