Effects of Hearing Preservation on Psychophysical Responses to Cochlear Implant Stimulation

Abstract

Previous studies have shown that residual acoustic hearing supplements cochlear implant function to improve speech recognition in noise as well as perception of music. The current study had two primary objectives. First, we sought to determine how cochlear implantation and electrical stimulation over a time period of 14 to 21 months influence cochlear structures such as hair cells and spiral ganglion neurons. Second, we sought to investigate whether the structures that provide acoustic hearing also affect the perception of electrical stimulation. We compared psychophysical responses to cochlear implant stimulation in two groups of adult guinea pigs. Group I (11 animals) received a cochlear implant in a previously untreated ear, while group II (ten animals) received a cochlear implant in an ear that had been previously infused with neomycin to destroy hearing. Psychophysical thresholds were measured in response to pulse-train and sinusoidal stimuli. Histological analysis of all group I animals and a subset of group II animals was performed. Nine of the 11 group I animals showed survival of the organ of Corti and spiral ganglion neurons adjacent to the electrode array. All group I animals showed survival of these elements in regions apical to the electrode array. Group II animals that were examined histologically showed complete loss of the organ of Corti in regions adjacent and apical to the electrode array and severe spiral ganglion neuron loss, consistent with previous reports for neomycin-treated ears. Behaviorally, group II animals had significantly lower thresholds than group I animals in response to 100 Hz sinusoidal stimuli. However, group I animals had significantly lower thresholds than group II animals in response to pulse-train stimuli (0.02 ms/phase; 156 to 5,000 pps). Additionally, the two groups showed distinct threshold versus pulse rate functions. We hypothesize that the differences in detection thresholds between groups are caused by the electrical activation of the hair cells in group I animals and/or differences between groups in the condition of the spiral ganglion neurons.

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.…”

“…In McKay and colleagues' model of temporal processing (McKay and McDermott, 1998;McKay et al, 2013), detection of pulsatile stimulation in electric hearing is influenced by a combined effect of reduced neural response due to refractory effects and temporal integration of neural activity in the integration window. Previous reports have shown that guinea pigs with sparse neural survival also tend to have low spontaneous activity in the remaining fibers, probably due to the loss of inner hair cells (Kang et al, 2010;Pfingst et al, 2011b). The lack of spontaneous activity can lead to abnormal across-fiber synchrony.…”

“…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.…”

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

“…However, previous studies have shown that the relationship between survival of hair cells and auditory neurons and psychophysical detection thresholds depends on the parameters of stimulation (Su et al, 2008;Kang et al, 2010). The relationship is positive for some types of stimuli and negative for others.…”