Acoustic–electric interactions in the guinea pig auditory nerve: Simultaneous and forward masking of the electrically evoked compound action potential

Abstract

The study investigated the time course of the effects of acoustic and electric stimulation on the electrically evoked compound action potential (ECAP). Adult guinea pigs were used in acute experimental sessions. Bursts of acoustic noise and high-rate (5000 pulses/s) electric pulse trains were used as maskers. Biphasic electric pulses were used as probes. ECAPs were recorded from the auditory nerve trunk.Simultaneous masking of the ECAP with acoustic noise featured an onset effect and a decrease in the amount of masking to a steady state. It was characterized by a two-component exponential function. The amount of masking increased with masker level and decreased with probe level. Poststimulatory ECAP recovery often featured a non-monotonic time course, described by a threecomponent exponent. Electric maskers produced similar post-stimulatory effects in hearing and acutely deafened subjects.Acoustic stimulation affects the ECAP in a level-and time-dependent manner. Simultaneous masking follows a time course comparable to that of adaptation to an acoustic stimulus. Refractoriness, spontaneous activity, and adaptation are suggested to play a role in ECAP recovery. Post-stimulatory changes in synchrony, possibly due to recovery of spontaneous activity and an additional hair-cell independent mechanism, are hypothesized to contribute to the observed non-monotonicity of recovery.

Auditory Nerve Fiber Responses to Combined Acoustic and Electric Stimulation

“…However, those data were obtained under different conditions, i.e., using single, electric, masking pulses and chemically deafened ears. The results obtained from hearing ears stimulated by relatively long-duration (400 ms) acoustic maskers, as in the Nourski et al (2007) study, could differ substantially. Clearly, ANF measures are needed to identify the specific ANF response properties that underlie changes in the gross potential.…”

“…Post-masking reductions in spike rate, within-fiber jitter and acrossfiber variance in latency were found, with the changes in temporal response properties limited to ANFs with high spontaneous rates. Thus, our results suggest how non-monotonic ECAP recovery occurs for ears with spontaneous activity, but cannot account for that pattern of recovery when there is no spontaneous activity, including the results from the presumably deafened ears used in the Nourski et al (2007) study. Finally, during simultaneous (electric+acoustic) stimulation, the degree of electrically driven spike activity had a strong influence on spike rate, but did not affect spike jitter, which apparently was determined by the acoustic noise stimulus or spontaneous activity.…”

“…However, even without acoustic stimulation, electrically evoked responses from acoustically sensitive ANFs likely differ from those of deaf ears, as inner hair cells produce spontaneous activity and, in some cases, may be depolarized by electric stimuli and produce δ responses (van den . Nourski et al (2007) used the electrically evoked compound action potential (ECAP) to study the influence of prior acoustic stimulation on ensemble responses to electric pulse trains. That study focused on the short-latency ECAP that is presumed to arise from α responses.…”

“…The magnitude of the non-monotonic portion of the recovery curve (i.e., the decrease in ECAP amplitude that preceded the expected, subsequent, recovery of the ECAP) could be as large as 25% of the unmasked amplitude. Nourski et al (2007) speculated that forward masking could transiently reduce the spontaneous rates (SRs) of ANFs and result in non-monotonic recovery. However, in further experiments using electric forward maskers, they reported non-monotonic functions from chemically deafened ears, suggesting that IHC activity was not essential to that result.…”

Loss of Afferent Vestibular Input Produces Central Adaptation and Increased Gain of Vestibular Prosthetic Stimulation

“…A second is that the electrical stimulation is driving depolarization and hair cell neurotransmitter release and that this contributes to the afferent fiber compound action potentials observed in the vECAP. Studies recording ECAPs from stimulation of the auditory nerve have suggested that hair cells themselves can contribute directly to ECAP amplitudes (Miller et al 2006;Nourski et al 2007;Van Den Honert and Stypulkowski 1984). It is also possible that the afferent fibers were at a lower galvanic threshold due to tonic unstimulated neurotransmitter release prior to gentamicin lesion of the hair cells and were therefore more responsive to direct galvanic stimulation.…”