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Biomedical subjects

Kirill V Nourski

Publications and source records attributed to Kirill V Nourski.

7 recordsLinked to original sources
PubMed2006

Electrical excitation of the acoustically sensitive auditory nerve: single-fiber responses to electric pulse trains.

Nearly all studies on auditory-nerve responses to electric stimuli have been conducted using chemically deafened animals so as to more realistically model the implanted human ear that has typically been profoundly deaf. However, clinical criteria for implantation have recently been relaxed. Ears with "residual" acoustic sensitivity are now being implanted, calling for the systematic evaluation of auditory-nerve responses to electric stimuli as well as combined electric and acoustic stimuli in acoustically sensitive ears. This article presents a systematic investigation of single-fiber responses to electric stimuli in acoustically sensitive ears. Responses to 250 pulse/s electric pulse trains were collected from 18 cats. Properties such as threshold, dynamic range, and jitter were found to differ from those of deaf ears. Other types of fiber activity observed in acoustically sensitive ears (i.e., spontaneous activity and electrophonic responses) were found to alter the temporal coding of electric stimuli. The electrophonic response, which was shown to greatly change the information encoded by spike intervals, also exhibited fast adaptation relative to that observed in the "direct" response to electric stimuli. More complex responses, such as "buildup" (increased responsiveness to successive pulses) and "bursting" (alternating periods of responsiveness and unresponsiveness) were observed. Our findings suggest that bursting is a response unique to sustained electric stimulation in ears with functional hair cells.

Animals↗
PubMed2005

Effects of acoustic noise on the auditory nerve compound action potentials evoked by electric pulse trains.

This study investigated the effects of acoustic noise on the auditory nerve compound action potentials in response to electric pulse trains. Subjects were adult guinea pigs, implanted with a minimally invasive electrode to preserve acoustic sensitivity. Electrically evoked compound action potentials (ECAP) were recorded from the auditory nerve trunk in response to electric pulse trains both during and after the presentation of acoustic white noise. Simultaneously presented acoustic noise produced a decrease in ECAP amplitude. The effect of the acoustic masker on the electric probe was greatest at the onset of the acoustic stimulus and it was followed by a partial recovery of the ECAP amplitude. Following cessation of the acoustic noise, ECAP amplitude recovered over a period of approximately 100-200 ms. The effects of the acoustic noise were more prominent at lower electric pulse rates (interpulse intervals of 3 ms and higher). At higher pulse rates, the ECAP adaptation to the electric pulse train alone was larger and the acoustic noise, when presented, produced little additional effect. The observed effects of noise on ECAP were the greatest at high electric stimulus levels and, for a particular electric stimulus level, at high acoustic noise levels.

Action Potentials↗
PubMed2004

Intracochlear and extracochlear ECAPs suggest antidromic action potentials.

With experimental animals, the electrically evoked compound action potential (ECAP) can be recorded from multiple sites (e.g., round window, intracranial and intracochlear sites). However, human ECAPs are typically recorded from intracochlear electrodes of the implanted array. To bridge this difference, we obtained ECAPs from cats using both intracochlear and nerve-trunk recording sites. We also sought to determine how recording the site influences the acquired evoked potential and how those differences may provide insight into basic excitation properties. In the main experiment, ECAPs were recorded from four acutely deafened cats after implanting a Nucleus-style banded electrode array. Potentials were recorded from an electrode positioned on the nerve trunk and an intracochlear electrode. We manipulated stimulus level, electrode configuration (monopolar vs bipolar) and stimulus polarity, variables that influence the site of excitation. Intracochlear ECAPs were found to be an order of magnitude greater than those obtained with the nerve-trunk electrode. Also, compared with the nerve-trunk potentials, the intracochlear ECAPs more closely resembled those obtained from humans in that latencies were shorter and the waveform morphology was typically biphasic (a negative peak followed by a positive peak). With anodic monophasic stimuli, the ECAP had a unique positive-to-negative morphology which we attributed to antidromic action potentials resulting from a relatively central site of excitation. We also collected intracochlear ECAPs from twenty Nucleus 24 implant users. Compared with the feline ECAPs, the human potentials had smaller amplitudes and longer latencies. It is not clear what underlies these differences, although several factors are considered.

Action Potentials↗
PubMed2004

Feasibility of using silicon-substrate recording electrodes within the auditory nerve.

The use of penetrating, silicon-substrate (i.e., "thin-film") probes within a cross-section of a sensory nerve offers the possibility of assessing the pattern and extent of fiber excitation within the nerve. We used acute cat preparations to assess the feasibility of this technique for recordings within the auditory nerve trunk. Four probe configurations fabricated by the University of Michigan Center for Neural Communication Technology were evaluated using acoustic and electric stimuli. Our main concerns were the nature of the recorded potentials and the degree of spatial selectivity provided by these probes. We also made some basic assessments of electrode-tissue compatibility. The recorded potentials were characterized as field potentials with varying degrees of spatial selectivity. In some cases, responses to pure tones demonstrated good spatial selectivity, with unique responses recorded by different electrode sites. When electrode sites were positioned at different longitudinal positions along the nerve trunk, responses with latencies characteristic of each site were recorded. These results indicate that thin-film electrodes are capable of providing spatially specific response information from sensory nerves. However, in the case of feline auditory nerves, place-specific responses were inconsistently observed, making it difficult to use this technique to obtain detailed cochleotopic maps of neural excitation. More productive results may be possible from other peripheral nerves with less complex spatial arrangements of fibers.

Acoustic Stimulation↗
PubMed2003

Electrode configuration influences action potential initiation site and ensemble stochastic response properties.

The configuration of intracochlear electrodes used to electrically stimulate the auditory nerve influences the ensemble fiber response. For example, monopolar stimulation produces lower thresholds and greater spread of excitation than does bipolar stimulation. We used two approaches to investigate how the ensemble of auditory-nerve fibers responds to stimulation delivered by different electrode configurations. As the electrically evoked compound action potential (ECAP) reflects the ensemble response of the nerve, we used its morphology and changes with stimulus level to assess issues related to site-of-excitation and fiber recruitment. In our first approach, feline ECAPs were obtained using a nucleus-style banded electrode array. ECAP latency functions indicated that bipolar stimulation can initiate action potentials at more peripheral sites than does monopolar stimulation. We observed double-peaked ECAPs with bipolar and tripolar stimulation, suggesting excitation of both peripheral and central neural processes. Finally, we observed in some cases a tendency for monopolar stimulation to produce wider ECAP potentials, consistent with the notion that monopolar stimulation excites a broader spatial extent of the fiber population. In our second approach, we applied a simple model to published surveys of single-fiber responses to provide insight into the stochastic properties of the ensemble response. Our results suggest that broader recruitment of fiber activity produced by monopolar stimulation results in a population response with more probabilistic response characteristics and ensemble spike jitter. These observations and our ECAP results are consistent with reports of perceptual advantages attributed to monopolar or other less-focused modes of stimulation.

Animals↗
PubMed2003

Auditory response to intracochlear electric stimuli following furosemide treatment.

The influence of functional hair cells on electrical stimulation of the auditory nerve is an important issue as individuals with significant residual hearing are now cochlear implant candidates. Previous work has shown that chemical deafening during the course of acute experiments changes the auditory nerve's responses to electrical stimulation [Third Quarterly Progress Report, NIH contract N01-DC-9-2106 (2000), Final Report, NIH Contract N01-DC-9-2106 (2002)]. This study extended that work by investigating the changes and subsequent recovery following furosemide injections which reversibly impair hair-cell function [Hear. Res. (1980) 79-89; Hear. Res. 14 (1984) 305-314, J. Physiol. 347 (1984) 685-696; Hear. Res. 71 (1993) 202-207]. Acoustic sensitivity of guinea pig subjects was repeatedly monitored with the click-evoked compound action potential. Responses to single biphasic electric pulses and biphasic electric pulse trains delivered by a monopolar intracochlear electrode were also repeatedly assessed using the electrically evoked compound action potential (ECAP). Our measures demonstrated a clear relationship between the state of hair-cell function and ECAP responses, as changes in the latter coincided with the loss or recovery of acoustic sensitivity. ECAP growth functions demonstrated increased slope and increased maximum (saturation) amplitude. Both trends were reversible and followed approximately the time course of post-furosemide hearing recovery. Additional changes were observed using electric pulse-train stimulation: (1) the magnitude of ECAP amplitude alternation (observed in response to successive stimulus pulses) increased, (2) the degree of ECAP adaptation (measured 80-100 ms after pulse-train onset) increased, and (3) the degree of refractoriness (measured by the ratio of ECAP amplitudes to the second and first pulses) tended to increase. All these trends are consistent with the hypothesis that functional hair cells desynchronize the population of auditory nerve fibers, thereby changing the electrically evoked responses. Viable hair cells may therefore provide positive effects on auditory response to electric stimuli delivered to implant patients with residual hearing, as they may enhance the random activity of the stimulated nerve.

Animals↗