Hugh S. Lusted

The nerve-electrode interface of the cochlear implant: current spread

One of the fundamental facets of the cochlear implant that must be understood to predict accurately the effect of an electrical stimulus on the auditory nerve is the nerve-electrode interface. One aspect of this interface is the degree to which current delivered by an electrode spreads to neurons distant from it. This paper reports a direct mapping of this current spread using recordings from single units from the cat auditory nerve. Large variations were seen in the degree to which the different units are selective in responding to electrodes at different positions within the scala tympani. Three types of units could be identified based on the selectiveness of their response to the different electrodes in a linear array. The first type of unit exhibited a gradual increase in threshold as the stimulating site was moved from more apical to more basal locations within the scala tympani. The second type of unit exhibited a sharp local minimum, with rapid increases in threshold in excess of 6 dB/mm in the vicinity of the minimum. At electrode sites distant from the local minima the rate of change of the threshold approached that of the first type of units. The final type of unit also demonstrated a gradual change in threshold with changing electrode position, however, two local minima, one apical and one basal, could be identified. These three types are hypothesized to correspond to units which originate apical to the electrode array, along the electrode array and basal to the electrode array.

Statistical quantification of 24-hour and monthly variabilities of spontaneous otoacoustic emission frequency in humans

Previous evidence has suggested a relationship between spontaneous otoacoustic emissions (SOAEs) and established, biological cycles, although detailed statistical quantifications of the suggested relationships do not exist in the literature. In an attempt to statistically quantify the purported circadian and monthly influences on this phenomenon, two experiments were undertaken. The first experiment was conducted over eight weeks, investigating 31 SOAEs recorded from eight women and two men. Time series statistical analysis examined whether daily, weekly, and/or monthly cycles characterized SOAE frequency variability. Results yielded a significant monthly cycle for the majority of SOAEs recorded from the women but for none of the SOAEs recorded from the men. These results suggest the possibility that SOAE frequency fluctuation in women may be entrained to the monthly menstrual cycle. In the second experiment, hourly SOAE frequency stability was examined over a 24-h period to ascertain the nature of the daily frequency variation as precisely as possible. Four SOAEs from two subjects were examined, and time series analysis of these data included (1) modelling the autocorrelation structure of the measurements, (2) resolving each 24-h series of measurements into cyclical components of various periodicities, and (3) testing the statistical significance of given cycles within the spectrum of each series. Findings included a significant 24-h variability of frequency for each SOAE, suggesting the possibility of a circadian influence on frequency fluctuation. Results from the two experiments provide quantitative evidence supporting a hypothetical relationship between SOAEs and established, biological cycles.

Role of perilymphatic fistula in sudden hearing loss: an animal model.

The electrophysiologic response of the guinea pig cochlea was monitored after sequential lesions to Reissners membrane and the round window (RW). Action potential (AP) responses to click stimuli were recorded from the RW before and after discrete puncture-type lesions were created in the cochlear partition of the second turn. Observed decrements were typically minor, comparable to no greater than 10 dB attenuation of stimulus intensity. The RW membranes then were perforated to create perilymphatic fistulas. Further monitoring demonstrated a rapid (within 5 to 10 minutes), severe decrement in AP amplitude and latency, with complete loss of the AP within 1 hour. Control animals with RW perforations alone did not show these decrements. Correct placement of the second turn lesions was documented by histology. We conclude that discrete lesions in the cochlear duct are not reflected in the AP input-output functions unless there is a fluid leak from the RW, and thus present a possible model for idiopathic sudden hearing loss.

Comparison of electrode sites in electrical stimulation of the cochlea

There is considerable controversy about the “best” location for single channel cochlear implant electrodes. We measured the electrically induced auditory brain stem response (EABR) in a series of normal to totally denervated cat ears in response to promontory (P), round window (RW) and scala tympani (ST) stimulation. The status of the ganglion cell population was then assessed by light microscopy. In ears with light to medium ganglion cell loss the ST EABR yielded the most definitive input‐output functions. RW responses were present at increased thresholds and smaller peak amplitudes. P responses were worse or even missing completely. In severely damaged ears, including some with no detectable ganglion cells, ST and RW EABRs were both markedly reduced with considerable overlap between the two sites. P responses, when present, were almost buried in the electrical noise near threshold. Extrapolating these results to humans suggests that when ganglion cell loss is very severe the RW or ST is an acceptable stimulation site. When ganglion cell loss is moderate or better, ST electrodes are superior.

Histological Reaction to Polyimide Films in the Cochlea

The biocompatibility of commercially available polyimides was examined in relation to the sensory and neural structures of the cochlea. Thin films of four different polyimides were prepared as substrates for photolithographically produced scala tympani electrode arrays and implanted in 32 cat cochleae. Following at least three months implantation time, the animals were sacrificed and the cochlea were evaluated histologically for toxicity, differences in the severity of ototoxic effects among the four polyimide groups, and relationships among the different kinds of histopathology secondary to implantation. Findings included the following. Inflammatory reaction to the implants, when present, was generally mild and confined to the immediate vicinity of the implant. Additionally, ototoxic effects were essentially the same across the four groups. Finally, a clear interdependence among the different kinds of cochlear damage secondary to implantation was demonstrated.

Comparison of electrophonic and auditory‐nerve electroneural responses

Electrophonic and auditory-nerve electroneural responses were recorded from the inferior colliculus of the cat. The electrophonic response appeared at a latency 1.0-1.5 ms later than the electroneural response, due to the time requirements for cochlear transduction. The electrophonic response also demonstrated very slow growth of response amplitude with increasing stimulus current as compared to the electroneural response. Aminoglycoside perfusion of the cochlea eliminated the electrophonic component from the evoked response record and left the electroneural component relatively unchanged, indicating that the electrophonic is an acoustic stimulus that requires an intact auditory end organ for transduction.

In Vivo Electrical Stimulation Using Multichannel Photolithographic Electrode Atrays

Multichannel electrical stimulation of the auditory nerve is demonstrated in a cat model using photolithographic electrode arrays. Evoked potentials from the auditory cortex are used to map the location of fibers activated by different electrodes in the array. The evoked responses obtained are equivalent to those produced by fine wire electrodes currently used for functional stimulation of the auditory system.

Interaction of cortical evoked potentials to electric and acoustic stimuli

Evoked potentials to a dichotic stimulus composed of either (1) two binaurally presented tone pips or (2) one tone pip and an electrical pulse to the auditory nerve are recorded from the primary auditory cortex of barbiturate anesthetized cats. The composite stimulus is delivered as a time delayed pair where the interstimulus interval (25 ms) is within the relative refractory period of the evoked potential to either stimulus alone. The amplitude of the cortical potential to the trailing stimulus is compared with its single amplitude as the frequency of the trailing tone pip is changed from 250 Hz through 40 kHz. There is an optimal frequency range over which the trailing stimulus is suppressed and this range appears directly related to the current of a preceding electrical pulse. The frequency of maximum suppression shifts according to the position of the electrode in the nerve. In some experiments secondary maxima develop, suggesting stimulus current spread from fibers of one cochlear turn into fibers from another turn.

Biomuse: Musical performance generated by human bioelectric signals

A new type of musical instrument is described which utilizes bioelectric signals to drive a keyboard synthesizer via standard musical instrument digital interface (MIDI) code. Electrical activity from muscle (EMG), brain (EEG), heart (EKG), and eye (EOG) is detected by small disk electrodes on the skin, analyzed by a digital signal processor, and used to control sound generation from the synthesizer. Signal processing algorithms are designed to extract useful control parameters from the given physiological signals. For instance, detection of amplitude changes in the EMG and spectral changes in the EEG can be used to generate MIDI commands. Applications for the device are discussed, such as dance generated music, musician self‐mixed performances, and music production by the handicapped.

Musical performance by the handicapped generated from bioelectric signals

Recently, the use of bioelectric signals to generate MIDI was reported [H. S. Lusted and R. B. Knapp, J. Acoust. Soc. Am. Suppl. 1 84, S179 (1988)]. Currently, the Biomuse system is being developed for use by movement‐impaired individuals to produce music from standard MIDI instruments. The system detects bioelectric signals that are then analyzed for specific intensity and spectral characteristics, and then mapped to MIDI output commands to produce desired effects from a synthesizer. The system can utilize the electrical signals from muscle (EMG), brain activity (EEG), and eye movements (EOG). Through the use of flexible front end software, the Biomuse can accommodate the needs of many diverse populations of handicapped users. In so doing, the Biomuse will open doors to independent music making and renewal of the creative process. A quadriplegic user, for example, can utilize residual upper extremity EMG and eye movements to directly control various MIDI parameters in order to produce synthesizer sounds ...