Ben M. Clopton

Multi-electrode cochlear implant and method of manufacturing the same

A multi-electrode cochlear implant is taught in which approximately twenty or more insulated metal wires are wound around a flexible tube. These wires are held in place with a further layer of dielectric insulating material. The insulation is selectively removed with a laser beam to form electrodes. Two or more layers or valences of wires can be used, with the inner layer of wires terminating distal to the outer layers to provide a stepwise approximation of the tapering of the scala tympani. A core of shape memory material may be introduced into the tube, so that the implant will retain an effective shape after implantation.

Neural responses in the inferior colliculus of albino rat to binaural stimuli

The responses of units in the inferior colliculus of rats were recorded while noise, tone, and click stimuli were presented binaurally. The interaural time and intensity disparities were varied; time to a maximum of 180 μsec in 30−μsec steps and intensity to a maximum of 20 dB in 4−dB steps. Total spike count, probability of firing, and average latency indicated that for most cells contralateral stimuli leading in time or of greater intensity relative to the ipsilateral ear produced increased spike counts and probability of firing and decreased latency of firing. While support is apparent for the influence of both time and intensity cues, the intensity cue would seem to predominate under naturally occuring conditions. A population model for the utilization of these two cues is supported by the data.Subject Classification: 65.44, 65.62.

Unit responses at cochlear nucleus to electrical stimulation through a cochlear prosthesis

Afferent auditory fibers of the guinea pig cochlea were electrically stimulated with current introduced through electrodes in the scala tympani. Thresholds were determined for unit responses recorded in the ventral cochlear nuclei to a sinusoid of 98 Hz from response-rate growth functions versus stimulus intensity. Suprathreshold response rates for most units grew rapidly from threshold to saturation at 2-15 dB above threshold. Peristimulus time histograms were collected for responses to single sinusoids and combinations of two and five sinusoids ranging from 86 to 134 Hz. Spike occurrences were highly synchronous with individual cycles of the pure sinusoids, but responses to the more complex waveforms occurred primarily to the more intense peaks. The amplitude envelope was thus a major contributor to responses to multiple sinusoids. Destruction of cochlear structures with neomycin increased unit thresholds and produced some changes in waveform encoding.

Unit responses in ventral cochlear nucleus reflect cochlear coding of rapid frequency sweeps

This study examines the encoding of rapid frequency sweeps in single units of the ventral cochlear nucleus (VCN). Sweeps were designed to explore the role of cochlear mechanics in shaping the temporal responses across cells in the VCN. The time course of frequency change for rapidly rising frequency sweeps theoretically produced simultaneous displacement maxima by cancelling travel time along the cochlear partition. Rising sweeps with longer time courses only partially canceled travel time, while falling sweeps had time courses of frequency change equal to or greater than travel time. Falling sweeps thus augmented normal travel time. Latency of unit firing to sweeps across unit characteristic frequency (CF) reflected cochlear delay-line mechanics. The latency-CF functions agreed with predictions from travel-time estimates for rising-frequency sweeps, but responses to falling sweeps were less predictable.

NEURAL ENCODING OF ELECTRICAL SIGNALSa

Research on the cochlear prosthesis is increasingly concerned with the nature of sounds that we encounter and how we might encode them for introduction through a cochlear prosthesis. As we progressively define the physiological and technical limitations on extracting, transforming, and introducing information from these sounds in electrical form, the rules relating the electrical waveforms used to drive a prosthesis and afferent neural activity become a central concern to prosthesis operation. These rules act at the interface between local electrical events and the response of neural membranes, giving rise to questions about stimulus encoding as well as the central processing leading to perception and responses. This is a selective review of published and ongoing research on some topics especially relevant to the encoding of signals presented through a cochlear prosthesis. Its purpose is to summarize in an organized manner information that we view as important for understanding and optimizing prosthesis operation. Data from our laboratories on animal and mathematical models form much of the matrix for the discussion that follows.

Effectiveness of Middle Ear Electrical Stimulation for Activating Central Auditory Pathways

Electrical stimulation of afferent auditory elements through electrodes placed in the middle ear was investigated in acute guinea pig preparations. Thresholds for auditory activation were current-dependent for low frequencies (<1 kHz) and charge-dependent at higher frequencies. Threshold currents were 3–5 times those for intracochlear stimulation. Mechanisms of activation were examined with removal of cochlear fluids and injection of neomycin, Xylocaine, saline, and artificial perilymph with different calcium concentrations. Neurons of the spiral ganglion are indicated as mediators of this stimulation.

Technology and the future of cochlear implants.

Cochlear implants are one of the dramatic success stories of the bioengineering enterprise. Although these prostheses are used extensively, they still can be improved substantially. We suggest that high-density electrode designs will permit field shaping and field steering to an extent not presently possible with the arrays that are used today. Those opportunities will make it possible to make use of the phase information that is richly available to normal listeners. Although this information makes possible more precise location of sound sources in the auditory environment, and will likely improve the recognition of intelligent sound in noise, it will require the consumption of additional power in future cochlear prostheses. Those opportunities and trade-offs provide the designers of cochlear implants with exciting goals for the future.

Motivated changes of auditory sensitivity in a simple detection task.

Recently both neurophysiological and psychophysical theories have suggested that the sensitivity of receptors, or of the whole organism, may change through “efferent control” or as a result of “motivation.” A psychophysical method has been devised to investigate changes in a listener’s ability to detect signals in a noisy background, which are elicited “on demand” by the E, and when the time course of the changes may be in the order of seconds rather than minutes orhours. Observed effects, which are consistent with the hypotheses of active control of sensitivity, are found to be orderly but quite small, generally less than that associated with a 1–2 dB increase in the level of a tonal signal. While the average increment in performance is thus slight, it is found that the variance between Ss is significantly reduced when strong motivating stimuli are introduced. This reduction in variance may reflect an upper limit on the performance of real listeners, which might be fruitfully compared with the theoretical limits proposed in the theory of signal detectability.

Biophysical Measurements in the Implanted Cochlea

Electrical stimulation via implanted electrodes has been used to produce perceptions of sound in human subjects. This study describes preliminary work needed to understand the implanted ear and the distribution of current within it so that a stimulus system can be designed that is optimal for longevity, information transfer, and safety.

Sensory Neurophysiology and Reaction Time Performance in Nonhuman Primates

It was clear from the conference that information is rapidly accumulating on sensory behavior in animals. Psychophysical relationships between stimulus and response parameters recently derived from animal subjects have been shown to be as reliable and precise as those obtained from man. Moreover, through analysis of these stimulus-response functions we are acquiring a better understanding of the influence of various stimulus parameters on behavior. One of the basic themes of the conference concerned the extension of our understanding of sensory functions to include the role of afferent neural structures in behavior. Contemporary behavioral procedures yielding psychophysical functions in animals provide a vehicle for such an extension. Simply stated, this approach suggests that we begin to study afferent neural activity in behaviorally trained animals from which precise measures of psychophysical relationships may be concurrently obtained.