HIGH levels of speech recognition have been achieved with a new sound processing strategy for multielectrode cochlear implants. A cochlear implant system consists of one or more implanted electrodes for direct electrical activation of the auditory nerve, an external speech processor that transforms a microphone input into stimuli for each electrode, and a transcutaneous (rf-link) or percutaneous (direct) connection between the processor and the electrodes. We report here the comparison of the new strategy and a standard clinical processor. The standard compressed analogue (CA) processor presented analogue waveforms simultaneously to all electrodes, whereas the new continuous interleaved sampling (CIS) strategy presented brief pulses to each electrode in a nonoverlapping sequence. Seven experienced implant users, selected for their excellent performance with the CA processor, participated as subjects. The new strategy produced large improvements in the scores of speech reception tests for all subjects. These results have important implications for the treatment of deafness and for minimal representations of speech at the auditory periphery.

Electric stimulation of the retina reliably elicits light percepts in patients blinded by outer retinal diseases. However, individual percepts are highly variable and do not readily assemble into more complex visual images. As a result, the quality of visual information conveyed to patients has been quite limited. To develop more effective stimulation methods that will lead to improved psychophysical outcomes, we are studying how retinal neurons respond to electric stimulation. The situation in the retina is analogous to other neural prosthetic applications in which a better understanding of the underlying neural response may lead to improved clinical outcomes. Here, we determined which element in retinal ganglion cells has the lowest threshold for initiating action potentials. Previous studies suggest multiple possibilities, although all were within the soma/proximal axon region. To determine the actual site, we measured thresholds in a dense two-dimensional grid around the soma/proximal axon region of rabbit ganglion cells in the flat mount preparation. In directionally selective (DS) ganglion cells, the lowest thresholds were found along a small section of the axon, about 40 microm from the soma. Immunochemical staining revealed a dense band of voltage-gated sodium channels centered at the same location, suggesting that thresholds are lowest when the stimulating electrode is closest to the sodium-channel band. The size and location of the low-threshold region was consistent within DS cells, but varied for other ganglion cell types. Analogously, the length and location of sodium channel bands also varied by cell type. Consistent with the differences in band properties, we found that the absolute (lowest) thresholds were also different for different cell types. Taken together, our results suggest that the sodium-channel band is the site that is most responsive to electric stimulation and that differences in the bands underlie the threshold differences we observed.

✓ The topographical anatomy of the primary visual cortex in man was studied by macroscopic identification and measurement of the line of Gennari on coronal sections from 52 hemispheres collected at autopsy. Computer analysis of these data provided detailed, quantitative information concerning the amount, variability, and distribution of the striate area on the surface of the occipital lobe, and within the calcarine fissure and other sulci. This information is particularly applicable to the development of a functional visual prosthesis for the blind based on stimulation of the visual cortex. Taking conservative values for two-point discrimination (3 mm) on the cortex based on previous observations made during neurosurgical operations, and the surgically accessible areas available for stimulation reported in this study, it seems clear that a useful number of electrodes could be implanted in virtually every patient.

These preliminary results suggest that the degree of damage to the lateral cochlear wall may play an important role in influencing the amount of new tissue formation following cochlear implantation. Intracochlear new tissue does not appear to be an important determinant of performance as measured by word recognition scores or the total number of remaining spiral ganglion cells.

Electric stimulation of the CNS is being evaluated as a treatment modality for a variety of neurological, psychiatric, and sensory disorders. Despite considerable success in some applications, existing stimulation techniques offer little control over which cell types or neuronal substructures are activated by stimulation. The ability to more precisely control neuronal activation would likely improve the clinical outcomes associated with these applications. Here, we show that specific frequencies of sinusoidal stimulation can be used to preferentially activate certain retinal cell types: photoreceptors are activated at 5 Hz, bipolar cells at 25 Hz, and ganglion cells at 100 Hz. In addition, low-frequency stimulation (≤25 Hz) did not activate passing axons but still elicited robust synaptically mediated responses in ganglion cells; therefore, elicited neural activity is confined to within a focal region around the stimulating electrode. Our results suggest that sinusoidal stimulation provides significantly improved control over elicited neural activity relative to conventional pulsatile stimulation.

Sensitivity to interaural time difference (ITD) in constant-amplitude pulse trains was measured in four sequentially implanted bilateral cochlear implant (CI) subjects. The sensitivity measurements were made as a function of time beginning directly after the second ear was implanted, continued for periods of months before subjects began wearing bilateral sound processors, and extended for months while the subjects used bilateral sound processors in day-to-day listening. Measurements were also made as a function of the relative position of the left/right electrodes. The two subjects with the shortest duration of binaural deprivation before implantation demonstrated ITD sensitivity soon after second-ear implantation (before receiving the second sound processor), while the other two did not demonstrate sensitivity until after months of daily experience using bilateral processors. The interaural mismatch in electrode position required to decrease ITD sensitivity by a factor of 2 (half-width) for CI subjects was five times greater than the half-width for interaural carrier-frequency disparity in normal-hearing subjects listening to sinusoidally amplitude-modulated high-frequency tones. This large half-width is likely to contribute to poor binaural performance in CI users, especially in environments with multiple broadband sound sources.

Experiments were conducted with a single, bilateral cochlear implant user to examine interaural level and time-delay cues that putatively underlie the design and efficacy of bilateral implant systems. The subject's two implants were of different types but custom equipment allowed presentation of controlled bilateral stimuli, particularly those with specified interaural time difference (ITD) and interaural level difference (ILD) cues. A lateralization task was used to measure the effect of these cues on the perceived location of the sensations elicited. For trains of fixed-amplitude, biphasic current pulses at 100 pps, the subject demonstrated sensitivity to an ITD of 300 micros, providing evidence of access to binaural information. The choice of bilateral electrode pair greatly influenced ITD sensitivity, suggesting that electrode pairings are likely to be an important consideration in the effort to provide binaural advantages. The selection of bilateral electrode pairs showing sensitivity to ITD was partially aided by comparisons of the pitch elicited by individual electrodes in each ear (when stimulated alone with fixed-amplitude current pulses at 813 pps): specifically, interaural electrodes with similar pitches were more likely (but not certain) to show ITD sensitivity. Significant changes in lateral position occurred with specific electrode pairs. With five bilateral electrode pairs of 14 tested, ITDs of 300 and 600 micros moved an auditory image significantly from right to left. With these same pairs, ILD changes of approximately 11% of the dynamic range (in microApp) moved an auditory image from the far left to the far right-significantly farther than the nine pairs not showing significant ITD sensitivity. However, even these nine pairs did show response changes as a function of the interaural (or confounding monaural) level cue. Overall, insofar as the access to bilateral cues demonstrated herein generalizes to other subjects, it provides hope that the normal binaural advantages for speech recognition and sound localization can be made available to bilateral implant users.

Many modern cochlear implants use sound processing strategies that stimulate the cochlea with modulated pulse trains. Rubinstein et al. [Hear. Res. 127, 108 (1999)] suggested that representation of the modulator in auditory nerve responses might be improved by the addition of a sustained, high-rate, desynchronizing pulse train (DPT). In addition, activity in response to the DPT may mimic the spontaneous activity (SA) in a healthy ear. The goals of this study were to compare responses of auditory nerve fibers in acutely deafened, anesthetized cats elicited by high-rate electric pulse trains delivered through an intracochlear electrode with SA, and to measure responses of these fibers to amplitude-modulated pulse trains superimposed upon a DPT. Responses to pulse trains showed variability from presentation to presentation, but differed from SA in the shape of the envelope of the interval histogram (IH) for pulse rates above 4.8 kpps (kilo pulses per second). These IHs had a prominent mode near 5 ms that was followed by a long tail. Responses to modulated biphasic pulse trains resembled responses to tones in intact ears for small (<10%) modulation depths, suggesting that acousticlike responses to sinusoidal stimuli might be obtained with a DPT. However, realistic responses were only observed over a narrow range of levels and modulation depths. Improved coding of complex stimulus waveforms may be achieved by signal processing strategies for cochlear implants that properly incorporate a DPT.