Evoked cortical activity and speech recognition as a function of the number of simulated cochlear implant channels.

OBJECTIVES: (1) To determine if consonant-vowel-consonant (CVC) syllables [Hillenbrand J, Getty L, Clark M, Wheeler K. Acoustic characteristics of American English vowels. J Acoust Soc Am 1995;97:3099-3111] could be used to evoke cortical far field response patterns in humans, (2) to characterize the effects of cochlear implant-simulated channel number on the perception and physiological detection of these same CVC stimuli, and (3) to define the relationship between perception and the morphology of the physiological responses evoked by these speech stimuli. METHODS: Ten normal hearing monolingual English speaking adults were tested. Unprocessed CVC naturally spoken syllables, containing medial vowels, as well as processed versions (2, 4, 8, 12, and 16 spectral channels) were used for behavioral and physiological testing. RESULTS: (1) CVC stimuli evoked a series of overlapping P1-N1-P2 cortical responses. (2) Amplitude of P1-N1-P2 responses increased as neural conduction time (latency) decreased with increases in the number of spectral channels. Perception of the CVC stimuli improved with increasing number of spectral channels. (3) Coinciding changes in P1-N1-P2 morphology did not significantly correlate with changes in perception. CONCLUSIONS: P1-N1-P2 responses can be recorded using CVC syllables and there is an effect of channel number on the latency and amplitude of these responses, as well as on vowel identification. However, the physiological detection of the acoustic changes does not fully account for the perceptual performance of these same syllables. SIGNIFICANCE: These results provide evidence that it is possible to use vocoded CVC stimuli to learn more about the physiological detection of acoustic changes contained within speech syllables, as well as to explore brain-behavior relationships.

Brainstem electronic implants for bilateral anacusis following surgical removal of cerebello pontine angle lesions.

The multichannel auditory brainstem implant (ABI) has been used successfully to treat deafness in individuals with neurofibromatosis type II. The device has been implanted in nearly 150 recipients worldwide, and clinical trials with the device are approaching completion. The implantation and fitting of the multichannel ABI differ significantly from cochlear implantation, and the processes are illustrated in a series of case studies. Performance data also are included from recipients with up to 7 years experience.

Effects of dynamic range and amplitude mapping on phoneme recognition in Nucleus-22 cochlear implant users.

OBJECTIVE: To determine the consequences for phoneme recognition of errors in setting threshold and loudness levels in cochlear implant listeners using a 4-channel continuous interleaved sampling (CIS) speech processor. DESIGN: Three Nucleus-22 cochlear implant listeners, who normally used the SPEAK speech processing strategy participated in this study. An experimental 4-channel CIS speech processor was implemented in each listener as follows. Speech signals were band-pass filtered into four broad frequency bands and the temporal envelope of the signal in each band was extracted by half-wave rectification and low-pass filtering. A power function was used to convert the extracted acoustic amplitudes to electric currents. The electric currents were dependent on the exponent of the mapping power function and the electrode dynamic range, which was determined by the minimum and maximum stimulation levels. In the baseline condition, the minimum and maximum stimulation levels were defined as the psychophysically measured threshold level (T-level) and maximum comfortable level (C-level). In the experimental conditions, the maximum stimulation levels were fixed at the C-level and the dynamic range (in dB) was changed by varying the minimum stimulation levels on all electrodes. This manipulation simulates the effect of an erroneous measurement of the T-level. Phoneme recognition was obtained as the dynamic range of electrodes was changed from 1 dB to 20 dB and as the exponent of the power-law amplitude mapping function was changed from 0.1 to 0.4. RESULTS: For each mapping condition, the electric dynamic range had a significant, but weak effect on vowel and consonant recognition. For a strong compression (p = 0.1), best vowel and consonant scores were obtained with a large dynamic range (12 dB). When the exponent of the mapping function was changed to 0.2 and 0.4, the dynamic range producing the highest scores decreased to 6 dB and 3 dB, respectively. CONCLUSIONS: Phoneme recognition with a 4-channel CIS strategy was only mildly affected by large changes in both electric threshold and loudness mapping. Errors in threshold by a factor of 2 (6 dB) and in the loudness mapping exponent by a factor of 2 were required to produce a significant decrease in performance. In these extreme conditions, the effect of the electric dynamic range on phoneme recognition could be due to two independent factors: abnormal loudness growth and a reduction in the number of discriminable intensity steps. The decrease in performance caused by a reduced electric dynamic range can be compensated by a more expansive power-law mapping function, as long as the number of discriminable intensity steps is moderately large (e.g., >8).

Gap detection as a measure of electrode interaction in cochlear implants.

Gap detection thresholds were measured as an indication of the amount of interaction between electrodes in a cochlear implant. The hypothesis in this study was as follows: when the two stimuli that bound the gap stimulate the same electrode, and thus the same neural population, the gap detection threshold will be short. As two stimuli are presented to two electrodes that are more widely separated, the amount of neural overlap of the two stimuli decreases, the stimuli sound more dissimilar, and the gap thresholds increase. Gap detection thresholds can thus be used to infer the amount of overlap in neural populations stimulated by two electrodes. Three users of the Nucleus cochlear implant participated in this study. Gap detection thresholds were measured as a function of the distance between the two electrode pairs and as a function of the spacing between the two electrodes of a bipolar pair (i.e., using different modes of stimulation). The results indicate that measuring gap detection thresholds may provide an estimate of the amount of electrode interaction. Gap detection thresholds were a function of the physical separation of the electrode pairs used for the two stimuli that bound the gap. Lower gap thresholds were observed when the two electrode pairs were closely spaced, and gap thresholds increased as the separation increased, resulting in a "psychophysical tuning curve" as a function of electrode separation. The sharpness of tuning varied across subjects, and for the three subjects in this study, the tuning was generally sharper for the subjects with better speech recognition. The data also indicate that increasing the separation between active and reference electrodes has limited effect on spatial selectivity (or tuning) as measured perceptually.

Multi-unit mapping of acoustic stimuli in gerbil inferior colliculus.

Multi-unit peristimulus time (MU-PST) histograms were recorded in the gerbil inferior colliculus (IC) in response to tone burst stimuli. Histograms were collected every 100 microns as the recording electrode was advanced along the tonotopic axis of the central nucleus of the IC. Space/time maps of neural activity were constructed from these data. In most of our sample the pattern of response changed systematically as the stimulating frequency was increased in octave steps. At low frequencies (< 500 Hz) the pattern of response was broadly distributed spatially and phase-locked to the stimulus frequency. At higher frequencies (> 1 kHz) the pattern of response was more localized and showed no evidence of phase locking. The location of the maximum response to tones from 1 to 32 kHz moved ventrally along the tonotopic axis at an approximate rate of 230 microns/stimulus octave. The patterns of response were localized near stimulus threshold and spread over a larger region as level increased. This method of collecting and displaying multi-unit response maps provides an overview of ensemble activity that allows concurrent observation of spatial and temporal variations in activity patterns. The quantitative analysis of components of MU-PST Maps are consistent with trends illustrated with single-unit tuning and level functions. This perspective of IC activity suggests potential processing mechanisms that are congruent with single-unit reconstructions.

Threshold-distance measures from electrical stimulation of human brainstem.

Threshold current levels for electrical stimulation of a single human brainstem via an auditory prosthesis are compared with postmortem measures of the distance between the electrode and stimulated structures. The results compare well with the summary of threshold-distance measures from animal experiments compiled by Ranck. The correspondence between the human and animal data gives confidence that the extent of current spread (distance to stimulable neural units) can be well estimated from the current level at threshold for 200 microseconds/phase biphasic pulses. This is of particular interest in electrical stimulation of the human central nervous system, where localization of stimulation is of paramount importance.

Possible origins of the non-monotonic intensity discrimination function in forward masking.

A non-monotonic intensity discrimination function in forward masking has been recently reported [Zeng et al. (1991) Hear. Res. 55, 223-230; Zeng and Turner (1992) J. Acoust Soc. Am. 92, 782-787] in which just-noticeable-differences (jnds) in intensity are largest for midlevel tones and smaller for soft and loud tones following an intense narrow-band noise. One hypothesis was that this midlevel hump reflects the contribution of low-spontaneous rate (SR) neurons to intensity coding, based on the differential recovery from forward masking of low-SR and high-SR neurons [Relkin and Doucet (1991) Hear. Res. 55, 215-222]. The present study conducted three experiments stimulating different stages of the auditory system in an attempt to determine the peripheral and central origins of the midlevel hump. First, in two cochlear implant (CI) listeners, the forward masker produced a midlevel hump on the intensity discrimination function, suggesting that the synapses between the hair cell and the eighth nerve are probably not responsible for the hump, as they are bypassed and the eighth nerve is stimulated directly. Second, in auditory brainstem implant (ABI) listeners, the forward masker produced no midlevel hump, but the masked jnds were larger than those without a masker. The absence of the midlevel hump in the ABI listeners suggests that the occurrence of the hump requires physiological mechanisms in the auditory nerve transmission, or the intrinsic processing circuits of the cochlear nuclei, or both. Third, in normal-hearing listeners, an ipsilateral, 90 dB SPL, pure-tone forward masker produced a midlevel hump, which is similar to that using a narrow-band noise masker; whereas a contralateral forward masker produced essentially no midlevel hump, suggesting that binaural interactions at superior olivary complex and more central sites are probably not responsible.

Quantitative comparison of electrically and acoustically evoked auditory perception: implications for the location of perceptual mechanisms.

Electrical stimulation of the human auditory system produces different patterns of spatial and temporal neural activity than those that occur in the normal, acoustically stimulated system. Quantitative comparison of psychophysical performance measured with acoustic and electrical stimulation may allow us to infer the physiological locus of perceptual mechanisms. In this paper we compare psychophysical data on temporal resolution from normal-hearing listeners, cochlear implant listeners, and patients electrically stimulated on the cochlear nucleus. Measures of gap detection, forward masking, and modulation detection will be compared. These comparisons demonstrate that temporal processing is relatively similar across these three groups once the obvious differences in dynamic range are taken into consideration. In addition, preliminary results with speech processors indicate that implant patients can utilize all temporal information in speech. Thus, implant patients have relatively normal temporal resolution and can integrate temporal cues normally for the recognition of complex acoustic patterns such as speech. These results imply that the central auditory systems of implant patients are able to fully utilize the non-natural patterns of temporal neural information produced by electrical stimulation. The differences in the microstructure of the neural pattern (phase locking, stochastic independence of fibers, spatial distribution of activity, etc.) between electrical and acoustic stimulation are apparently not necessary for temporal processing. Thus, the physiological locus of temporal processing mechanisms must be more central in the auditory system than the cochlea and cochlear nucleus.

Growth of pulsation threshold of a suppressed tone as a function of its level.

The pulsation threshold (PT) was measured at the frequency of a probe tone in a two-tone stimulus. A suppressor tone was higher in frequency than the probe tone and was fixed in level. As the level of the probe tone was increased, three regions of performance were observed: (1) for probe tone levels below simultaneous masked threshold (SMT), PT was the same as that measured for the suppressor alone, (2) for levels above SMT, PT increased linearly with level, indicating a constant amount of suppression in dB, and (3) for higher levels a recruitment-like phenomenon was observed, in which the PT increased faster than the probe level. The maximum amount of suppression observed was equal to the difference between the PT and SMT for the suppressor alone. One interpretation is that the suppressor reduces excitation on the slopes of its own excitation pattern by the same amount that it reduces the additional excitation from a probe tone. These results are consistent with physiological data, where the amount of suppression is determined by the suppressor and is independent of the level of the probe tone.