Nelson Y. S. Kiang

Physiological Considerations in Artificial Stimulation of the Inner Ear

It has long been established that electric stimulation of the inner ear can produce auditory sensations in severely deaf humans.lV2 However, repeated attempts to develop prosthetic devices based on electric stimulation of the auditory nerve have yielded disappointing result^.^-^ To date, no experiment on any deaf human subject has produced hearing that could be considered adequate for conventional speech communication.

Auditory-nerve activity in cats exposed to ototoxic drugs and high-intensity sounds.

The response characteristics of auditory-nerve fibers in normal cats are compared with those in cats exposed to kanamycin and high-intensity sounds. The pathophysiology is characterized by an elevation of the tuning-curve “tips,” which is sometimes associated with hypersensitivity of the “tails.” Plots of unit thresholds are correlated with patterns of sensory-cell losses in the cochlea. There can be significant shifts in unit threshold without significant loss of hair cells; however, significant hair cell loss is always accompanied by highly abnormal unit thresholds. The presence of inner hair cells seems to be essential for the long-term survival of spiral ganglion cells. An incidental observation is that in the “normal” animal there is almost always a prominent “notch” at 3–4 kHz in the plots of threshold at characteristic frequency, which may have been produced by environmental noise.

Imaging Subcortical Auditory Activity in Humans

There is a lack of physiological data pertaining to how listening humans process auditory information. Functional magnetic resonance imaging (fMRI) has provided some data for the auditory cortex in awake humans, but there is still a paucity of comparable data for subcortical auditory areas where the early stages of processing take place, as amply demonstrated by single‐unit studies in animals. It is unclear why fMRI has been unsuccessful in imaging auditory brain‐stem activity, but one problem may be cardiac‐related, pulsatile brain‐stem motion. To examine this, a method eliminating such motion (using cardiac gating) was applied to map sound‐related activity in the auditory cortices and inferior colliculi in the brain stem. Activation in both the colliculi and cortex became more discernible when gating was used. In contrast with the cortex, the improvement in the colliculi resulted from a reduction in signal variability, rather than from an increase in percent signal change. This reduction is consistent with the hypothesis that motion or pulsatile flow is a major factor in brain‐stem imaging. The way now seems clear to studying activity throughout the human auditory pathway in listening humans. Hum. Brain Mapping 6:33–41, 1998.

Synchrony of Neural Activity in Electric Responses Evoked by Transient Acoustic Stimuli

Electric responses evoked by transient acoustic stimuli were recorded by gross electrodes from the periphery and cortex of the cat. The responses to clicks and to the rapid onset of bursts of noise or tones are characterized by a sharp N1 peak in the peripheral response and by a broader cortical response. When the rise time of the onset of bursts of noise (or tones) is lengthened, or when a burst of sudden onset is presented in a noise background, it is possible to obtain cortical responses under conditions for which the N1 component of the peripheral response is undetectable. These results and associated findings are interpreted by considering the degree of synchronization of the discharge and the wave forms of the activity of units that contribute to responses recorded by gross electrodes. A simple statistical interpretation is presented.

Spontaneous spike discharges from single units in the cochlear nucleus after destruction of the cochlea

Abstract Spike discharges occur in single units in the cochlear nucleus of anesthetized cats in the absence of controlled acoustic stimuli. The origins of this spontaneous activity are unknown. The present paper compares the spontaneous activity in the cochlear nucleus of intact animals with that found in animals with destroyed cochleas. Cochlear destruction results in an immediate disappearance of almost all activity in the ventral cochlear nucleus, while the activity in the dorsal cochlear nucleus is relatively unaffected even in chronic preparations. Thus most of the spontaneous activity in the ventral nucleus appears to be dependent on the spontaneous activity of the auditory nerve. The spontaneous activity in the dorsal nucleus, on the other hand, appears either to be a result of activity in other pathways to the cochlear nucleus or to originate in the units themselves. These physiological results are consistent with anatomical descriptions of the effects of cochlear destruction on the neurons in the cochlear nucleus.

Generators of the brainstem auditory evoked potential in cat III: identified cell populations

This paper examines the relationship between different brainstem cell populations and the brainstem auditory evoked potential (BAEP). First, we present a mathematical model relating the BAEP to underlying cellular activity. Then, we identify specific cellular generators of the click-evoked BAEP in cats by combining model-derived insights with key experimental data. These data include (a) a correspondence between particular brainstem regions and specific extrema in the BAEP waveform, determined from lesion experiments, and (b) values for model parameters derived from published physiological and anatomical information. Ultimately, we conclude (with varying degrees of confidence) that: (1) the earliest extrema in the BAEP are generated by spiral ganglion cells, (2) P2 is mainly generated by cochlear nucleus (CN) globular cells, (3) P3 is partly generated by CN spherical cells and partly by cells receiving inputs from globular cells, (4) P4 is predominantly generated by medial superior olive (MSO) principal cells, which are driven by spherical cells, (5) the generators of P5 are driven by MSO principal cells, and (6) the BAEP, as a whole, is generated mainly by cells with characteristic frequencies above 2 kHz. Thus, the BAEP in cats mainly reflects cellular activity in two parallel pathways, one originating with globular cells and the other with spherical cells. Since the globular cell pathway is poorly represented in humans, we suggest that the human BAEP is largely generated by brainstem cells in the spherical cell pathway. Given our conclusions, it should now be possible to relate activity in specific cell populations to psychophysical performance since the BAEP can be recorded in behaving humans and animals.

STIMULUS CODING IN THE COCHLEAR NUCLEUS.

Stimulus coding in cochlear nucleus, noting different discharge patterns of units in different subdivisions

Acoustic noise during functional magnetic resonance imaging.

Functional magnetic resonance imaging (fMRI) enables sites of brain activation to be localized in human subjects. For studies of the auditory system, acoustic noise generated during fMRI can interfere with assessments of this activation by introducing uncontrolled extraneous sounds. As a first step toward reducing the noise during fMRI, this paper describes the temporal and spectral characteristics of the noise present under typical fMRI study conditions for two imagers with different static magnetic field strengths. Peak noise levels were 123 and 138 dB re 20 microPa in a 1.5-tesla (T) and a 3-T imager, respectively. The noise spectrum (calculated over a 10-ms window coinciding with the highest-amplitude noise) showed a prominent maximum at 1 kHz for the 1.5-T imager (115 dB SPL) and at 1.4 kHz for the 3-T imager (131 dB SPL). The frequency content and timing of the most intense noise components indicated that the noise was primarily attributable to the readout gradients in the imaging pulse sequence. The noise persisted above background levels for 300-500 ms after gradient activity ceased, indicating that resonating structures in the imager or noise reverberating in the imager room were also factors. The gradient noise waveform was highly repeatable. In addition, the coolant pump for the imagers permanent magnet and the room air-handling system were sources of ongoing noise lower in both level and frequency than gradient coil noise. Knowledge of the sources and characteristics of the noise enabled the examination of general approaches to noise control that could be applied to reduce the unwanted noise during fMRI sessions.

Speech coding in the auditory nerve: IV. Sounds with consonant‐like dynamic characteristics

Discharge patterns of auditory-nerve fibers in anesthetized cats were obtained for two stimulus levels in response to synthetic stimuli with dynamic characteristics appropriate for selected consonants. A set of stimuli was constructed by preceding a signal that was identified as /da/by another sound that was systematically manipulated so that the entire complex would sound like either /da/, /ada/, /na/, /sa/, /sa/, or others. Discharge rates of auditory-nerve fibers in response to the common /da/-like formant transitions depended on the preceding context. Average discharge rates during these transitions decreased most for fibers whose CFs were in frequency regions where the context had considerable energy. Some effect of the preceding context on fine time patterns of response to the transitions was also found, but the identity of the largest response components (which often corresponded to the formant frequencies) was in general unaffected. Thus the response patterns during the formant transitions contain cues about both the nature of the transitions and the preceding context. A second set of stimuli sounding like /s/ and /c/ was obtained by varying the duration of the rise in amplitude at the onset of a filtered noise burst. At both 45 and 60 dB SPL, there were fibers which showed a more prominent peak in discharge rate at stimulus onset for /c/ than for /s/, but the CF regions that reflected the clearest distinctions depended on stimulus level. The peaks in discharge rate that occur in response to rapid changes in amplitude or spectrum might be used by the central processor as pointers to portions of speech signals that are rich in phonetic information.

Evoked responses from the auditory cortex.

Abstract Evoked responses were recorded from the auditory cortex of unanesthetized cats. With clicks of moderate intensity, the waveforms of the responses were highly repeatable among awake cats. Introduction of certain variables such as localized injury to cortex, anesthesia, or sleep resulted in more or less characteristic changes in the waveform of the evoked responses. In particular, the later components of the responses seemed to be more sensitive to changes in the state of the animal, while the early components seemed to be abolished by local injury to the cortex. The evoked responses and particularly the later components did not always behave in a reliably repeatable fashion as click intensity or click rate was changed. When broad-band background noise was introduced, the click evoked responses always showed a characteristic decrease in the earliest component. The later components were not substantially affected until the noise was raised to an intensity that was sufficient to mask the click for a listener. These results are interpreted in terms of multiple projection pathways to cortex with possible functional significance.