Cochlear Position Research Articles

Application of loudness models to sound processing for cochlear implants.

A new paradigm for processing sound signals for multiple-electrode cochlear implants is introduced, and results are presented from an initial psychophysical evaluation of its effect on the perceived loudness of complex sounds. A real-time processing scheme based on this paradigm, called SpeL, has been developed primarily to improve control of loudness for implant users. SpeL differs from previous schemes in several ways. Most importantly, it incorporates a published numerical model which predicts the loudness perceived by implant users for complex patterns of pulsatile electric stimulation as a function of the pulses' physical parameters. This model is controlled by the output of a corresponding model that estimates the loudness perceived by normally hearing listeners for complex sounds. The latter model produces an estimate of the specific loudness arising from an acoustic signal. In SpeL, the specific loudness function, which describes the contribution to total loudness of each of a number of frequency bands (or cochlear positions), is converted to a pattern of electric stimulation on an appropriate set of electrodes. By application of the loudness model for electric stimulation, this pattern is designed to produce a specific loudness function for the implant user which approximates that produced by the normal-hearing model for the same input signal. The results of loudness magnitude estimation experiments with five users of the SpeL scheme confirmed that the psychophysical functions relating overall loudness perceived to input sound level for five complex acoustic signals were, on average, very similar to those for normal hearing.

Spiral model of pitch

A spiral model of pitch interrelates tone chroma, tone height, equal temperament scales, and a cochlear map. Donkin suggested in 1870 that the pitch of tones could be well represented by an equiangular spiral. More recently, the cylindrical helix has been popular for representing tone chroma and tone height. Here it is shown that tone chroma, tone height, and cochlear position can be conveniently related to tone frequency via a planar spiral. For this ‘‘equal-temperament spiral,’’ (ET Spiral) tone chroma is conceived as a circular array with semitones at 30° intervals. The frequency of sound on the cent scale (re 16.351 Hz) is represented by the radius of the spiral defined by r=(1200/2π)θr, where θr is in radians. By these definitions, one revolution represents one octave, 1200 cents, 30° represents a semitone, the radius relates θ to cents in accordance with equal temperament (ET) tuning, and the arclength of the spiral matches the mapping of sound frequency to the basilar membrane. Thus, the ET Spiral gives tone chroma as θ, tone height as the cent scale, and the cochlear map as the arclength. The possible implications and directions for further work are discussed.

Opposite Actions of Brain-Derived Neurotrophic Factor and Neurotrophin-3 on Firing Features and Ion Channel Composition of Murine Spiral Ganglion Neurons

It is now well established that sensory neurons and receptors display characteristic morphological and electrophysiological properties tailored to their functions. This is especially evident in the auditory system, where cells are arranged tonotopically and are highly specialized for precise coding of frequency- and timing-dependent auditory information. Less well understood, however, are the mechanisms that give rise to these biophysical properties. We have provided insight into this issue by using whole-cell current-clamp recordings and immunocytochemistry to show that BDNF and NT-3, neurotrophins found normally in the cochlea, have profound effects on the firing properties and ion channel distribution of spiral ganglion neurons in the murine cochlea. Exposure of neurons to BDNF caused all neurons, regardless of their original cochlear position, to display characteristics of the basal neurons. Conversely, NT-3 caused cells to show the properties of apical neurons. These results are consistent with oppositely oriented gradients of these two neurotrophins and/or their high-affinity receptors along the tonotopic map, and they suggest that a combination of neurotrophins are necessary to establish the characteristic firing features of postnatal spiral ganglion neurons.

Neuronal responses in cat primary auditory cortex to electrical cochlear stimulation. III. Activation patterns in short- and long-term deafness.

The effects of auditory deprivation on the spatial distribution of cortical response thresholds to electrical stimulation of the adult cat cochlea were evaluated. Threshold distributions for single- and multiple-unit responses from the middle cortical layers were obtained on the ectosylvian gyrus in three groups of animals: adult, acutely implanted animals ("acute group"); adult animals, 2 wk after deafening and implantation ("short-term group"); adult, neonatally deafened animals ("long-term group") implanted after 2-5 years of deafness. For all three groups, we observed similar patterns of circumscribed regions of low response thresholds in the region of primary auditory cortex (AI). A dorsal and a ventral region of low response thresholds were found separated by a narrow, anterior-posterior strip of elevated thresholds. The two low-threshold regions in the acute and the short-term group were arranged cochleotopically. This was reflected in a systematic shift of the cortical locations with minimum thresholds as a function of cochlear position of the radial and monopolar stimulation electrodes. By contrast, the long-term deafened animals maintained only weak or no signs of cochleotopicity. In some cases of this group, significant deviations from a simple tri-partition of the dorsoventral axis of AI was observed. Analysis of the spatial extent of the low-threshold regions revealed that the activated area in acute cases was significantly smaller than the long- and the short-term cases for both dorsal and ventral AI. There were no significant differences in the rostrocaudal extent of activation between long- and short-term deafening, although the total activated area in the short-term cases was larger than in long-term deafened animals. The width of the narrow high-threshold ridge that separated the dorsal and ventral low-threshold regions was the widest for the acute cases and the narrowest for the short-term deafened animals. The findings of relative large differences in cortical response distributions between the acute and short-term animals suggests that the effects observed in long-term deafened animals are not solely a consequence of loss of peripheral innervation density. The effects may reflect electrode-specific effects or reorganizational changes based on factors such as differences in excitatory and inhibitory balance.

Derived-Band Auditory Brain-Stem Response Estimates of Traveling Wave Velocity in Humans: II. Subjects With Noise-Induced Hearing Loss and Meniere’s Disease

Estimates of cochlear traveling wave velocity (TWV) were computed from derived-band auditory brain-stem response (ABR) latencies in subjects with noise-induced hearing loss (NIHL) or Meniére's disease (MD). ABR wave V latencies were determined for each of six derived frequency bands (unmasked-8 kHz, 8-4 kHz, 4-2 kHz, 2-1 kHz, 1 kHz-500 Hz, and 500-250 Hz). Representative frequencies were assigned to the derived bands by estimating their energy midpoints, and cochlear positions corresponding to these frequencies were determined using Greenwood's (1961) place-frequency function for humans. An exponential function of the form I = A + BeCd was fitted to each subject's latency-by-distance data using a least-squares algorithm, and a TWV function was generated by taking the reciprocal of the derivative of the latency function with respect to distance [v = 1/(BCeCd)]. Expected values for subjects' TWV functions were compared to normative data from Donaldson and Ruth (1993) at five cochlear loci. NIHL subjects' TWV estimates fell within normal limits at all cochlear loci, and no relation between severity of high-frequency hearing loss and TWV could be discerned. MD subjects with good low-frequency hearing sensitivity generally yielded normal TWV estimates, whereas MD subjects with low-frequency hearing loss yielded either normal or elevated TWVs. MD subjects' data generally support the hypothesis that endolymphatic hydrops results in increased TWV or, alternatively, a basalward shift in the peak of the traveling wave, in cochleas with presumed normal basilar membrane elasticity.

The tonotopic map in the embryonic chicken cochlea

The purpose of the present study was to determine the tonotopic map in the chicken cochlea at 19 days of incubation (E19) by obtaining characteristic frequencies (CFs) for primary afferents, labeling the characterized neurons, and documenting their projections to the papilla. The lowest and highest CFs recorded were 188 and 1623 Hz respectively. The embryonic tonotopic map coincided with maps reported for post-hatch chicks. There was no evidence that neurons selective to low frequencies project inappropriately to more basal locations of the embryonic papilla. Linear regression was used to estimate the frequency gradient ( b = 0.037 ± 0.012 ln Hz/% [ b ± SE b ]) and intercept ( ln C, where C = 111 Hz) of the semilog plot of frequency versus cochlear position (in % distance from apex). From these estimates the octave distribution was calculated to be 18.7%/octave or 0.58 mm/octave. These quantities were not significantly different from those found in post hatch chickens. We conclude that the tonotopic map of the avian cochlea for CFs between 100 and 1700 Hz is stable and relatively mature from age E19 to post-hatch day 21 (P21). The most striking sign of immaturity in the E19 embryo is the limited range of high CFs. We offer the hypothesis that, between the ages of E19 and P21, improvements in middle ear admittance alone or in combination with functional maturation of the cochlear base may be the principal factors responsible for the appearance of adult-like high CF limits and not an apically shifting tonotopic map.

Erratum and comments re: Critical bandwidth and consonance: Their operational definitions in relation to cochlear nonlinearity and combination tones (Hear. Res. 54, 209–246, 1991)

In a part of the paper cited above, it was suggested that detection of combination components might well determine the initial fall in signal threshold when an experimeter separates two bands of noise masking a signal centered between them-the notched-noise paradigm developed by Patterson (1976). Unfortunately, I mis-stated the bandwidths of the noise bands used by Patterson despite his clear description of their widths and despite the fact that I have also generated noises in the same way. I erred by stating the bandwidths to be half of what they were. Given this error, I calculated examples of the cochlear distribution of the combination components that would be produced by Patterson's noise bands, by themselves and in combination with the signal. The calculated combination-band frequency limits and their cochlear positions, using the wrong primary-noise bandwidth, therefore do not apply to Patterson's experiment. Here the errors are corrected. The modified distribution is not affected in principle. However, for some readers it might seem less probable that detection of combination components in frequency regions lower than the lower band of masking noise could determine the initial fall in threshold (rather than detection of events created by the signal in a cochlear region given by the notch in the two-band masker). But in any case this is an empirical question requiring the direct study that was being urged by the reasoning and calculations presented. There is still reason to suspect that combination tones may have played a role in the two-band experiments, as is illustrated here by figures from Greenwood (1972b) and Krammer (1973) in which the masking stimuli were much narrower. In one figure, given a tone and noise-band masker, a wide array of separated combination bands is generated at lower frequencies, which can mask signals of external origin. They will also either mask, or fail to mask, signal-masker generated combination bands created when an external signal is added in the gap or notch within the two-part primary masker. If signal-masker combination bands are not absolutely everywhere masked by the two-part masker and its combination bands, the signal-masker bands may permit detection of the added signal's presence (thus lowering ‘signal’ threshold). If the masker bands are widened (as by the above corrections, with notch constant), the combination bands generated by them will join and overlap, to mask more uniformly in the lower frequencies, but their distribution will not be the same as if the notch were filled. Whether they will be able everywhere to mask also-wider signal-masker combination bands is the critical question posed by these figures, which indicate that an experimental study, varying notch and bandwidth parameters, would be of considerable interest whatever answer emerged.

Speech recognition in analog multichannel cochlear prostheses: initial experiments in controlling classifications

Computer-synthesized vowels were used to examine methods for controlling and measuring the perceptions elicited during electrical stimulation of the human cochlea. In the first experiment, we measured the importance of the second formant (F2) in the identification of vowels, matched for duration, in a single subject with a multichannel cochlear implant. The subject never confused vowels having a "low" frequency F2 with those having a "high" frequency F2. In the second experiment, identification functions were generated for a series of vowels varying only in F2. When the pattern of F2 stimulation at the basilar membrane was manipulated, vowel identification functions were altered. For the categorization of vowels, the data indicate that the relative cochlear position of F2 stimulation was more important than fine-grain temporal waveform cues. The data are supportive of cochlear implant coding strategies that make use of cochlear place information. In the later experiments, we manipulated filter passbands and channel gains to explore their effect on these classifications. These preliminary studies indicate that it is possible to "fine-tune" such classifications.

Responses of Cat Auditory Nerve Fibers to Biphasic Electrical Current Pulses

Discharge patterns of single auditory nerve fibers were recorded from normal-hearing cats implanted with a 12-band intracochlear electrode array. Stimuli were biphasic current pulses of specifiable width, amplitude, and rate. Acoustic tuning curves were obtained to determine the cochlear positions of the fibers. Response latencies to electrical stimuli formed two groups. Short latency (0.3 to 0.7 ms) responses were attributed to direct activation of spiral ganglion neurons. At high stimulus intensities, these often exhibited abrupt shifts toward even shorter latencies. Long latency (> 1.5 ms) responses were probably caused by electrophonic activation of functional hair cells. Response thresholds to electrical stimuli depended on a fiber's proximity to the stimulating electrodes, and they did not depend on a fiber's acoustic response threshold or spontaneous discharge rate. High intensity (> 1.5 mA) stimuli could excite fibers over a wide range of characteristic frequencies, even for the narrowest (0.45 mm) electrode separations. Response threshold was an exponentially decreasing function of pulse width for widths up to 300 μs/phase. Fiber discharges were highly phase-locked at all suprathreshold intensities, and saturation discharge rates usually equaled stimulus pulse rates for rates up to at least 800 pulses/s. Dynamic ranges were small (1 to 6 dB), increased with pulse rate, and were uncorrelated with electrical response threshold. Within the dynamic range, shapes of poststimulus time and interspike interval histograms resembled those obtained in response to acoustic stimuli. Depolarization block caused fiber activity to cease in 2 to 5 seconds for sustained stimuli presented at high (> 600 pulses/s) pulse rates and intensities.

Preliminary psychophysical results of electrically stimulating the auditory nerve in man

A series of psychophysical tests and speech tests were performed on a patient implanted with a cochlear prosthesis. The device was a multi‐electrode scala tympani insert connected to a percutaneous plug. Threshold data and growth of loudness data will be presented for sinusoidal and pulsatile waveforms and several electrical configurations, i.e., monopolar referenced to an external ground or bipolar pairs of electrodes. The dynamic range for electrical stimulation was about 15 dB, composed of two segments. From threshold to about 10 dB above threshold loudness grows very slowly. In the next 5 dB the loudness grows from very soft to uncomfortably loud. This growth is the same for sinusoidal and pulsatile stimuli, and for apical or basal cochlear stimulation. Pitch rankings were obtained for different electrodes and were consistent with the ranking expected on the basis of cochlear position. Speech reception tests run with single channel stimulation of the multi‐electrode array showed 40% reception of sentence material and 50% reception of open‐set spondee words without lip‐reading cues. [Work supported by grants NS 11804, NS 72367, and NS12998.]

Representation of the cochlea within the anterior auditory field (AAF) of the cat

The anterior auditory field (AAF) is a large auditory representation in the cerebral cortex of the cat rostral to the ‘primary’ auditory cortex (AI) on the middle and anterior ectosylvian gyri. Fine-grained microelectrode maps of unit best frequencies were made in AAF of 16 ketamine anesthetized cats. Among the results were the following. (1) Most units in AAF that were driven by tonal stimuli had tuning curves with a single, sharply definable minimum (the best frequency). Occasionally, units were found to have multiple tuning curve minima. (2) In penetrations normal to the cortical surface of AAF, isolated units had remarkably similar tuning curves througout the middle and deep cortical layers. (3) In most of AAF, a restricted sector of the cochlear partition is represented by a straight belt of cortex crossing the entire field. (4) There is a highly ordered representation of the cochlea within AAF. The cortical belt representing the most basal sector of the cochlea (highest best frequencies) is oriented dorsoventrally along the border with the high frequency region of AI. Proceeding rostrally for some distance with AAF, lower frequency cortical belts are parallel to one another and maintain a highly ordered representation of progressively more apical cochlear sectors. The orientation of the lowest frequency representation usually rotates following the ventral curvature of the anterior ectosylvian gyrus. (5) In the portion of the field caudal to the axis of rotation of the representation, best frequency (represented cochlear position) is a simple (and apparently relatively constant) function of distance from the mutual high frequency AI-AAF border. (6) There is a propootionately larger representation of the highest frequency octaves within AAF. (7) The results of penetrations down the banks of the suprasylvian sulcus indicate that there may be units of vertical organization within AAF similar to those in AI, and in somatosensory and visual cortical fields. (8) The boundaries of AAF as well as the frequency representation within it are highly variable when referenced to cortical surface landmarks. (9) The cytoarchitectonic boundaries of AAF approximately correspond with the physiologically defined boundaries. (10) There were no cells driven to discharge by tonal stimuli in the fields dorsal and ventral to AAF. Comparison of the properties of AAF and AI show that these two fields are remarkably similar in many important features including unit response properties under ketamine anesthesia, short latency to earliest unit discharge, organization in depth, units of vertical organization, size, spatial representation of frequency and proportionately greater representation of higher frequency octaves. They also share some common thalamocortical inputs. These similarities suggest that AAF is not a ‘secondary’ cortical field, but, rather, that AAF and AI are virtually mirror images of one another and are co-participants in the earliest and fundamental processing of acoustic information at the cortical level.

Transmission of radial shear forces to cochlear hair cells.

The radial shear forces transmitted to the cuticular plates of cochlear sensory cells are investigated by means of a mathematical model. The model is based on the fine anatomy of the hair cells and their supporting structures. It predicts that the inner hair cells are stimulated by a viscous force which is linearly proportional to, and in phase with, basilar membrane velocity. The outer hair cells are stimulated by a shear force which is linearly proportional to, and in phase with, basilar membrane displacement at low frequencies (e.g., less than 700 Hz for a midbasal turn cell). At higher frequencies, the shear force transmitted to the outer hair cells is a function of both displacement and velocity. This outer hair-cell shear force is at least an order of magnitude greater than the corresponding inner hair cell force for the frequencies and cochlear positions studied. These results compare favorably to recent cochlear microphonic data from normal and kanamycin-treated guinea pigs for frequencies less than 4000 Hz. Shear force amplitude envelopes are presented for 800- and 1600-Hz pure tones. The results suggest that the “velocity-sensitive” inner hair cells are candidates for “place-mechanism” receptors while the outer hail cells are candidates for “frequency-mechanism” receptors. The model predicts no mechanical fine tuning in the force transmission.

Representation of the Cochlea within the Inferior Colliculus of the Cat: Isofrequency Contours

The three-dimensional pattern of representation of the cochlea within the inferior colliculus has been investigated by determining the “best frequency” of 1846 neurons and 546 neuron clusters studied in 111 penetrations in 20 anesthetized cats. All penetrations were histologically reconstructed. These studies have revealed the following: (1) Auditory responses were encountered in the pericentral nucleus, and a crude tonotopic organization could be demonstrated within it. A reversal or discontinuity in the orderly progression of best frequencies was frequently seen across the boundary of the pericentral and central nucleus, with lowest frequencies encountered along their mutual border. (2) The central nucleus is exquisitely tonotopically organized. Functions relating represented cochlear position to penetration depth derived for penetrations all across the central nucleus were parallel. Reconstruction of rows of penetrations allowed for definition of isofrequency contours and reconstruction of cochlear place representation within the three dimensions of the central nucleus. (3) In penetrations perpendicular to defined isofrequency contours, represented cochlear place was seen to be a near-linear function of penetration depth. These experiments provide the framework for investigation of the functional significance of the highly ordered representation of the cochlear across the three dimensions of the central nucleus of the inferior colliculus. [Supported by NIH Grant NS10414-01.]