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PubMed1983

Auditory cortical field projections to the basal ganglia of the cat.

Projections to the basal ganglia from four auditory cortical fields in the cat were studied by combining microelectrode-mapping of the neurons' best frequencies with autoradiographic and histochemical tract-tracing techniques. Each auditory field is a source of projections to the homolateral basal ganglia. The distribution of labeling within the basal ganglia is related to the cortical field in which the injection site is located. The dorsal portion of the putamen and adjacent caudate nucleus are connected with cortical fields situated anteriorly and dorsally, while the ventral portion of the putamen and adjacent lateral amygdaloid nucleus are related to auditory fields situated posteriorly and ventrally. Injections of two different tracers into different best-frequency loci of one cortical field provided evidence that low best-frequency neurons project medially within the basal ganglia while high best-frequency neurons project more laterally. We concluded that there was a basic similarity among patterns of terminations in the basal ganglia from axons that originate in different auditory cortical fields. When the source of a projection was confined to a restricted portion of an auditory cortical field, labeling appeared as dense patches of silver grains separated from each other by areas of less dense labeling. Often, these patches were distributed within a sheet of tissue, elongated both dorsoventrally and anteroposteriorly. Loci having the same best-frequency representation, but situated in different auditory cortical fields, project upon overlapping but not coextensive portions of a single sheet of tissue. Thus the projections from geographically distant cortical loci possessing similar best-frequency representations are notably distinguished on a topographic basis. By comparison, two adjacent sheets of tissue were labeled when two injections were made into the low best-frequency and high best-frequency representations of the same auditory field. Double-injection, double-tracer experiments revealed that adjacent sheets of tissue received projections from different best-frequency loci. These observations suggested a degree of tonotopic organization to this projection system which was equipoise to the evidence obtained for a topographic organization.

Animals↗
PubMed1979

Scaling of pitch strength.

Pitch strength of the following equally loud sounds was scaled by magnitude estimation procedures: pure and complex tones, band-pass filtered complex tones, AM-tones, low-pass, high-pass and band-pass noise, comb-filtered noise. AM-noise and Zwicker-tone. At the test frequencies 125, 250 and 500 Hz pure tones were assigned a pitch strength of 100%. Relative to this value spectral pitches reach 100--75%, virtual pitches 50% and noise pitches 25--0% pitch strength. At 125 Hz no consistent data were found for the Zwicker-tone, while at 500 Hz it elicits on the average the same pitch strength as a pure tone (100%). Implications concerning pitch mechanisms would seem to suggest a prevalence of the 'place principle' in comparison to the 'time principle'.

Adult↗
PubMed1980

Growth of L(f2--f1) and L(2f1--f2) with input level: influence of f2/f1.

The present investigation examined the effect of f2/f1 (where f1 and f2 are the frequencies of a two-tone input, f2 greater than f1) exerted on the slopes of the functions relating simple difference tone level (L(f2-f1)) and cubic difference tone level (L(2f1-f2)) to input level (L1 = L2, where L1 and L2 represent the levels of the tones at f1 and f2, respectively). Two normal-hearing young adults served as subjects. An adaptive 2AFC nonsimultaneous gap-masking paradigm was utilized to obtain estimates of distortion product magnitude. Parameters of the two-tone input were: f1 = 1550 Hz; f2/f1 = 1.08 and 1.41; and L1 = L2 = 35 to 85 dB SL. Results revealed that slopes for the functions relating L(f2-f1) and L(2f1-f2) to L1 = L2 were approximately 1.0 dB/dB for f2/f1 = 1.08. For f2/f1 = 1.41, however, a slope of 2.0 dB/dB was observed for L(f2-f1) while a slope of 3.0 dB/dB was obtained for L(2f1-f2). It is suggested that some combination of nonlinearities is needed to account for these data.

Acoustic Stimulation↗
PubMed1983

Effects on guinea pig cochlea from exposure to moderately intense broad-band noise.

Guinea pigs, which were either anesthetized (A) or conscious (U), were exposed to four 2 h sessions of broad-band noise of 96 dB SPL. Cochlear microphonics and N1 thresholds were measured prior to killing from 1 to 13 days later. The cochleas were examined by SEM and by section. The U series suffered less N1 threshold loss and recovered within 10-13 days, while the large initial loss in the A series did not completely reverse within the period of study. Initially, the IHC stereocilia in the basal half of the cochlea showed marked bending, the affected area being somewhat more extensive in the A group. These hairs gradually recovered, although not progressively. On the other hand, the disturbance to OHC stereocilia, which appeared to be less pronounced early on and was more apically centered, developed over time into marked permanent damage. The outermost row consistently showed the greatest effect with hairs becoming elongated or fused, and occasionally lost altogether. Susceptibility to noise varied between individuals of both groups. The recovery of N1 threshold was concomitant with the recovery of the erectness of the IHC hairs.

Animals↗
PubMed1983

A quantitative light microscopic study of the bullfrog amphibian papilla tectorium: correlation with the tonotopic organization.

The height, width and cross-sectional area of the bullfrog amphibian papilla tectorial membrane were quantitatively analysed from frontal serial sections. The cross-sectional area (which is a measure of mass) of the tectorium does not appear to be linearly graded along the length of the papilla, but rather spatial gradations occur in more or less a step-wise manner. The spatially graded area (or mass) is well correlated with the width of the tectorium and their relationships with the tonotopic organization of the amphibian papilla are discussed.

Animals↗
PubMed1983

Endolymphatic hydrops in the rabbit: auditory brainstem responses and cochlear morphology.

A rabbit model of endolymphatic hydrops was studied using detailed functional and cytohistologic methods. Immediately following surgical destruction of the endolymphatic sac and the distal portion of the duct, measures of the evoked auditory brainstem response (ABR) revealed mild to profound losses specific to low- and high-frequency test stimuli while responses to mid-frequency signals remained unchanged for the majority of animals. Rabbits exhibited varying degrees of vestibular upset involving both overt behavior and reduced responses to caloric stimulation. Histologic processing of the plastic embedded cochleae demonstrated distended Reissner's membranes along with extensive damage to apical and basal turn sensory cells and myelinated afferent nerve fibers while the middle portion of the cochlear duct remained relatively unaltered. An atypical pattern of hair cell lesions involving a greater loss for inner than for outer hair cells was identified at the interface between damaged apical sensorineural elements and the normal appearing organ of Corti of the middle turns.

Animals↗
PubMed1983

An active process in cochlear mechanics.

A model for cochlear mechanics is proposed to take account of its two systems, one passive and one active. The classical passive system stimulates the inner hair cells directly at levels above about 40 dB SL. At intensities below about 60 dB an active process, the 'cochlear amplifier' (CA), somehow provides additional energy that enhances the vibration of a narrow segment of the basilar membrane near the apical foot of the familiar, traveling wave envelope. The outer hair cells are essential for CA. The active system acts like a high-Q acoustic resonator, and it accounts for the great sensitivity and sharp tuning expressed by the 'tips' of neural tuning curves. The tips are selectively vulnerable to anoxia, noise exposure and other trauma. The CA model explains the detection of small differences in time as well as in frequency, the dual character of the electrocochleogram, recruitment of loudness in cochlear hearing impairment, the long latency of normal neural responses near threshold, acoustic emissions (both stimulated and spontaneous) and the locus of TTS in the frequency range above the exposure tone. Both the classical high-intensity system and the active low-level CA system are highly nonlinear and they combine to compress the great dynamic range of hearing into a much narrower range of mechanical movement of the cilia of the inner hair cells. The mechanism of CA is unknown, and the problem remains of how its action can be triggered by submolecular movements near threshold.

Auditory Threshold↗
PubMed1983

Tonotopic organisation of the auditory midbrain nuclei of the midwife toad (Alytes obstetricans).

The torus semicircularis (TS) of Alytes obstetricans is tonotopically organized. A stereotactic system was used to obtain isointensity responses with a few-units recording method; at each recording site the dominant frequency (that eliciting maximal discharge) was noted. Neurons activated by acoustic stimuli were found in the laminar, principal and magnocellular nuclei; they were rare in the commissural nucleus, and in the subependymal nucleus no stimulus-correlated responses were recorded. High-frequency dominance (greater than or equal to 900 Hz) was found only in a particular region of the torus, extending from caudomedial to rostrolateral, and it was restricted to ventral sites in the caudal and lateral parts of this region. In a few of the more rostral penetrations high-frequency dominance was found at dorsal as well as ventral positions. In a rostromedial area high-frequency neurons predominated over the entire dorsoventral extent of the torus, and in a caudolateral area low-frequency (less than 500 Hz) neurons were similarly distributed. The maximal discharge elicited by high tones proved, almost without exception, to derive from neurons of the laminar and magnocellular nuclei. Low-frequency dominance was found at some positions in these nuclei as well as in the principal and commissural nuclei.

Animals↗
PubMed1984

Single-neuron labeling and chronic cochlear pathology. I. Threshold shift and characteristic-frequency shift.

Iontophoretic injections of horseradish peroxidase were used to label single auditory-nerve fibers in cats with permanent, noise-induced or drug-induced threshold shifts. Using these labeling techniques, it was possible to estimate the degree to which the characteristic frequency (CF) of a neuron shifts as its threshold shifts in cases of chronic cochlear pathology. The original CF of an abnormal neuron was estimated by measuring the location of its peripheral terminal along the cochlear spiral and comparing that to data from normal animals on the relation between CF and cochlear location [12]. Data from 25 labeled neurons with estimated threshold shifts varying from 0 to 70 dB suggest that, at least for units with original CF above 1 kHz, the measured CF shifts down as the threshold rises. The largest CF shifts (0.26 to 0.66 octave) were seen in units contacting inner hair cells within 1 mm of cochlear regions in which the entire organ of Corti (including supporting cells) was destroyed. For the abnormal units located more than 1 mm from such regions, the discrepancy between the CF predicted on the basis of fiber location and the CF measured from the tuning curve was less than 0.18 octave even though threshold shifts were comparable in the two groups. Units with the largest CF shifts showed abnormally low slopes on the portion of the tuning curve immediately above CF. This observation suggested a means of identifying abnormal units with significant CF shifts by simple inspection of the tuning curve. The possibility that the degree, and even the direction, of the CF shift varies from high- to low-CF units is discussed.

Animals↗
PubMed1984

Single-neuron labeling and chronic cochlear pathology. II. Stereocilia damage and alterations of spontaneous discharge rates.

The spontaneous discharge rates (SRs) sampled from auditory-nerve fibers in cases of chronic cochlear pathology are often abnormally low [17]. The application of intracellular labeling techniques to noise-exposed ears makes it possible to accurately correlate fiber populations showing SR abnormalities with the cochlear locations from which these responses originate. The correlations reveal that a decrease in the mean rates of spontaneous discharge is typically associated with selective loss of the tallest row of stereocilia from the inner hair cells. In cochlear regions where virtually all of the tall stereocilia are missing from the inner hair cells, the maximum rates of spontaneous discharge are less than 1/3 normal values. We suggest that the loss of tall stereocilia causes the decrease in SR because much of the resting current in the inner hair cell normally flows through the stereocilia membrane. Thus, the loss of that membrane leads to a hyperpolarization of the inner hair cell which, in turn, decreases the spontaneous release of vesicles at the synapse. An interpretation is also suggested for the "compression" of the SR distribution commonly seen among high-frequency neurons in normal animals [9].

Animals↗
PubMed1984

Theoretical and experimental analysis of a central optimal processor for pitch of multicomponent inharmonic tones.

This paper describes some experiments on the pitch produced by multicomponent tones. Stimuli used were the sum of a pure tone at frequency f = 1 kHz and of the first six successive subharmonics at frequencies psi i = f/i (i = 2, 3, 4, 5, 6, 7) or, in separate experiments, the sum of six (i = 2, 3, 4, 5, 6, 7), five (i = 3, 4, 5, 6, 7) or four (i = 4, 5, 6, 7) subharmonics, respectively. Pitch data produced by these stimuli have been compared with the predictions of the optimum processor theory developed by Goldstein and coworkers (Goldstein, J.L. (1973): J. Acoust. Soc. Am. 54, 1496-1516; Goldstein, J.L. et al. (1978): J. Acoust. Soc. Am. 63, 486-497; Gerson, A. and Goldstein, J.L. (1978): J. Acoust. Soc. Am. 63, 498-510). Model predictions are shown to deviate significantly from the experimental results. By assuming the existence of an additional transformation to be included into the original mathematical model, without any other modification, predictions are in complete agreement with pitch data.

Acoustic Stimulation↗
PubMed1985

Progressive ototoxicity of neomycin monitored using derived brainstem response audiometry.

Progressive hearing loss following the systemic administration of neomycin was investigated using derived brainstem response audiometry. Cats were given three to five times the maximum recommended clinical dose of neomycin over a period of 10 days. Their hearing was monitored prior to and during the administration of the drug, and periodically following its completion. The results of this study showed that the induced hearing loss generally proceeded from high to low frequencies as an advancing lesion, with regions apical to the lesion functioning normally. Although considerable variability in response to the drug existed among animals, the evoked responses from both ears of each animal showed close bilateral symmetry during the deafening process. Furthermore, the present results highlight the long-term ongoing ototoxicity associated with neomycin, and the importance of monitoring high frequencies for initial signs of an aminoglycoside induced hearing loss.

Animals↗
PubMed1985

Role of inner and outer hair cells in mechanical frequency selectivity of the cochlea.

Resonant frequencies of inner (IHC) and outer (OHC) hair cell systems in the guinea pig cochlea were computed using data on sensory-hair stiffness obtained from in vitro organ of Corti preparations (D. Strelioff and A. Flock (1984): Hearing Res. 15, 19-28). IHC stereocilia were modelled as stiff, free-standing, uniform cylinders which rotate about their elastic attachments to the apical surfaces of IHC. OHC, with the overlying tectorial membrane (TM), were modelled as a resonant mechanical system with the TM providing mass and the three rows of OHC sensory-hair bundles providing elasticity for linear, simple harmonic motion parallel to the reticular lamina. Since computed IHC resonant frequencies increase from 128 kHz at the apex to 300 kHz at the base, it is unlikely that they contribute to frequency selectivity. In contrast, computed frequencies of the OHC-TM system are within the audio range, increasing from 1.2 kHz at the apex to 22 kHz at the base. The results of these computations support the hypothesis that the OHC-TM system contributes to mechanical frequency selectivity of the cochlea whereas IHC are passive receptors which respond to mechanical movements of the cochlear partition.

Animals↗
PubMed1985

Ontogenic changes in cochlear characteristic frequency at a basal turn location as reflected in the summating potential.

In these experiments the development of summating potential (SP) responses in gerbils from neonates to adults was followed. Special recording techniques were used to eliminate maturational effects associated with the middle ear so that developmental changes in cochlear physiology were isolated for study. Results indicate that as development proceeds the frequency that maximally excites the basilar membrane (BM) of the gerbil at a specific mid-basal turn electrode location progresses from low to high, demonstrating a 1.5 octave shift from the onset of the generation of electrical activity until adult-like response are obtained. These findings support the theory proposed by E.W. Rubel (in: Handbook of Sensory Physiology, Vol. IX: Development of Sensory Systems, pp. 135-237. Editor: M. Jacobsen. Springer-Verlag, New York) which explains the observed development of physiological responses measured in the cochlea and higher centers in terms of changing micromechanical transduction properties of the BM.

Aging↗
PubMed1985

Brainstem, whole-nerve AP and single-fiber suppression in the gerbil: normative data.

Suppression of gerbil brainstem responses (BSRs) and whole-nerve AP responses was studied by means of a forward masking procedure in which a tone-burst probe was preceded by a narrow-band masker. The effectiveness of the masker in reducing the brainstem response to the probe can be diminished by presenting a tone burst simultaneously with the masker. By varying the frequency and intensity of the third stimulus, BSR suppression areas can be determined. These flank the tails and high-frequency sides of BSR tuning curves in a manner similar to the suppression areas of AP tuning curves. The shapes and sizes of the BSR and AP suppression areas vary greatly across probe frequencies and animals. However, the lower boundaries of suppression areas associated with the tails of the tuning curves occur at similar absolute levels regardless of probe frequency or tuning curve shape. The BSR and AP suppression areas are in some respects similar to areas of two-tone rate suppression in single auditory nerve fibers of the gerbil.

Animals↗
PubMed1985

Single-unit characteristics in the auditory midbrain of the immobilized grassfrog.

The anuran auditory midbrain of the grassfrog (Rana temporaria L.) was studied by a combined spectro-temporal analysis of sound preceding neural events. From the spectro-temporal sensitivities (STS) estimates of best frequencies (BF) and latencies (LT) were derived. Several types of STSs were observed: monomodal excitatory STSs comprised about half of the cases. Bimodal excitatory STSs, i.e. STSs with two discrete excitation regions, were observed in about 25%. Trimodal and broadly tuned STSs comprised about 5%. The remaining 20% of the STSs were characterized by inhibitory phenomena such as pure inhibition, sideband inhibition and post-activation inhibition. The distribution of best frequencies matches the frequency spectrum of the animal's vocalizations. A relative absence of monomodal units was noted in the mid frequency range. The distribution of latencies was bimodal over the range 7-108 ms. For each unit 6 functional parameters were determined; besides BF and LT these were: form of the STS (i.e. monomodality versus multimodality), spontaneous activity, binaural interaction, and firing mode (i.e. sustained versus transient) upon continuous noises stimulation. In addition, two structural parameters were considered: location in the torus and action potential waveform. Large correlations appeared between LT and action potential waveform, and between BF and binaural interaction type. Tonotopy was not found. A comparison was made between results from this study with a previous study on lightly anesthetized grassfrogs, using the same stimulus paradigms (D.J. Hermes et al. (1981): Hearing Res. 5, 147-178; D.J. Hermes et al. (1982): Hearing Res. 6, 103-126). Spontaneous activity, inhibitory phenomena and complex STSs were common using immobilization, whereas these have hardly been observed using anesthesia. Furthermore, interdependencies between the neural characteristics are substantially weaker for the immobilized preparation.

Animals↗