Afferent Auditory Pathway Research Articles

Topographic and temporal indices of vowel spectral envelope extraction in the human auditory cortex.

The auditory-evoked neuromagnetic field elicited by single vowel formants and two-formant vowels was recorded under active listening conditions using a 37-channel magnetometer. There were three single formants with formant frequencies of 200, 400, and 800 Hz, another single formant with a formant frequency of 2600 Hz, and three vowels that were constructed by linear superimposition of the high- onto one of the low-frequency formants. P50 m and N100 m latency values were inversely correlated with the formant frequency of single formants. A strong effect of formant frequency on source location was obtained along the postero-anterior axis, which is orthogonal to the well-established latero-medial tonotopic gradient. Regardless of whether single formants or first formants of vowels were considered, N100 m sources were more anterior and sustained field sources were more posterior for higher-frequency than for lower-frequency formants. The velocity of the apparent posterior-to-anterior movement across cortical surface of N100 m sources first reported by Rogers et al. [Rogers, R. L., Papanicolaou, A. C., Baumann, S. B., Saydjari, C., & Eisenberg, H. M. (1990). Neuromagnetic evidence of a dynamic excitation pattern generating the N100 auditory response. Electroencephalography and Clinical Neurophysiology,77, 237-240] decreased as a function of latency. The amount of deceleration was positively correlated with formant frequency. Responses to the vowels were superadditive, indicating that the processes elicited by the constituents of composite stimuli interact at one or more stages of the afferent auditory pathway. Such interaction may account for the absence of a lateral-to-medial tonotopic mapping of first formant frequency. The source topography found may reflect activity in auditory fields adjacent to AI with the strength of the contribution varying with formant frequency. Alternatively, it may reflect sharpness-of-tuning and inhibitory response-area asymmetry gradients along isofrequency stripes within AI. Either alternative may be interpreted in terms of a spectral blurring mechanism that abstracts spectral envelope information from the details of spectral composition, an important step towards the formation of invariant phonetic percepts.

A gain-control mechanism for processing of chorus sounds in the afferent auditory pathway of the bushcricket Tettigonia viridissima (Orthoptera; Tettigoniidae).

The representation of alternative conspecific acoustic signals in the responses of a pair of local interneurons of the bushcricket Tettigonia viridissima was studied with variation in intensity and the direction of sound signals. The results suggest that the auditory world of the bushcricket is rather sharply divided into two azimuthal hemispheres, with signals arriving from any direction within one hemisphere being predominantly represented in the discharge of neurons of this side of the auditory pathway. In addition, each pathway also selects for the most intense of several alternative sounds. A low-intensity signal at 45 dB sound pressure level is quite effective when presented alone, but completely suppressed when given simultaneously with another signal at 60 dB sound pressure level. In a series of intracellular experiments the synaptic nature of the intensity-dependent suppression of competitive signals was investigated in a number of interneurons. The underlying synaptic mechanism is based on a membrane hyperpolarization with a time-constant in the order of 5-10 s. The significance of this mechanism for hearing in choruses, and for the evolution of acoustic signals and signalling behaviour is discussed.

Strategies for hearing in noise: peripheral control over auditory sensitivity in the bushcricket sciarasaga quadrata (Austrosaginae: tettigoniidae)

The carrier frequency of the call of the Australian bushcricket Sciarasaga quadrata is unusually low for tettigoniids at 5 kHz. The sound transmission measured in the habitat of the insect reflects the advantages of producing low- rather than high-frequency signals; attenuation is explained almost entirely by the spherical spreading of sound. The natural vibration frequency of the wings is controlled by size and stiffness. The small tegmina are unusually fleshy, with an order-of-magnitude higher water content than in comparable sagine species. Reduced stiffness allows the insect to call at low carrier frequencies, albeit at lower intensity levels (60 dB SPL at 1 m), than bushcrickets of comparable size (80-90 dB SPL). The responses of the tympanic nerve and a first-order interneurone (omega neurone) in the afferent auditory pathway showed that the hearing system is most sensitive to frequencies of 15-20 kHz, an effective mismatch to the conspecific call resulting in a reduced sensitivity of approximately 20 dB at the carrier frequency of the call. S. quadrata can occlude its spiracular opening, which increases the sensitivity of the ear to lower frequencies. Under such conditions, the best frequency of the ear matched that of the carrier frequency of the call. We measured the activity of auditory neurones in the field, noting the ability of the open and partially closed ear to filter out potentially masking calls of congenerics. At the same time, the directionality of the system was only slightly reduced in the closed relative to the open spiracle status. We discuss the evolutionary advantages of an insect both calling with a low carrier frequency and having the ability to close down the tracheal system to avoid the effects of masking. In addition, we consider the advantages of such a signalling system in avoiding the most significant predator of the male, the ormine fly Homotrixa alleni. <P>

Effect of contralateral auditory stimuli on active cochlear micro-mechanical properties in human subjects

The present study investigates the possibility that contralateral auditory stimulation along medial efferent system pathways may alter active cochlear micromechanics and hence affect evoked oto-acoustic emissions in humans. A first experiment, involving 21 healthy subjects showed reduction of oto-acoustic emission amplitude under low intensity contralateral white noise (from 30 dB SPL, 10 dB SL, upwards). The effect is found for intensities below the acoustic reflex threshold (85.2 dB HL). A second experiment, involving 10 of the above 21 subjects, sought to rule out any technical artefact. Recording was again carried out, but after sealing of the contralateral ear with a silicon putty plug. No contralateral intensity effect on oto-acoustic emission amplitude was found for contralateral intensities below 65 dB SPL. In subjective perception terms (dB SL) an effect was found under sealing when the sound reached or passed above the 10 dB SL level. These two findings confirm the preceding experiment. The third experiment investigated the role of transcranial transmission of the contralateral auditory stimulus. 16 subjects having total unilateral deafness and one healthy ear were tested by the same procedure as above. No fall-off in oto-acoustic emission amplitude was found for contralateral stimuli equal to or less than 80 dB SPL. There is thus a contralateral auditory stimulus effect on active cochlear micromechanics. The most appropriate explanation involves the medial cochlear efferent system, excited at brainstem level via the afferent auditory pathways. Alteration of active cochlear micromechanics seems promising at a basic level, pointing, as it does, to an interactive cochlear functioning which can be investigated by simple, non-intrusive, objective techniques which can be used with human subjects. We have here a model for functional exploration of the medial olivocochlear efferent system.

Effects of increased ICP on auditory and cardiovascular responses

Background: Auditory brainstem responses (ABRs) have been previously investigated in subjects with Down's syndrome (DS), but the published data are generally from children with hearing loss. The aim of this study was to evaluate the hearing pathway in normal hearing adult DS patients. Methods: We used ABRs to analyze absolute and interpeak latencies in 19 adult DS patients aged 18–45 years whose pure tone audiometry (PTA) test results indicated thresholds within normal limits, and 19 normal controls. Results: The DS sample showed statistically significant gender-related differences in interpeak interval III–V (p = 0.015). The latencies of waves III and V, and interpeak intervals III–V and I–V, were significantly shortened in the DS patients than in the controls. Conclusions: Our findings may be due to the smaller brain sizes and simpler afferent auditory pathways of DS subjects.

The Physiological Basis of Hearing: A Review

This is a unique book of outstanding importance to students, researchers, and, especially, otologists. It crystallizes electrophysiology from the cochlea to the cortex and provides a solid foundation for understanding the auditory system. The authors have put together and simplified complex concepts and research, including 1,269 references. The major chapters discuss the physiology of the inner ear, the physiology of the afferent auditory pathway, and slow evoked potentials. This last chapter is exceptionally important for practicing otolaryngologists and may provide ideas on which to base future research for new developments on tinnitus, nerve deafness, and other problems still confronting otologists. The numerous figures included are clear and illustrate the text most effectively. Anyone interested in the ear will benefit by repeated reading of this excellent textbook.

The physiology of ascending auditory interneurons in the tettigoniidCaedicia simplex (Orthoptera: Ensifera): response properties and a model of integration in the afferent auditory pathway

1. Single unit recording from the oesophageal connectives was used to characterise the physiological responses to monaural stimulation of ascending auditory interneurons in the tettigoniidCaedicia simplex. 2. In terms of characteristic patterns of spike discharge, ascending auditory interneurons have been classified as tonic, phasic-tonic or phasic (Fig. 1). 3. According to suprathreshold, isoresponse criteria, tonic interneurons either are broadlytuned between 10–16 kHz or are more narrowlytuned to a single sound frequency below 10 kHz (Fig. 2A). All tonic interneurons recorded have a roll-off in sensitivity of 20–40 dB/octave above and below their frequencies of maximum sensitivity. This corresponds with the roll-offs applicable to primary afferent fibres. 4. Most phasic-tonic interneurons display uniform sensitivity to one of three different frequency bands (about 3 kHz in width) in the range 10-18 kHz (Fig. 2B). These three classes of phasictonic interneuron have roll-off in sensitivity of more than 50 dB/octave for sound frequencies below their range of maximum sensitivity. 5. In contrast, a distinct class of phasic-tonic interneuron is sharply-tuned to 16 kHz, the species' call frequency (Fig. 3 A). The roll-off in sensitivity of this class of interneuron 40–50 dB/octave for frequencies above and greater than 50 dB/octave for frequencies below 16 kHz. For frequencies below 16 kHz, this roll-off in sensitivity exceeds that of primary afferent fibres by a factor of two (Fig. 3B). 6. Phasic interneurons each are tuned to a single sound frequency in the range 10–16 kHz and have roll-off's in sensitivity of 40–50 dB/octave above and 20–30 dB/octave below their frequency of maximum sensitivity (Fig. 2C). 7. For tonic interneurons, a positive, approximately linear relationship exists between sound intensity and spike response that is similar for different sound frequencies (Fig. 4 A). 8. Phasic-tonic interneurons have a complex intensity-response characteristic that is dependent on sound frequency (Fig. 4B). For frequencies within or above the range of maximum sensitivity, the response increases at a rate of 6 spikes/20 dB, saturates at 10–12 spikes/stimulus and may then decline to as little as 2–3 spikes/stimulus at the higher sound intensities. For frequencies below the band of highest sensitivity the gradient, maximum response and dynamic range of the intensity characteristic progressively decrease. 9. Phasic interneurons produce a maximum of 2–4 spikes/stimulus and, as with tonic interneurons, the intensity characteristic is independent of sound frequency (Fig. 4C). 10. Two-tone experiments demonstrate that phasic-tonic interneurons are inhibited by a sensory response to sound frequencies below their band of highest sensitivity (Fig. 5) and that the onset of this inhibition is delayed by 20 ms, relative to that of excitation (Fig. 6). Similar two-tone experiments conducted on primary afferent fibres demonstrate that no such inhibition occurs at the periphery (Fig. 7). 11. Based on idealized frequency and intensity characteristics of primary afferent fibres (Fig. 8) derived from empirical data (Oldfield 1982, 1983) and on the characteristics of monaurally-stimulated ascending interneurons, a simple model of integration is developed, in which the response of each class of ascending interneuron is attributed to the sum of the inferred excitatory and inhibitory inputs provided by an array of separately-tuned primary afferent fibres. The model accurately predicts the responses of ascending auditory interneurons inC. simplex for the monaural case (Fig. 9).

Estimates of essential neural elements for stimulation through a cochlear prosthesis.

Electrical stimulation of afferent auditory pathways through electrodes placed within and outside of the cochlea were used to study stimulation and design parameters relevant to a cochlear prosthesis. In the acute guinea pig preparation, the tract response evoked in brachium of the inferior colliculus by electrical stimulation to an ear provided estimates of the effectiveness of various electrode placements. Stimulation between an electrode in the cochlea and a site along the eighth nerve was characterized by the lowest thresholds. Stimulation between intracochlear electrodes was somewhat less effective, and stimulation between external electrodes at the nerve, cochlear nucleus, or distant point was least effective. Thresholds, expressed as current, rose at approximately 6 dB per octave for stimulus frequencies from 1 kHz to 16 kHz. Thresholds below 10 microA rms were seen for optimal placements. These observations suggest that the neural elements being stimulated are the cell bodies of the spiral ganglion cells.

The response of the acoustic reflex as a function of the intensity and temporal characteristics of pulsed stimuli

The response of the acoustic reflex in normal human subjects was measured as a change in the complex acoustic admittance of the eardrum for pulsed stimuli at various intensities and with various durations. Both rectangular and exponentially decaying pulse shapes were used. Thus, detailed quantitative information was obtained regarding the relative effects of stimulus intensity, duration and pulse shape on the magnitude of the reflex response. The magnitude of the acoustic reflex response was found to increase monotonically with increasing stimulus intensity and duration, as might be anticipated from previous studies of the reflex threshold alone. The response was found to be greater for rectangular pulses than for exponentially decaying pulses with the same peak intensity and total duration. The dependence of the acoustic reflex response magnitude on both stimulus intensity and duration was compared with the temporal summation of loudness which has been observed in many past psychoacoustical experiments. The two phenomena were found to be quantitatively different, suggesting that, contrary to previous assumptions, they are not the same process. Temporal summation of the acoustic reflex response was also investigated by analysis of the time-course of the response, using a modelling approach. Comparison of this analysis with previous electromyographic data indicated that the temporal summation of the reflex is probably mainly a function of the transduction of neural excitation into mechanical force within the stapedius muscle itself, rather than summation of neural excitation within the afferent auditory pathway. The absolute measurements of the conductance and susceptance of the eardrum at 220 and 660 Hz allowed a comprehensive description to be made of the changes in eardrum admittance caused by the various levels of reflex response. This information had not been available previously. At both frequencies, the effect of the reflex could be represented approximately by an increase in eardrum reactance, with resistance remaining constant. This simplified description should benefit the diagnostic interpretation of acoustic reflex responses.

Effects of visual attention on the intensity of auditory evoked potentials

Click-evoked potentials were recorded from the round window (cochlear microphonic and auditory nerve), cochlear nucleus, and auditory cortex of unanesthetized cats during periods of visual attention and increased auditory intensity. The clicks (irrelevant stimuli) were presented continuously as background before, during, and after the presentation of a visual discrimination task (relevant stimuli) which attempted to alter the attentive state of the animals. At all electrode sites, the mean peak-to-peak amplitudes of click-evoked potentials were significantly smaller during attention to the visual discrimination stimuli when compared with the pretest and posttest control periods. Although the amplitudes of the click-evoked potentials were suppressed at all intensities during visual attention, much greater suppression occurred at low auditory intensities than at high auditory intensities. The results suggest that during attention, a central inhibitory mechanism suppresses irrelevant auditory stimuli presumably via the olivo-cochlear bundle at the peripheral stages in the afferent auditory pathways.

The afferent auditory pathway in the ventral cord oflocusta migratoria (Acrididae)

The afferent auditory pathway in the region of the ventral cord ofLocusta migratoria has been studied electrophysiologically. Various recording, stimulating and surgical techniques have been used to infer the circuitry underlying the response patterns of the ventral-cord neurons ascending to the supraesophageal ganglion. The stimuli used were artificial sounds (white noise and pure tones varying in frequency, intensity, duration, source direction and repetition rate). The responses of the auditory ventralcord neurons over the range of frequencies and intensities are described. Presentation of two sound stimuli occurring simultaneously or displaced in time permits the demonstration of inhibitory and subthreshold facilitatory influences which, with extracellular recording, cannot be observed directlyvia changes in membrane potential (Figs. 3 and 4). On the basis of comparisons of responses before and after inactivation of a tympanal organ additional inferences are made concerning the relative contributions of the two tympanal organs to the formation of the response-patterns (Figs. 6, 11, 16, 21, 25). The geometry, location and course of certain auditory ventral-cord neurons have been revealed by a combined recording and staining technique (Fig. 7); transection of connectives has made it possible to determine the origin of certain response-patterns and whether these change at various stations of the ventral cord as a result of corresponding synaptic input. The results of such experiments, together with the knowledge of the course and location of endings of the tympanal receptor fibers in the ventral cord, have been used to construct sufficient connectivity diagrams for the ascending ventral-cord neurons (Figs. 8, 12, 17, 22, 26). The following general properties have been revealed: 1. Between receptor cells and ascending auditory ventral-cord neurons, as a rule, interneurons are interposed; this accounts for the observation that the postsynaptic structures of the ventral-cord neurons are for the most part situated outside the auditory neuropile. 2. Six basic connectivity patterns underlie the responses of the 14 ventral-cord neurons described; that is, the same response patterns are produced in sets of two or three ventral-cord neurons, owing to synaptic connectivities involving the same interneurons. The distinctions between the response patterns of the 14 neurons are interpretable as stemming from the addition of another interneuron to, or by elimination of a facilitatory or inhibitory connection from, the basic connectivity patterns.

Degenerative alterations in the ventral cochlear nucleus of the guinea pig after impulse noise exposure

Guinea pigs were exposed to the noise of 40 shots of an alarm pistol held at a distance of about 60 cm. The ventral cochlear nuclei were studied in phase contrast and electron microscopy after both survival periods and longer periods of up to 55 days survival. Marked degeneration of primary cochlear nerve endings and of synapting secondary neurons of the posterior caudal part of the ventral cochlear nucleus (AVCN) and the octupus cell area (OCA) of the posterior ventral cochlear nucleus (PVCN) was found most distinctly after 5-55 days. As criteria of degeneration of the second neuron of the afferent auditory pathway we used: 1. The loss of the synapting nerve endings, mainly 'shrinking". 2. The formation of huge mitochondria in the second order neurons and their dendrites. 3. The phagocytosis by glial cells of nerve endings, of the second order neurons and of their dendrites. After 5 days survival time no distinct changes were found in the granular cell area of PVCN, where as all stages of degeneration could be found in OCA at this time. In the discussion of these findings it is concluded that additional studies of the morphology of the cochlear nuclei seem necessary, as these may lead to a better understanding of the pathology of hearing following heavy noise exposure.

Cytoarchitecture of the cochlear nuclei. Report of an illustrative case of erythroblastosis.

The cochlear nucleus in the human is divided into dorsal, superior ventral, and inferior ventral. Six component nerve cell types are recognized. Spheroid cells populate mainly the superior ventral nucleus. They appear to constitute the second-order neurons of the main afferent auditory pathway. They are especially sensitive to pathogenic factors. Their injury appears to be the pathologic basis for sensorineural hearing loss of central or so-called nuclear type. The superior ventral nucleus appears to show high-frequency dorsal to low-frequency ventral tonotopicity. The nucleus exhibits a high-tone vulnerability comparable to that of the organ of Corti. This nucleus may be used histopathologically as an index of injury of the cochlear nuclear complex. It can be divided into dorsally progressive strata whose degree of injury can be correlated with audiogram.