Integrative Functions in the Mammalian Auditory Pathway (Springer Handbook of Auditory Research)

Donata Oertel
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Book Binding:Hardback. World of Books USA was. Fay from Waterstones today! Click and Collect from. A summary. Webster, Arthur N. Synapses in the outermost layer of the dorsal cochlear nucleus in contrast, are weakened or strengthened by coordinated activity. In mutant mice, the function of these neuronal circuits is altered.

We are using mutant mice to learn how these neuronal networks change in animals. In the cochlear nuclei we address questions at several different levels. At the cellular and molecular levels we seek to understand how synaptic function and biophysical properties of neurons are regulated.

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At the systems level we want to understand how the brain begins to extract information about the location and meaning of sounds. We also seek to gain a better understanding of how hearing loss affects the function of the neuronal networks in the cochlear nuclei.

U niversity of W isconsin —Madison. Golding N, Oertel D. Synaptic integration in dendrites: Exceptional need for speed. J Physiol. Hearing Res Oertel D. GluA4 sustains sensing of sounds through stable, speedy, sumptuous, spineless synapses. J Physiol Lond Young,E.

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Cochlear Nucleus. In Handbook of Brain Microcircuits , G. Shepherd, and S. Grillner, eds. New York: Oxford , pp.

Auditory Transduction (2002)

Temporal processing in the auditory pathway. Oertel D, Young ED. TINS Young ED, Oertel D.

The Cochlear Nucleus. In addition to a strong rate profile that encodes changes in fluctuation amplitudes across frequency channels, many IC neurons have exceptional phase-locking to the fluctuation frequency Joris et al. Midbrain rate profiles may be adequate to encode spectral peaks for mid-level sounds in quiet. However, in background noise, the phase-locked responses may improve coding of complex sounds.

For example, the phase-locking thresholds of budgerigar midbrain neurons to fluctuations provide the best neural correlate for their human-like behavioral thresholds for formant frequency discrimination in background noise Henry et al.

Because these profiles involve the level-dependence of IHC transduction, enhancing them would require control of gain in the cochlea. The auditory efferent system provides just such a control system. The auditory efferent system has typically been considered to be an SPL-driven gain control system that modulates cochlear amplification via MOC projections to outer hair cells and manipulates afferent sensitivity via the LOCs. This hypothesis differs from the usual functions ascribed to the efferent system: protection from loud sounds e.

Therefore, this system cannot provide precise gain control for rate-based coding across a significant portion of the auditory dynamic range. However, by always pulling the operating point of IHC transduction towards the sweet spot, the efferent system could enhance fluctuation profiles over a relatively wide range of levels.

The proposed function for the auditory efferent system is consistent with evidence that it may improve detection in noise e. To enhance fluctuation profiles, the MOC system would function as follows: in the presence of high-level stimuli or background noise, where differences in fluctuation amplitudes across frequency channels would be reduced by increased IHC saturation, the MOC signals act to reduce cochlear gain. Reducing the gain would reduce the amount of IHC saturation in frequency channels with relatively low spectral amplitudes, acting to increase the fluctuation amplitude differences across channels.

In the presence of very low-level sounds, maximal cochlear gain is the best strategy for increasing fluctuation differences. If the efferent system acts to maintain or enhance fluctuation profiles, especially in the presence of background noise and changes in overall level, then deficits to this system could explain reported problems in supra-threshold speech intelligibility in background noise.

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Reduction of the differences in fluctuation amplitude across frequency channels would decrease the information encoded in fluctuation profiles. This deficit would apply to listeners with reduced numbers of AN fibers due to synaptopathy, whether due to age or noise exposure. Identification of a deficit related to cochlear gain control would require a paradigm that explicitly stresses this control system, perhaps explaining the difficulty of identifying synaptopathy using standard psychophysical tasks. Broadly speaking, the hypothesized efferent mechanism requires the known ascending and descending anatomical connections Fig.

Schematic of auditory efferent pathways, illustrating descending connections from the IC and auditory cortex onto the regions of the brainstem where MOC and LOC cell bodies are clustered from Terreros and Delano ; reprinted with permission. Uncrossed projections descending from the auditory cortex are indicated by solid lines and crossed projections by dotted lines. For the efferent system to act as a profile-driven feedback system requires not only information about the overall stimulus sound level but also signals representing the fluctuation profile itself. Indeed, the IC, with discharge rates that are sensitive to low-frequency fluctuations, provides a major input to the MOC region of the brainstem Thompson and Thompson ; Schofield and Cant ; Terreros and Delano ; Cant and Oliver , reviews: Warr ; Schofield MOC neurons have bandpass modulation transfer functions Gummer et al.

These two actions would tend to increase the differences across channels in fluctuation amplitudes, enhancing fluctuation profiles. Thus, although much detail about the specifics of projection neurons and brainstem targets remains unknown, the general circuitry is in place for the MOC component of the efferent system to act as a fluctuation profile-driven system.

The effects of the efferent system on fluctuation profiles during running speech are constrained by the dynamics of this system. Durations of phonemes vary considerably, e. Thus, the efferent system could affect fluctuation profiles within the timecourse of the phoneme, especially for longer phonemes, and changes in cochlear gain in response to any phoneme would be expected to affect fluctuation profiles in response to the subsequent phoneme. Most tests of the auditory efferent system have used electrical stimulation, tones, or noise review: Guinan , and these stimuli would elicit relatively impoverished fluctuation profiles.

Several psychophysical studies have used precursor stimuli to explore the relatively long-latency effect of the efferent system on detection or discrimination tasks e. A few of these studies have used notched noise precursors in studies of masked detection Carlyon ; Strickland or amplitude modulation AM detection Almishaal and Jennings Another strategy for testing the efferent system takes advantage of the ability of contralateral stimuli to suppress otoacoustic emissions via the efferent system.

Suppression studies using AM noise Maison et al. The influence of attention on the action of the efferent system has been suggested in several studies e. Fluctuation profiles provide a new framework for approaching this topic. The anatomical pathways are in place: in addition to direct projections from cortex to brainstem, the cortex also projects to the IC and thus could potentially gate or modulate the response profiles in the midbrain that in turn descend to the brainstem-level efferent system e.

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Acoustic chiasm IV: Eight midbrain decussations of the auditory system in the cat. Lai, J. Brain 3 , — Search within As mentioned earlier, some interval-counting neurons show band-suppression characteristics when tested with sinusoidal AM stimuli.

That is, the presence of different modulation frequency channels, consistent with both psychophysical e. Further work is required to test hypotheses related to the interactions of fluctuation profiles in attention and speech segregation. Such studies are especially challenging as they require systematic manipulation of attention. Indeed, commonly occurring hearing deficits would be expected to influence fluctuation profiles in different ways Fig.

Reduced cochlear sensitivity would reduce the differences in fluctuation amplitudes across channels in the peripheral representation of sounds Carney et al.

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Finally, central mechanisms at the midbrain level that convert fluctuation profiles into rate and envelope-locked representations likely involve interactions between excitation and inhibition Nelson and Carney ; Gai and Carney , ; review: Davis et al. Schematic illustration of hypothesized fluctuation-profile-driven gain control system.

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Effects of various impairments are indicated in red boxes. All projections would be tonotopically organized not shown. Details of crossed and uncrossed MOC projections are not shown see Fig.

While challenges in supra-threshold hearing for listeners with normal thresholds have received recent attention, it has long been known that listeners with measurably elevated thresholds, as in sensorineural hearing loss SNHL , have difficulty with supra-threshold sounds, especially speech in background noise. For example, SNHL would reduce differences in fluctuation amplitudes across channels in peripheral responses: for a stimulus at a given level, decreased cochlear gain due to OHC impairment would decrease the degree of IHC saturation near spectral peaks and thus reduce the differences across channels tuned near spectral peaks.

Reduced endocochlear potential EP would result in decreased sensitivity of both OHCs and IHCs, further reducing cochlear gain and shifting the operating point of the IHC saturating nonlinearity to higher sound levels, again reducing the differences in fluctuation amplitudes across frequency channels Fig.