Auditory Afferents Research Articles

Contralateral Noise Stimulation Delays P300 Latency in School-Aged Children.

Background and ObjectiveThe auditory cortex modulates auditory afferents through the olivocochlear system, which innervates the outer hair cells and the afferent neurons under the inner hair cells in the cochlea. Most of the studies that investigated the efferent activity in humans focused on evaluating the suppression of the otoacoustic emissions by stimulating the contralateral ear with noise, which assesses the activation of the medial olivocochlear bundle. The neurophysiology and the mechanisms involving efferent activity on higher regions of the auditory pathway, however, are still unknown. Also, the lack of studies investigating the effects of noise on human auditory cortex, especially in peadiatric population, points to the need for recording the late auditory potentials in noise conditions. Assessing the auditory efferents in schoolaged children is highly important due to some of its attributed functions such as selective attention and signal detection in noise, which are important abilities related to the development of language and academic skills. For this reason, the aim of the present study was to evaluate the effects of noise on P300 responses of children with normal hearing.MethodsP300 was recorded in 27 children aged from 8 to 14 years with normal hearing in two conditions: with and whitout contralateral white noise stimulation.ResultsP300 latencies were significantly longer at the presence of contralateral noise. No significant changes were observed for the amplitude values.ConclusionContralateral white noise stimulation delayed P300 latency in a group of school-aged children with normal hearing. These results suggest a possible influence of the medial olivocochlear activation on P300 responses under noise condition.

What the Toadfish Ear Tells the Toadfish Brain About Sound.

Of the three, paired otolithic endorgans in the ear of teleost fishes, the saccule is the one most often demonstrated to have a major role in encoding frequencies of biologically relevant sounds. The toadfish saccule also encodes sound level and sound source direction in the phase-locked activity conveyed via auditory afferents to nuclei of the ipsilateral octaval column in the medulla. Although paired auditory receptors are present in teleost fishes, binaural processes were believed to be unimportant due to the speed of sound in water and the acoustic transparency of the tissues in water. In contrast, there are behavioral and anatomical data that support binaural processing in fishes. Studies in the toadfish combined anatomical tract-tracing and physiological recordings from identified sites along the ascending auditory pathway to document response characteristics at each level. Binaural computations in the medulla and midbrain sharpen the directional information provided by the saccule. Furthermore, physiological studies in the central nervous system indicated that encoding frequency, sound level, temporal pattern, and sound source direction are important components of what the toadfish ear tells the toadfish brain about sound.

The Auditory System of the Dipteran Parasitoid Emblemasoma auditrix (Sarcophagidae).

Several taxa of insects evolved a tympanate ear at different body positions, whereby the ear is composed of common parts: a scolopidial sense organ, a tracheal air space, and a tympanal membrane. Here, we analyzed the anatomy and physiology of the ear at the ventral prothorax of the sarcophagid fly, Emblemasoma auditrix (Soper). We used micro-computed tomography to analyze the ear and its tracheal air space in relation to the body morphology. Both tympana are separated by a small cuticular bridge, face in the same frontal direction, and are backed by a single tracheal enlargement. This enlargement is connected to the anterior spiracles at the dorsofrontal thorax and is continuous with the tracheal network in the thorax and in the abdomen. Analyses of responses of auditory afferents and interneurons show that the ear is broadly tuned, with a sensitivity peak at 5 kHz. Single-cell recordings of auditory interneurons indicate a frequency- and intensity-dependent tuning, whereby some neurons react best to 9 kHz, the peak frequency of the host’s calling song. The results are compared to the convergently evolved ear in Tachinidae (Diptera).

InVivo Ca(2+) Imaging Reveals that Decreased Dendritic Excitability Drives Startle Habituation.

Exposure to repetitive startling stimuli induces habitation, a simple form of learning. Despite its simplicity, the precise cellular mechanisms by which repeated stimulation converts a robust behavioral response to behavioral indifference are unclear. Here, we use head-restrained zebrafish larvae to monitor subcellular Ca(2+) dynamics in Mauthner neurons, the startle command neurons, during startle habituation in vivo. Using the Ca(2+) reporter GCaMP6s, we find that the amplitude of Ca(2+) signals in the lateral dendrite of the Mauthner neuron determines startle probability and that depression of this dendritic activity rather than downstream inhibition mediates glycine and N-methyl-D-aspartate (NMDA)-receptor-dependent short-term habituation. Combined, our results suggest a model for habituation learning in which increased inhibitory drive from feedforward inhibitory neurons combined with decreased excitatory input from auditory afferents decreases dendritic and Mauthner neuron excitability.

Basic response properties of auditory nerve fibers: a review.

All acoustic information from the periphery is encoded in the timing and rates of spikes in the population of spiral ganglion neurons projecting to the central auditory system. Considerable progress has been made in characterizing the physiological properties of type-I and type-II primary auditory afferents and understanding the basic properties of type-I afferents in response to sounds. Here, we review some of these properties, with emphasis placed on issues such as the stochastic nature of spike timing during spontaneous and driven activity, frequency tuning curves, spike-rate-versus-level functions, dynamic-range and spike-rate adaptation, and phase locking to stimulus fine structure and temporal envelope. We also review effects of acoustic trauma on some of these response properties.

Calcium-Induced Calcium Release during Action Potential Firing in Developing Inner Hair Cells

In the mature auditory system, inner hair cells (IHCs) convert sound-induced vibrations into electrical signals that are relayed to the central nervous system via auditory afferents. Before the cochlea can respond to normal sound levels, developing IHCs fire calcium-based action potentials that disappear close to the onset of hearing. Action potential firing triggers transmitter release from the immature IHC that in turn generates experience-independent firing in auditory neurons. These early signaling events are thought to be essential for the organization and development of the auditory system and hair cells. A critical component of the action potential is the rise in intracellular calcium that activates both small conductance potassium channels essential during membrane repolarization, and triggers transmitter release from the cell. Whether this calcium signal is generated by calcium influx or requires calcium-induced calcium release (CICR) is not yet known. IHCs can generate CICR, but to date its physiological role has remained unclear. Here, we used high and low concentrations of ryanodine to block or enhance CICR to determine whether calcium release from intracellular stores affected action potential waveform, interspike interval, or changes in membrane capacitance during development of mouse IHCs. Blocking CICR resulted in mixed action potential waveforms with both brief and prolonged oscillations in membrane potential and intracellular calcium. This mixed behavior is captured well by our mathematical model of IHC electrical activity. We perform two-parameter bifurcation analysis of the model that predicts the dependence of IHCs firing patterns on the level of activation of two parameters, the SK2 channels activation and CICR rate. Our data show that CICR forms an important component of the calcium signal that shapes action potentials and regulates firing patterns, but is not involved directly in triggering exocytosis. These data provide important insights into the calcium signaling mechanisms involved in early developmental processes.

Dynamic firing properties of type I spiral ganglion neurons.

Spiral ganglion neurons, the first neural element in the auditory system, possess complex intrinsic properties, possibly required to process frequency-specific sensory input that is integrated with extensive efferent regulation. Together with their tonotopically-graded sizes, the somata of these neurons reveal a sophisticated electrophysiological profile. Type I neurons, which make up ~95 % of the ganglion, have myriad voltage-gated ion channels that not only vary along the frequency contour of the cochlea, but also can be modulated by regulators such as voltage, calcium, and second messengers. The resultant developmentally- and tonotopically-regulated neuronal firing patterns conform to three distinct response modes (unitary, rapid, and slow) based on threshold and accommodation. This phenotype, however, is not static for any individual type I neuron. Recent observations have shown that, as neurons become less excitable with age, they demonstrate enhanced plasticity enabling them to change from one response mode to another depending upon resting membrane potential and the presence of neurotrophin-3. Thus, the primary auditory afferents utilized to encode dynamic acoustic stimuli possess the intrinsic specializations that allow them dynamically to alter their firing pattern.

Loss of auditory activity modifies the location of potassium channel KCNQ5 in auditory brainstem neurons.

KCNQ5/Kv7.5, a low-threshold noninactivating voltage-gated potassium channel, is preferentially targeted to excitatory endings of auditory neurons in the adult rat brainstem. Endbulds of Held from auditory nerve axons on the bushy cells of the ventral cochlear nucleus (VCN) and calyces of Held around the principal neurons in the medial nucleus of the trapezoid body (MNTB) are rich in KCNQ5 immunoreactivity. We have previously shown that this synaptic distribution occurs at about the time of hearing onset. The current study tests whether this localization in excitatory endings depends on the peripheral activity carried by the auditory nerve. Auditory nerve activity was abolished by cochlear removal or intracochlear injection of tetrodotoxin (TTX). Presence of KCNQ5 was analyzed by immunocytochemistry, Western blotting, and quantitative reverse transcription polymerase chain reaction. After cochlear removal, KCNQ5 immunoreactivity was virtually undetectable at its usual location in endbulbs and calyces of Held in the anteroventral CN and in the MNTB, respectively, although it was found in cell bodies in the VCN. The results were comparable after intracochlear TTX injection, which drastically reduced KCNQ5 immunostaining in MNTB calyces and increased immunolabeling in VCN cell bodies. Endbulbs of Held in the VCN also showed diminished KCNQ5 labeling after intracochlear TTX injection. These results show that peripheral activity from auditory nerve afferents is necessary to maintain the subcellular distribution of KCNQ5 in synaptic endings of the auditory brainstem. This may contribute to adaptations in the excitability and neurotransmitter release properties of these presynaptic endings under altered input conditions. © 2014 The Authors. Journal of Neuroscience Research Published by Wiley Periodicals, Inc.

Origin and function of short-latency inputs to the neural substrates underlying the acoustic startle reflex.

The acoustic startle reflex (ASR) is a survival mechanism of alarm, which rapidly alerts the organism to a sudden loud auditory stimulus. In rats, the primary ASR circuit encompasses three serially connected structures: cochlear root neurons (CRNs), neurons in the caudal pontine reticular nucleus (PnC), and motoneurons in the medulla and spinal cord. It is well-established that both CRNs and PnC neurons receive short-latency auditory inputs to mediate the ASR. Here, we investigated the anatomical origin and functional role of these inputs using a multidisciplinary approach that combines morphological, electrophysiological and behavioral techniques. Anterograde tracer injections into the cochlea suggest that CRNs somata and dendrites receive inputs depending, respectively, on their basal or apical cochlear origin. Confocal colocalization experiments demonstrated that these cochlear inputs are immunopositive for the vesicular glutamate transporter 1 (VGLUT1). Using extracellular recordings in vivo followed by subsequent tracer injections, we investigated the response of PnC neurons after contra-, ipsi-, and bilateral acoustic stimulation and identified the source of their auditory afferents. Our results showed that the binaural firing rate of PnC neurons was higher than the monaural, exhibiting higher spike discharges with contralateral than ipsilateral acoustic stimulations. Our histological analysis confirmed the CRNs as the principal source of short-latency acoustic inputs, and indicated that other areas of the cochlear nucleus complex are not likely to innervate PnC. Behaviorally, we observed a strong reduction of ASR amplitude in monaural earplugged rats that corresponds with the binaural summation process shown in our electrophysiological findings. Our study contributes to understand better the role of neuronal mechanisms in auditory alerting behaviors and provides strong evidence that the CRNs-PnC pathway mediates fast neurotransmission and binaural summation of the ASR.

Superficial stellate cells of the dorsal cochlear nucleus.

The dorsal cochlear nucleus (DCN) integrates auditory and multisensory signals at the earliest levels of auditory processing. Proposed roles for this region include sound localization in the vertical plane, head orientation to sounds of interest, and suppression of sensitivity to expected sounds. Auditory and non-auditory information streams to the DCN are refined by a remarkably complex array of inhibitory and excitatory interneurons, and the role of each cell type is gaining increasing attention. One inhibitory neuron that has been poorly appreciated to date is the superficial stellate cell. Here we review previous studies and describe new results that reveal the surprisingly rich interactions that this tiny interneuron has with its neighbors, interactions which enable it to respond to both multisensory and auditory afferents.

Effects of furosemide on cochlear neural activity, central hyperactivity and behavioural tinnitus after cochlear trauma in guinea pig.

Cochlear trauma causes increased spontaneous activity (hyperactivity) to develop in central auditory structures, and this has been suggested as a neural substrate for tinnitus. Using a guinea pig model we have previously demonstrated that for some time after cochlear trauma, central hyperactivity is dependent on peripheral afferent drive and only later becomes generated intrinsically within central structures. Furosemide, a loop diuretic, reduces spontaneous firing of auditory afferents. We investigated in our guinea pig model the efficacy of furosemide in reducing 1) spontaneous firing of auditory afferents, using the spectrum of neural noise (SNN) from round window recording, 2) hyperactivity in inferior colliculus, using extracellular single neuron recordings and 3) tinnitus at early time-points after cochlear trauma. Tinnitus was assessed using gap prepulse inhibition of acoustic startle (GPIAS). Intraperitoneal furosemide, but not saline, caused a marked decrease in both SNN and central hyperactivity. Intracochlear perfusion with furosemide similarly reversed central hyperactivity. In animals in which GPIAS measurements suggested the presence of tinnitus (reduced GPIAS), this could be reversed with an intraperitoneal injection with furosemide but not saline. The results are consistent with furosemide reducing central hyperactivity and behavioural signs of tinnitus by acting peripherally to decrease spontaneous firing of auditory afferents. The data support the notion that hyperactivity may be involved in the generation of tinnitus and further suggest that there may be a therapeutic window after cochlear trauma using drug treatments that target peripheral spontaneous activity.

Tracing and 3-dimensional representation of the primary afferents from the moth ear

Heliothine moths perceive acoustic information via two auditory sensory neurons only. Previous cobalt staining experiments have described the projection pattern of the two auditory neurons, called the A1 and the A2 cell, plus one additional neuron, the so-called B cell, up to the prothorax. We have obtained new and improved data about the projection pattern of the three sensory afferents by means of fluorescent staining experiments combined with scanning confocal microscopy. The present data show the fine structure of each sensory axon that arises from the moth ear and its ascending pathway relative to that of the others. In accordance with the previous data, the A2 auditory cell was found to extend projections in the pterothorax only. A novel finding is that terminal branches of the A2 cell cross the midline. The staining pattern of the two remaining neurons, the A1 and B cell, which project tightly together in the thoracic ganglia, differ somewhat from that previously described. As demonstrated here, one of these two neurons, the A1 cell, terminates in the prothoracic ganglion whereas the other, the B cell, projects further on via the cervical connectives to the subesophageal ganglion. The current data, therefore, indicate that none of the auditory afferents in the heliothine moth projects to the brain.

Phase shifts in binaural stimuli provide directional cues for sound localisation in the field cricket Gryllus bimaculatus.

The cricket's auditory system is a highly directional pressure difference receiver whose function is hypothesised to depend on phase relationships between the sound waves propagating through the auditory trachea that connects the left and right hearing organs. We tested this hypothesis by measuring the effect of experimentally constructed phase shifts in acoustic stimuli on phonotactic behavior of Gryllus bimaculatus, the oscillatory response patterns of the tympanic membrane, and the activity of the auditory afferents. The same artificial calling song was played simultaneously at the left and right sides of the cricket, but one sound pattern was shifted in phase by 90 deg (carrier frequencies between 3.6 and 5.4 kHz). All three levels of auditory processing are sensitive to experimentally induced acoustic phase shifts, and the response characteristics are dependent on the carrier frequency of the sound stimulus. At lower frequencies, crickets steered away from the sound leading in phase, while tympanic membrane vibrations and auditory afferent responses were smaller when the ipsilateral sound was leading. In contrast, opposite responses were observed at higher frequencies in all three levels of auditory processing. Minimal responses occurred near the carrier frequency of the cricket's calling song, suggesting a stability at this frequency. Our results indicate that crickets may use directional cues arising from phase shifts in acoustic signals for sound localisation, and that the response properties of pressure difference receivers may be analysed with phase-shifted sound stimuli to further our understanding of how insect auditory systems are adapted for directional processing.

Differential Roles for EphA and EphB Signaling in Segregation and Patterning of Central Vestibulocochlear Nerve Projections

Auditory and vestibular afferents enter the brainstem through the VIIIth cranial nerve and find targets in distinct brain regions. We previously reported that the axon guidance molecules EphA4 and EphB2 have largely complementary expression patterns in the developing avian VIIIth nerve. Here, we tested whether inhibition of Eph signaling alters central targeting of VIIIth nerve axons. We first identified the central compartments through which auditory and vestibular axons travel. We then manipulated Eph-ephrin signaling using pharmacological inhibition of Eph receptors and in ovo electroporation to misexpress EphA4 and EphB2. Anterograde labeling of auditory afferents showed that inhibition of Eph signaling did not misroute axons to non-auditory target regions. Similarly, we did not find vestibular axons within auditory projection regions. However, we found that pharmacologic inhibition of Eph receptors reduced the volume of the vestibular projection compartment. Inhibition of EphB signaling alone did not affect auditory or vestibular central projection volumes, but it significantly increased the area of the auditory sensory epithelium. Misexpression of EphA4 and EphB2 in VIIIth nerve axons resulted in a significant shift of dorsoventral spacing between the axon tracts, suggesting a cell-autonomous role for the partitioning of projection areas along this axis. Cochlear ganglion volumes did not differ among treatment groups, indicating the changes seen were not due to a gain or loss of cochlear ganglion cells. These results suggest that Eph-ephrin signaling does not specify auditory versus vestibular targets but rather contributes to formation of boundaries for patterning of inner ear projections in the hindbrain.

Molecular and Functional Asymmetry at a Vertebrate Electrical Synapse

Electrical synapses are abundant in the vertebrate brain, but their functional and molecular complexities are still poorly understood. We report here that electrical synapses between auditory afferents and goldfish Mauthner cells are constructed by apposition of hemichannels formed by two homologs of mammalian connexin 36 (Cx36) and that, while Cx35 is restricted to presynaptic hemiplaques, Cx34.7 is restricted to postsynaptic hemiplaques, forming heterotypic junctions. This molecular asymmetry is associated with rectification of electrical transmission that may act to promote cooperativity between auditory afferents. Our data suggest that, in similarity to pre- and postsynaptic sites at chemical synapses, one side in electrical synapses should not necessarily be considered the mirror image of the other. While asymmetry based on the presence of two Cx36 homologs is restricted to teleost fish, it might also be based on differences in posttranslational modifications of individual connexins or in the complement of gap junction-associated proteins.

A perceptual model of auditory deafferentation

Overexposure to intense sound produces temporary threshold shifts but permanent loss of afferent nerve terminals. Here, we present a vocoder designed to explore the perceptual consequences of this type of damage. The basic idea is that the spike train produced by an individual auditory afferent resembles a stochastically digitized binary version of the stimulus waveform and that the quality of the waveform representation in the whole nerve depends on the number of aggregated spike trains. Sounds were processed by filtering them into ten adjacent frequency bands. For the signal in each band, multiple spike trains were then obtained in an attempt to mimic the different representations of that signal conveyed by different auditory afferents innervating a given cochlear region. The aggregated spike train was multiplied by the original signal to obtain an acoustic version of the simulated nerve waveform. Tone-in-noise and speech-in-noise perception tests were performed by young, normal-hearing listeners using different numbers of afferents per frequency band. Results support that deafferentation impairs perception in noise more than in quiet. The proposed vocoder may be extended to model other types of hearing damage and to guide the design of hearing aids and cochlear implants.

Perception of stochastically undersampled sound waveforms: a model of auditory deafferentation

Auditory deafferentation, or permanent loss of auditory nerve afferent terminals, occurs after noise overexposure and aging and may accompany many forms of hearing loss. It could cause significant auditory impairment but is undetected by regular clinical tests and so its effects on perception are poorly understood. Here, we hypothesize and test a neural mechanism by which deafferentation could deteriorate perception. The basic idea is that the spike train produced by each auditory afferent resembles a stochastically digitized version of the sound waveform and that the quality of the waveform representation in the whole nerve depends on the number of aggregated spike trains or auditory afferents. We reason that because spikes occur stochastically in time with a higher probability for high- than for low-intensity sounds, more afferents would be required for the nerve to faithfully encode high-frequency or low-intensity waveform features than low-frequency or high-intensity features. Deafferentation would thus degrade the encoding of these features. We further reason that due to the stochastic nature of nerve firing, the degradation would be greater in noise than in quiet. This hypothesis is tested using a vocoder. Sounds were filtered through ten adjacent frequency bands. For the signal in each band, multiple stochastically subsampled copies were obtained to roughly mimic different stochastic representations of that signal conveyed by different auditory afferents innervating a given cochlear region. These copies were then aggregated to obtain an acoustic stimulus. Tone detection and speech identification tests were performed by young, normal-hearing listeners using different numbers of stochastic samplers per frequency band in the vocoder. Results support the hypothesis that stochastic undersampling of the sound waveform, inspired by deafferentation, impairs speech perception in noise more than in quiet, consistent with auditory aging effects.

Quantification of dendritic and axonal growth after injury to the auditory system of the adult cricket Gryllus bimaculatus

Dendrite and axon growth and branching during development are regulated by a complex set of intracellular and external signals. However, the cues that maintain or influence adult neuronal morphology are less well understood. Injury and deafferentation tend to have negative effects on adult nervous systems. An interesting example of injury-induced compensatory growth is seen in the cricket, Gryllus bimaculatus. After unilateral loss of an ear in the adult cricket, auditory neurons within the central nervous system (CNS) sprout to compensate for the injury. Specifically, after being deafferented, ascending neurons (AN-1 and AN-2) send dendrites across the midline of the prothoracic ganglion where they receive input from auditory afferents that project through the contralateral auditory nerve (N5). Deafferentation also triggers contralateral N5 axonal growth. In this study, we quantified AN dendritic and N5 axonal growth at 30 h, as well as at 3, 5, 7, 14, and 20 days after deafferentation in adult crickets. Significant differences in the rates of dendritic growth between males and females were noted. In females, dendritic growth rates were non-linear; a rapid burst of dendritic extension in the first few days was followed by a plateau reached at 3 days after deafferentation. In males, however, dendritic growth rates were linear, with dendrites growing steadily over time and reaching lengths, on average, twice as long as in females. On the other hand, rates of N5 axonal growth showed no significant sexual dimorphism and were linear. Within each animal, the growth rates of dendrites and axons were not correlated, indicating that independent factors likely influence dendritic and axonal growth in response to injury in this system. Our findings provide a basis for future study of the cellular features that allow differing dendrite and axon growth patterns as well as sexually dimorphic dendritic growth in response to deafferentation.

Visual Input Modulates Audiomotor Function via Hypothalamic Dopaminergic Neurons through a Cooperative Mechanism

Visual cues often modulate auditory signal processing, leading to improved sound detection. However, the synaptic and circuit mechanism underlying this cross-modal modulation remains poorly understood. Using larval zebrafish, we first established a cross-modal behavioral paradigm in which a preceding flash enhances sound-evoked escape behavior, which is known to be executed through auditory afferents (VIII(th) nerves) and command-like neurons (Mauthner cells). In vivo recording revealed that the visual enhancement of auditory escape is achieved by increasing sound-evoked Mauthner cell responses. This increase in Mauthner cell responses is accounted for by the increase in the signal-to-noise ratio of sound-evoked VIII(th) nerve spiking and efficacy of VIII(th) nerve-Mauthner cell synapses. Furthermore, the visual enhancement of Mauthner cell response and escape behavior requires light-responsive dopaminergic neurons in the caudal hypothalamus and D1 dopamine receptor activation. Our findings illustrate a cooperative neural mechanism for visual modulation of audiomotor processing that involves dopaminergic neuromodulation.

Adaptive auditory plasticity for social communication in the plainfin midshipman fish (Porichthys notatus)

Acoustic communication plays an important role in the social behaviours of vocal teleost fishes in the family Batrachoididae (midshipman and toad fishes). The midshipman and toadfishes have become good models for investigating the neural and endocrine mechanisms of auditory perception and vocal production shared by all vertebrates (Bass and Lu 2006), in part, because the reproductive success of these batrachoidid fishes is highly dependent on acoustic communication. Recent neuroethological studies of acoustic communication in the plainfin midshipman fish (Porichthys notatus) have provided strong evidence for the related reproductive-state and steroid-dependent modulation of hearing sensitivity in this species that leads to enhanced coupling of sender and receiver in this communication system (Sisneros and Bass 2003; Sisneros et al. 2004a). The plainfin midshipman fish is a vocal marine teleost fish found along the west coast of the USA. During the late spring and summer, midshipman fish migrate from deep water into the rocky intertidal zone to court and spawn. Midshipman fish are known to have three adult reproductive morphs that include female and two male morphs known as type I and II. Type I or ‘singing’ males acoustically court females with their seasonal advertisement calls or ‘hums’, which is a multiharmonic acoustic signal with a fundamental frequency (F0) that ranges from 90 to 100Hz (Brantley and Bass 1994). The dominant harmonics of the hum range up to 400Hz and can contain as much or more spectral energy than the F0. These harmonics have been hypothesized to be important for the detection and localization of the advertisement signal during the reproductive season (Sisneros and Bass 2003). Type II males or ‘sneaker’ males use an alternative reproductive tactic that does not require them to build nests or acoustically court females. Instead type II males satellite and/or ‘sneak’ spawn in the nests of type I males to steal fertilizations from ‘singing’ males when they are actively courting females. Females rely heavily on their auditory sense to detect and locate advertising males that produce the multiharmonic hums during the breeding season and will often exhibit robust phonotaxis to a synthesized hum (and even to a low frequency tone near the F0 of the male’s advertisement call). Previous work showed that the frequency sensitivity of the auditory afferents that innervated the saccule, the main organ of hearing in the midshipman, changed seasonally with reproductive state such that reproductive females were better suited than nonreproductive females to encode the dominant harmonics of the male’s advertisement call (Sisneros and Bass 2003). An investigation of the seasonal periodicity of reproduction