Peripheral control of acoustic signals in the auditory system of echolocating bats

Echolocating Bats Have Evolved Decreased Susceptibility to Noise-Induced Temporary Hearing Losses

Glenis Long championed the application of quantitative psychophysical methods to understand comparative hearing abilities across species. She contributed the first psychophysical studies of absolute and masked hearing sensitivities in an auditory specialist, the echolocating horseshoe bat. Her data demonstrated that this bat has hyperacute frequency discrimination in the 83-kHz range of its echolocation broadcast. This specialization facilitates the bat's use of Doppler shift compensation to separate echoes of fluttering insects from concurrent echoes of non-moving objects. In this review, we discuss another specialization for hearing in a species of echolocating bat that contributes to perception of echoes within a complex auditory scene. Psychophysical and behavioral studies with big brown bats show that exposures to long duration, intense wideband or narrowband ultrasonic noise do not induce significant increases in their thresholds to echoes and do not impair their ability to orient through a naturalistic sonar scene containing multiple distracting echoes. Thresholds of auditory brainstem responses also remain low after intense noise exposures. These data indicate that big brown bats are not susceptible to temporary threshold shifts as measured in comparable paradigms used with other mammals, at least within the range of stimulus parameters that have been tested so far. We hypothesize that echolocating bats have evolved a decreased susceptibility to noise-induced hearing losses as a specialization for echolocation in noisy environments.

Multiple processes of vocal sensory-motor interaction in primate auditory cortex

Sensory-motor interactions in the auditory system play an important role in vocal self-monitoring and control. These result from top-down corollary discharges, relaying predictions about vocal timing and acoustics. Recent evidence suggests such signals may be two distinct processes, one suppressing neural activity during vocalization and another enhancing sensitivity to sensory feedback, rather than a single mechanism. Single-neuron recordings have been unable to disambiguate due to overlap of motor signals with sensory inputs. Here, we sought to disentangle these processes in marmoset auditory cortex during production of multi-phrased 'twitter' vocalizations. Temporal responses revealed two timescales of vocal suppression: temporally-precise phasic suppression during phrases and sustained tonic suppression. Both components were present within individual neurons, however, phasic suppression presented broadly regardless of frequency tuning (gating), while tonic was selective for vocal frequencies and feedback (prediction). This suggests that auditory cortex is modulated by concurrent corollary discharges during vocalization, with different computational mechanisms.

Organization of an ascending circuit that conveys flight motor state in Drosophila

Natural behaviors are a coordinated symphony of motor acts that drive reafferent (self-induced) sensory activation. Individual sensors cannot disambiguate exafferent (externally induced) from reafferent sources. Nevertheless, animals readily differentiate between these sources of sensory signals to carry out adaptive behaviors through corollary discharge circuits (CDCs), which provide predictive motor signals from motor pathways to sensory processing and other motor pathways. Yet, how CDCs comprehensively integrate into the nervous system remains unexplored. Here, we use connectomics, neuroanatomical, physiological, and behavioral approaches to resolve the network architecture of two pairs of ascending histaminergic neurons (AHNs) in Drosophila, which function as a predictive CDC in other insects. Both AHN pairs receive input primarily from a partially overlapping population of descending neurons, especially from DNg02, which controls wing motor output. Using Ca2+ imaging and behavioral recordings, we show that AHN activation is correlated to flight behavior and precedes wing motion. Optogenetic activation of DNg02 is sufficient to activate AHNs, indicating that AHNs are activated by descending commands in advance of behavior and not as a consequence of sensory input. Downstream, each AHN pair targets predominantly non-overlapping networks, including those that process visual, auditory, and mechanosensory information, as well as networks controlling wing, haltere, and leg sensorimotor control. These results support the conclusion that the AHNs provide a predictive motor signal about wing motor state to mostly non-overlapping sensory and motor networks. Future work will determine how AHN signaling is driven by other descending neurons and interpreted by AHN downstream targets to maintain adaptive sensorimotor performance.

Organization of an Ascending Circuit that Conveys Flight Motor State

Natural behaviors are a coordinated symphony of motor acts which drive self-induced or reafferent sensory activation. Single sensors only signal presence and magnitude of a sensory cue; they cannot disambiguate exafferent (externally-induced) from reafferent sources. Nevertheless, animals readily differentiate between these sources of sensory signals to make appropriate decisions and initiate adaptive behavioral outcomes. This is mediated by predictive motor signaling mechanisms, which emanate from motor control pathways to sensory processing pathways, but how predictive motor signaling circuits function at the cellular and synaptic level is poorly understood. We use a variety of techniques, including connectomics from both male and female electron microscopy volumes, transcriptomics, neuroanatomical, physiological and behavioral approaches to resolve the network architecture of two pairs of ascending histaminergic neurons (AHNs), which putatively provide predictive motor signals to several sensory and motor neuropil. Both AHN pairs receive input primarily from an overlapping population of descending neurons, many of which drive wing motor output. The two AHN pairs target almost exclusively non-overlapping downstream neural networks including those that process visual, auditory and mechanosensory information as well as networks coordinating wing, haltere, and leg motor output. These results support the conclusion that the AHN pairs multi-task, integrating a large amount of common input, then tile their output in the brain, providing predictive motor signals to non-overlapping sensory networks affecting motor control both directly and indirectly.

Bats experience age-related hearing loss (presbycusis)

Hearing loss is a hallmark of aging, typically initially affecting the higher frequencies. In echolocating bats, the ability to discern high frequencies is essential. However, nothing is known about age-related hearing loss in bats, and they are often assumed to be immune to it. We tested the hearing of 47 wild Egyptian fruit bats by recording their auditory brainstem response and cochlear microphonics, and we also assessed the cochlear histology in four of these bats. We used the bats' DNA methylation profile to evaluate their age and found that bats exhibit age-related hearing loss, with more prominent deterioration at the higher frequencies. The rate of the deterioration was ∼1 dB per year, comparable to the hearing loss observed in humans. Assessing the noise in the fruit bat roost revealed that these bats are exposed to continuous immense noise-mostly of social vocalizations-supporting the assumption that bats might be partially resistant to loud noise. Thus, in contrast to previous assumptions, our results suggest that bats constitute a model animal for the study of age-related hearing loss.

Adaptive echolocation behavior of bats and toothed whales in dynamic soundscapes

Journal of Experimental Biology has a long history of reporting research discoveries on animal echolocation, the subject of this Centenary Review. Echolocating animals emit intense sound pulses and process echoes to localize objects in dynamic soundscapes. More than 1100 species of bats and 70 species of toothed whales rely on echolocation to operate in aerial and aquatic environments, respectively. The need to mitigate acoustic clutter and ambient noise is common to both aerial and aquatic echolocating animals, resulting in convergence of many echolocation features, such as directional sound emission and hearing, and decreased pulse intervals and sound intensity during target approach. The physics of sound transmission in air and underwater constrains the production, detection and localization of sonar signals, resulting in differences in response times to initiate prey interception by aerial and aquatic echolocating animals. Anti-predator behavioral responses of prey pursued by echolocating animals affect behavioral foraging strategies in air and underwater. For example, many insect prey can detect and react to bat echolocation sounds, whereas most fish and squid are unresponsive to toothed whale signals, but can instead sense water movements generated by an approaching predator. These differences have implications for how bats and toothed whales hunt using echolocation. Here, we consider the behaviors used by echolocating mammals to (1) track and intercept moving prey equipped with predator detectors, (2) interrogate dynamic sonar scenes and (3) exploit visual and passive acoustic stimuli. Similarities and differences in animal sonar behaviors underwater and in air point to open research questions that are ripe for exploration.

Tyraminergic corollary discharge filters reafferent perception in a chemosensory neuron

Interpreting sensory information requires its integration with the current behavior of the animal. However, how motor-related circuits influence sensory information processing is incompletely understood. Here, we report that current locomotor state directly modulates the activity of BAG CO₂ sensory neurons in Caenorhabditis elegans. By recording neuronal activity in animals freely navigating CO₂ landscapes, we found that during reverse crawling states, BAG activity is suppressed by tyraminergic corollary discharge signaling. We provide genetic evidence that tyramine released from the RIM reversal interneurons extrasynaptically activates the inhibitory chloride channel LGC-55 in BAG. Disrupting this pathway genetically leads to excessive behavioral responses to CO₂ stimuli. Moreover, we find that LGC-55 signaling cancels out perception of self-produced CO₂ and O₂ stimuli when animals reverse into their own gas plume in ethologically relevant aqueous environments. Our results show that sensorimotor integration involves corollary discharge signals directly modulating chemosensory neurons.

Efference copies: Context matters when ignoring self-induced motion

Across the animal kingdom, efference copies of neuronal motor commands are used to ensure our senses ignore stimuli generated by our own actions. New work shows that the underlying motivation for an action affects whether visual neurons are responsive to self-generated stimuli.

Functional Analyses of Peripheral Auditory System Adaptations for Echolocation in Air vs. Water

The similarity of acoustic tasks performed by odontocete (toothed whale) and microchiropteran (insectivorous bat) biosonar suggests they may have common ultrasonic signal reception and processing mechanisms. However, there are also significant media and prey dependent differences, notably speed of sound and wavelengths in air vs. water, that may be reflected in adaptations in their auditory systems and peak spectra of out-going signals for similarly sized prey. We examined the anatomy of the peripheral auditory system of two species of FM bat (big brown bat Eptesicus fuscus; Japanese house bat Pipistrellus abramus) and two toothed whales (harbor porpoise Phocoena phocoena; bottlenose dolphin Tursiops truncatus) using ultra high resolution (11–100 micron) isotropic voxel computed tomography (helical and microCT). Significant differences were found for oval and round window location, cochlear length, basilar membrane gradients, neural distributions, cochlear spiral morphometry and curvature, and basilar membrane suspension distributions. Length correlates with body mass, not hearing ranges. High and low frequency hearing range cut-offs correlate with basilar membrane thickness/width ratios and the cochlear radius of curvature. These features are predictive of high and low frequency hearing limits in all ears examined. The ears of the harbor porpoise, the highest frequency echolocator in the study, had significantly greater stiffness, higher basal basilar membrane ratios, and bilateral bony support for 60% of the basilar membrane length. The porpoise’s basilar membrane includes a “foveal” region with “stretched” frequency representation and relatively constant membrane thickness/width ratio values similar to those reported for some bat species. Both species of bats and the harbor porpoise displayed unusual stapedial input locations and low ratios of cochlear radii, specializations that may enhance higher ultrasonic frequency signal resolution and deter low frequency cochlear propagation.

Cochlear hair cells of echolocating bats are immune to intense noise

Exposure to intense noise can damage cochlear hair cells, leading to hearing loss in mammals. To avoid this constraint, most mammals have evolved in relatively quiet environments. Echolocating bats, however, are naturally exposed to continuous intense sounds from their own and neighboring sonar emissions for maintaining sonar directionality and range. Here, we propose the presence of intense noise resistance in cochlear hair cells of echolocating bats against noise-induced hearing loss (NIHL). To test this hypothesis, we performed noise exposure experiments for laboratory mice, one nonecholocating bat species, and five echolocating bat species. Contrary to nonecholocating fruit bats and mice, the hearing and the cochlear hair cells of echolocating bats remained unimpaired after continuous intense noise exposure. The comparative analyses of cochleae transcriptomic data showed that several genes protecting cochlear hair cells from intense sounds were overexpressed in echolocating bats. Particularly, the experimental examinations revealed that ISL1 overexpression significantly improved the survival of cochlear hair cells. Our findings support the existence of protective effects in cochlear hair cells of echolocating bats against intense noises, which provides new insight into understanding the relationship between cochlear hair cells and intense noises, and preventing or ameliorating NIHL in mammals.

Efficient high-voltage protection in the electric catfish

For thousands of years, starting with detailed accounts from ancient Egypt, the African electric catfish (Malapteruridae) has been renowned for its ability to hunt and to defend itself with powerful electric shocks. Surprisingly, the degree to which electric catfish are protected against their own or external electric shocks, how specific any protection would be to the species-specific waveform and whether a discharging catfish has to actively prepare for the onset of its high-voltage discharges has never been analysed. Here, we used digital high-speed video to record catfish during their own discharges or as they were exposed to external discharges, employing goldfish to carefully calibrate the efficiency of all discharges. Electric catfish show a remarkable degree of protection against high voltages: both self-produced and external electric shocks that heavily affected control goldfish failed to evoke involuntary muscle contraction or to affect sensorimotor processing. Even a commercial electrofishing device, set to efficiently immobilise and narcotise fish, failed to have any effect on the electric catfish. Our findings rule out several protective mechanisms and demonstrate a highly efficient and versatile shielding whose nature is presently unclear.

Lack of reproducibility of temporary hearing threshold shifts in a harbor porpoise after exposure to repeated airgun sounds

Noise-induced temporary hearing threshold shift (TTS) was studied in a harbor porpoise exposed to impulsive sounds of scaled-down airguns while both stationary and free-swimming for up to 90 min. In a previous study, ∼4 dB TTS was elicited in this porpoise, but despite 8 dB higher single-shot and cumulative exposure levels (up to 199 dB re 1 μPa²s) in the present study, the porpoise showed no significant TTS at hearing frequencies 2, 4, or 8 kHz. There were no changes in the study animal's audiogram between the studies or significant differences in the fatiguing sound that could explain the difference, but audible and visual cues in the present study may have allowed the porpoise to predict when the fatiguing sounds would be produced. The discrepancy between the studies may have resulted from self-mitigation by the porpoise. Self-mitigation, resulting in reduced hearing sensitivity, can be achieved via changes in the orientation of the head, or via alteration of the hearing threshold by processes in the ear or central nervous system.

Corollary Discharge Mechanisms During Vocal Production in Marmoset Monkeys

Interactions between motor systems and sensory processing are ubiquitous throughout the animal kingdom and play an important role in many sensorimotor behaviors, including both human speech and animal vocalization. During vocal production, the auditory system plays important roles in both encoding feedback of produced sounds, allowing one to self-monitor for vocal errors, and simultaneously maintaining sensitivity to the outside acoustic environment. Supporting these roles is an efferent motor-to-sensory signal known as a corollary discharge. This review summarizes recent work on the role of such signaling during vocalization in the marmoset monkey, a nonhuman primate model of social vocal communication.

Echolocation: a personal historical perspective

The remarkable ability of echolocating bats to use echoes of emitted sounds as a substitute for vision is one of the most fascinating stories in neuroethology. Based in part on personal experience, the author discusses key discoveries in the early decades after the discovery of the phenomenon by Griffin and Galambos in 1941. Advances in our understanding of this remarkable phenomenon illustrate well the importance of comparative approaches, technological advances, and the synergistic interaction between behavior and electrophysiology.

Non-auditory, electrophysiological potentials preceding dolphin biosonar click production

The auditory brainstem response to a dolphin’s own emitted biosonar click can be measured by averaging epochs of the instantaneous electroencephalogram (EEG) that are time-locked to the emitted click. In this study, averaged EEGs were measured using surface electrodes placed on the head in six different configurations while dolphins performed an echolocation task. Simultaneously, biosonar click emissions were measured using contact hydrophones on the melon and a hydrophone in the farfield. The averaged EEGs revealed an electrophysiological potential (the pre-auditory wave, PAW) that preceded the production of each biosonar click. The largest PAW amplitudes occurred with the non-inverting electrode just right of the midline—the apparent side of biosonar click generation—and posterior of the blowhole. Although the source of the PAW is unknown, the temporal and spatial properties rule out an auditory source. The PAW may be a neural or myogenic potential associated with click production; however, it is not known if muscles within the dolphin nasal system can be actuated at the high rates reported for dolphin click production, or if sufficiently coordinated and fast motor endplates of nasal muscles exist to produce a PAW detectable with surface electrodes.

The Middle Ear Muscle Reflex in Rat: Developing a Biomarker of Auditory Nerve Degeneration

OBJECTIVES

The long-term goal of this research is to determine whether the middle ear muscle reflex can be used to predict the number of healthy auditory nerve fibers in hearing-impaired ears. In this study, we develop a high-impedance source and an animal model of the middle ear muscle reflex and explore the influence of signal frequency and level on parameters of the reflex to determine an optimal signal to examine auditory nerve fiber survival.

DESIGN

A high-impedance source was developed using a hearing aid receiver attached to a 0.06 diameter 10.5-cm length tube. The impedance probe consisted of a microphone probe placed near the tip of a tube coupled to a sound source. The probe was calibrated by inserting it into a syringe of known volumes and impedances. The reflex in the anesthetized rat was measured with elicitor stimuli ranging from 3 to 16 kHz presented at levels ranging from 35 to 100 dB SPL to one ear while the reflex was measured in the opposite ear containing the probe and probe stimulus.

RESULTS

The amplitude of the reflex increased with elicitor level and was largest at 3 kHz. The lowest threshold was approximately 54 dB SPL for the 3-kHz stimulus. The rate of decay of the reflex was greatest at 16 kHz followed by 10 and 3 kHz. The rate of decay did not change significantly with elicitor signal level for 3 and 16 kHz, but decreased as the level of the 10-kHz elicitor increased. A negative feedback model accounts for the reflex decay by having the strength of feedback dependent on auditory nerve input. The rise time of the reflex varied with frequency and changed with level for the 10- and 16-kHz signals but not significantly for the 3-kHz signal. The latency of the reflex increased with a decrease in elicitor level, and the change in latency with level was largest for the 10-kHz stimulus.

CONCLUSIONS

Because the amplitude of the reflex in rat was largest with an elicitor signal at 3 kHz, had the lowest threshold, and yielded the least amount of decay, this may be the ideal frequency to estimate auditory nerve survival in hearing-impaired ears.

Time-varying auditory gain control in response to double-pulse stimuli in harbour porpoises is not mediated by a stapedial reflex

Echolocating animals reduce their output level and hearing sensitivity with decreasing echo delays, presumably to stabilize the perceived echo intensity during target approaches. In bats, this variation in hearing sensitivity is formed by a call-induced stapedial reflex that tapers off over time after the call. Here, we test the hypothesis that a similar mechanism exists in toothed whales by subjecting a trained harbour porpoise to a series of double sound pulses varying in delay and frequency, while measuring the magnitudes of the evoked auditory brainstem responses (ABRs). We find that the recovery of the ABR to the second pulse is frequency dependent, and that a stapedial reflex therefore cannot account for the reduced hearing sensitivity at short pulse delays. We propose that toothed whale auditory time-varying gain control during echolocation is not enabled by the middle ear as in bats, but rather by frequency-dependent mechanisms such as forward masking and perhaps higher-order control of efferent feedback to the outer hair cells.

Contributions of sensory tuning to auditory-vocal interactions in marmoset auditory cortex

During speech, humans continuously listen to their own vocal output to ensure accurate communication. Such self-monitoring is thought to require the integration of information about the feedback of vocal acoustics with internal motor control signals. The neural mechanism of this auditory-vocal interaction remains largely unknown at the cellular level. Previous studies in naturally vocalizing marmosets have demonstrated diverse neural activities in auditory cortex during vocalization, dominated by a vocalization-induced suppression of neural firing. How underlying auditory tuning properties of these neurons might contribute to this sensory-motor processing is unknown. In the present study, we quantitatively compared marmoset auditory cortex neural activities during vocal production with those during passive listening. We found that neurons excited during vocalization were readily driven by passive playback of vocalizations and other acoustic stimuli. In contrast, neurons suppressed during vocalization exhibited more diverse playback responses, including responses that were not predictable by auditory tuning properties. These results suggest that vocalization-related excitation in auditory cortex is largely a sensory-driven response. In contrast, vocalization-induced suppression is not well predicted by a neuron's auditory responses, supporting the prevailing theory that internal motor-related signals contribute to the auditory-vocal interaction observed in auditory cortex.

Conditioned hearing sensitivity change in the harbor porpoise (Phocoena phocoena)

Hearing sensitivity, during trials in which a warning sound preceding a loud sound, was investigated in two harbor porpoises (Phocoena phocoena). Sensitivity was measured using pip-train test stimuli and auditory evoked potential recording. When a hearing test/warning stimulus, with a frequency of either 45 or 32 kHz, preceded a loud 32 kHz tone with a sound pressure level of 152 dB re 1 μPa root mean square, lasting 2 s yielding an sound exposure level (SEL) of 155 dB re 1 μPa(2)s, pooled hearing thresholds measured just before the loud sound increased relative to baseline thresholds. During two experimental sessions the threshold increased up to 17 dB for the test frequency of 45 kHz and up to 11 dB for the test frequency of 32 kHz. An extinction test revealed very rapid threshold recovery within the first two experimental sessions. The SEL producing the hearing dampening effect was low compared to previous other odontocete hearing change efforts with each individual trial equal to 155 dB re 1 μPa(2) but the cumulative SEL for each subsession may have been as high as 168 dB re 1 μPa(2). Interpretations of conditioned hearing sensation change and possible change due to temporary threshold shifts are considered for the harbor porpoise and discussed in the light of potential mechanisms and echolocation.

Short-term enhancement and suppression of dolphin auditory evoked responses following echolocation click emission

Biosonar gain control mechanisms in a bottlenose dolphin were investigated by measuring the auditory steady-state response (ASSR) to an external tone while the animal echolocated. The dolphin performed an echo change-detection task that utilized electronically synthesized echoes with echo delays corresponding to 25- and 50-m target range. During the task, amplitude modulated tones with carrier frequencies from 25 to 125 kHz were continuously presented and the instantaneous electroencephalogram stored for later analysis. ASSRs were extracted from the electroencephalogram by synchronously averaging time epochs temporally aligned with the onset of the external tone modulation cycle nearest to each of the dolphin's echolocation clicks. Results showed an overall suppression of the ASSR amplitude for tones with frequencies near the click center frequencies. A larger, temporary suppression of the ASSR amplitude was also measured at frequencies above 40-50 kHz, while a temporary enhancement was observed at lower frequencies. Temporal patterns for ASSR enhancement or suppression were frequency-, level-, and range-dependent, with recovery to pre-click values occurring within the two-way travel time. Suppressive effects fit the patterns expected from forward masking by the emitted biosonar pulse, while the specific mechanisms responsible for the frequency-dependent enhancement are unknown.

An auditory-neuroscience perspective on the development of selective mutism

Selective mutism (SM) is a relatively rare psychiatric disorder of childhood characterized by consistent inability to speak in specific social situations despite the ability to speak normally in others. SM typically involves severe impairments in social and academic functioning. Common complications include school failure, social difficulties in the peer group, and aggravated intra-familial relationships. Although SM has been described in the medical and psychological literatures for many years, the potential underlying neural basis of the disorder has only recently been explored. Here we explore the potential role of specific auditory neural mechanisms in the psychopathology of SM and discuss possible implications for treatment.

Echo amplitude sensitivity of bat auditory neurons improves with decreasing pulse-echo gap

During hunting, insectivorous bats systematically vary the parameters of emitted pulses and analyze the returning echoes to extract prey features. As such, the duration of the pulse (P) and echo (E), the P-E gap, and the P-E amplitude difference progressively decrease throughout the prey-approach sequence. Our previous studies have shown that most inferior collicular neurons of bats discharge maximally to a best duration, and they have the sharpest echo frequency and amplitude sensitivity when stimulated with P-E pairs with duration the same as the best duration. Furthermore, their echo duration and frequency sensitivity improves with decreasing P-E duration and P-E gap. The present study shows that this is also true in the amplitude domain. Thus, all these data indicate that bats can better extract multiple parameters of expected rather than unexpected echo after pulse emission. They also support the hypothesis that a bat's inferior collicular neurons improve the response sensitivity in multiple parametric domains as the prey is approached to increase the success of hunting.

Cognitive adaptation of sonar gain control in the bottlenose dolphin

Echolocating animals adjust the transmit intensity and receive sensitivity of their sonar in order to regulate the sensation level of their echoes; this process is often termed automatic gain control. Gain control is considered not to be under the animal's cognitive control, but previous investigations studied animals ensonifying targets or hydrophone arrays at predictable distances. To test whether animals maintain gain control at a fixed level in uncertain conditions, we measured changes in signal intensity for a bottlenose dolphin (Tursiops truncatus) detecting a target at three target distances (2.5, 4 and 7 m) in two types of sessions: predictable and unpredictable. Predictable sessions presented the target at a constant distance; unpredictable sessions moved the target randomly between the three target positions. In the predictable sessions the dolphin demonstrated intensity distance compensation, increasing the emitted click intensity as the target distance increased. Additionally, as trials within sessions progressed, the animal adjusted its click intensity even from the first click in a click train, which is consistent with the animal expecting a target at a certain range. In the unpredictable sessions there was no significant difference of intensity with target distance until after the 7th click in a click train. Together, these results demonstrate that the bottlenose dolphin uses learning and expectation for sonar gain control.

Conditioned hearing sensitivity reduction in a bottlenose dolphin (Tursiops truncatus)

The conditioned change in hearing sensitivity during a warning sound preceding a loud sound was investigated in the bottlenose dolphin. Hearing sensitivity was measured using pip-train test stimuli and auditory evoked potential recording. When the test/warning stimulus with a frequency of 22.5 or 32 kHz preceded the loud sound with a frequency of 22.5 kHz and a sound pressure level of 165 dB re 1 μPa rms, hearing thresholds before the loud sound increased relative to the baseline. The threshold increased up to 15 dB. In order to further investigate whether the observed threshold increase was due to conditioning, the dependence of the effect on warning duration and inter-trial interval was investigated. The duration of the warning substantially influenced the effect. Shorter warnings resulted in deeper suppression of responses and higher threshold increases than longer warnings. Alternatively, the effect was nearly independent of the duration of the inter-trial interval, i.e. independent of the delay from the loud sound to the test/warning sound in the subsequent trial. These data are considered as evidence that the observed hearing threshold increases were not a result of the unconditioned effect of the loud sound and were instead a manifestation of a conditioned dampening of hearing when the subject anticipated the quick appearance of a loud sound in the bottlenose dolphin in the same way as previously demonstrated in the false killer whale.

Auditory evoked potentials in a bottlenose dolphin during moderate-range echolocation tasks

Studies with echolocating odontocetes have suggested that forms of automatic gain control mediate auditory electrophysiological responses to target-related echoes. This study used a phantom echo generator and auditory evoked potential measurements to examine automatic gain control in a bottlenose dolphin. Auditory evoked potentials to outgoing clicks and incoming echoes were recorded for simulated ranges from 2.5 to 80 m. When geometric spreading loss was simulated, echo-evoked potential amplitudes were essentially constant up to 14 m and progressively decreased with increasing range. When the echo levels were held constant relative to clicks, echo-evoked potential amplitudes increased with increasing range up to 80 m. These results suggest that automatic gain control maintains distance-independent echo-evoked potential amplitudes at close range, but does not fully compensate for attenuation due to spreading loss at longer ranges. The automatic gain control process appears to arise from an interaction of transmitter and receiver based processes, resulting in a short-range region of distance-independent echo-evoked potential amplitudes for relevant targets, and a longer-range region in which echo-evoked potential amplitudes are reduced.

New model for gain control of signal intensity to object distance in echolocating bats

Echolocating bats emit ultrasonic calls and listen for the returning echoes to orient and localize prey in darkness. The emitted source level, SL (estimated signal intensity 10 cm from the mouth), is adjusted dynamically from call to call in response to sensory feedback as bats approach objects. A logarithmic relationship of SL=20 log(10)(x), i.e. 6 dB output reduction per halving of distance, x, has been proposed as a model for the relationship between emitted intensity and object distance, not only for bats but also for echolocating toothed whales. This logarithmic model suggests that the approaching echolocator maintains a constant intensity impinging upon the object, but it also implies ever-increasing source levels with distance, a physical and biological impossibility. We developed a new model for intensity compensation with an exponential rise to the maximum source level: SL=SL(max)-ae(-)(bx). In addition to providing a method for estimating maximum output, the new model also offers a tool for estimating a minimum detection distance where intensity compensation starts. We tested the new exponential model against the 'conventional' logarithmic model on data from five bat species. The new model performed better in 77% of the trials and as good as the conventional model in the rest (23%). We found much steeper rates of compensation when fitting the model to individual rather than pooled data, with slopes often steeper than -20 dB per halving of distance. This emphasizes the importance of analyzing individual events. The results are discussed in light of habitat constraints and the interaction between bats and their eared prey.

Articulatory movements modulate auditory responses to speech

Production of actions is highly dependent on concurrent sensory information. In speech production, for example, movement of the articulators is guided by both auditory and somatosensory input. It has been demonstrated in non-human primates that self-produced vocalizations and those of others are differentially processed in the temporal cortex. The aim of the current study was to investigate how auditory and motor responses differ for self-produced and externally produced speech. Using functional neuroimaging, subjects were asked to produce sentences aloud, to silently mouth while listening to a different speaker producing the same sentence, to passively listen to sentences being read aloud, or to read sentences silently.

We show that that separate regions of the superior temporal cortex display distinct response profiles to speaking aloud, mouthing while listening, and passive listening. Responses in anterior superior temporal cortices in both hemispheres are greater for passive listening compared with both mouthing while listening, and speaking aloud. This is the first demonstration that articulation, whether or not it has auditory consequences, modulates responses of the dorsolateral temporal cortex. In contrast posterior regions of the superior temporal cortex are recruited during both articulation conditions. In dorsal regions of the posterior superior temporal gyrus, responses to mouthing and reading aloud were equivalent, and in more ventral posterior superior temporal sulcus, responses were greater for reading aloud compared with mouthing while listening. These data demonstrate an anterior–posterior division of superior temporal regions where anterior fields are suppressed during motor output, potentially for the purpose of enhanced detection of the speech of others. We suggest posterior fields are engaged in auditory processing for the guidance of articulation by auditory information.

Source level reduction and sonar beam aiming in landing big brown bats (Eptesicus fuscus)

Reduction of echolocation call source levels in bats has previously been studied using set-ups with one microphone. By using a 16 microphone array, sound pressure level (SPL) variations, possibly caused by the scanning movements of the bat, can be excluded and the sonar beam aiming can be studied. During the last two meters of approach flights to a landing platform in a large flight room, five big brown bats aimed sonar beams at the landing site and reduced the source level on average by 7 dB per halving of distance. Considerable variation was found among the five individuals in the amount of source level reduction ranging from 4 to 9 dB per halving of distance. These results are discussed with respect to automatic gain control and intensity compensation and the combination of the two effects. It is argued that the two effects together do not lead to a stable echo level at the cochlea. This excludes a tightly coupled closed loop feed back control system as an explanation for the observed reduction of signal SPL in landing big brown bats.

Biosonar adjustments to target range of echolocating bottlenose dolphins(Tursiops sp.) in the wild

Toothed whales use echolocation to locate and track prey. Most knowledge of toothed whale echolocation stems from studies on trained animals, and little is known about how toothed whales regulate and use their biosonar systems in the wild. Recent research suggests that an automatic gain control mechanism in delphinid biosonars adjusts the biosonar output to the one-way transmission loss to the target, possibly a consequence of pneumatic restrictions in how fast the sound generator can be actuated and still maintain high outputs. This study examines the relationships between target range (R), click intervals,and source levels of wild bottlenose dolphins (Tursiops sp.) by recording regular (non-buzz) echolocation clicks with a linear hydrophone array. Dolphins clicked faster with decreasing distance to the array,reflecting a decreasing delay between the outgoing echolocation click and the returning array echo. However, for interclick intervals longer than 30–40 ms, source levels were not limited by the repetition rate. Thus,pneumatic constraints in the sound-production apparatus cannot account for source level adjustments to range as a possible automatic gain control mechanism for target ranges longer than a few body lengths of the dolphin. Source level estimates drop with reducing range between the echolocating dolphins and the target as a function of 17 log(R). This may indicate either(1) an active form of time-varying gain in the biosonar independent of click intervals or (2) a bias in array recordings towards a 20 log(R) relationship for apparent source levels introduced by a threshold on received click levels included in the analysis.

Echo frequency selectivity of duration-tuned inferior collicular neurons of the big brown bat, Eptesicus fuscus, determined with pulse-echo pairs

During hunting, insectivorous bats such as Eptesicus fuscus progressively vary the repetition rate, duration, frequency and amplitude of emitted pulses such that analysis of an echo parameter by bats would be inevitably affected by other co-varying echo parameters. The present study is to determine the variation of echo frequency selectivity of duration-tuned inferior collicular neurons during different phases of hunting using pulse-echo (P-E) pairs as stimuli. All collicular neurons discharge maximally to a tone at a particular frequency which is defined as the best frequency (BF). Most collicular neurons also discharge maximally to a BF pulse at a particular duration which is defined as the best duration (BD). A family of echo iso-level frequency tuning curves (iso-level FTC) of these duration-tuned collicular neurons is measured with the number of impulses in response to the echo pulse at selected frequencies when the P-E pairs are presented at varied P-E duration and gap. Our data show that these duration-tuned collicular neurons have narrower echo iso-level FTC when measured with BD than with non-BD echo pulses. Also, IC neurons with low BF and short BD have narrower echo iso-level FTC than IC neurons with high BF and long BD have. The bandwidth of echo iso-level FTC significantly decreases with shortening of P-E duration and P-E gap. These data suggest that duration-tuned collicular neurons not only can facilitate bat's echo recognition but also can enhance echo frequency selectivity for prey feature analysis throughout a target approaching sequence during hunting. These data also support previous behavior studies showing that bats prepare their auditory system to analyze expected returning echoes within a time window to extract target features after pulse emission.

A false killer whale adjusts its hearing when it echolocates

The use of auditory evoked potential (AEP) measurements has added considerably to knowledge of the hearing mechanisms of marine mammals. We have recently measured the hearing of a stranded infant Risso's dolphin, the audiograms of white-beaked dolphins temporarily caught and released, and the hearing of anaesthetized polar bears. Most small toothed whales echolocate and hear very high frequency sounds underwater. While much has previously been learned about the echolocation performance and characteristics of the outgoing signals of echolocating dolphins and small whales, the hearing processes occurring while these animals actively echolocate have not previously been examined. Working with a well-trained echolocating false killer whale (Pseudorca crassidens) wearing latex surface suction cup electrodes, we have measured echolocation hearing AEPs in response to outgoing echolocation clicks, returning echoes, and comparable simulated whale clicks and echoes in a variety of situations. We have found that: (1) the whale may hear her loud outgoing clicks and much quieter returning echoes at comparable levels, (2) the whale has protective mechanisms that dampen the intensity of her outgoing signals - she hears her outgoing signals at a level about 40 dB lower than similar signals presented directly in front of her, (3) when echo return levels are lowered either by making the targets smaller or by placing the targets farther away - without changing the levels of her outgoing signals - the hearing of these echoes remains at almost the same level, (4) if targets are made much smaller and harder to echolocate, the animal will modify what she hears of her outgoing signal - as if to heighten overall hearing sensitivity to keep the echo level hearable, (5) the animal has an active 'automatic gain control' mechanism in her hearing based on both forward masking that balances outgoing pulse intensity and time between pulse and echo, and active hearing control. Overall, hearing during echolocation appears to be a very active process.

Bat echolocation calls: adaptation and convergent evolution

Bat echolocation calls provide remarkable examples of 'good design' through evolution by natural selection. Theory developed from acoustics and sonar engineering permits a strong predictive basis for understanding echolocation performance. Call features, such as frequency, bandwidth, duration and pulse interval are all related to ecological niche. Recent technological breakthroughs have aided our understanding of adaptive aspects of call design in free-living bats. Stereo videogrammetry, laser scanning of habitat features and acoustic flight path tracking permit reconstruction of the flight paths of echolocating bats relative to obstacles and prey in nature. These methods show that echolocation calls are among the most intense airborne vocalizations produced by animals. Acoustic tracking has clarified how and why bats vary call structure in relation to flight speed. Bats using broadband echolocation calls adjust call design in a range-dependent manner so that nearby obstacles are localized accurately. Recent phylogenetic analyses based on gene sequences show that particular types of echolocation signals have evolved independently in several lineages of bats. Call design is often influenced more by perceptual challenges imposed by the environment than by phylogeny, and provides excellent examples of convergent evolution. Now that whole genome sequences of bats are imminent, understanding the functional genomics of echolocation will become a major challenge.

Source-to-sensation level ratio of transmitted biosonar pulses in an echolocating false killer whale

Transmitted biosonar pulses, and the brain auditory evoked potentials (AEPs) associated with those pulses, were synchronously recorded in a false killer whale Pseudorca crassidens trained to accept suction-cup EEG electrodes and to detect targets by echolocation. AEP amplitude was investigated as a function of the transmitted biosonar pulse source level. For that, a few thousand of the individual AEP records were sorted according to the spontaneously varied amplitude of synchronously recorded biosonar pulses. In each of the sorting bins (in 5-dB steps) AEP records were averaged to extract AEP from noise; AEP amplitude was plotted as a function of the biosonar pulse source level. For comparison, AEPs were recorded to external (in free field) sound pulses of a waveform and spectrum similar to those of the biosonar pulses; amplitude of these AEPs was plotted as a function of sound pressure level. A comparison of these two functions has shown that, depending on the presence or absence of a target, the sensitivity of the whale's hearing to its own transmitted biosonar pulses was 30 to 45 dB lower than might be expected in a free acoustic field.

Audio-vocal interaction in the pontine brainstem during self-initiated vocalization in the squirrel monkey

The adjustment of the voice by auditory input happens at several brain levels. The caudal pontine brainstem, though rarely investigated, is one candidate area for such audio-vocal integration. We recorded neuronal activity in this area in awake, behaving squirrel monkeys (Saimiri sciureus) during vocal communication, using telemetric single-unit recording techniques. We found audio-vocal neurons at locations not described before, namely in the periolivary region of the superior olivary complex and the adjacent pontine reticular formation. They showed various responses to external sounds (noise bursts) and activity increases (excitation) or decreases (inhibition) to self-produced vocalizations, starting prior to vocal onset and continuing through vocalizations. In most of them, the responses to noise bursts and self-produced vocalizations were similar, with the only difference that neuronal activity started prior to vocal onset. About one-third responded phasically to noise bursts, independent of whether they increased or decreased their activity to vocalization. The activity of most audio-vocal neurons correlated with basic acoustic features of the vocalization, such as call duration and/or syllable structure. Auditory neurons near audio-vocal neurons showed significantly more frequent phasic response patterns than those in areas without audio-vocal activity. Based on these findings, we propose that audio-vocal neurons showing similar activity to external acoustical stimuli and vocalization play a role in olivocochlear regulation. Specifically, audio-vocal neurons with a phasic response to external auditory stimuli are candidates for the mediation of basal audio-vocal reflexes such as the Lombard reflex. Thus, our findings suggest that complex audio-vocal integration mechanisms exist in the ventrolateral pontine brainstem.

The effect of monaural middle ear destruction on response properties of neurons in the auditory midbrain of juvenile and adult mice

This article reviews our studies of the effect of monaural middle ear destruction on midbrain auditory response properties of the laboratory mouse, Mus musculus. Monaural middle ear destruction was performed on juvenile and adult mice and the auditory sensitivity of neurons in the midbrain inferior colliculus (IC) ipsilateral and contralateral to the intact ear was examined 4 weeks later. When stimulated with sound pulses, IC neurons of the control mice typically had lower minimum threshold, larger dynamic range, and sharper frequency tuning curve than IC neurons of the experimental juvenile and adult mice. In the experimental mice, neurons in the ipsilateral IC had significantly longer latency, higher minimum threshold, and smaller dynamic range than neurons in the contralateral IC. When determined at two sound directions (ipsilateral 40 degrees and contralateral 40 degrees to the recording site), IC neurons of the control mice had higher minimum threshold, sharper frequency tuning curve but smaller dynamic range at I-40 degrees than at C-40 degrees . However, these direction-dependent response properties were not observed for IC neurons of the experimental juvenile and adult mice. Clear tonotopic organization was only observed in the IC of the control mice and experimental adult mice but not in the IC of experimental juvenile mice. These different response properties are discussed in relation to the effect of monaural middle ear destruction.

Corollary discharge inhibition and audition in the stridulating cricket

The romantic notion of crickets singing on a warm summer's evening is quickly dispelled when one comes ear to ear with a stridulating male. Remarkably, stridulating male crickets are able to hear sounds from the environment despite generating a 100 db song (Heiligenberg 1969; Jones and Dambach 1973). This review summarises recent work examining how they achieve this feat of sensory processing. While the responsiveness of the crickets' peripheral auditory system (tympanic membrane, tympanic nerve, state of the acoustic spiracle) is maintained during sound production, central auditory neurons are inhibited by a feedforward corollary discharge signal precisely timed to coincide with the auditory neurons' maximum response to self-generated sound. In this way, the corollary discharge inhibition prevents desensitisation of the crickets' auditory pathway during sound production.

Dynamics of auditory-vocal interaction in monkey auditory cortex

Single neurons in the primate auditory cortex exhibit vocalization-related modulations (excitatory or inhibitory) during self-initiated vocal production. Previous studies have shown that these modulations of cortical activity are variable in individual neurons' responses to multiple instances of vocalization and diverse between different cortical neurons. The present study investigated dynamic patterns of vocalization-related modulations and demonstrated that much of the variability in cortical modulations was related to the acoustic structures of self-produced vocalization. We found that suppression of single unit activity during multi-phrased vocalizations was temporally specific in that it was maintained during each phrase, but was released between phrases. Furthermore, the degree of suppression or excitation was correlated to the mean energy and frequency of the produced vocalizations, accounting for much of the response variability between multiple instances of vocalization. Simultaneous recordings of pairs of neurons from a single electrode revealed that the modulations by self-produced vocalizations in nearby neurons were largely uncorrelated. Additionally, vocalization-induced suppression was found to be preferentially distributed to upper cortical layers. Finally, we showed that the summation of all auditory cortical activity during vocalization, including both single and multi-unit responses, was weakly excitatory, consistent with observations from studies of the human brain during speech.

Biosonar performance of foraging beaked whales (Mesoplodon densirostris)

Toothed whales (Cetacea, odontoceti) emit sound pulses to probe their surroundings by active echolocation. Non-invasive, acoustic Dtags were placed on deep-diving Blainville's beaked whales (Mesoplodon densirostris) to record their ultrasonic clicks and the returning echoes from prey items, providing a unique view on how a whale operates its biosonar during foraging in the wild. The process of echolocation during prey capture in this species can be divided into search, approach and terminal phases, as in echolocating bats. The approach phase, defined by the onset of detectable echoes recorded on the tag for click sequences terminated by a buzz, has interclick intervals (ICI) of 300-400 ms. These ICIs are more than a magnitude longer than the decreasing two-way travel time to the targets, showing that ICIs are not given by the two-way-travel times plus a fixed, short lag time. During the approach phase, the received echo energy increases by 10.4(±2) dB when the target range is halved, demonstrating that the whales do not employ range-compensating gain control of the transmitter, as has been implicated for some bats and dolphins. The terminal/buzz phase with ICIs of around 10 ms is initiated when one or more targets are within approximately a body length of the whale (2-5 m), so that strong echo returns in the approach phase are traded for rapid updates in the terminal phase. It is suggested that stable ICIs in the search and approach phases facilitate auditory scene analysis in a complex multi-target environment, and that a concomitant low click rate allows the whales to maintain high sound pressure outputs for prey detection and discrimination with a pneumatically driven,bi-modal sound generator.

Sound-evoked efferent effects on cochlear mechanics of the mustached bat

The influence of the crossed medial efferent system on cochlear mechanics of the mustached bat was tested by measuring delayed evoked otoacoustic emissions (DEOAEs), cochlear microphonics, distortion product otoacoustic emissions (DPOAEs) and stimulus frequency otoacoustic emissions. Contralaterally delivered sinusoids, broadband noise and bat echolocation calls were used for acoustic stimulation of the efferent system. With all four measures we found a level-dependent suppression under stimulation with both broadband noise and echolocation calls. In addition, the sharply tuned cochlear resonance of the mustached bat which is involved in processing echolocation signals at 61 kHz shifted upward in frequency by several 100 Hz. Presentation of sinusoids did not have any significant effect. DEOAEs and DPOAEs were in some cases enhanced during contralateral presentation of the bat calls at moderate intensities. The most important function of the efferent system in the mustached bat might be the control of the extraordinarily fine-tuned resonator of this species, which is close to instability as evident from the very pronounced evoked otoacoustic emissions which sometimes convert into spontaneous otoacoustic emissions of high level.

Automatic gain control in the echolocation system of dolphins

In bats and technological sonars, the gain of the receiver is progressively increased with time after the transmission of a signal to compensate for acoustic propagation loss. The current understanding of dolphin echolocation indicates that automatic gain control is not a part of their sonar system. In order to test this understanding, we have performed field measurements of free-ranging echolocating dolphins. Here we show that dolphins do possess an automatic gain control mechanism, but that it is implemented in the transmission phase rather than the receiving phase of a sonar cycle. We find that the amplitude of the dolphins' echolocation signals are highly range dependent; this amplitude increases with increasing target range, R, in a 20 log(R) fashion to compensate for propagation loss. If the echolocation target is a fish school with many sound scatterers, the echoes from the school will remain nearly constant with range as the dolphin closes in on it. This characteristic has the same effect as time-varying gain in bats and technological sonar when considered from a sonar system perspective.

Sensory-motor interaction in the primate auditory cortex during self-initiated vocalizations

Little is known about sensory-motor interaction in the auditory cortex of primates at the level of single neurons and its role in supporting vocal communication. The present study investigated single-unit activities in the auditory cortex of a vocal primate, the common marmoset (Callithrix jacchus), during self-initiated vocalizations. We found that 1) self-initiated vocalizations resulted in suppression of neural discharges in a majority of auditory cortical neurons. The vocalization-induced inhibition suppressed both spontaneous and stimulus-driven discharges. Suppressed units responded poorly to external acoustic stimuli during vocalization. 2) Vocalization-induced suppression began several hundred milliseconds prior to the onset of vocalization. 3) The suppression of cortical discharges reduced neural firings to below the rates expected from a unit's rate-level function, adjusted for known subcortical attenuation, and therefore was likely not entirely caused by subcortical attenuation mechanisms. 4) A smaller population of auditory cortical neurons showed increased discharges during self-initiated vocalizations. This vocalization-related excitation began after the onset of vocalization and is likely the result of acoustic feedback. Units showing this excitation responded nearly normally to external stimuli during vocalization. Based on these findings, we propose that the suppression of auditory cortical neurons, possibly originating from cortical vocal production centers, acts to increase the dynamic range of cortical responses to vocalization feedback for self monitoring. The excitatory responses, on the other hand, likely play a role in maintaining hearing sensitivity to the external acoustic environment during vocalization.

A corollary discharge mechanism modulates central auditory processing in singing crickets

Crickets communicate using loud (100 dB SPL) sound signals that could adversely affect their own auditory system. To examine how they cope with this self-generated acoustic stimulation, intracellular recordings were made from auditory afferent neurons and an identified auditory interneuron-the Omega 1 neuron (ON1)-during pharmacologically elicited singing (stridulation). During sonorous stridulation, the auditory afferents and ON1 responded with bursts of spikes to the crickets' own song. When the crickets were stridulating silently, after one wing had been removed, only a few spikes were recorded in the afferents and ON1. Primary afferent depolarizations (PADs) occurred in the terminals of the auditory afferents, and inhibitory postsynaptic potentials (IPSPs) were apparent in ON1. The PADs and IPSPs were composed of many summed, small-amplitude potentials that occurred at a rate of about 230 Hz. The PADs and the IPSPs started during the closing wing movement and peaked in amplitude during the subsequent opening wing movement. As a consequence, during silent stridulation, ON1's response to acoustic stimuli was maximally inhibited during wing opening. Inhibition coincides with the time when ON1 would otherwise be most strongly excited by self-generated sounds in a sonorously stridulating cricket. The PADs and the IPSPs persisted in fictively stridulating crickets whose ventral nerve cord had been isolated from muscles and sense organs. This strongly suggests that the inhibition of the auditory pathway is the result of a corollary discharge from the stridulation motor network. The central inhibition was mimicked by hyperpolarizing current injection into ON1 while it was responding to a 100 dB SPL sound pulse. This suppressed its spiking response to the acoustic stimulus and maintained its response to subsequent, quieter stimuli. The corollary discharge therefore prevents auditory desensitization in stridulating crickets and allows the animals to respond to external acoustic signals during the production of calling song.

Monaural middle ear destruction in juvenile and adult mice: effects on responses to sound direction in the inferior colliculus ipsilateral to the intact ear

This study examined the effect of monaural middle ear destruction on auditory responses to sound direction in the inferior colliculus (IC) of the laboratory mice, Mus musculus. Monaural middle ear destruction was performed on juvenile and adult mice (the experimental mice). Auditory response properties of neurons to ipsilateral and contralateral sounds (I-40 degrees and C-40 degrees ) were examined in the IC ipsilateral to the intact ear 4 weeks later. IC neurons of control mice had higher minimum thresholds (MTs), larger Q(n) (Q(10), Q(30)) values but smaller dynamic ranges at I-40 degrees than at C-40 degrees. These direction-dependent response properties were not observed for IC neurons of experimental juvenile and adult mice. However, Q(n) values of IC neurons were significantly smaller in experimental juvenile than in control and experimental adult mice. Normal tonotopic organization in terms of positive correlation between recording depth and best frequency (BF) was observed in the IC of control and experimental adult mice at both sound directions but not in the IC of experimental juvenile mice. A positive correlation of increasing MT with BF was only observed for IC neurons in control mice but not in both experimental mice. Possible mechanisms for these different response properties are discussed.

Interaction between excitation and inhibition affects frequency tuning curve, response size and latency of neurons in the auditory cortex of the big brown bat, Eptesicus fuscus

Neurons in the auditory cortex (AC) receive convergent excitatory and inhibitory inputs from the lower auditory nuclei. Interaction between these two opposing inputs shapes different response properties of AC neurons. In this study, we examined how this interaction might affect the frequency tuning curves (FTCs), number of impulses and latency of AC neurons in the big brown bat, Eptesicus fuscus, using a probe (excitatory tone) and a masker (inhibitory tone) under different stimulation conditions. Excitatory FTCs of AC neurons were either V-shaped, closed (i.e. upper threshold) or double-peaked. Inhibitory FTCs were obtained either at both flanks or only at the low or high flank of excitatory FTCs. Application of bicuculline, an antagonist for gamma-aminobutyric acid A receptors, produced expansion of excitatory FTCs into predrug inhibitory FTCs. Inhibition of probe-elicited responses occurred when a masker was presented at certain intertone intervals. Maximal inhibition typically took place when a masker was presented within 4 ms prior to the probe. During maximal inhibition, a neuron had the minimal number of impulses and the longest response latency. Inhibition became stronger with increasing masker intensity but became weaker with increasing intertone interval. Biological significance of these data is discussed.