Faster Repetition Rate Sharpens the Cortical Representation of Echo Streams in Echolocating Bats.

There is consensus that primary auditory cortex (A1) utilizes a combination of rate codes and temporally precise population codes to represent discreet auditory objects. During the response to auditory streams, forward suppression constrains cortical rate coding strategies, but it may also be well positioned to enhance temporal coding strategies that rely on synchronized firing across neural ensembles. Here, we exploited the rapid temporal dynamics of bat echolocation to investigate how forward suppression modulates the cortical ensemble representation of complex acoustic signals embedded in echo streams. We recorded from auditory cortex of anesthetized free-tailed bats while stimulating the auditory system with naturalistic biosonar pulse-echo sequences covering a range of pulse emission rates. As expected, increasing pulse repetition rate significantly reduced the number of spikes per echo stimulus, but it also increased spike timing precision and doubled the information gain. This increased spike-timing precision translated into more robust inter-neuronal synchronization patterns with >10-dB higher signal-to-noise ratios (SNRs) at the ensemble level. We propose that forward suppression dynamically mediates a trade-off between the sensitive detection of isolated sounds versus precise spatiotemporal encoding of ongoing sound sequences in auditory cortex.

Temporal coding of echo spectral shape in the bat auditory cortex.

Echolocating bats rely upon spectral interference patterns in echoes to reconstruct fine details of a reflecting object's shape. However, the acoustic modulations required to do this are extremely brief, raising questions about how their auditory cortex encodes and processes such rapid and fine spectrotemporal details. Here, we tested the hypothesis that biosonar target shape representation in the primary auditory cortex (A1) is more reliably encoded by changes in spike timing (latency) than spike rates and that latency is sufficiently precise to support a synchronization-based ensemble representation of this critical auditory object feature space. To test this, we measured how the spatiotemporal activation patterns of A1 changed when naturalistic spectral notches were inserted into echo mimic stimuli. Neurons tuned to notch frequencies were predicted to exhibit longer latencies and lower mean firing rates due to lower signal amplitudes at their preferred frequencies, and both were found to occur. Comparative analyses confirmed that significantly more information was recoverable from changes in spike times relative to concurrent changes in spike rates. With this data, we reconstructed spatiotemporal activation maps of A1 and estimated the level of emerging neuronal spike synchrony between cortical neurons tuned to different frequencies. The results support existing computational models, indicating that spectral interference patterns may be efficiently encoded by a cascading tonotopic sequence of neural synchronization patterns within an ensemble of network activity that relates to the physical features of the reflecting object surface.

Hearing sensitivity evaluated by the auditory brainstem response in Miniopterus fuliginosus.

This study evaluated the hearing sensitivity of Miniopterus fuliginosus, a frequency-modulating (FM) bat species, by measuring the auditory brainstem responses in the inferior colliculus. The average audiogram was U-shaped. The mean threshold decreased gradually as the frequency increased from 16 to 40 kHz and then decreased rapidly as the frequency reached 46 kHz, with the peak sensitivity occurring at the terminal portion of the echolocation pulse between frequencies of 44 and 56 kHz. The shape of audiogram of M. fuliginosus is consistent with other FM bats, and is compared with its vocalization behavior.

Processing of Natural Echolocation Sequences in the Inferior Colliculus of Seba's Fruit Eating Bat, Carollia perspicillata.

For the purpose of orientation, echolocating bats emit highly repetitive and spatially directed sonar calls. Echoes arising from call reflections are used to create an acoustic image of the environment. The inferior colliculus (IC) represents an important auditory stage for initial processing of echolocation signals. The present study addresses the following questions: (1) how does the temporal context of an echolocation sequence mimicking an approach flight of an animal affect neuronal processing of distance information to echo delays? (2) how does the IC process complex echolocation sequences containing echo information from multiple objects (multiobject sequence)? Here, we conducted neurophysiological recordings from the IC of ketamine-anaesthetized bats of the species Carollia perspicillata and compared the results from the IC with the ones from the auditory cortex (AC). Neuronal responses to an echolocation sequence was suppressed when compared to the responses to temporally isolated and randomized segments of the sequence. The neuronal suppression was weaker in the IC than in the AC. In contrast to the cortex, the time course of the acoustic events is reflected by IC activity. In the IC, suppression sharpens the neuronal tuning to specific call-echo elements and increases the signal-to-noise ratio in the units' responses. When presenting multiple-object sequences, despite collicular suppression, the neurons responded to each object-specific echo. The latter allows parallel processing of multiple echolocation streams at the IC level. Altogether, our data suggests that temporally-precise neuronal responses in the IC could allow fast and parallel processing of multiple acoustic streams.

Sensorimotor integration on a rapid time scale.

Sensing is fundamental to the control of movement: From grasping objects to speech production, sensing guides action. So far, most of our knowledge about sensorimotor integration comes from visually guided reaching and oculomotor integration, in which the time course and trajectories of movements can be measured at a high temporal resolution. By contrast, production of vocalizations by humans and animals involves complex and variable actions, and each syllable often lasts a few hundreds of milliseconds, making it difficult to infer underlying neural processes. Here, we measured and modeled the transfer of sensory information into motor commands for vocal amplitude control in response to background noise, also known as the Lombard effect. We exploited the brief vocalizations of echolocating bats to trace the time course of the Lombard effect on a millisecond time scale. Empirical studies revealed that the Lombard effect features a response latency of a mere 30 ms and provided the foundation for the quantitative audiomotor model of the Lombard effect. We show that the Lombard effect operates by continuously integrating the sound pressure level of background noise through temporal summation to guide the extremely rapid vocal-motor adjustments. These findings can now be extended to models and measures of audiomotor integration in other animals, including humans.

Human Exploration of Enclosed Spaces through Echolocation.

Some blind humans have developed echolocation, as a method of navigation in space. Echolocation is a truly active sense because subjects analyze echoes of dedicated, self-generated sounds to assess space around them. Using a special virtual space technique, we assess how humans perceive enclosed spaces through echolocation, thereby revealing the interplay between sensory and vocal-motor neural activity while humans perform this task. Sighted subjects were trained to detect small changes in virtual-room size analyzing real-time generated echoes of their vocalizations. Individual differences in performance were related to the type and number of vocalizations produced. We then asked subjects to estimate virtual-room size with either active or passive sounds while measuring their brain activity with fMRI. Subjects were better at estimating room size when actively vocalizing. This was reflected in the hemodynamic activity of vocal-motor cortices, even after individual motor and sensory components were removed. Activity in these areas also varied with perceived room size, although the vocal-motor output was unchanged. In addition, thalamic and auditory-midbrain activity was correlated with perceived room size; a likely result of top-down auditory pathways for human echolocation, comparable with those described in echolocating bats. Our data provide evidence that human echolocation is supported by active sensing, both behaviorally and in terms of brain activity. The neural sensory-motor coupling complements the fundamental acoustic motor-sensory coupling via the environment in echolocation.SIGNIFICANCE STATEMENT Passive listening is the predominant method for examining brain activity during echolocation, the auditory analysis of self-generated sounds. We show that sighted humans perform better when they actively vocalize than during passive listening. Correspondingly, vocal motor and cerebellar activity is greater during active echolocation than vocalization alone. Motor and subcortical auditory brain activity covaries with the auditory percept, although motor output is unchanged. Our results reveal behaviorally relevant neural sensory-motor coupling during echolocation.

Functional segregation of monaural and binaural selectivity in the pallid bat auditory cortex.

Different fields of the auditory cortex can be distinguished by the extent and level tolerance of spatial selectivity. The mechanisms underlying the range of spatial tuning properties observed across cortical fields are unclear. Here, this issue was addressed in the pallid bat because its auditory cortex contains two segregated regions of response selectivity that serve two different behaviors: echolocation for obstacle avoidance and localization of prey-generated noise. This provides the unique opportunity to examine mechanisms of spatial properties in two functionally distinct regions. Previous studies have shown that spatial selectivity of neurons in the region selective for noise (noise-selective region, NSR) is level tolerant and shaped by interaural level difference (ILD) selectivity. In contrast, spatial selectivity of neurons in the echolocation region ('FM sweep-selective region' or FMSR) is strongly level dependent with many neurons responding to multiple distinct spatial locations for louder sounds. To determine the mechanisms underlying such level dependence, frequency, azimuth, rate-level responses and ILD selectivity were measured from the same FMSR neurons. The majority (∼75%) of FMSR neurons were monaural (ILD insensitive). Azimuth tuning curves expanded or split into multiple peaks with increasing sound level in a manner that was predicted by the rate-level response of neurons. These data suggest that azimuth selectivity of FMSR neurons depends more on monaural ear directionality and rate-level responses. The pallid bat cortex utilizes segregated monaural and binaural regions to process echoes and prey-generated noise. Together the pallid bat FMSR/NSR data provide mechanistic explanations for a broad range of spatial tuning properties seen across species.

People's Ability to Detect Objects Using Click-Based Echolocation: A Direct Comparison between Mouth-Clicks and Clicks Made by a Loudspeaker.

Echolocation is the ability to use reflected sound to obtain information about the spatial environment. Echolocation is an active process that requires both the production of the emission as well as the sensory processing of the resultant sound. Appreciating the general usefulness of echo-acoustic cues for people, in particular those with vision impairments, various devices have been built that exploit the principle of echolocation to obtain and provide information about the environment. It is common to all these devices that they do not require the person to make a sound. Instead, the device produces the emission autonomously and feeds a resultant sound back to the user. Here we tested if echolocation performance in a simple object detection task was affected by the use of a head-mounted loudspeaker as compared to active clicking. We found that 27 sighted participants new to echolocation did generally better when they used a loudspeaker as compared to mouth-clicks, and that two blind participants with experience in echolocation did equally well with mouth clicks and the speaker. Importantly, performance of sighted participants' was not statistically different from performance of blind experts when they used the speaker. Based on acoustic click data collected from a subset of our participants, those participants whose mouth clicks were more similar to the speaker clicks, and thus had higher peak frequencies and sound intensity, did better. We conclude that our results are encouraging for the consideration and development of assistive devices that exploit the principle of echolocation.

Broadband noise exposure does not affect hearing sensitivity in big brown bats (Eptesicus fuscus).

In many vertebrates, exposure to intense sounds under certain stimulus conditions can induce temporary threshold shifts that reduce hearing sensitivity. Susceptibility to these hearing losses may reflect the relatively quiet environments in which most of these species have evolved. Echolocating big brown bats (Eptesicus fuscus) live in extremely intense acoustic environments in which they navigate and forage successfully, both alone and in company with other bats. We hypothesized that bats may have evolved a mechanism to minimize noise-induced hearing losses that otherwise could impair natural echolocation behaviors. The hearing sensitivity of seven big brown bats was measured in active echolocation and passive hearing tasks, before and after exposure to broadband noise spanning their audiometric range (10-100 kHz, 116 dB SPL re. 20 µPa rms, 1 h duration; sound exposure level 152 dB). Detection thresholds measured 20 min, 2 h or 24 h after exposure did not vary significantly from pre-exposure thresholds or from thresholds in control (sham exposure) conditions. These results suggest that big brown bats may be less susceptible to temporary threshold shifts than are other terrestrial mammals after exposure to similarly intense broadband sounds. These experiments provide fertile ground for future research on possible mechanisms employed by echolocating bats to minimize hearing losses while orienting effectively in noisy biological soundscapes.

Temporal encoding precision of bat auditory neurons tuned to target distance deteriorates on the way to the cortex.

During echolocation, bats estimate distance to avoid obstacles and capture moving prey. The primary distance cue is the delay between the bat's emitted echolocation pulse and the return of an echo. In the bat's auditory system, echo delay-tuned neurons that only respond to pulse-echo pairs having a specific echo delay serve target distance calculation. Accurate prey localization should benefit from the spike precision in such neurons. Here we show that delay-tuned neurons in the inferior colliculus of the mustached bat respond with higher temporal precision, shorter latency and shorter response duration than those of the auditory cortex. Based on these characteristics, we suggest that collicular neurons are best suited for a fast and accurate response that could lead to fast behavioral reactions while cortical neurons, with coarser temporal precision and longer latencies and response durations could be more appropriate for integrating acoustic information over time. The latter could be important for the formation of biosonar images.

Synaptic mechanisms shaping delay-tuned combination-sensitivity in the auditory thalamus of mustached bats.

For the processing of target-distance information, delay-tuned auditory neurons of the mustached bat show facilitative responses to a combination of signal elements of a biosonar pulse-echo pair with a specific echo delay. They are initially produced in the inferior colliculus by facilitative responses based on the coincidence of the rebound response following glycinergic inhibition to the first harmonic of the pulse and a short-latency response to the 2nd-4th harmonics of its echo. Here, we report that further facilitative responses to pulse-echo pairs of thalamic delay-tuned neurons are mediated by glutamate receptors (NMDA and non-NMDA receptors), and that GABAergic inhibition shortens the duration of facilitative responses mediated by NMDA-receptors, without changing the delay tuning of thalamic delay-tuned neurons. Different from collicular delay-tuned neurons, thalamic ones respond much more to pulse-echo pairs than individual signal elements. The neural mechanisms involved in shaping thalamic delay-tuning support a model of hierarchical signal processing in the auditory system.

Directional Hearing and Sound Source Localization in Fishes.

Evidence suggests that the capacity for sound source localization is common to mammals, birds, reptiles, and amphibians, but surprisingly it is not known whether fish locate sound sources in the same manner (e.g., combining binaural and monaural cues) or what computational strategies they use for successful source localization. Directional hearing and sound source localization in fishes continues to be important topics in neuroethology and in the hearing sciences, but the empirical and theoretical work on these topics have been contradictory and obscure for decades. This chapter reviews the previous behavioral work on directional hearing and sound source localization in fishes including the most recent experiments on sound source localization by the plainfin midshipman fish (Porichthys notatus), which has proven to be an exceptional species for fish studies of sound localization. In addition, the theoretical models of directional hearing and sound source localization for fishes are reviewed including a new model that uses a time-averaged intensity approach for source localization that has wide applicability with regard to source type, acoustic environment, and time waveform.

Revisiting Psychoacoustic Methods for the Assessment of Fish Hearing.

Behavioral methods have been critical in the study of auditory perception and discrimination in fishes. In this chapter, we review some of the common methods used in fish psychoacoustics. We discuss associative methods, such as operant, avoidance, and classical conditioning, and their use in constructing audiograms, measuring frequency selectivity, and auditory stream segregation. We also discuss the measurement of innate behavioral responses, such as the acoustic startle response (ASR), prepulse inhibition (PPI), and phonotaxis, and their use in the assessment of fish hearing to determine auditory thresholds and in the testing of mechanisms for sound source localization. For each psychoacoustic method, we provide examples of their use and discuss the parameters and situations where such methods can be best utilized. In the case of the ASR, we show how this method can be used to construct and compare audiograms between two species of larval fishes, the three-spined stickleback (Gasterosteus aculeatus) and the zebrafish (Danio rerio). We also discuss considerations for experimental design with respect to stimulus presentation and threshold criteria and how these techniques can be used in future studies to investigate auditory perception in fishes.

Mechanisms of Sound Localization in Two Functionally Distinct Regions of the Auditory Cortex.

The auditory cortex is necessary for sound localization. The mechanisms that shape bicoordinate spatial representation in the auditory cortex remain unclear. Here, we addressed this issue by quantifying spatial receptive fields (SRFs) in two functionally distinct cortical regions in the pallid bat. The pallid bat uses echolocation for obstacle avoidance and listens to prey-generated noise to localize prey. Its cortex contains two segregated regions of response selectivity that serve echolocation and localization of prey-generated noise. The main aim of this study was to compare 2D SRFs between neurons in the noise-selective region (NSR) and the echolocation region [frequency-modulated sweep-selective region (FMSR)]. The data reveal the following major differences between these two regions: (1) compared with NSR neurons, SRF properties of FMSR neurons were more strongly dependent on sound level; (2) as a population, NSR neurons represent a broad region of contralateral space, while FMSR selectivity was focused near the midline at sound levels near threshold and expanded considerably with increasing sound levels; and (3) the SRF size and centroid elevation were correlated with the characteristic frequency in the NSR, but not the FMSR. These data suggest different mechanisms of sound localization for two different behaviors. Previously, we reported that azimuth is represented by predictable changes in the extent of activated cortex. The present data indicate how elevation constrains this activity pattern. These data suggest a novel model for bicoordinate spatial representation that is based on the extent of activated cortex resulting from the overlap of binaural and tonotopic maps. SIGNIFICANCE STATEMENT: Unlike the visual and somatosensory systems, spatial information is not directly represented at the sensory receptor epithelium in the auditory system. Spatial locations are computed by integrating neural binaural properties and frequency-dependent pinna filtering, providing a useful model to study how neural properties and peripheral structures are adapted for sensory encoding. Although auditory cortex is necessary for sound localization, our understanding of how the cortex represents space remains rudimentary. Here we show that two functionally distinct regions of the pallid bat auditory cortex represent 2D space using different mechanisms. In addition, we suggest a novel hypothesis on how the nature of overlap between systematic maps of binaural and frequency selectivity leads to representation of both azimuth and elevation.

Neural processing of auditory signals in the time domain: delay-tuned coincidence detectors in the mustached bat.

The central auditory system produces combination-sensitive neurons tuned to a specific combination of multiple signal elements. Some of these neurons act as coincidence detectors with delay lines for the extraction of spectro-temporal information from sounds. "Delay-tuned" neurons of mustached bats are tuned to a combination of up to four signal elements with a specific delay between them and form a delay map. They are produced in the inferior colliculus by the coincidence of the rebound response following glycinergic inhibition to the first harmonic of a biosonar pulse with the short-latency response to the 2nd-4th harmonics of its echo. Compared with collicular delay-tuned neurons, thalamic and cortical ones respond more to pulse-echo pairs than individual sounds. Cortical delay-tuned neurons are clustered in the three separate areas. They interact with each other through a circuit mediating positive feedback and lateral inhibition for adjustment and improvement of the delay tuning of cortical and subcortical neurons. The current article reviews the mechanisms for delay tuning and the response properties of collicular, thalamic and cortical delay-tuned neurons in relation to hierarchical signal processing.

Parvalbumin and calbindin expression in parallel thalamocortical pathways in a gleaning bat, Antrozous pallidus.

The pallid bat (Antrozous pallidus) listens to prey-generated noise to localize and hunt terrestrial prey while reserving echolocation to avoid obstacles. The thalamocortical connections in the pallid bat are organized as parallel pathways that may serve echolocation and prey localization behaviors. Thalamic inputs to the cortical echolocation call- and noise-selective regions originate primarily in the suprageniculate nucleus (SG) and ventral division of medial geniculate body (MGBv), respectively. Here we examined the distribution of parvalbumin (PV) and calbindin (CB) expression in cortical regions and thalamic nuclei of these pathways. Electrophysiology was used to identify cortical regions selective for echolocation calls and noise. Immunohistochemistry was used to stain for PV and CB in the auditory cortex and MGB. A higher percentage (relative to Nissl-stained cells) of PV(+) cells compared with CB(+) cells was found in both echolocation call- and noise-selective regions. This was due to differences in cortical layers V-VI, but not layers I-IV. In the MGB, CB(+) cells were present across all divisions of the MGB, with a higher percentage in the MGBv than the SG. Perhaps the most surprising result was the virtual absence of PV staining in the MGBv. PV staining was present only in the SG. Even in the SG, the staining was mostly diffuse in the neuropil. These data support the notion that calcium binding proteins are differentially distributed in different processing streams. Our comparative data, however, do not support a general mammalian pattern of PV/CB staining that distinguishes lemniscal and nonlemniscal pathways.

Blurry topography for precise target-distance computations in the auditory cortex of echolocating bats.

Echolocating bats use the time from biosonar pulse emission to the arrival of echo (defined as echo delay) to calculate the space depth of targets. In the dorsal auditory cortex of several species, neurons that encode increasing echo delays are organized rostrocaudally in a topographic arrangement defined as chronotopy. Precise chronotopy could be important for precise target-distance computations. Here we show that in the cortex of three echolocating bat species (Pteronotus quadridens, Pteronotus parnellii and Carollia perspicillata), chronotopy is not precise but blurry. In all three species, neurons throughout the chronotopic map are driven by short echo delays that indicate the presence of close targets and the robustness of map organization depends on the parameter of the receptive field used to characterize neuronal tuning. The timing of cortical responses (latency and duration) provides a binding code that could be important for assembling acoustic scenes using echo delay information from objects with different space depths.

Discovering your inner bat: echo-acoustic target ranging in humans.

Echolocation is typically associated with bats and toothed whales. To date, only few studies have investigated echolocation in humans. Moreover, these experiments were conducted with real objects in real rooms; a configuration in which features of both vocal emissions and perceptual cues are difficult to analyse and control. We investigated human sonar target-ranging in virtual echo-acoustic space, using a short-latency, real-time convolution engine. Subjects produced tongue clicks, which were picked up by a headset microphone, digitally delayed, convolved with individual head-related transfer functions and played back through earphones, thus simulating a reflecting surface at a specific range in front of the subject. In an adaptive 2-AFC paradigm, we measured the perceptual sensitivity to changes of the range for reference ranges of 1.7, 3.4 or 6.8 m. In a follow-up experiment, a second simulated surface at a lateral position and a fixed range was added, expected to act either as an interfering masker or a useful reference. The psychophysical data show that the subjects were well capable to discriminate differences in the range of a frontal reflector. The range-discrimination thresholds were typically below 1 m and, for a reference range of 1.7 m, they were typically below 0.5 m. Performance improved when a second reflector was introduced at a lateral angle of 45°. A detailed analysis of the tongue clicks showed that the subjects typically produced short, broadband palatal clicks with durations between 3 and 15 ms, and sound levels between 60 and 108 dB. Typically, the tongue clicks had relatively high peak frequencies around 6 to 8 kHz. Through the combination of highly controlled psychophysical experiments in virtual space and a detailed analysis of both the subjects' performance and their emitted tongue clicks, the current experiments provide insights into both vocal motor and sensory processes recruited by humans that aim to explore their environment by echolocation.

The sonar aperture and its neural representation in bats.

As opposed to visual imaging, biosonar imaging of spatial object properties represents a challenge for the auditory system because its sensory epithelium is not arranged along space axes. For echolocating bats, object width is encoded by the amplitude of its echo (echo intensity) but also by the naturally covarying spread of angles of incidence from which the echoes impinge on the bat's ears (sonar aperture). It is unclear whether bats use the echo intensity and/or the sonar aperture to estimate an object's width. We addressed this question in a combined psychophysical and electrophysiological approach. In three virtual-object playback experiments, bats of the species Phyllostomus discolor had to discriminate simple reflections of their own echolocation calls differing in echo intensity, sonar aperture, or both. Discrimination performance for objects with physically correct covariation of sonar aperture and echo intensity ("object width") did not differ from discrimination performances when only the sonar aperture was varied. Thus, the bats were able to detect changes in object width in the absence of intensity cues. The psychophysical results are reflected in the responses of a population of units in the auditory midbrain and cortex that responded strongest to echoes from objects with a specific sonar aperture, regardless of variations in echo intensity. Neurometric functions obtained from cortical units encoding the sonar aperture are sufficient to explain the behavioral performance of the bats. These current data show that the sonar aperture is a behaviorally relevant and reliably encoded cue for object size in bat sonar.

Regulation of bat echolocation pulse acoustics by striatal dopamine.

The ability to control the bandwidth, amplitude and duration of echolocation pulses is a crucial aspect of echolocation performance but few details are known about the neural mechanisms underlying the control of these voice parameters in any mammal. The basal ganglia (BG) are a suite of forebrain nuclei centrally involved in sensory-motor control and are characterized by their dependence on dopamine. We hypothesized that pharmacological manipulation of brain dopamine levels could reveal how BG circuits might influence the acoustic structure of bat echolocation pulses. A single intraperitoneal injection of a low dose (5 mg kg(-1)) of the neurotoxin 1-methyl-4-phenylpyridine (MPTP), which selectively targets dopamine-producing cells of the substantia nigra, produced a rapid degradation in pulse acoustic structure and eliminated the bat's ability to make compensatory changes in pulse amplitude in response to background noise, i.e. the Lombard response. However, high-performance liquid chromatography (HPLC) measurements of striatal dopamine concentrations revealed that the main effect of MPTP was a fourfold increase rather than the predicted decrease in striatal dopamine levels. After first using autoradiographic methods to confirm the presence and location of D(1)- and D(2)-type dopamine receptors in the bat striatum, systemic injections of receptor subtype-specific agonists showed that MPTP's effects on pulse acoustics were mimicked by a D(2)-type dopamine receptor agonist (Quinpirole) but not by a D(1)-type dopamine receptor agonist (SKF82958). The results suggest that BG circuits have the capacity to influence echolocation pulse acoustics, particularly via D(2)-type dopamine receptor-mediated pathways, and may therefore represent an important mechanism for vocal control in bats.