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This study examines binaural response properties and sensitivity to interaural level difference of single neurons in the primary auditory cortex (AC) of the big brown bat, Eptesicus fuscus under earphone stimulation conditions. Contralateral sound stimulation always evoked response from all 306 AC neurons recorded but ipsilateral sound stimulation either excited, inhibited or did not affect their responses. High best frequency (BF) neurons typically had high minimum threshold (MT) and low BF neurons had low MT. However, both BF and MT did not correlate with their recording depth. The BF of these AC neurons progressively changed from high to low along the anteromedial-posterolateral axis of the AC. Their number of impulses and response latency varied with sound level and inter-aural level differences (ILD). Their number of impulses typically increased either monotonically or non-monotonically to a maximum and the latency shortened to a minimum at a specific sound level. Among 205 AC neurons studied at varied ILD, 178 (87%) and 127 (62%) neurons discharged maximally and responded with the shortest response latency at a specific ILD, respectively. Neurons sequentially isolated within an orthogonal electrode puncture shared similar BF, MT, binaurality and ILD curves. However, the response latency of these AC neurons progressively shortened with recording depth. Species-specific difference among this bat, the mustached bat and the pallid bat is discussed in terms of frequency and binaurality representation in the AC.

Background: This study compares the mental health and psychological response of students with or without hearing loss during the recurrence of the COVID-19 pandemic in Beijing, the capital of China. It explores the relevant factors affecting mental health and provides evidence-driven strategies to reduce adverse psychological impacts during the COVID-19 pandemic. Methods: We used the Chinese version of depression, anxiety, and stress scale 21 (DASS-21) to assess the mental health and the impact of events scale—revised (IES-R) to assess the COVID-19 psychological impact. Results: The students with hearing loss are frustrated with their disability and particularly vulnerable to stress symptoms, but they are highly endurable in mitigating this negative impact on coping with their well-being and responsibilities. They are also more resilient psychologically but less resistant mentally to the pandemic impacts than the students with normal hearing. Their mental and psychological response to the pandemic is associated with more related factors and variables than that of the students with normal hearing is. Conclusions: To safeguard the welfare of society, timely information on the pandemic, essential services for communication disorders, additional assistance and support in mental counseling should be provided to the vulnerable persons with hearing loss that are more susceptible to a public health emergency.

In the ascending auditory pathway, the central nucleus of the inferior colliculus (IC) receives and integrates excitatory and inhibitory inputs from many bilateral lower auditory nuclei, intrinsic projections within the IC, contralateral IC through the commissure of the IC and from the auditory cortex. All these presynaptic excitatory and inhibitory inputs dynamically shape and modulate the auditory response properties of individual IC neurons. For this reason, acoustic response properties vary among individual IC neurons due to different activity pattern of presynaptic inputs. The present study examines modulation of auditory response properties of IC neurons by combining sound stimulation with focal electrical stimulation of the contralateral dorsal nucleus of the lateral lemniscus (referred to as ESDNLL) in the albino mouse. Brief ESDNLL produces variation (increase or decrease) in the number of impulses, response latency and discharge duration of modulated IC neurons. Additionally, 30-minute short-term ESDNLL alone produces variation in the best frequency (BF) and minimum threshold (MT) of modulated IC neurons. These varied response parameters recover in different manner and time course among individual modulated IC neurons. Possible pathways and neural mechanisms underlying these findings are discussed.

The present study aimed to investigate how bats protect their brain in a hypothermic situation. Formosan leaf-nosed bats (Hipposideros terasensis) were used in this study and treated under three conditions: room temperature (25±1°C), low temperature (4±1°C), and hibernation. The reactive oxygen species (ROS) levels in the blood and apoptosis-related proteins in the brain tissue were assessed and then compared among those bats under three conditions. Our results showed that the blood ROS levels of bats treated under conditions of low temperature and hibernation were significantly reduced compared with bats treated under the condition of room temperature. Both immunohistochemistry and immunoblotting expressions of hypoxia, inflammation, and apoptosis-related proteins in the brain tissue of bats treated under the condition of hibernation were significantly reduced compared with those bats treated under conditions of room temperature and low temperature. Thus, we suggested that bats can protect the brain in cold environment by reducing blood ROS levels and decreasing expressions of hypoxia, inflammation, and apoptosis-related proteins in the brain. Possible protection mechanisms involved in hypothermic adaptations need to be further clarified.

Although echolocating bats and other mammals share the basic design of laryngeal apparatus for sound production and auditory system for sound reception, they have a specialized laryngeal mechanism for ultrasonic sound emissions as well as a highly developed auditory system for processing species-specific sounds. Because the sounds used by bats for echolocation and rodents for communication are quite different, there must be differences in the central nervous system devoted to producing and processing species-specific sounds between them. The present study examines the difference in the relative size of several brain structures and expression of auditory-related and vocal-related proteins in the central nervous system of echolocation bats and rodents. Here, we report that bats using constant frequency-frequency-modulated sounds (CF-FM bats) and FM bats for echolocation have a larger volume of midbrain nuclei (inferior and superior colliculi) and cerebellum relative to the size of the brain than rodents (mice and rats). However, the former have a smaller volume of the cerebrum and olfactory bulb, but greater expression of otoferlin and forkhead box protein P2 than the latter. Although the size of both midbrain colliculi is comparable in both CF-FM and FM bats, CF-FM bats have a larger cerebrum and greater expression of otoferlin and forkhead box protein P2 than FM bats. These differences in brain structure and protein expression are discussed in relation to their biologically relevant sounds and foraging behavior.

The inferior colliculus (IC) receives and integrates excitatory and inhibitory inputs from many bilateral lower auditory nuclei, intrinsic projections within IC, contralateral IC through the commissure of IC and from the auditory cortex (AC). These excitatory and inhibitory inputs from both ascending and descending auditory pathways contribute significantly to auditory response properties and temporal signal processing in IC. The present study examines the contribution of gamma-aminobutyric acid-ergic (GABAergic) inhibition of dorsal nucleus of the lateral lemniscus (DNLL) in influencing the response properties and amplitude sensitivity of contralateral IC neurons using focal electrical stimulation of contralateral DNLL and by the application of bicuculline to the recording site of modulated IC neurons. Focal electrical stimulation of contralateral DNLL produces inhibition (78.1%), facilitation (7.1%) or no effect (14.8%) in the number of spikes, firing duration and the first-spike latency of modulated IC neurons. The degree of modulation is inversely correlated to the difference in best frequency (BF) between electrically stimulated DNLL neurons and modulated IC neurons (p 0.01). The application of bicuculline to the recording site of modulated IC neurons abolishes the inhibitory effect of focal electrical stimulation of DNLL neurons. DNLL inhibition also modulates the amplitude sensitivity of IC neurons by changing the dynamic range (DR) and the slope of rate-amplitude function (RAF) of modulated IC neurons. Possible biological significance of these findings in relation to auditory signal processing is discussed.

The auditory system of echolocating bats is adapted for processing species-specific ultrasonic signals. While FM (frequency modulation) bats are strictly sensitive to the frequency ranges of their orientation signals or prey-generated noise, CF-FM (constant frequency-FM) bats have a disproportionate number of neurons tuned to frequencies near the CF component of their orientation sounds, and most of them are on–off responders. Furthermore, the inferior collicular neurons of the CF-FM bats discharged as single-on or double-on responders to CF-FM stimuli. To further study the differences in auditory signal processing of these two types of bats, as the first step we conducted an evoked potential response study in the inferior colliculus of the CF-FM bat, Hipposideros pratti and the FM bat, Pipistrellus abramus using CF, FM and CF-FM stimuli. The results showed that the CF sounds always evoked collicular on- and off-responses in CF-FM bats, but the FM bats only had on-responses to both CF and FM sounds, indicting species-specific neural circuits. However, when stimulated with CF-FM sounds, collicular responses were evoked by both the CF and FM components from both FM and CF-FM bats, suggesting they have some generic neural circuit.

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.

Previous studies show that when stimulated with constant frequency-frequency modulated (CF-FM) sounds, the inferior collicular neurons of the leaf-nosed bat, Hipposideros armiger, either discharge impulses only to the CF component (single-on, SO neurons) or to both CF and FM components (double-on, DO neurons). In this study, we specifically determine the role of the FM component in shaping the number of impulses and response latency of these two types of neurons in response to CF-FM sounds. Adding the FM component to the CF sounds significantly decreases the number of impulses of both SO and DO neurons but shortens the response latency of DO neurons in response to the CF component of the CF-FM sounds. The possible neural mechanisms underlying these seemingly paradoxical observations are briefly discussed based on our preliminary intracellular recording studies. Biological relevance of these findings in relation to different phases of bats' hunting is also discussed.

During hunting, bats of suborder Microchiropetra emit intense ultrasonic pulses and analyze the weak returning echoes with their highly developed auditory system to extract the information about insects or obstacles. These bats progressively shorten the duration, lower the frequency, decrease the intensity and increase the repetition rate of emitted pulses as they search, approach, and finally intercept insects or negotiate obstacles. This dynamic variation in multiple parameters of emitted pulses predicts that analysis of an echo parameter by the bat would be inevitably affected by other co-varying echo parameters. The progressive increase in the pulse repetition rate throughout the entire course of hunting would presumably enable the bat to extract maximal information from the increasing number of echoes about the rapid changes in the target or obstacle position for successful hunting. However, the increase in pulse repetition rate may make it difficult to produce intense short pulse at high repetition rate at the end of long-held breath. The increase in pulse repetition rate may also make it difficult to produce high frequency pulse due to the inability of the bat laryngeal muscles to reach its full extent of each contraction and relaxation cycle at a high repetition rate. In addition, the increase in pulse repetition rate increases the minimum threshold (i.e. decrease auditory sensitivity) and the response latency of auditory neurons. In spite of these seemingly physiological disadvantages in pulse emission and auditory sensitivity, these bats do progressively increase pulse repetition rate throughout a target approaching sequence. Then, what is the adaptive value of increasing pulse repetition rate during echolocation? What are the underlying mechanisms for obtaining maximal information about the target features during increasing pulse repetition rate? This article reviews the electrophysiological studies of the effect of pulse repetition rate on multiple-parametric selectivity of neurons in the central nucleus of the inferior colliculus of the big brown bat, Eptesicus fuscus using single repetitive sound pulses and temporally patterned trains of sound pulses. These studies show that increasing pulse repetition rate improves multiple-parametric selectivity of inferior collicular neurons. Conceivably, this improvement of multiple-parametric selectivity of collicular neurons with increasing pulse repetition rate may serve as the underlying mechanisms for obtaining maximal information about the prey features for successful hunting by bats.

For survival, bats of the suborder Microchiropetra emit intense ultrasonic pulses and analyze the weak returning echoes to extract the direction, distance, velocity, size, and shape of the prey. Although these bats and other mammals share the common layout of the auditory pathway and sound coding mechanism, they have highly developed auditory systems to process biologically relevant pulses at the expense of a reduced visual system. During this active biosonar behavior, they progressively shorten the pulse duration, decrease the amplitude and pulse-echo gap as they search, approach and finally intercept the prey. Presumably, these changes in multiple pulse parameters throughout the entire course of hunting enable them to extract maximal information about localized prey from the returning echoes. To hunt successfully, the auditory system of these bats must be less sensitive to intense emitted pulses but highly sensitive to weak returning echoes. They also need to recognize and differentiate the echoes of their emitted pulses from echoes of pulses emitted by other conspecifics. Past studies have shown the following mechanical and neural adaptive mechanisms underlying the successful bat biosonar behavior: (1) Forward orienting and highly mobile pinnae for effective scanning, signal reception, sound pressure transformation and mobile auditory sensitivity; (2) Avoiding and detecting moving targets more successfully than stationary ones; (3) Coordinated activity of highly developed laryngeal and middle ear muscles during pulse emission and reception; (4) Mechanical and neural attenuation of intense emitted pulses to prepare for better reception of weak returning echoes; (5) Increasing pulse repetition rate to improve multiple-parametric selectivity to echoes; (6) Dynamic variation of duration selectivity and recovery cycle of auditory neurons with hunting phase for better echo analysis; (7) Maximal multiple-parametric selectivity to expected echoes returning within a time window after pulse emission; (8) Pulse-echo delaysensitive neurons in higher auditory centers for echo ranging; (9) Corticofugal modulation to improve on-going multiple-parametric signal processing and reorganize signal representation, and (10) A large area of the superior colliculus, pontine nuclei and cerebellum that is sensitive to sound for sensori-motor integration. All these adaptive mechanisms facilitate the bat to effectively extract prey features for successful hunting.

A previous study shows that most inferior collicular neurons of the bat discharge maximally to a best duration and these duration-selective neurons have better echo frequency selectivity when the duration of both echo and pulse matches the best duration. In this study, we show that these duration-selective collicular neurons also have the sharpest echo amplitude selectivity when the duration of both echo and pulse matches the best duration. These data indicate that bats can better extract multiple parameters of expected than unexpected echo within the same time window after pulse emission. These data also support the previous studies showing that bats prepare their auditory system to analyze expected returning echoes within a time window after pulse emission.

In central auditory signal processing, neural inhibition plays an important role in sharpening the selectivity of auditory neurons. The present study examines the involvement of GABA-mediated inhibition in shaping the frequency selectivity of neurons in the bat inferior colliculus (IC) using forward masking paradigm and bicuculline application. At each study session, we recorded two IC neurons with a pair of electrodes and reciprocally studied whether a sound that served as a probe to elicit response of one neuron might serve as a masker to affect the frequency tuning curve (FTC) of the other paired neuron. Among the 33 pairs of IC neurons recorded, this forward masking paradigm produces sharpening of the FTC in 29 (88%) pairs of IC neurons and broadening of the FTC in 4 (12%) pairs of IC neurons. The degree of sharpening of FTC decreases with recording depth as well as with the difference in the best frequency and recording depth between each pair of IC neurons. Although bicuculline application broadens the FTC of all IC neurons, forward masking still produces sharpening of the FTC in most IC neurons. These data suggest that population of IC neurons are highly correlated during frequency analysis such that frequency selectivity of some groups of IC neurons is improved through inhibition while the spectrum of frequency sensitivity of other groups of IC neurons is enhanced through excitation. Biological significance of these data relevant to acoustic signal processing is discussed.

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.

During hunting, insectivorous bats such as Eptesicus fuscus progressively increase the pulse repetition rate, shorten the pulse duration, and lower the frequency and amplitude of emitted pulses as they search, approach and finally intercept insects or negotiate obstacles. As such, analysis of an echo parameter by the bat is inevitably affected by other co-varying echo parameters. The present study examined the effect of pulse duration on frequency selectivity of neurons in the central nucleus of the inferior colliculus (IC) of the big brown bat. A family of iso-level frequency tuning curves of each IC neuron was first measured with tone bursts of different durations. The bandwidth of iso-level frequency tuning curves within each family was then compared. Our data show that most IC neurons discharge maximally to a particular pulse duration which is defined as the best duration (BDu). The iso-level frequency tuning curves of these duration-selective neurons have the narrowest bandwidth when measured with the BDu pulse than with non-BDu pulses. They also have the narrowest bandwidth when measured with a short than with a long BDu pulse. These data suggest that frequency selectivity of duration-selective IC neurons becomes sharper when short echo duration at the final phase of hunting is encoded by IC neurons that have short BDu.

From the evolutionary perspective, the ear of each animal species is built for effective processing of the biologically relevant signals used for communication and acoustically guided orientation. Because the sound pulses used by echolocating bats for orientation and rodents for communication are quite different, the basic design of the mammalian auditory system commonly shared by echolocating bats must be specialized in some manner to effectively process their species-specific sounds. The present study examines the difference in the cochlea of these animal species using MRI images and histological techniques. We report here that, although all these animal species share a similar cochlear structure, they vary in their cochlear size and turns. Bats using constant frequency-frequency-modulated pulses (CF-FM bats) and frequency-modulated pulses (FM bats) for echolocation have a larger cochlear size and more cochlear turns than rodents (mice and rats). However, CF-FM bats have the largest cochlear size and most cochlear turns. This difference in cochlear size and turns of these animal species is discussed in relation to their biologically relevant sounds and acoustic behavior.

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.

Recent studies have shown that the corticofugal system systematically modulates and improves subcortical signal processing in the frequency, time and spatial domains. The present study examined corticofugal modulation of amplitude sensitivity of 113 corticofugally inhibited neurons in the central nucleus of the inferior colliculus (IC) of the big brown bat, Eptesicus fuscus. Cortical electrical stimulation decreased the number of impulses and increased the response latency of these neurons. They had an average of 5.9+/-4.4 kHz best frequency (BF) differences between collicular and electrically stimulated cortical neurons. Cortical electrical stimulation synchronized with sound stimulation for 30 min compressed the rate-amplitude functions of half (56, 49.6%) of these collicular neurons and shifted their minimum thresholds (MT) and dynamic ranges (DR) toward that of electrically stimulated cortical neurons for as long as 40 min. These collicular neurons had an average of 1.6+/-1.4 kHz BF differences. The shift in collicular MT and DR significantly increased with differences in MT and DR between collicular and cortical neurons. Cortical electrical stimulation also shifted the BF and best amplitude (BA) of collicular neurons toward that of cortical neurons. The BF shift increased with BF differences and the BA shift increased with BA differences. These data suggest that the corticofugal system modulates collicular responses on the basis of topographic projections between the IC and auditory cortex. However, corticofugal modulation of collicular amplitude sensitivity is primarily dependent upon the difference but not the absolute amplitude sensitivity between collicular and cortical neurons.

The recovery cycle of auditory neurons is an important neuronal property, which determines a neuron's ability to respond to pairs of sounds presented at short inter-sound intervals. This property is particularly important for bats, which rely upon analysis of returning echoes to extract the information about targets after emission of intense orientation sounds. Because target direction often changes throughout the course of hunting, the changing echo direction may affect the recovery cycle and thus temporal processing of auditory neurons. In this study, we examined the effect of sound azimuth on the recovery cycle of inferior collicular (IC) neurons in the big brown bat, Eptesicus fuscus, under free-field stimulation conditions. Our study showed that the recovery cycle of most IC neurons (42/49, 86%) was longer when determined with sounds delivered at 40 degrees ipsilateral (i40 degrees ) than at 40 degrees contralateral (c40 degrees ) to the recording site. To study the contribution of GABAergic inhibition to sound azimuth-dependent recovery cycle, we compared the recovery cycle of IC neurons determined at two sound azimuths before and during iontophoretic application of bicuculline, an antagonist for GABA(A) receptors. Bicuculline application produced a greater decrease of the recovery cycle of these neurons at i40 degrees than at c40 degrees. As a result, the azimuth-dependent recovery cycle of these neurons was abolished or greatly reduced. Possible mechanisms underlying these observations and biological relevance to bat echolocation are discussed.

Neurons in the inferior colliculus (IC) of the midbrain receive convergent excitatory and inhibitory inputs from both lower and higher auditory nuclei. Interaction of these two opposing inputs shapes different response properties of IC neurons. In this study, we examined this interaction of excitation and inhibition in IC neurons using a probe (excitatory pulse) and a masker (inhibitory pulse) under different stimulation conditions. Inhibition of probe-elicited responses by a masker, i.e. masking, occurred when the masker was presented at certain inter-pulse intervals (the temporal window) in relation to the probe. At the best inter-pulse interval, masking was maximal such that a neuron had the minimal number of impulses, the longest response latency, and the smallest excitatory frequency tuning curve. The temporal window for masking expanded with increasing masker duration. The inhibition decreased with increasing probe intensity but increased with increasing masker intensity. Increasing masker intensity also produced progressive shrinkage in excitatory frequency tuning curves. Similarly, increasing probe intensity produced progressive shrinkage of inhibitory frequency tuning curves. Possible mechanisms underlying the time and intensity dependence of inhibition are discussed.

This study examined the effect of GABAergic and glycinergic inhibition on excitatory frequency tuning curves (FTCs) of inferior collicular (IC) neurons of the big brown bat, Eptesicus fuscus. The excitatory FTCs of 70 IC neurons were either V-shaped (57, 81%), closed (11, 16%), or double-peaked (2, 3%). By means of a two-tone stimulation paradigm, inhibitory FTCs were obtained at one frequency flank only (low-frequency flank: 11, 16%; high-frequency flank: 7, 10%), at both frequency flanks (36, 51%) of excitatory FTCs, or between two excitatory FTCs (2, 3%). IC neurons that had inhibitory FTCs typically had larger Q 10 and Q 30 values (i.e., sharper excitatory FTCs) than neurons that did not have inhibitory FTCs. Neurons with inhibitory FTCs at both frequency flanks had larger Q 10 and Q 30 values than neurons with inhibitory FTCs at one frequency flank only. IC neurons with a small difference between excitatory and inhibitory best frequencies typically had sharper excitatory frequency tuning. Bicuculline (an antagonist for GABAA) application produced a greater degree of abolishing inhibitory FTCs than strychnine (an antagonist for glycine) application. Application of both drugs was most effective in abolishing the inhibitory FTCs of IC neurons. The implications of these findings for bat echolocation are discussed.

Recent studies have shown that the auditory corticofugal system modulates and improves signal processing in the frequency, time and spatial domains. In this study, we examine corticofugal modulation of rate-intensity functions of inferior collicular (IC) neurons of the big brown bat, Eptesicus fuscus, by electrical stimulation in the primary auditory cortex (AC). Cortical electrical stimulation compressed all types of rate-intensity functions so as to increase the slope but decrease the dynamic range of IC neurons. Cortical electrical stimulation also shifts the responsive intensity of IC neurons to higher levels. These data indicate that corticofugal modulation also improves subcortical signal processing in intensity domain. The implication of these findings to bat echolocation is discussed.

This study examined the role of GABAergic inhibition on direction-dependent sharpening of frequency tuning curves (FTCs) in bat inferior collicular (IC) neurons under free field stimulation conditions. The minimum threshold (MT) at the neurons best frequency (BF) and the sharpness (Q(10), Q(20), Q(30)) of FTCs of most IC neurons increased as the sound direction changed from contralateral azimuths to ipsilateral azimuths. The application of GABA(A) antagonist, bicuculline, lowered all MTs but the application did not abolish direction-dependent variation in MT. MTs determined during bicuculline application at 40 ipsilateral were still significantly higher than those determined at 40 degrees contralateral (two-tailed paired t-test, P

Corticofugal regulation of excitatory and inhibitory frequency tuning curves (FTCs) of neurons in the central nucleus of bat inferior colliculus (ICc) was studied by electrical stimulation of the primary auditory cortex (AC stimulation) under free field stimulation conditions using a two-tone inhibition paradigm. AC stimulation narrowed the excitatory FTCs and asymmetrically expanded the lateral inhibitory FTCs of corticofugally inhibited ICc neurons. The opposite results were observed for excitatory and inhibitory FTCs of corticofugally facilitated ICc neurons. These data support previous reports that corticofugal systems work together with widespread lateral inhibition to regulate subcortical frequency processing.

This study examines the contribution of GABAergic inhibition to the discharge pattern and pulse duration tuning characteristics of 101 bat inferior collicular neurons by means of bicuculline application to their recording sites. When stimulated with single pulses, 56 (55%) neurons discharged 1 or 2 impulses (phasic responders), 42 (42%) discharged 3–10 impulses (phasic bursters) and 3 (3%) discharged impulses throughout the stimulus duration (tonic responders). Bicuculline application increased the number of impulses and changed the discharge patterns of 66 neurons. Using 50% difference between maximal and minimal responses as a criterion, the duration tuning characteristics of these neurons can be described as band-pass (20, 20%), long-pass (17, 17%), short-pass (33, 32%), and all-pass (31, 31%). Each band-pass neuron discharged maximally to a specific duration (the best duration) which was at least 50% larger than the neuron's responses to a long-duration pulse and a short-duration pulse. In contrast, each long- or short-pass neuron discharged maximally to a range of long or short duration pulses. Bicuculline application changed the duration tuning characteristics of 65 neurons. Possible mechanisms underlying duration tuning characteristics and the behavioral relevance to bat echolocation are discussed.

This study examines the effect of pulse repetition rate (PRR), pulse intensity, and bicuculline on the minimum threshold (MT) and latency of inferior collicular neurons of the big brown bat, Eptesicus fuscus, under free-field stimulation conditions. It tests the hypothesis that changes in MT and latency of collicular neurons are co-dependent on PRR. The number of impulses in inferior collicular neurons (n = 245) increased either monotonically (25%) or non-monotonically (75%) with pulse intensity. Latencies either decreased to a plateau (72%), fluctuated unpredictably within 3 ms (21%) or changed very little (7%) with increasing pulse intensity. Latencies and MTs of most collicular neurons increased by 1.5-24 ms (mean +/- SD = 4.8 +/- 3.3 ms) and 4-75 dB (mean +/- SD = 22.1 +/- 16.2 dB) with increasing PRR. In most neurons (94%), the latency increase was completely (42%) or partially (52%) eliminated when pulse intensity was compensated for the MT increase with PRR. Complete elimination of latency was achieved by bicuculline application. In a few neurons (6%), the latency increase with PRR was not affected by compensated pulse intensity or bicuculline application.

This study examines the contribution of GABAergic inhibition to the discharge pattern and recovery properties of 110 bat inferior collicular neurons by means of bicuculline application to their recording sites. When stimulated with single pulses, 74 (67%) neurons discharged one or two impulses (phasic responders), 19 (17%) discharged three to ten impulses (phasic bursters) and 17 (16%) discharged impulses throughout the entire stimulus duration (tonic responders). Bicuculline application changed phasic responders into phasic bursters or tonic responders, increased the number of impulses by 10–2000% and shortened the response latency of most neurons. When stimulated with pairs of sound pulses, the recovery cycles of these neurons can be described as: (1) long inhibition (n = 49, 45%); (2) short inhibition (n = 41, 37%); and (3) fast recovery (n = 20, 18%) based upon the 50% recovery time that was either longer than 20 ms, between 10 and 20 ms or shorter than 10 ms. Bicuculline application shortened the 50% recovery time of most neurons by 11–2350% allowing them to respond to pairs of sound pulses at very short interpulse intervals. These data demonstrate that GABAergic inhibition contributes significantly to auditory temporal processing.

Using bats as a model auditory system, we studied corticofugal control of auditory sensitivity of neurons in the inferior colliculus. We demonstrate for the first time that the corticocollicular pathway continuously regulates acoustic signal processing in the inferior colliculus by increasing the threshold, reducing the auditory spatial response area, and sharpening the frequency tuning curve of recorded inferior collicular neurons. Regulation of auditory sensitivity of recorded inferior collicular neurons was observed when the corticocollicular pathway was activated by electrical stimulation in the auditory cortex. The effect of this corticofugal regulation of auditory sensitivity in inferior collicular neurons can also be produced by ionophoretical application of GABA to the collicular recording site. This regulation of ascending acoustic information by commands originating from higher brain centers may provide the bat with a mechanism to actively control acoustic signal processing and thus optimize acoustic signal analysis.

In nature, communication sounds among animal species including humans are typical complex sounds that occur in sequence and vary with time in several parameters including amplitude, frequency, duration as well as separation, and order of individual sounds. Among these multiple parameters, sound duration is a simple but important one that contributes to the distinct spectral and temporal attributes of individual biological sounds. Likewise, the separation of individual sounds is an important temporal attribute that determines an animal's ability in distinguishing individual sounds. Whereas duration selectivity of auditory neurons underlies an animal's ability in recognition of sound duration, the recovery cycle of auditory neurons determines a neuron's ability in responding to closely spaced sound pulses and therefore, it underlies the animal's ability in analyzing the order of individual sounds. Since the multiple parameters of naturally occurring communication sounds vary with time, the analysis of a specific sound parameter by an animal would be inevitably affected by other co-varying sound parameters. This is particularly obvious in insectivorous bats, which rely on analysis of returning echoes for prey capture when they systematically vary the multiple pulse parameters throughout a target approach sequence. In this review article, we present our studies of dynamic variation of duration selectivity and recovery cycle of neurons in the central nucleus of the inferior colliculus of the frequency-modulated bats to highlight the dynamic temporal signal processing of central auditory neurons. These studies use single pulses and three biologically relevant pulse-echo (P-E) pairs with varied duration, gap, and amplitude difference similar to that occurring during search, approach, and terminal phases of hunting by bats. These studies show that most collicular neurons respond maximally to a best tuned sound duration (BD). The sound duration to which these neurons are tuned correspond closely to the behaviorally relevant sounds occurring at different phases of hunting. The duration selectivity of these collicular neurons progressively increases with decrease in the duration of pulse and echo, P-E gap, and P-E amplitude difference. GABAergic inhibition plays an important role in shaping the duration selectivity of these collicular neurons. The duration selectivity of these neurons is systematically organized along the tonotopic axis of the inferior colliculus and is closely correlated with the graded spatial distribution of GABA(A) receptors. Duration-selective collicular neurons have a wide range of recovery cycle covering the P-E intervals occurring throughout the entire target approaching sequences. Collicular neurons with low best frequency and short BD recover rapidly when stimulated with P-E pairs with short duration and small P-E amplitude difference, whereas neurons with high best frequency and long BD recover rapidly when stimulated with P-E pairs with long duration and large P-E amplitude difference. This dynamic variation of echo duration selectivity and recovery cycle of collicular neurons may serve as the neural basis underlying successful hunting by bats. Conceivably, high best frequency neurons with long BD would be suitable for echo recognition during search and approach phases of hunting when the returning echoes are high in frequency, large in P-E amplitude difference, long in duration but low in repetition rate. Conversely, low best frequency neurons with shorter BD and sharper duration selectivity would be suitable for echo recognition during the terminal phase of hunting when the highly repetitive echoes are low in frequency, small in P-E amplitude difference, and short in duration. Furthermore, the tonotopically organized duration selectivity would make it possible to facilitate the recruitment of different groups of collicular neurons along the tonotopic axis for effective processing of the returning echoes throughout the entire course of hunting.

C-fos immunocytochemistry was used as a rapid and sensitive technique for identification of sound activated neurons in the cerebral cortex, the cerebellum and subcortical nuclei of the big brown bat, Eptesicus fuscus. When bats were stimulated with sounds under the both-ears opened conditions, Fos-like immunoreactive neurons were bilaterally and symmetrically distributed in all subcortical auditory nuclei, the auditory cortex, the superior colliculus, the pontine nuclei and the cerebellar deep nuclei. Interestingly, when bats were stimulated with sounds under the monaurally plugged conditions, a larger (31–74% more) number of Fos-like immunoreactive neurons were observed. They were predominantly distributed in all contralateral auditory nuclei from the level of the nucleus of the lateral lemniscus down and in all ipsi lateral auditory nuclei from the level of inferior colliculus up as well as in the contralateral superior colliculus, pontine nuclei and cerebellar deep nuclei. Implications of these observations in relation to known mammalian auditory pathways and electrophysiological studies are discussed.

Under free field stimulation conditions, we studied the responses of inferior collicular neurons of the FM bat, Eptesicus fuscus , to pulse trains with varied pulse amplitudes. Each pulse train consisted of 7 pulses of 4 ms delivered at 24 ms interpulse-intervals (i.e. 42 pulses/s). For a control pulse train, all pulse amplitudes were equal to a neuron's best amplitude which, when delivered in single pulses, elicited maximal number of impulses from the neuron. The amplitudes of individual pulses of the remaining pulse trains were linearly increased or decreased at a slope of 0, 14, 28, 42, 56 and 69 dB/s. All 56 inferior collicular neurons discharged to pulse trains were of two main types. Type I ( N 43, 77%) neurons discharged to each pulse within a train while type II ( N 11, 20%) neurons discharged to the first pulse of a train stimulus only. Discharge patterns of the remaining ( N 2, 3%) neurons changed between type I and type II when stimulated with different pulse trains. The number of impulses discharged by a neuron varied with different pulse trains. In addition, the number of impulses discharged to each pulse by type I neurons also varied among individual pulses within the train. Only 14 neurons (25%) discharged maximally to the control pulse train. Responses of the remaining neurons to other pulse trains were either 30%–120% larger than ( N 17, 30%) or within 30% ( N 25, 45%) of the control pulse train response. Furthermore, half of 56 neurons selectively discharged to a most preferred pulse train with a response magnitude which was at least 50% larger than the response to the least preferred pulse train. Possible mechanisms underlying the different discharge patterns are discussed in terms of a neuron's recovery cycle, minimum threshold and inhibitory period relative to the temporal characteristics (pulse repetition rate and amplitude) of the pulse trains.

The effect of pulse repetition rate on auditory sensitivity of the big brown bat, Eptesicus fuscus , was studied by determining the minimum threshold, response latency and recovery cycle of inferior collicular neurons at different repetition rates under free field stimulation conditions. In general, collicular neurons shortened the response latency and increased the number of impulses monotonically or non-monotonically with stimulus intensity. They recovered at least 50% when the interpulse interval was 10–57 ms. In addition, they increased the minimum threshohold, lengthened the response latency, and reduced the number of impulses discharged to each pulse with increasing repetition rate. The increase in minimum threshold with repetition rate is partly because the neuron can not recover from previous stimulation when the interpulse interval is shortened. This increase reduces a neuron's response sensitivity and thus diminishes its number of impulses to each presented pulse. This increase also reduces the effectiveness of a given stimulus intensity which contributes to the lengthening of the neuron's response latency. Data obtained from single neuron recordings are used to highlight these observations. Implications of present findings regarding the bat's echolocation are also discussed.

The recovery cycle of auditory neurons is an important neuronal property which underlies a bat's ability in analyzing returning echoes and to determine target distance (i.e., echo ranging). In the same token, duration selectivity of auditory neurons plays an important role in pulse recognition in bat echolocation. Because insectivorous bats progressively vary the pulse parameters (repetition rate, duration, and amplitude) during hunting, the recovery cycle of auditory neurons is inevitably affected by their selectivity to other co-varying echo parameters. This study examines the effect of pulse duration and amplitude on recovery cycle of neurons in the central nucleus of the inferior colliculus (IC) of the FM bat, Pipistrellus abramus, using biologically relevant pulse-echo (P-E) pairs with varied duration and amplitude difference. We specifically examine how duration selectivity may affect a neuron's recovery cycle. IC neurons have wide range of recovery cycle and best duration (BD) covering P-E intervals and duration occurring different phases of hunting. The recovery cycle of most IC neurons increases with P-E duration and amplitude difference. Most duration-selective IC neurons recover rapidly when stimulated with biologically relevant P-E pairs. As such, neurons with short BD recover rapidly when stimulated with P-E pairs of short duration and small P-E amplitude difference. Conversely, neurons with long BD recover rapidly when stimulated with P-E pairs of long duration and large P-E amplitude difference. These data suggest that bats may potentially utilize the response of IC neurons with different BD and recovery cycle to effectively perform echo detection, recognition of echo duration and echo ranging throughout a target approaching sequence.

Single-neuron responses to pulse repetition rate in the inferior colliculus, auditory cortex and pontine nuclei of the FM bat, Eptesicus fuscus were studied under free-field stimulation conditions. The best frequency (BF) and minimum threshold (MT) of each neuron were first determined with a 4 ms pulse broadcast from a specific point (response center) of the bat's frontal auditory space at which the neuron had maximal spatial sensitivity. The neuron's intensity-rate function was then studied with a 4 ms BF pulse delivered at 10 dB increments above its MT in order to determine the best intensity to which the neuron discharged maximally. The neuron's discharge pattern and number of impulses to 32 trials of 300 ms train stimuli, which consisted of different number of 4 ms BF and best intensity pulses (1, 2, 3, 8, 12, 19, 24, 29 pulses/train) and delivered at an interpulse interval of 1000, 250, 170, 100, 40, 25, 15, 12 and 10 ms (i.e. at a pulse repetition rate of 1, 4, 6, 10, 25, 40, 67, 83, 100 pulses/s), were sequentially recorded. All neurons recorded from the inferior colliculus, auditory cortex and pontine nuclei discharged phasically (1-3 impulses) but they responded to the pulse repetition rate in different manners. More than 63% of 38 inferior collicular and 65 pontine neurons studied discharged impulses to each pulse within a train stimulus when the pulse repetition rate was up to 40 pulses/s.(ABSTRACT TRUNCATED AT 250 WORDS)

Our previous study showed that when stimulated with constant frequency–frequency modulation (CF–FM) sounds, neurons in the central nucleus of the inferior colliculus of the CF–FM bat, Hipposideros armiger, either only discharged impulses to the onset of CF–FM sounds (76%, single-on neurons) or to the onset of both CF and FM components of CF–FM sounds (24%, double-on neuron) (Fu et al., 2010). The present paper reports the recovery cycles of these two types of neurons using paired CF, FM and CF–FM sounds as stimuli. Both types of neurons had similar recovery cycle for CF sounds but had the shortest recovery cycle for FM sounds. Whereas single-on neurons had similar recovery cycle for CF and CF–FM sounds, double-on neurons had longer recovery cycle for CF sounds than for CF–FM sounds. In addition, double-on neurons had significantly shorter recovery cycles than single-on neurons for FM and CF–FM sounds. Most neurons did not respond to the second sound when each pair of sounds overlapped. However, when stimulated with paired CF–FM sounds, 3 single-on and 7 double-on neurons discharged to the second sound even when both sounds overlapped. As such, they had “cyclic” recovery cycles that varied between maximum and minimum with inter-pulse intervals. Possible mechanisms underlying the different recovery cycles of these neurons are proposed. Possible biological significance of these neurons in relation to responding to varied pulse repetition rate during hunting is discussed.

Using free-field acoustic stimulation conditions, we studied the response properties and spatial sensitivity of 146 pontine neurons of the big brown bat, Eptesicus fuscus. The best frequency (BF) and minimum threshold (MT) of a pontine neuron were first determined with a sound broadcast from a loudspeaker placed ahead of the bat. A BF sound was delivered from the loudspeaker as it moved across the frontal auditory space in order to locate the response center at which the neuron had its lowest MT. Then the basic response properties of the neuron to a sound delivered from the response center were studied. As in inferior collicular and auditory cortical neurons, pontine neurons can be characterized as phasic responders, phasic bursters and tonic responders. They have both monotonic and non-monotonic intensity-rate functions. However, most of them are broadly tuned as are cerebellar neurons. Auditory spatial sensitivity was studied for 144 pontine neurons. In 9 neurons, variation of MT with a BF sound delivered from several azimuthal and elevational angles along the horizontal and vertical planes crossing the neuron's response center was measured. In addition, variation in the number of impulses with several stimulus intensities at 10 dB increments above a neuron's MT delivered from each angle was also studied. The auditory spatial sensitivity of other pontine neurons was studied by measuring the response area of each neuron with stimulus intensities at 3, 5, 10, 15 or 40 dB above its lowest MT. The response areas of pontine neurons expanded asymmetrically with stimulus intensity, but the size of the response area was not correlated with either MT or BF. In half of the pontine neurons studied, the response area expanded greatly and eventually covered almost the entire frontal auditory space. The response areas of the other half of the pontine neurons only expanded to a restricted area of frontal auditory space. Two possible neural mechanisms underlying these two types of response areas are hypothesized. The response centers of all 144 neurons were located within a small area of the frontal auditory space. The locations of response centers of these neurons are not correlated with their BFs. The distribution pattern of these response centers is comparable to that of superior collicular and cerebellar neurons but is different from that of inferior collicular and auditory cortical neurons. The results of our study suggest that auditory information is integrated in the pontine nuclei before being further sent into the cerebellum.

The present study examines the response properties of neurons in the central nucleus of the inferior colliculus (IC) of the CF-FM (constant frequency-frequency-modulated) bat, Hipposideros armiger using CF, FM and CF-FM sounds as stimuli. All 169 IC neurons recorded are tonotopically organized along the dorsoventral axis of the IC. Collicular neurons have V-shaped or upper-threshold frequency tuning curves. Those neurons tuned at the predominant second harmonic have extremely sharp frequency tuning curves and low minimum thresholds. Collicular neurons typically discharge impulses to both CF and FM sounds. However, when stimulated with CF-FM sounds, most (76%) neurons only discharge impulses to the onset of CF-FM sounds (single-on responders). The remaining neurons (24%) discharge impulses to both CF and FM components (double-on responders) of CF-FM sounds. The double-on responders have higher minimum threshold and longer latency to the FM component than to the CF component of CF-FM sounds. Our data show that the FM component of the CF-FM sounds contributes significantly in shaping the discharge pattern, latency and number of impulses of IC neurons. The present study suggests that using CF-FM sounds to study auditory response properties of the CF-FM bat may be essential for a better understanding of echo analysis by the CF-FM in the real world. Because the double-on responders have shorter response latency than single-on responders, we speculate that these two types of responders may be best suited for echo analysis during different phases of hunting.

During hunting, duration selectivity and recovery cycle underlie a bat's ability to determine echo duration and target distance (echo ranging). This study shows that the recovery cycle of most duration-selective neurons in the bat central nucleus of the inferior colliculus neurons varies with biologically relevant pulse-echo (P-E) duration and amplitude. As such, neurons with short best duration recover rapidly when stimulated with P-E pairs with short duration and small P-E amplitude difference, whereas neurons with long best duration recover rapidly when stimulated with P-E pairs with long duration and large P-E amplitude difference. These data indicate that different groups of duration-selective neurons underlie the bat's ability to effectively perform echo recognition and ranging during different phases of hunting.

When insectivorous bats such as Eptesicus fuscus emit ultrasonic signals and analyze the returning echoes to hunt insects, duration selectivity of auditory neurons plays an important role in echo recognition. The success of prey capture indicates that they can eVectively encode progressively shortened echo dura- tion throughout the hunting process. The present study examines the echo duration selectivity of neurons in the central nucleus of the bat inferior colliculus (IC) under stimulation conditions of single pulses and pulse-echo (P-E) pairs. This study also examines the role of gamma-aminobutyric acid (GABA)ergic inhibi- tion in shaping echo duration selectivity of IC neurons. The data obtained show that the echo duration selec- tivity of IC neurons is sharper when determined with P-E pairs than with single pulses. Echo duration selec- tivity also sharpens with shortening of pulse duration and P-E gap. Bicuculline application decreases and GABA application increases echo duration selectivity of IC neurons. The degree of change in echo duration selectivity progressively increases with shortening of pulse duration and P-E gap during bicuculline applica- tion while the opposite is observed during the GABA application. These data indicate that the GABAergic inhibition contributes to sharpening of echo duration selectivity of IC neurons and facilitates echo recogni- tion by bats throughout diVerent phases of hunting.

The auditory brainstem response (ABR) is an evoked potential response of auditory activity in the auditory nerve and subsequent fiber tracts and nuclei within the auditory brainstem pathways. The threshold, amplitude, and latency analysis of the ABR provides information on the peripheral hearing status and the integrity of brainstem pathways. In this study, we compared the threshold, amplitude, and latency of ABRs recorded from 149 mice of 10 commonly used inbred strains (BALB/cJ, C3HeB/FeJ, C3H/HeJ, CAST/EiJ, CBA/CaJ, CBA/J, FVB/ NJ, MRL/MpJ, NZB/BlNJ, and SJL/J) using clicks of different intensities. The ABR thresholds of these strains ranged from 32 to 43 dB SPL. The amplitude of both waves I and IV of ABRs, which increased monotonically with click intensity in most strains, differed significantly among different strains at each intensity tested. Moreover, the amplitude of both waves was inversely correlated with the body weight of each strain at most intensities tested. In general, the amplitude of wave IV was smaller than that of wave I resulting in the IV/I amplitude ratio of

A previous study has shown that duration selectivity of neurons in the inferior colliculus (IC) of the big brown bat, Eptesicus fuscus becomes sharper with increasing pulse repetition rate (PRR). The present study examines the role of GABAergic inhibition in improving duration selectivity of bat IC neurons with PRR by means of iontophoretic application of GABA as well as its antagonist, bicuculline. Duration selectivity of IC neurons is studied by plotting the duration tuning curves with the number of impulses per pulse against the pulse duration. Duration tuning curves of IC neurons are described as band-, short-, long- and all-pass in terms of filtering properties to sound duration. Bicuculline application produces more pronounced broadening of duration tuning curves at high than at low PRR. Conversely, GABA application produces more pronounced narrowing of duration tuning curves at low than at high PRR. In either case, sharpening of duration selectivity of IC neurons with increasing PRR is abolished during drug application. The duration tuning curves of IC neurons progressively broadens with recording depth. Broadening of duration tuning curves during bicuculline application is more pronounced for neurons at upper than at deep IC. This progressive decrease in duration selectivity with recording depth is discussed in relation to spatial distribution gradient of GABAA receptors in the IC. Possible biological significance of these findings relevant to bat echolocation is discussed.

In our recent study of corticofugal modulation of collicular amplitude sensitivity of the big brown bat, Eptesicus fuscus, we suggested that the corticofugal modulation is based upon the best frequency (BF) differences and the relative amplitude sensitivity difference between collicular (IC) and cortical (AC) neurons but not the absolute amplitude sensitivity of IC and AC neurons. To show that corticofugal modulation is systematic and multiparametric, we studied corticofugal modulation of directional sensitivity in 89 corticofugally inhibited IC neurons in the same bat species under free field stimulation conditions. A neuron’s directional sensitivity was expressed with the azimuthal range (AR) at 50% below the maximum of each directional sensitivity curve and the best azimuth (BAZ) at which the neuron discharged maximally. Cortical electrical stimulation did not affect the directional sensitivity of 40 (45%) neurons with BFIC–AC differences of 7.3 ± 4.4 kHz but sharpened the directional sensitivity of other 49 (55%) neurons with BFIC–AC differences of 2.3 ± 1.8 kHz. Corticofugal modulation sharpened directional sensitivity curves of IC neurons by decreasing the AR and shifting collicular BAZ toward cortical BAZ. The decrease in AR and the shift in BAZ increased significantly with ARIC–AC and BAZIC–AC differences but not with absolute AR and BAZ of IC and AC neurons or BFIC–AC differences. Corticofual modulation also shifted collicular BF toward cortical BF. The shift in BF increased significantly with BFIC–AC differences but not with the BF of IC and AC neurons or BAZ shift. Consonant with our previous study, these data indicate that corticofugal modulation of collicular directional sensitivity is based on topographic projections between the IC and the AC and the difference in directional sensitivity but not the absolute directional sensitivity of IC or AC neurons.

Natural sounds, such as vocal communication sounds of many animal species typically occur as sequential sound pulses. Therefore, the response size of auditory neurons to a sound pulse would be inevitably affected when the sound pulse is preceded and succeeded by another sound pulse (i.e., forward and backward masking). The present study presents data to show that increasing strength of GABAergic inhibition relative to excitation contributes to decreasing response size and sharpening of duration selectivity of bat inferior collicular (IC) neurons to sound pulses in rapid sequences. The response size in number of impulses and duration selectivity of IC neurons were studied with a pulse train containing 9 sound pulses. A family of duration tuning curves was plotted for IC neurons using the number of impulses discharged to each presented sound pulse against pulse duration. Our data show that the response size of IC neurons progressively decreased and duration selectivity increased when determined with sequentially presented sound pulses. This variation in the response size and duration selectivity of IC neurons with sequentially presented sound pulses was abolished or reduced during bicuculline and GABA application. Bicuculline application increased the response size and broadened the duration tuning curve of IC neurons while GABA application produced opposite results. Possible mechanisms underlying increasing strength of GABAergic inhibition with sequentially presented sound pulses are presented. Biological significance of these findings in relation to acoustic signal processing is also discussed.

In our previous study, we have shown that the recovery cycle of most neurons in the inferior colliculus (IC) of the big brown bat, Eptesicus fuscus, is typically longer at ipsilateral azimuth than at contralateral azimuth under free-field stimulation conditions. The present study is to test the hypothesis that this azimuth-dependent recovery cycle may contribute to the variation of directional selectivity of IC neurons with sequential presented sound pulses within a pulse train. A 300-ms pulse train containing nine sound pulses of 4-ms with an inter-pulse interval of 33.3 ms was delivered at several selected azimuthal angles between +/-80 degrees lateral in the frontal auditory space of a bat. A family of nine directional selectivity curves was plotted with a neuron's number of impulses in response to each individual pulse against the azimuthal angles. The type and sharpness of these directional selectivity curves were then compared in relation to pulse position within the pulse train. All 675 directional selectivity curves obtained from 75 IC neurons could be described as directionally selective (423, 63%), hemifield (220, 32%), or non-directional (32, 5%). The directional selectivity curves of 45 (60%) neurons did not vary with pulse position. However, those of the remaining neurons (30, 40%) changed from one type to another such that the number of neurons with directionally selective curves progressively increased and the number of neurons with hemifield and non-directional selectivity curves decreased with increasing pulse position within the pulse train. Among 68 IC neurons whose directional selectivity curves were compared quantitatively, directional selectivity determined with sequentially presented sound pulses significantly increased in 38 (56%) neurons; decreased in 18 (26%) neurons but did not change in 12 (18%) neurons. This change of directional selectivity was due to the variation in recovery cycle of these IC neurons with azimuthal angle as we hypothesized.

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.

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.

This study examined the effect of bicuculline application on sharpness of frequency tuning curves (FTCs) of bat inferior collicular neurons plotted under three different pulse repetition rates (PRRs) of 10, 30 and 90 pulses per second. The sharpness of FTCs of collicular neurons, which was expressed in Q n ( Q 10 , Q 20 , Q 30 ) and bandwidths (90, 75 and 50% of the maximal response at the best frequency), improved with increasing PRR. However, this PRR-dependent frequency selectivity of collicular neurons was abolished during bicuculline application. This observation suggests that GABAergic inhibition contributes more effectively to sharpening of FTCs at higher than at lower PRRs.

This study examined auditory responses of two simultaneously recorded neurons in the central nucleus of bat inferior colliculus (IC) under two-tone stimulation conditions. We specifically examined how a sound within the excitatory frequency tuning curve (FTC) of one IC neuron might affect responses of the other IC neuron in amplitude and frequency domains. Under this specific two-tone stimulation condition, responses of 82% neurons were suppressed and their excitatory FTCs sharpened. Responses of the other 18% neurons were facilitated and their excitatory FTCs broadened. Two-tone suppression was greater at low than at high stimulus amplitudes. Two-tone suppression also decreased with increasing recording depth and best frequency (BF) difference between each pair of neurons. The suppressive or facilitatory FTC of a neuron plotted under two-tone stimulation conditions was always within the excitatory FTC of the other neuron. Two-tone suppression or two-tone facilitation was weak near the BF but became increasingly strong with frequencies away from the BF. Biological significance of these findings is discussed.