Frequency tuning properties of neurons in the inferior colliculus of an FM bat

Spectrally non-overlapping background noise disturbs echolocation via acoustic masking in the CF-FM bat, Hipposideros pratti

The environment noise may disturb animal behavior and echolocation via three potential mechanisms: acoustic masking, reduced attention and noise avoidance. Compared with the mechanisms of reduced attention and noise avoidance, acoustic masking is thought to occur only when the signal and background noise overlap spectrally and temporally. In this study, we investigated the effects of spectrally non-overlapping noise on echolocation pulses and electrophysiological responses of a constant frequency-frequency modulation (CF-FM) bat, Hipposideros pratti. We found that H. pratti called at higher intensities while keeping the CFs of their echolocation pulses consistent. Electrophysiological tests indicated that the noise could decrease auditory sensitivity and sharp intensity tuning, suggesting that spectrally non-overlapping noise imparts an acoustic masking effect. Because anthropogenic noises are usually concentrated at low frequencies and are spectrally non-overlapping with the bat's echolocation pulses, our results provide further evidence of negative consequences of anthropogenic noise. On this basis, we sound a warning against noise in the foraging habitats of echolocating bats.

Urethane Improves the Response of Auditory Neurons to Tone

Urethane has little effect on nervous system and is often used in neuroscience studies. However, the effect of urethane in neurons is not thoroughly clear. In this study, we investigated changes in neuron responses to tones in inferior colliculus during urethane anesthesia. As urethane was metabolized, the best and characteristic frequencies did not obviously change, but the minimal threshold (MT) remained relatively stable or was elevated. The frequency tuning bandwidth at 60 dB SPL (BW_60dBSPL) remained unchanged or decreased, and the average evoked spike of effective frequencies at 60 dB SPL (ES_60dBSPL) gradually decreased. Although the average evoked spike of effective frequencies at a tone intensity of 20 dB SPL above MT (ES_{20dBSPLaboveMT}) decreased, the frequency tuning bandwidth at a tone intensity of 20 dB SPL above MT (BW_{20dBSPLaboveMT}) did not change. In addition, the changes in MT, ES_60dBSPL, BW_60dBSPL, and ES_{20dBSPLaboveMT} increased with the MT in pre-anesthesia awake state (MT_{pre−anesthesiaawake}). In some neurons, the MT was lower, BW_60dBSPL was broader, and ES_60dBSPL and ES_{20dBSPLaboveMT} were higher in urethane anesthesia state than in pre-anesthesia awake state. During anesthesia, the inhibitory effect of urethane reduced the ES_{20dBSPLaboveMT}, but did not change the MT, characteristic frequency, or BW_{20dBSPLaboveMT}. In the recording session with the strongest neuron response, the first spike latency did not decrease, and the spontaneous spike did not increase. Therefore, we conclude that urethane can reduce/not change the MT, increase the evoked spike, or broaden/not change the frequency tuning range, and eventually improve the response of auditory neurons to tone with or without “pushing down” the tonal receptive field in thresholding model. The improved effect increases with the MT_{pre−anesthesiaawake} of neurons. The changes induced by the inhibitory and improved effects of urethane abide by similar regularities, but the change directions are contrary. The improvement mechanism may be likely due to the increase in the ratio of excitatory/inhibitory postsynaptic inputs to neurons.

Effect of background clutter on neural discrimination in the bat auditory midbrain

The discrimination of complex sounds is a fundamental function of the auditory system. This operation must be robust in the presence of noise and acoustic clutter. Echolocating bats are auditory specialists that discriminate sonar objects in acoustically complex environments. Bats produce brief signals, interrupted by periods of silence, rendering echo snapshots of sonar objects. Sonar object discrimination requires that bats process spatially and temporally overlapping echoes to make split-second decisions. The mechanisms that enable this discrimination are not well understood, particularly in complex environments. We explored the neural underpinnings of sonar object discrimination in the presence of acoustic scattering caused by physical clutter. We performed electrophysiological recordings in the inferior colliculus of awake big brown bats, to broadcasts of prerecorded echoes from physical objects. We acquired single unit responses to echoes and discovered a subpopulation of IC neurons that encode acoustic features that can be used to discriminate between sonar objects. We further investigated the effects of environmental clutter on this population's encoding of acoustic features. We discovered that the effect of background clutter on sonar object discrimination is highly variable and depends on object properties and target-clutter spatiotemporal separation. In many conditions, clutter impaired discrimination of sonar objects. However, in some instances clutter enhanced acoustic features of echo returns, enabling higher levels of discrimination. This finding suggests that environmental clutter may augment acoustic cues used for sonar target discrimination and provides further evidence in a growing body of literature that noise is not universally detrimental to sensory encoding.NEW & NOTEWORTHY Bats are powerful animal models for investigating the encoding of auditory objects under acoustically challenging conditions. Although past work has considered the effect of acoustic clutter on sonar target detection, less is known about target discrimination in clutter. Our work shows that the neural encoding of auditory objects was affected by clutter in a distance-dependent manner. These findings advance the knowledge on auditory object detection and discrimination and noise-dependent stimulus enhancement.

Enhanced representation of natural sound sequences in the ventral auditory midbrain

The auditory midbrain (inferior colliculus, IC) plays an important role in sound processing, acting as hub for acoustic information extraction and for the implementation of fast audio-motor behaviors. IC neurons are topographically organized according to their sound frequency preference: dorsal IC regions encode low frequencies while ventral areas respond best to high frequencies, a type of sensory map defined as tonotopy. Tonotopic maps have been studied extensively using artificial stimuli (pure tones) but our knowledge of how these maps represent information about sequences of natural, spectro-temporally rich sounds is sparse. We studied this question by conducting simultaneous extracellular recordings across IC depths in awake bats (Carollia perspicillata) that listened to sequences of natural communication and echolocation sounds. The hypothesis was that information about these two types of sound streams is represented at different IC depths since they exhibit large differences in spectral composition, i.e., echolocation covers the high-frequency portion of the bat soundscape (> 45 kHz), while communication sounds are broadband and carry most power at low frequencies (20–25 kHz). Our results showed that mutual information between neuronal responses and acoustic stimuli, as well as response redundancy in pairs of neurons recorded simultaneously, increase exponentially with IC depth. The latter occurs regardless of the sound type presented to the bats (echolocation or communication). Taken together, our results indicate the existence of mutual information and redundancy maps at the midbrain level whose response cannot be predicted based on the frequency composition of natural sounds and classic neuronal tuning curves.

Frequency-modulated up-chirps produce larger evoked responses than down-chirps in the big brown bat auditory brainstem

In many mammals, upward-sweeping frequency-modulated (FM) sounds (up-chirps) evoke larger auditory brainstem responses than downward-sweeping sounds (down-chirps). To determine if similar effects occur in FM echolocating bats, auditory evoked responses (AERs) in big brown bats in response to up-chirps and down-chirps at different chirp durations and levels were recorded. Even though down-chirps are the biologically relevant stimulus for big brown bats, up-chirps typically evoked larger peaks in the AER, but with some exceptions at the shortest chirp durations. The up-chirp duration that produced the largest AERs and the greatest differences between up-chirps and down-chirps varied between individual bats and stimulus levels. Cross-covariance analyses using the entire AER waveform confirmed that amplitudes were typically larger to up-chirps than down-chirps at supra-threshold levels, with optimal durations around 0.5-1 ms. Changes in response latencies with stimulus levels were consistent with previous estimates of amplitude-latency trading. Latencies tended to decrease with increasing up-chirp duration and increase with increasing down-chirp duration. The effects of chirp direction on AER waveforms are generally consistent with those seen in other mammals but with small differences in response patterns that may reflect specializations for FM echolocation.

Organization of subcortical auditory nuclei of Japanese house bat (Pipistrellus abramus) identified with cytoarchitecture and molecular expression

The auditory system of echolocating bats shows remarkable specialization likely related to analyzing echoes of sonar pulses. However, significant interspecies differences have been observed in the organization of auditory pathways among echolocating bats, and the homology of auditory nuclei with those of non-echolocating species has not been established. Here, in order to establish the homology and specialization of auditory pathways in echolocating bats, the expression of markers for glutamatergic, GABAergic, and glycinergic phenotypes in the subcortical auditory nuclei of Japanese house bat (Pipistrellus abramus) was evaluated. In the superior olivary complex, we identified the medial superior olive and superior paraolivary nuclei as expressing glutamatergic and GABAergic phenotypes, respectively, suggesting these nuclei are homologous with those of rodents. In the nuclei of the lateral lemniscus (NLL), the dorsal nucleus was found to be purely GABAergic, the intermediate nucleus was a mixture of glutamatergic and inhibitory neurons, the compact part of the ventral nucleus was purely glycinergic, and the multipolar part of the ventral nucleus expressed both GABA and glycine. In the inferior colliculus (IC), the central nucleus was found to be further subdivided into dorsal and ventral parts according to differences in the density of terminals and the morphology of large GABAergic neurons, suggesting specialization to sonar pulse structure. Medial geniculate virtually lacked GABAergic neurons, suggesting that the organization of the tectothalamic pathway is similar with that of rodents. Taken together, our findings revealed that specialization primarily occurs with regard to nuclei size and organization of the NLL and IC.

Tuning for rate and duration of frequency-modulated sweeps in the mammalian inferior colliculus

Responses of auditory duration-tuned neurons (DTNs) are selective for stimulus duration. We used single-unit extracellular recording to investigate how the inferior colliculus (IC) encodes frequency-modulated (FM) sweeps in the big brown bat. It was unclear whether the responses of so-called “FM DTNs” encode signal duration, like classic pure-tone DTNs, or the FM sweep rate. Most FM cells had spiking responses selective for downward FM sweeps. We presented cells with linear FM sweeps whose center frequency (CEF) was set to the best excitatory frequency and whose bandwidth (BW) maximized the spike count. With these baseline parameters, we stimulated cells with linear FM sweeps randomly varied in duration to measure the range of excitatory FM durations and/or sweep rates. To separate FM rate and FM duration tuning, we doubled (and halved) the BW of the baseline FM stimulus while keeping the CEF constant and then recollected each cell’s FM duration tuning curve. If the cell was tuned to FM duration, then the best duration (or range of excitatory durations) should remain constant despite changes in signal BW; however, if the cell was tuned to the FM rate, then the best duration should covary with the same FM rate at each BW. A Bayesian model comparison revealed that the majority of neurons were tuned to the FM sweep rate, although a few cells showed tuning for FM duration. We conclude that the dominant parameter for temporal tuning of FM neurons in the IC is FM sweep rate and not FM duration.

NEW & NOTEWORTHY Reports of inferior colliculus neurons with response selectivity to the duration of frequency-modulated (FM) stimuli exist, yet it remains unclear whether such cells are tuned to the FM duration or the FM sweep rate. To disambiguate these hypotheses, we presented neurons with variable-duration FM signals that were systematically manipulated in bandwidth. A Bayesian model comparison revealed that most temporally selective midbrain cells were tuned to the FM sweep rate and not the FM duration.

Natural echolocation sequences evoke echo-delay selectivity in the auditory midbrain of the FM bat, Eptesicus fuscus

Echolocating bats must process temporal streams of sonar sounds to represent objects along the range axis. Neuronal echo-delay tuning, the putative mechanism of sonar ranging, has been characterized in the inferior colliculus (IC) of the mustached bat, an insectivorous species that produces echolocation calls consisting of constant frequency and frequency modulated (FM) components, but not in species that use FM signals alone. This raises questions about the mechanisms that give rise to echo-delay tuning in insectivorous bats that use different signal designs. To investigate whether stimulus context may account for species differences in echo-delay selectivity, we characterized single-unit responses in the IC of awake passively listening FM bats, Eptesicus fuscus, to broadcasts of natural sonar call-echo sequences, which contained dynamic changes in signal duration, interval, spectrotemporal structure, and echo-delay. In E. fuscus, neural selectivity to call-echo delay emerges in a population of IC neurons when stimulated with call-echo pairs presented at intervals mimicking those in a natural sonar sequence. To determine whether echo-delay selectivity also depends on the spectrotemporal features of individual sounds within natural sonar sequences, we studied responses to computer-generated echolocation signals that controlled for call interval, duration, bandwidth, sweep rate, and echo-delay. A subpopulation of IC neurons responded selectively to the combination of the spectrotemporal structure of natural call-echo pairs and their temporal patterning within a dynamic sonar sequence. These new findings suggest that the FM bat's fine control over biosonar signal parameters may modulate IC neuronal selectivity to the dimension of echo-delay. NEW & NOTEWORTHY Echolocating bats perform precise auditory temporal computations to estimate their distance to objects. Here, we report that response selectivity of neurons in the inferior colliculus of a frequency modulated bat to call-echo delay, or target range tuning, depends on the temporal patterning and spectrotemporal features of sound elements in a natural echolocation sequence. We suggest that echo responses to objects at different distances are gated by the bat's active control over the spectrotemporal patterning of its sonar emissions.

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.

Frequency tuning of synaptic inhibition underlying duration-tuned neurons in the mammalian inferior colliculus

Inhibition plays an important role in creating the temporal response properties of duration-tuned neurons (DTNs) in the mammalian inferior colliculus (IC). Neurophysiological and computational studies indicate that duration selectivity in the IC is created through the convergence of excitatory and inhibitory synaptic inputs offset in time. We used paired-tone stimulation and extracellular recording to measure the frequency tuning of the inhibition acting on DTNs in the IC of the big brown bat (Eptesicus fuscus). We stimulated DTNs with pairs of tones differing in duration, onset time, and frequency. The onset time of a short, best-duration (BD), probe tone set to the best excitatory frequency (BEF) was varied relative to the onset of a longer-duration, nonexcitatory (NE) tone whose frequency was varied. When the NE tone frequency was near or within the cell's excitatory bandwidth (eBW), BD tone-evoked spikes were suppressed by an onset-evoked inhibition. The onset of the spike suppression was independent of stimulus frequency, but both the offset and duration of the suppression decreased as the NE tone frequency departed from the BEF. We measured the inhibitory frequency response area, best inhibitory frequency (BIF), and inhibitory bandwidth (iBW) of each cell. We found that the BIF closely matched the BEF, but the iBW was broader and usually overlapped the eBW measured from the same cell. These data suggest that temporal selectivity of midbrain DTNs is created and preserved by having cells receive an onset-evoked, constant-latency, broadband inhibition that largely overlaps the cell's excitatory receptive field. We conclude by discussing possible neural sources of the inhibition.NEW & NOTEWORTHY Duration-tuned neurons (DTNs) arise from temporally offset excitatory and inhibitory synaptic inputs. We used single-unit recording and paired-tone stimulation to measure the spectral tuning of the inhibitory inputs to DTNs. The onset of inhibition was independent of stimulus frequency; the offset and duration of inhibition systematically decreased as the stimulus departed from the cell's best excitatory frequency. Best inhibitory frequencies matched best excitatory frequencies; however, inhibitory bandwidths were more broadly tuned than excitatory bandwidths.

Effects of Electrical Stimulation in the Inferior Colliculus on Frequency Discrimination by Rhesus Monkeys and Implications for the Auditory Midbrain Implant

Understanding the relationship between the auditory selectivity of neurons and their contribution to perception is critical to the design of effective auditory brain prosthetics. These prosthetics seek to mimic natural activity patterns to achieve desired perceptual outcomes. We measured the contribution of inferior colliculus (IC) sites to perception using combined recording and electrical stimulation. Monkeys performed a frequency-based discrimination task, reporting whether a probe sound was higher or lower in frequency than a reference sound. Stimulation pulses were paired with the probe sound on 50% of trials (0.5-80 μA, 100-300 Hz, n = 172 IC locations in 3 rhesus monkeys). Electrical stimulation tended to bias the animals' judgments in a fashion that was coarsely but significantly correlated with the best frequency of the stimulation site compared with the reference frequency used in the task. Although there was considerable variability in the effects of stimulation (including impairments in performance and shifts in performance away from the direction predicted based on the site's response properties), the results indicate that stimulation of the IC can evoke percepts correlated with the frequency-tuning properties of the IC. Consistent with the implications of recent human studies, the main avenue for improvement for the auditory midbrain implant suggested by our findings is to increase the number and spatial extent of electrodes, to increase the size of the region that can be electrically activated, and to provide a greater range of evoked percepts.

SIGNIFICANCE STATEMENT

Patients with hearing loss stemming from causes that interrupt the auditory pathway after the cochlea need a brain prosthetic to restore hearing. Recently, prosthetic stimulation in the human inferior colliculus (IC) was evaluated in a clinical trial. Thus far, speech understanding was limited for the subjects and this limitation is thought to be partly due to challenges in harnessing the sound frequency representation in the IC. Here, we tested the effects of IC stimulation in monkeys trained to report the sound frequencies they heard. Our results indicate that the IC can be used to introduce a range of frequency percepts and suggest that placement of a greater number of electrode contacts may improve the effectiveness of such implants.

Somatosensory organ topography across the star of the star-nosed mole (Condylura cristata)

Quantifying somatosensory receptor distribution in glabrous skin is usually difficult because of the diversity of skin receptor subtypes and their location within the dermis and epidermis. However, the glabrous noses of moles are an exception. In most species of moles, the skin on the nose is covered with domed mechanosensory units known as an Eimer's organs. Eimer's organs contain a stereotyped array of different mechanosensory neurons, meaning that the distribution of mechanosensitive nerve endings can be inferred by visual inspection of the skin surface. Here we detail the distribution of Eimer's organs on the highly derived somatosensory star on the rostrum of the star-nosed mole (Condylura cristata). The star consists of 22 fleshy appendages, or rays, that are covered in Eimer's organs. We find that the density of Eimer's organs increases from proximal to distal locations along the length of the star's rays with a ratio of 1:2.3:3.1 from the surface nearest to the nostril, to the middle part of ray, to the ray tip, respectively. This ratio is comparable to the increase in receptor unit density reported for the human hand, from the palm, to the middle of the digits, to the distal fingertips. We also note that the tactile fovea of the star-nosed mole, located on the medial ventral ray, does not have increased sensory organ density, and we describe these findings in comparison with other sensory fovea.

The aftereffect of perceived duration is contingent on auditory frequency but not visual orientation

Recent sensory history plays a critical role in duration perception. It has been established that after adapting to a particular duration, the test durations within a certain range appear to be distorted. To explore whether the aftereffect of perceived duration can be constrained by sensory modality and stimulus feature within a modality, the current study applied the technique of simultaneous sensory adaptation, by which observers were able to simultaneously adapt to two durations defined by two different stimuli. Using both simple visual and auditory stimuli, we found that the aftereffect of perceived duration is modality specific and contingent on auditory frequency but not visual orientation of the stimulus. These results demonstrate that there are independent timers responsible for the aftereffects of perceived duration in each sensory modality. Furthermore, the timer for the auditory modality may be located at a relatively earlier stage of sensory processing than the timer for the visual modality.

Effect of echolocation behavior-related constant frequency-frequency modulation sound on the frequency tuning of inferior collicular neurons in Hipposideros armiger

In constant frequency-frequency modulation (CF-FM) bats, the CF-FM echolocation signals include both CF and FM components, yet the role of such complex acoustic signals in frequency resolution by bats remains unknown. Using CF and CF-FM echolocation signals as acoustic stimuli, the responses of inferior collicular (IC) neurons of Hipposideros armiger were obtained by extracellular recordings. We tested the effect of preceding CF or CF-FM sounds on the shape of the frequency tuning curves (FTCs) of IC neurons. Results showed that both CF-FM and CF sounds reduced the number of FTCs with tailed lower-frequency-side of IC neurons. However, more IC neurons experienced such conversion after adding CF-FM sound compared with CF sound. We also found that the Q 20 value of the FTC of IC neurons experienced the largest increase with the addition of CF-FM sound. Moreover, only CF-FM sound could cause an increase in the slope of the neurons' FTCs, and such increase occurred mainly in the lower-frequency edge. These results suggested that CF-FM sound could increase the accuracy of frequency analysis of echo and cut-off low-frequency elements from the habitat of bats more than CF sound.

Organization and trade-off of spectro-temporal tuning properties of duration-tuned neurons in the mammalian inferior colliculus

Neurons throughout the mammalian central auditory pathway respond selectively to stimulus frequency and amplitude, and some are also selective for stimulus duration. First found in the auditory midbrain or inferior colliculus (IC), these duration-tuned neurons (DTNs) provide a potential neural mechanism for encoding temporal features of sound. In this study, we investigated how having an additional neural response filter, one selective to the duration of an auditory stimulus, influences frequency tuning and neural organization by recording single-unit responses and measuring the dorsal-ventral position and spectral-temporal tuning properties of auditory DTNs from the IC of the awake big brown bat (Eptesicus fuscus). Like other IC neurons, DTNs were tonotopically organized and had either V-shaped, U-shaped, or O-shaped frequency tuning curves (excitatory frequency response areas). We hypothesized there would be an interaction between frequency and duration tuning in DTNs, as electrical engineering theory for resonant filters dictates a trade-off in spectral-temporal resolution: sharp tuning in the frequency domain results in poorer resolution in the time domain and vice versa. While the IC is a more complex signal analyzer than an electrical filter, a similar operational trade-off could exist in the responses of DTNs. Our data revealed two patterns of spectro-temporal sensitivity and spatial organization within the IC: DTNs with sharp frequency tuning and broad duration tuning were located in the dorsal IC, whereas cells with wide spectral tuning and narrow temporal tuning were found in the ventral IC.

Classification of frequency response areas in the inferior colliculus reveals continua not discrete classes

A differential response to sound frequency is a fundamental property of auditory neurons. Frequency analysis in the cochlea gives rise to V-shaped tuning functions in auditory nerve fibres, but by the level of the inferior colliculus (IC), the midbrain nucleus of the auditory pathway, neuronal receptive fields display diverse shapes that reflect the interplay of excitation and inhibition. The origin and nature of these frequency receptive field types is still open to question. One proposed hypothesis is that the frequency response class of any given neuron in the IC is predominantly inherited from one of three major afferent pathways projecting to the IC, giving rise to three distinct receptive field classes. Here, we applied subjective classification, principal component analysis, cluster analysis, and other objective statistical measures, to a large population (2826) of frequency response areas from single neurons recorded in the IC of the anaesthetised guinea pig. Subjectively, we recognised seven frequency response classes (V-shaped, non-monotonic Vs, narrow, closed, tilt down, tilt up and double-peaked), that were represented at all frequencies. We could identify similar classes using our objective classification tools. Importantly, however, many neurons exhibited properties intermediate between these classes, and none of the objective methods used here showed evidence of discrete response classes. Thus receptive field shapes in the IC form continua rather than discrete classes, a finding consistent with the integration of afferent inputs in the generation of frequency response areas. The frequency disposition of inhibition in the response areas of some neurons suggests that across-frequency inputs originating at or below the level of the IC are involved in their generation.

Conserved mechanisms of vocalization coding in mammalian and songbird auditory midbrain

The ubiquity of social vocalizations among animals provides the opportunity to identify conserved mechanisms of auditory processing that subserve communication. Identifying auditory coding properties that are shared across vocal communicators will provide insight into how human auditory processing leads to speech perception. Here, we compare auditory response properties and neural coding of social vocalizations in auditory midbrain neurons of mammalian and avian vocal communicators. The auditory midbrain is a nexus of auditory processing because it receives and integrates information from multiple parallel pathways and provides the ascending auditory input to the thalamus. The auditory midbrain is also the first region in the ascending auditory system where neurons show complex tuning properties that are correlated with the acoustics of social vocalizations. Single unit studies in mice, bats and zebra finches reveal shared principles of auditory coding including tonotopy, excitatory and inhibitory interactions that shape responses to vocal signals, nonlinear response properties that are important for auditory coding of social vocalizations and modulation tuning. Additionally, single neuron responses in the mouse and songbird midbrain are reliable, selective for specific syllables, and rely on spike timing for neural discrimination of distinct vocalizations. We propose that future research on auditory coding of vocalizations in mouse and songbird midbrain neurons adopt similar experimental and analytical approaches so that conserved principles of vocalization coding may be distinguished from those that are specialized for each species. This article is part of a Special Issue entitled "Communication Sounds and the Brain: New Directions and Perspectives".

Spectral coding in auditory midbrain neurons

The data of frequency properties of single neurons in the auditory midbrain center, central nucleus of the inferior colliculus and morphologic aspects of its functional ordering are reviewed. On the basis of reconstruction of single units frequency receptive fields and morphophysiologic mapping of their location within the frequency-band lamina of the central nucleus the model of spectral coding by auditory midbrain neurons is developed. The main structural basement of spectral coding in the auditory midbrain is the tonotopic organization of its central nucleus as well as the order in its morphological structure expressed in alternation of layers of disc-shaped neurons and neuropil. The main neural mechanism of spectral coding is a critical bands mechanism. The critical bandwidth is controlled by inhibitory disc-shaped neurons which constitute an essential part of the population of the central nucleus disc-shaped cells.

Systematic mapping of the monkey inferior colliculus reveals enhanced low frequency sound representation

We investigated the functional architecture of the inferior colliculus (IC) in rhesus monkeys. We systematically mapped multiunit responses to tonal stimuli and noise in the IC and surrounding tissue of six rhesus macaques, collecting data at evenly placed locations and recording nonresponsive locations to define boundaries. The results show a modest tonotopically organized region (17 of 100 recording penetration locations in 4 of 6 monkeys) surrounded by a large mass of tissue that, although vigorously responsive, showed no clear topographic arrangement (68 of 100 penetration locations). Rather, most cells in these recordings responded best to frequencies at the low end of the macaque auditory range. The remaining 15 (of 100) locations exhibited auditory responses that were not sensitive to sound frequency. Potential anatomical correlates of functionally defined regions and implications for midbrain auditory prosthetic devices are discussed.

Frequency tuning and latency organization of responses in the inferior colliculus of Japanese house bat, Pipistrellus abramus

Pipistrellus abramus emits quasi-constant frequency pulses during search, which extend the end frequency portion of the downward frequency-modulated sweep (terminal frequency; TF). If the narrowed frequency range is important for detecting a small frequency change caused by insect fluttering, the bats may need much finer frequency resolution at the TF. To test this hypothesis, the distribution of the best frequencies (BFs) in the inferior colliculus (IC) was electrophysiologically measured. The TF of the echolocation pulse was 41.44+/-2.62 kHz. The frequency range of 35-45 kHz was overrepresented in the IC (n=50/105; 48%), and a faint second peak was seen at 75-85 kHz (the second harmonic of the TF) in the BF distribution. The BF increased as a function of recording depth along the dorsoventral axis, except for the BFs of 35-45 and 75-85 kHz, which were found at a wide range of depths. The response latency ranged between 3.7 and 23.2 ms for the BFs of 35-45 kHz, and the maximum target range was estimated to be 3.3 m from the delay line observed in the IC. These electrophysiological measures suggest the importance of a target distance within approximately 3 m, which is consistent with behavioral measures during foraging in this species.

Tone-specific and nonspecific plasticity of inferior colliculus elicited by pseudo-conditioning: role of acetylcholine and auditory and somatosensory cortices

Experience-dependent plasticity in the central sensory systems depends on activation of both the sensory and neuromodulatory systems. Sensitization or nonspecific augmentation of central auditory neurons elicited by pseudo-conditioning with unpaired conditioning tonal (CS) and unconditioned electric leg (US) stimuli is quite different from tone-specific plasticity, called best frequency (BF) shifts, of the neurons elicited by auditory fear conditioning with paired CS and US. Therefore the neural circuits eliciting the nonspecific augmentation must be different from that eliciting the BF shifts. We first examined plastic changes in the response properties of collicular neurons of the big brown bat elicited by pseudo-conditioning and found that it elicited prominent nonspecific augmentation-an auditory response increase, a frequency-tuning broadening, and a threshold decreas-and that, in addition, it elicited a small short-lasting BF shift only when the CS frequency was 5 kHz lower than the BF of a recorded neuron. We examined the role of acetylcholine and the auditory and somatosensory cortices in these collicular changes. The development of the nonspecific augmentation was affected little by a muscarinic acetylcholine receptor antagonist applied to the inferior colliculus and by a GABA(A) receptor agonist applied to the auditory or somatosensory cortex. However, these drugs abolished the small short-lasting BF shift as they abolished the large long-lasting cortical and short-lasting collicular BF shifts elicited by the conditioning. These results indicate that, different from the BF shift, the nonspecific augmentation of the inferior colliculus depends on neither the cholinergic neuromodulator nor the auditory and somatosensory cortices.

Binaural interactions shape binaural response structures and frequency response functions in primary auditory cortex

The overall purpose of this study is to examine the behavior of primary auditory cortex (AI) units in the three-dimensional stimulus space that resembles normal listening conditions, viz., level at the two ears and frequency. A binaural-level response area (LRA) is the response to a matrix of contralateral and ipsilateral stimuli presented at a single frequency. LRAs have been examined in the inferior colliculus and AI and found to be highly organized response patterns that are shaped by binaural interactions. The aggregate of LRAs across frequency is the binaural response structure (BRS), a new concept that captures unit behavior in this three-dimensional stimulus space. Since binaural interactions contribute greatly to configuring component LRAs, it is clear that binaural interactions help shape the aggregate BRS. The BRS contains the data required to generate binaural frequency response functions. The frequency range and magnitude of these functions depend on the level of the stimulus at each ear and the configuration of the BRS. Changing either level can greatly alter the binaural frequency response function. Thus, in addition to their classic role in localization, binaural interactions play a fundamentally important role in determining the frequency domain of units in AI.

Rethinking tuning: in vivo whole-cell recordings of the inferior colliculus in awake bats

Tuning curves were recorded with patch electrodes from the inferior colliculus (IC) of awake bats to evaluate the tuning of the inputs to IC neurons, reflected in their synaptic tuning, compared with the tuning of their outputs, expressed in their discharge tuning. A number of unexpected features were revealed with whole-cell recordings. Among these was that most neurons responded to tones with inhibition and/or subthreshold excitation over a surprisingly broad frequency range. The synaptic tuning in many cells was at least 1.5-2.0 octaves wide and, on average, was more than twice as wide as the frequency range that evoked discharges even after inhibition was blocked. In most cells, tones evoked complex synaptic response configurations that varied with frequency, suggesting that these cells were not innervated by congruent excitatory and inhibitory projections. Synaptic tuning was not only wide but was also diverse, in which some cells were dominated by excitation (n = 20), others were dominated by excitation with sideband inhibition (n = 21), but most were dominated by inhibition with little evidence of excitation (n = 31). Another unexpected finding was that some cells responded with inhibition to the onset and offset of tones over a wide frequency range, in which the patterns of synaptic responses changed markedly with frequency. These cells never fired to tones at 50 dB sound pressure level but fired to frequency-modulated sweeps at that intensity and were directionally selective. Thus, the features revealed by whole-cell recordings show that the processing in many IC cells results from inputs spectrally broader and more complex than previously believed.

Response Properties and Location of Neurons Selective for Sinusoidal Frequency Modulations in the Inferior Colliculus of the Big Brown Bat

Most animal vocalizations, including echolocation signals used by bats, contain frequency-modulated (FM) components. Previous studies have described a class of neurons in the inferior colliculus (IC) of the big brown bat that respond exclusively to sinusoidally frequency modulated (SFM) signals and fail to respond to pure tones, noise, amplitude-modulated tones, or single FM sweeps. The aims of this study were to further characterize these neurons' response properties and to determine whether they are localized within a specific area of the IC. We recorded extracellularly from 214 neurons throughout the IC. Of these, 47 (22%) responded exclusively to SFM. SFM-selective cells were tuned to relatively low carrier frequencies (9–50 kHz), low modulation rates (20–210 Hz), and shallow modulation depths (3–10 kHz). Most had extremely low thresholds, with an average of 16.5 ± 7.6 dB SPL, and 89% had upper thresholds and closed response areas. For SFM-selective cells with spontaneous activity, the spontaneous activity was eliminated when sound amplitude exceeded their upper threshold and resumed after the stimulus was over. These findings suggest that SFM-selective cells receive low-threshold excitatory inputs and high-threshold inhibitory inputs. SFM-selective cells were clustered in the rostrodorsal part of the IC. Within this area, best modulation rate appeared to be correlated with best carrier frequency and depth within the IC.

Comparison of responses of neurons in the mouse inferior colliculus to current injections, tones of different durations, and sinusoidal amplitude-modulated tones

We made in vivo whole cell patch-clamp recordings from the inferior colliculus of young-adult, anesthetized C57/Bl6 mice to compare the responses to constant-current injections with the responses to tones of different duration or to sinusoidal amplitude-modulated (SAM) tones. We observed that voltage-dependent ion channels contributed in several ways to the response to tones. A sustained response to long tones was observed only in cells showing little accommodation during current injection. Cells showing burst-onset firing during current injection showed a small response to SAM tones, whereas burst-sustained cells showed a good response to SAM tones. The hyperpolarization-activated nonselective cation channel I(h) had a special role in shaping the responses: I(h) was associated with an increased excitability, with chopper and pauser responses, and with an afterhyperpolarization following tones. Synaptic properties were more important in determining the responses to tones of different durations. A short-latency inhibitory response appeared to contribute to the long-pass response in some cells and short-pass and band-pass neurons were characterized by their slow recovery from synaptic adaptation. Cells that recovered slowly from synaptic adaptation showed a relatively small response to SAM tones. Our results show an important role for both intrinsic membrane properties -- most notably the presence of I(h) and the extent of accommodation -- and synaptic adaptation in shaping the response to tones in the inferior colliculus.

The serotonin releaser fenfluramine alters the auditory responses of inferior colliculus neurons

Local direct application of the neuromodulator serotonin strongly influences auditory response properties of neurons in the inferior colliculus (IC), but endogenous stores of serotonin may be released in a distinct spatial or temporal pattern. To explore this issue, the serotonin releaser fenfluramine was iontophoretically applied to extracellularly recorded neurons in the IC of the Mexican free-tailed bat (Tadarida brasiliensis). Fenfluramine mimicked the effects of serotonin on spike count and first spike latency in most neurons, and its effects could be blocked by co-application of serotonin receptor antagonists, consistent with fenfluramine-evoked serotonin release. Responses to fenfluramine did not vary during single applications or across multiple applications, suggesting that fenfluramine did not deplete serotonin stores. A predicted gradient in the effects of fenfluramine with serotonin fiber density was not observed, but neurons with fenfluramine-evoked increases in latency occurred at relatively greater recording depths compared to other neurons with similar characteristic frequencies. These findings support the conclusion that there may be spatial differences in the effects of exogenous and endogenous sources of serotonin, but that other factors such as the identities and locations of serotonin receptors are also likely to play a role in determining the dynamics of serotonergic effects.

Intracellular recording reveals temporal integration in inferior colliculus neurons of awake bats

The central nucleus of the inferior colliculus (IC) is a major integrative center in the central auditory system. It receives information from both the ascending and descending auditory pathways. To determine how single IC neurons integrate information over a wide range of sound frequencies and sound levels, we examined their intracellular responses to frequency-modulated (FM) sounds in awake little brown bats (Myotis lucifugus). Postsynaptic potentials were recorded in response to downward FM sweeps of the range typical for little brown bats (80-20 kHz) and to three FM subcomponents (80-60, 60-40, and 40-20 kHz). The majority of recorded neurons responded to the 80- to 20-kHz downward FM sweep with a complex response. In this response an initial hyperpolarization was followed by depolarization with or without spike followed by hyperpolarization. Intracellular recordings in response to three FM subcomponents revealed that these neurons receive excitatory and inhibitory inputs from a wide range of sound frequencies. One third of IC neurons performed nearly linear temporal summation across a wide range of sound frequencies, whereas two thirds of IC neurons exhibited nonlinear summation with different degrees of nonlinearity. Some IC neurons showed different latencies of postsynaptic potentials in response to different FM subcomponents. Often responses to the later FM subcomponent occurred before responses to the earlier ones. This phenomenon may be responsible for response selectivity of IC neurons to FM sweeps.

FM signals produce robust paradoxical latency shifts in the bat's inferior colliculus

Previous studies in echolocating bats, Myotis lucifugus, showed that paradoxical latency shift (PLS) is essential for neural computation of target range and that a number of neurons in the inferior colliculus (IC) exhibit unit-specific PLS (characterized by longer first-spike latency at higher sound levels) in response to tone pulses at the unit's best frequency. The present study investigated whether or not frequency-modulated (FM) pulses that mimic the bat's echolocation sonar signals were equally effective in eliciting PLS. For two-thirds of PLS neurons in the IC, both FM and tone pulses could elicit PLS, but only FM pulses consistently produced unit-specific PLS. For the remainder of PLS neurons, only FM pulses effectively elicited PLS; these cells showed either no PLS or no response, to tone pulses. PLS neurons generally showed more pronounced PLS in response to narrow-band FM (each sweeping 20 kHz in 2 ms) pulse that contained the unit's best frequency. In addition, almost all PLS neurons showed duration-independent PLS to FM pulses, but the same units exhibited duration-dependent PLS to tone pulses. Taken together, when compared to tone pulses, FM stimuli can provide more reliable estimates of target range.

Responses of inferior colliculus neurons to SAM tones located in inhibitory response areas

In order to examine the effect of inhibition on processing auditory temporal information, responses of single neurons in the inferior colliculus of the chinchilla to sinusoidally amplitude-modulated (SAM) tones alone and the presence of a steady-state tone were obtained. The carrier frequency of the SAM tone was either the characteristic frequency (CF) or a frequency in the inhibitory response area of a studied neuron. When the carrier frequency was set to the neuron's CF, neurons responded in synchrony to the SAM-tone envelope, as expected. When the carrier frequency was set to a frequency at which pure tones produced inhibition, SAM tones elicited little or no response, also as expected. However, when the same SAM tone was paired with a pure tone whose frequency was set to the neuron's CF, responses synchronized to the SAM tone envelope were obtained. These modulated responses were typically one-half cycle out-of-phase with the response to the SAM tone at CF, suggesting that they arose from cyclic inhibition and release from inhibition by the SAM tone. The results demonstrate that the representation of temporal information by inferior colliculus neurons is influenced by temporally-patterned inhibition arising from locations remote from CF.

Different serotonin receptor agonists have distinct effects on sound-evoked responses in inferior colliculus

The neuromodulator serotonin has a complex set of effects on the auditory responses of neurons within the inferior colliculus (IC), a midbrain auditory nucleus that integrates a wide range of inputs from auditory and nonauditory sources. To determine whether activation of different types of serotonin receptors is a source of the variability in serotonergic effects, four selective agonists of serotonin receptors in the serotonin (5-HT) 1 and 5-HT2 families were iontophoretically applied to IC neurons, which were monitored for changes in their responses to auditory stimuli. Different agonists had different effects on neural responses. The 5-HT1A agonist had mixed facilitatory and depressive effects, whereas 5-HT1B and 5-HT2C agonists were both largely facilitatory. Different agonists changed threshold and frequency tuning in ways that reflected their effects on spike count. When pairs of agonists were applied sequentially to the same neurons, selective agonists sometimes affected neurons in ways that were similar to serotonin, but not to other selective agonists tested. Different agonists also differentially affected groups of neurons classified by the shapes of their frequency-tuning curves, with serotonin and the 5-HT1 receptors affecting proportionally more non-V-type neurons relative to the other agonists tested. In all, evidence suggests that the diversity of serotonin receptor subtypes in the IC is likely to account for at least some of the variability of the effects of serotonin and that receptor subtypes fulfill specialized roles in auditory processing.

Neural mechanisms underlying selectivity for the rate and direction of frequency-modulated sweeps in the inferior colliculus of the pallid bat

This study describes mechanisms that underlie neuronal selectivity for the direction and rate of frequency-modulated sweeps in the central nucleus of the inferior colliculus (ICC) of the pallid bat (Antrozous pallidus). This ICC contains a high percentage of neurons (66%) that respond selectively to the downward sweep direction of the bat's echolocation pulse. Some (19%) are specialists that respond only to downward sweeps. Most neurons (83%) are also tuned to sweep rates. A two-tone inhibition paradigm was used to describe inhibitory mechanisms that shape selectivity for sweep direction and rate. Two different mechanisms can create similar rate tuning. The first is an early on-best frequency inhibition that shapes duration tuning, which in turn determines rate tuning. In most neurons that are not duration tuned, a delayed high-frequency inhibition creates rate tuning. These neurons respond to fast sweep rates, but are inhibited as rate slows, and delayed inhibition overlaps excitation. In these neurons, starting a downward sweep within the excitatory tuning curve eliminates rate tuning. However, if rate tuning is shaped by duration tuning, this manipulation has no effect. Selectivity for the downward sweep direction is created by an early low-frequency inhibition that prevents responses to upward sweeps. In addition to this asymmetry in arrival times of low- and high-frequency inhibitions, the bandwidth of the low-frequency sideband was broader. Bandwidth influences the arrival time of inhibition during an FM sweep because a broader sideband will be encountered sooner. These findings show that similar spectrotemporal filters can be created by different mechanisms.

Relation between intrinsic connections and isofrequency contours in the inferior colliculus of the big brown bat, Eptesicus fuscus

Information processing in the inferior colliculus depends on interactions between ascending pathways and intrinsic circuitry, both of which exist within a functional tonotopic organization. To determine how local projections of neurons in the inferior colliculus are related to tonotopy, we placed a small iontophoretic injection of biodextran amine at a physiologically characterized location in the inferior colliculus. We then used electrophysiological recording to place a grid of small deposits of Chicago Sky Blue throughout the same frequency range to specify an isofrequency contour. Using three-dimensional computer reconstructions, we analyzed patterns of transport relative to the physiologically determined isofrequency contour to quantify the extent of the intrinsic connection lamina in all three dimensions. We also performed a quantitative analysis of the numbers of cells in different regions relative to the biodextran amine injection. Biodextran amine-labeled fibers were mainly located dorsomedial to the injection site, confined within the isofrequency contour, but biodextran amine-labeled cells were mainly located ventrolateral to the injection site. When we counted numbers of labeled cells classified by morphological type, we found that both elongate and multipolar cells were labeled within the isofrequency contour. Because the dendrites of multipolar cells typically extend outside the isofrequency lamina, it is likely that they receive input from other isofrequency contours and relay it to more dorsomedial portions of their specific isofrequency contour, along with the frequency-specific projections of the elongate cells. Within a given isofrequency contour, there is a consistent organization in which intrinsic connections ascend from the ventrolateral portion to more dorsomedial points along the contour, forming a cascaded system of intrinsic feedforward connections that seem ideally suited to provide the delay lines necessary to produce several forms of selectivity for temporal patterns in inferior colliculus neurons.

On the prediction of sweep rate and directional selectivity for FM sounds from two-tone interactions in the inferior colliculus

Two-tone stimuli have traditionally been used to reveal regions of inhibition in auditory spectral receptive fields, particularly for neurons with low spontaneous rates. These techniques reveal how different frequencies excite or suppress the response to an excitatory frequency of a cell, but have often been assessed at a fixed masker-probe time interval. We used a variation of this methodology to determine whether two-tone spectrotemporal interactions can account for rate-dependent directional selectivity for frequency modulations (FM) in the mustached bat inferior colliculus (IC). First, we quantified the response to upward and downward sweeping, linear, fixed-bandwidth FM tones centered at a unit's characteristic frequency (CF) at 6 sweep durations ranging from 2 to 64 ms. Then, to examine how responses to instantaneous frequencies contained within the sweeps might interact in time, we varied the frequency and relative onset of a brief (4 ms) "conditioner" tone paired with a fixed 4-ms CF probe tone. We constructed "conditioned response areas" (CRA) depicting regions of suppression and facilitation of the probe tone caused by the conditioning tone. We classified the CRAs as predominantly excitatory (40.9%), inhibitory (22.7%), or mixed (36.4%). To generate FM response predictions, the CRAs were multiplied with spectrograms of the same sweeps used to assess response to FM. The predictions of FM rate and directionality were accurate by our criteria in approximately 20% of units. Conversely, the CRAs from the remaining units failed to predict FM responses as accurately, suggesting that most IC units respond differently to FM sweeps than they do to tone-pairs matched to the instantaneous frequencies contained in those sweeps. The implications of these results for models of FM directionality are discussed.

Spectral integration in the inferior colliculus of the CBA/CaJ mouse

The inferior colliculus receives a massive convergence of inputs and in the mustached bat, this convergence leads to the creation of neurons in the inferior colliculus that integrate information across multiple frequency bands. These neurons are tuned to multiple frequency bands or are combination-sensitive; responding best to the combination of two signals of different frequency composition. The importance of combination-sensitive neurons in processing echolocation signals is well described, and it has been thought that combination sensitivity is a neural specialization for echolocation behaviors. Combination sensitivity and other response properties indicative of spectral integration have not been thoroughly examined in the inferior colliculus of non-echolocating mammals. In this study we tested the hypothesis that integration across frequencies occurs in the inferior colliculus of mice. We tested excitatory frequency response areas in the inferior colliculus of unanesthetized mice by varying the frequency of a single tone between 6 and 100 kHz. We then tested combination-sensitive responses by holding one tone at the unit's best frequency, and varying the frequency and intensity of a second tone. Thirty-two percent of the neurons were tuned to multiple frequency bands, 16% showed combination-sensitive facilitation and another 12% showed combination-sensitive inhibition. These findings suggests that the neural mechanisms underlying processing of complex sounds in the inferior colliculus share some common features among mammals as different as the bat and the mouse.

Duration selective neurons in the inferior colliculus of the rat: topographic distribution and relation of duration sensitivity to other response properties

Many animals use duration to help them identify the source and meaning of a sound. Duration-sensitive neurons have been found in the auditory midbrain of mammals and amphibians, where their selectivity seems to correspond to the lengths of species-specific vocalizations. In this study, single neurons in the rat inferior colliculus (IC) were tested for sensitivity to sound duration. About one-half (54%) of the units sampled showed some form of duration selectivity. The majority of these (76%) were long-pass neurons that responded to sounds exceeding some duration threshold (range: 5-60 ms). Band-pass neurons, which only responded to a restricted range of durations, made up 13% of duration-sensitive neurons (best durations: 15-120 ms). Other units displayed short-pass (2%) or mixed (9%) response patterns. The majority of duration-sensitive neurons were localized outside the central nucleus of the IC, especially in the dorsal cortex, where more than one-half of the neurons sampled had long-pass selectivity for duration. Band-pass duration tuned neurons were only found outside the central nucleus. Characteristics of duration-sensitive neurons in the rat support the idea that this filtering arises through an interaction of excitatory and inhibitory inputs that converge in the IC. Band-pass neurons typically responded at sound offset, suggesting that their tuning is created through the same mechanisms that have been described in echolocating bats. The finding that the first-spike latencies of all long-pass neurons were longer than the shortest duration to which they responded supports the idea that they receive transient inhibition before, or simultaneously with, a sustained excitatory input. The ranges of selectivity in rat IC neurons are within the range of durations of rat vocalizations. These data suggest that a population of neurons in the rat IC have evolved to transmit information about behaviorally relevant sound durations using mechanisms that are common to all mammals, with an emphasis on long-pass tuning characteristics.

Corticofugal feedback for collicular plasticity evoked by electric stimulation of the inferior colliculus

Focal electric stimulation of the auditory cortex, 30-min repetitive acoustic stimulation, and auditory fear conditioning each evoke shifts of the frequency-tuning curves [hereafter, best frequency (BF) shifts] of cortical and collicular neurons. The short-term collicular BF shift is produced by the corticofugal system and primarily depends on the relationship in BF between a recorded collicular and a stimulated cortical neuron or between the BF of a recorded collicular neuron and the frequency of an acoustic stimulus. However, it has been unknown whether focal electric stimulation of the inferior colliculus evokes the collicular BF shift and whether the collicular BF shift, if evoked, depends on corticofugal feedback. In our present research with the awake big brown bat, we found that focal electric stimulation of collicular neurons evoked the BF shifts of collicular neurons located near the stimulated ones; that there were two types of BF shifts: centripetal and centrifugal BF shifts, i.e., shifts toward and shifts away from the BF of stimulated neurons, respectively; and that the development of these collicular BF shifts was blocked by inactivation of the auditory cortex. Our data indicate that the collicular BF shifts (plasticity) evoked by collicular electric stimulation depended on corticofugal feedback. It should be noted that collicular BF shifts also depend on acetylcholine because it has been demonstrated that atropine (an antagonist of muscarinic acetylcholine receptors) applied to the IC blocks the development of collicular BF shifts.

Duration selectivity of neurons in the inferior colliculus of the big brown bat: tolerance to changes in sound level

At and above the level of the inferior colliculus (IC), some neurons respond maximally to a limited range of sound durations, with little or no excitatory response to durations outside of this range. Such neurons have been termed "duration tuned" or "duration selective." In this study we examined the effects of varying signal amplitude on best duration, width of tuning, and first spike latency of duration tuned neurons in the IC of the big brown bat, Eptesicus fuscus. Response areas as a function of stimulus duration and intensity took a variety of forms, including open (V-shaped), narrow and level tolerant (U-shaped), or closed (O-shaped). The majority (82%) of duration tuned neurons had narrow U-shaped or O-shaped duration response areas. Those with narrow U-shaped response areas retained their duration tuning across a broad dynamic range, < or = 50 dB above threshold, whereas those with O-shaped response areas were narrowly tuned to both stimulus duration and amplitude. For about one-half (55%) of the neurons with either a U- or O-shaped response areas, best duration (BD) changed by <1 ms across the range of suprathreshold amplitudes tested. Changes in BD most often took the form of a shift to slightly shorter durations as stimulus level increased. For the majority (65%) of U- and O-shaped neurons, 50% width of duration tuning changed by <2 ms with increasing amplitude. Latency of response at BD remained stable across changes in sound level, suggesting that the relative strengths of excitatory and inhibitory inputs to duration tuned neurons remain in balance over a wide dynamic range of sound pressure levels.

Processing of modulated sounds in the zebra finch auditory midbrain: responses to noise, frequency sweeps, and sinusoidal amplitude modulations

The avian auditory midbrain nucleus, the mesencephalicus lateralis, dorsalis (MLd), is the first auditory processing stage in which multiple parallel inputs converge, and it provides the input to the auditory thalamus. We studied the responses of single MLd neurons to four types of modulated sounds: 1) white noise; 2) band-limited noise; 3) frequency modulated (FM) sweeps, and 4) sinusoidally amplitude-modulated tones (SAM) in adult male zebra finches. Responses were compared with the responses of the same neurons to pure tones in terms of temporal response patterns, thresholds, characteristic frequencies, frequency tuning bandwidths, tuning sharpness, and spike rate/intensity relationships. Most neurons responded well to noise. More than one-half of the neurons responded selectively to particular portions of the noise, suggesting that, unlike forebrain neurons, many MLd neurons can encode specific acoustic components of highly modulated sounds such as noise. Selectivity for FM sweep direction was found in only 13% of cells that responded to sweeps. Those cells also showed asymmetric tuning curves, suggesting that asymmetric inhibition plays a role in FM directional selectivity. Responses to SAM showed that MLd neurons code temporal modulation rates using both spike rate and synchronization. Nearly all cells showed low-pass or band-pass filtering properties for SAM. Best modulation frequencies matched the temporal modulations in zebra finch song. Results suggest that auditory midbrain neurons are well suited for encoding a wide range of complex sounds with a high degree of temporal accuracy rather than selectively responding to only some sounds.

The inferior colliculus of the rat: A quantitative analysis of monaural frequency response areas

Frequency response areas (FRAs) were measured for 237 single units in the inferior colliculus (IC) of urethane-anesthetized pigmented rats using monaural pure-tone stimulation. Based on qualitative criteria [J Neurosci 21 (2001) 7303], FRAs were classified as V-shaped in 69% of neurons, non-V-shaped in 29%, and unclassifiable in the remaining 2%. Non-V-shaped FRAs were heterogeneous, comprising a number of subtypes including narrow, closed, low- and high-tilt, multipeaked, U-shaped, mosaic and inhibitory. To complement this subjective classification, we applied quantitative measures used by others (e.g. [J Neurophysiol 84 (2000) 1012]), including the inverse slope of the upper and lower FRA borders, Q-values, and other measures of bandwidth. The results suggest that FRAs in the rat IC are best described as forming a continuous distribution among subtypes, rather than clustering into discrete categories. Moreover, there is a broad range of frequency tuning characteristics and FRA types across the entire frequency spectrum. Within this general pattern, however, there are some frequency-specific differences in FRA type distribution. The relative proportion of V-shaped FRAs was greatest at the high and low ends of the auditory range, with the highest proportion of non-V-shaped FRAs in the mid-range from 6 to 12 kHz. For most neurons with multipeaked FRAs, the peak frequencies were not harmonically related. Frequency tuning in the pigmented rat IC is generally similar to that in other species. Comparison of Q values across auditory nuclei shows little evidence that FRAs are sharpened at levels above the auditory nerve. Rather, there is a broad range of frequency tuning properties at each level.

Neurobiological specializations in echolocating bats

Although the bat's nervous system follows the general mammalian plan in both its structure and function, it has undergone a number of modifications associated with flight and echolocation. The most obvious neuroanatomical specializations are seen in the cochleas of certain species of bats and in the lower brainstem auditory pathways of all microchiroptera. This article is a review of peripheral and central auditory neuroanatomical specializations in echolocating bats. Findings show that although the structural features of the central nervous system of echolocating microchiropteran bats are basically the same as those of more generalized mammals, certain pathways, mainly those having to do with accurate processing of temporal information and auditory control of motor activity, are hypertrophied and/or organized somewhat differently from those same pathways in nonecholocating species. Through the resulting changes in strengths and timing of synaptic inputs to neurons in these pathways, bats have optimized the mechanisms for analysis of complex sound patterns to derive accurate information about objects in their environment and direct behavior toward those objects.

Azimuth-dependent recovery cycle affects directional selectivity of bat inferior collicular neurons determined with sound pulses within a pulse train

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.

Corticofugal shaping of frequency tuning curves in the central nucleus of the inferior colliculus of mice

Plasticity of the auditory cortex can be induced by conditioning or focal cortical stimulation. The latter was used here to measure how stimulation in the tonotopy of the mouse primary auditory cortex influences frequency tuning in the midbrain central nucleus of the inferior colliculus (ICC). Shapes of collicular frequency tuning curves (FTCs) were quantified before and after cortical activation by measuring best frequencies, FTC bandwidths at various sound levels, level tolerance, Q-values, steepness of low- and high-frequency slopes, and asymmetries. We show here that all of these measures were significantly changed by focal cortical activation. The changes were dependent not only on the relationship of physiological properties between the stimulated cortical neurons and recorded collicular neurons but also on the tuning curve class of the collicular neuron. Cortical activation assimilated collicular FTC shapes; sharp and broad FTCs were changed to the shapes comparable to those of auditory nerve fibers. Plasticity in the ICC was organized in a center (excitatory)-surround (inhibitory) way with regard to the stimulated location (i.e., the frequency) of cortical tonotopy. This ensures, together with the spatial gradients of distribution of collicular FTC shapes, a sharp spectral filtering at the core of collicular frequency-band laminae and an increase in frequency selectivity at the periphery of the laminae. Mechanisms of FTC plasticity were suggested to comprise both corticofugal and local ICC components of excitatory and inhibitory modulation leading to a temporary change of the balance between excitation and inhibition in the ICC.

Ambiguities in sound-duration selectivity by neurons in the inferior colliculus of the bat Molossus molossus from Cuba

This study examines duration selectivity in auditory neurons of the inferior colliculus of the bat Molossus molossus (Molossidae, Chiroptera) from Cuba. Three main types of duration selectivity, short-, band-, and long-pass, as previously described in other species, are present in M. molossus. The range of best durations in the inferior colliculus of this species approximates the durations of their echolocation calls, suggesting that, as has been shown in other species of bats and frogs, the filter mechanism that produces duration tuning is selective for species-specific sounds relevant to behavior. Duration coding in M. molossus is not unambiguous because approximately 30% of the short- and band-pass neurons respond best to two different stimulus durations. This bimodal duration selectivity could be explained by time delayed excitatory inputs that coincide with an inhibitory rebound. In addition, the effect of stimulus intensity on duration selectivity was tested. For most of the neurons (78%), duration selectivity was affected by absolute sound pressure level and/or small changes of sound pressure. In this respect, the processing of stimulus duration by collicular neurons seems to be more complex in M. molossus than in other species studied so far.

Response properties of single neurons in the zebra finch auditory midbrain: response patterns, frequency coding, intensity coding, and spike latencies

The avian mesencephalicus lateralis, dorsalis (MLd) is the auditory midbrain nucleus in which multiple parallel inputs from lower brain stem converge and through which most auditory information passes to reach the forebrain. Auditory processing in the MLd has not been investigated in songbirds. We studied the tuning properties of single MLd neurons in adult male zebra finches. Pure tones were used to examine tonotopy, temporal response patterns, frequency coding, intensity coding, spike latencies, and duration tuning. Most neurons had no spontaneous activity. The tonotopy of MLd is like that of other birds and mammals; characteristic frequencies (CFs) increase in a dorsal to ventral direction. Four major response patterns were found: 1) onset (49% of cells); 2) primary-like (20%); 3) sustained (19%); and 4) primary-like with notch (12%). CFs ranged between 0.9 and 6.1 kHz, matching the zebra finch hearing range and the power spectrum of song. Tuning curves were generally V-shaped, but complex curves, with multiple peaks or noncontiguous excitatory regions, were observed in 22% of cells. Rate-level functions indicated that 51% of nononset cells showed monotonic relationships between spike rate and sound level. Other cells showed low saturation or nonmonotonic responses. Spike latencies ranged from 4 to 40 ms, measured at CF. Spike latencies generally decreased with increasing sound pressure level (SPL), although paradoxical latency shifts were observed in 16% of units. For onset cells, changes in SPL produced smaller latency changes than for cells showing other response types. Results suggest that auditory midbrain neurons may be particularly suited for processing temporally complex signals with a high degree of precision.

Expression of the Kv1.1 ion channel subunit in the auditory brainstem of the big brown bat, Eptesicus fuscus

Voltage-gated potassium channels play an important role in shaping membrane properties that underlie neurons' discharge patterns and the ways in which they transform their input. In the auditory system, low threshold potassium currents such as those created by Kv1.1 subunits contribute to precise phaselocking and to transient onset responses that provide time markers for temporal features of sounds. The purpose of the present study was to compare information about the distribution of neurons expressing the KV 1.1 in the brainstem auditory nuclei with the distribution of neurons with known functional properties in the auditory system of the big brown bat, Eptesicus fuscus. We used immunocytochemistry and light microscopy to look at the distribution of Kv1.1 subunits in the brainstem auditory nuclei. There was prominent expression in cell types known to contain high levels of Kv1.1 in other species and known to respond to auditory signals with high temporal precision. These included octopus cells and spherical bushy cells of the cochlear nucleus and principal neurons of the medial nucleus of the trapezoid body. In addition, we found high levels of Kv1.1 in neurons of the columnar subdivision of the ventral nucleus of the lateral lemniscus and in ventral periolivary cell groups. Neurons with high levels of Kv1.1 were differentially distributed in the intermediate nucleus of the lateral lemniscus and in the inferior colliculus, suggesting that these structures contain functionally distinct cell populations, some of which may be involved in high-precision temporal processing.