Duration selectivity organization in the inferior colliculus of the big brown bat, Eptesicus fuscus

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.

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.

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

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

Decoding stimulus duration from neural responses in the auditory midbrain

Neurons with responses selective for the duration of an auditory stimulus are called duration-tuned neurons (DTNs). Temporal specificity in their spiking suggests that one function of DTNs is to encode stimulus duration; however, the efficacy of duration encoding by DTNs has yet to be investigated. Herein, we characterize the information content of individual cells and a population of DTNs from the mammalian inferior colliculus (IC) by measuring the stimulus-specific information (SSI) and estimated Fisher information (FI) of spike count responses. We found that SSI was typically greatest for those stimulus durations that evoked maximum spike counts, defined as best duration (BD) stimuli, and that FI was maximal for stimulus durations off BD where sensitivity to a change in duration was greatest. Using population data, we demonstrate that a maximum likelihood estimator (MLE) can accurately decode stimulus duration from evoked spike counts. We also simulated a two-alternative forced choice task by having MLE models decide whether two durations were the same or different. With this task we measured the just-noticeable difference threshold for stimulus duration and calculated the corresponding Weber fractions across the stimulus domain. Altogether, these results demonstrate that the spiking responses of DTNs from the mammalian IC contain sufficient information for the CNS to encode, decode, and discriminate behaviorally relevant auditory signal durations.

Dichotic sound localization properties of duration-tuned neurons in the inferior colliculus of the big brown bat

Electrophysiological studies on duration-tuned neurons (DTNs) from the mammalian auditory midbrain have typically evoked spiking responses from these cells using monaural or free-field acoustic stimulation focused on the contralateral ear, with fewer studies devoted to examining the electrophysiological properties of duration tuning using binaural stimulation. Because the inferior colliculus (IC) receives convergent inputs from lower brainstem auditory nuclei that process sounds from each ear, many midbrain neurons have responses shaped by binaural interactions and are selective to binaural cues important for sound localization. In this study, we used dichotic stimulation to vary interaural level difference (ILD) and interaural time difference (ITD) acoustic cues and explore the binaural interactions and response properties of DTNs and non-DTNs from the IC of the big brown bat (Eptesicus fuscus). Our results reveal that both DTNs and non-DTNs can have responses selective to binaural stimulation, with a majority of IC neurons showing some type of ILD selectivity, fewer cells showing ITD selectivity, and a number of neurons showing both ILD and ITD selectivity. This study provides the first demonstration that the temporally selective responses of DTNs from the vertebrate auditory midbrain can be selective to binaural cues used for sound localization in addition to having spiking responses that are selective for stimulus frequency, amplitude, and duration.

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.

Multiple mechanisms shape FM sweep rate selectivity: complementary or redundant?

Auditory neurons in the inferior colliculus (IC) of the pallid bat have highly rate selective responses to downward frequency modulated (FM) sweeps attributable to the spectrotemporal pattern of their echolocation call (a brief FM pulse). Several mechanisms are known to shape FM rate selectivity within the pallid bat IC. Here we explore how two mechanisms, stimulus duration and high-frequency inhibition (HFI), can interact to shape FM rate selectivity within the same neuron. Results from extracellular recordings indicated that a derived duration-rate function (based on tonal response) was highly predictive of the shape of the FM rate response. Longpass duration selectivity for tones was predictive of slowpass rate selectivity for FM sweeps, both of which required long stimulus durations and remained intact following iontophoretic blockade of inhibitory input. Bandpass duration selectivity for tones, sensitive to only a narrow range of tone durations, was predictive of bandpass rate selectivity for FM sweeps. Conversion of the tone duration response from bandpass to longpass after blocking inhibition was coincident with a change in FM rate selectivity from bandpass to slowpass indicating an active inhibitory component to the formation of bandpass selectivity. Independent of the effect of duration tuning on FM rate selectivity, the presence of HFI acted as a fastpass FM rate filter by suppressing slow FM sweep rates. In cases where both mechanisms were present, both had to be eliminated, by removing inhibition, before bandpass FM rate selectivity was affected. It is unknown why the auditory system utilizes multiple mechanisms capable of shaping identical forms of FM rate selectivity though it may represent distinct but convergent modes of neural signaling directed at shaping response selectivity for important biologically relevant sounds.

Dynamic temporal signal processing in the inferior colliculus of echolocating bats

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.

Tuning shifts of the auditory system by corticocortical and corticofugal projections and conditioning

The central auditory system consists of the lemniscal and nonlemniscal systems. The thalamic lemniscal and nonlemniscal auditory nuclei are different from each other in response properties and neural connectivities. The cortical auditory areas receiving the projections from these thalamic nuclei interact with each other through corticocortical projections and project down to the subcortical auditory nuclei. This corticofugal (descending) system forms multiple feedback loops with the ascending system. The corticocortical and corticofugal projections modulate auditory signal processing and play an essential role in the plasticity of the auditory system. Focal electric stimulation - comparable to repetitive tonal stimulation - of the lemniscal system evokes three major types of changes in the physiological properties, such as the tuning to specific values of acoustic parameters of cortical and subcortical auditory neurons through different combinations of facilitation and inhibition. For such changes, a neuromodulator, acetylcholine, plays an essential role. Electric stimulation of the nonlemniscal system evokes changes in the lemniscal system that is different from those evoked by the lemniscal stimulation. Auditory signals ascending from the lemniscal and nonlemniscal thalamic nuclei to the cortical auditory areas appear to be selected or adjusted by a "differential" gating mechanism. Conditioning for associative learning and pseudo-conditioning for nonassociative learning respectively elicit tone-specific and nonspecific plastic changes. The lemniscal, corticofugal and cholinergic systems are involved in eliciting the former, but not the latter. The current article reviews the recent progress in the research of corticocortical and corticofugal modulations of the auditory system and its plasticity elicited by conditioning and pseudo-conditioning.

Duration tuning in the auditory midbrain of echolocating and non-echolocating vertebrates

Neurons tuned for stimulus duration were first discovered in the auditory midbrain of frogs. Duration-tuned neurons (DTNs) have since been reported from the central auditory system of both echolocating and non-echolocating mammals, and from the central visual system of cats. We hypothesize that the functional significance of auditory duration tuning likely varies between species with different evolutionary histories, sensory ecologies, and bioacoustic constraints. For example, in non-echolocating animals such as frogs and mice the temporal filtering properties of auditory DTNs may function to discriminate species-specific communication sounds. In echolocating bats duration tuning may also be used to create cells with highly selective responses for specific rates of frequency modulation and/or pulse-echo delays. The ability to echolocate appears to have selected for high temporal acuity in the duration tuning curves of inferior colliculus neurons in bats. Our understanding of the neural mechanisms underlying sound duration selectivity has improved substantially since DTNs were first discovered almost 50 years ago, but additional research is required for a comprehensive understanding of the functional role and the behavioral significance that duration tuning plays in sensory systems.

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

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

The recovery cycle of bat duration-selective collicular neurons varies with hunting phase

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.