Temporal features of spectral integration in the inferior colliculus: effects of stimulus duration and rise time

Footprints of inhibition in the response of cortical delay-tuned neurons of bats

Responses of echo-delay-tuned neurons that encode target distance were investigated in the dorsal auditory cortex of anesthetized short-tailed fruit bats (Carollia perspicillata). This species echolocates using short downward frequency-modulated (FM) biosonar signals. In response to FM sweeps of increasing level, 60 out of 131 studied neurons (47%) displayed a "paradoxical latency shift," i.e., longer response latency to loud sounds and shorter latency to faint sounds. In addition, a disproportionately large number of neurons (80%) displayed nonmonotonic responses, i.e., weaker responses to loud sounds and stronger responses to faint sounds. We speculate that the observed paradoxical latency shift and nonmonotonic responses are extracellular footprints of inhibitory processes evoked by loud sounds and that they could represent a specialization for the processing of the emitted loud biosonar pulse. Supporting this idea is the fact that all studied neurons displayed strong response suppression when an artificial loud pulse and a faint echo were presented together at a nonoptimal delay. In 24 neurons, iontophoresis of bicuculline (an antagonist of A-type γ-aminobutyric acid receptors) did not remove inhibitory footprints but did increase the overall spike output, and in some cases it also modified the response bandwidth and shifted the neuron's "best delay." We suggest that inhibition could play a dual role in shaping delay tuning in different auditory stations. Below the cortex it participates in delay-tuning implementation and leaves a footprint that is measurable in cortical responses, while in the cortex it provides a substrate for an in situ control of neuronal selectivity.

Circuitry underlying spectrotemporal integration in the auditory midbrain

Combination sensitivity in central auditory neurons is a form of spectrotemporal integration in which excitatory responses to sounds at one frequency are facilitated by sounds within a distinctly different frequency band. Combination-sensitive neurons respond selectively to acoustic elements of sonar echoes or social vocalizations. In mustached bats, this response property originates in high-frequency representations of the inferior colliculus (IC) and depends on low and high frequency-tuned glycinergic inputs. To identify the source of these inputs, we combined glycine immunohistochemistry with retrograde tract tracing. Tracers were deposited at high-frequency (>56 kHz), combination-sensitive recording sites in IC. Most glycine-immunopositive, retrogradely labeled cells were in ipsilateral ventral and intermediate nuclei of the lateral lemniscus (VNLL and INLL), with some double labeling in ipsilateral lateral and medial superior olivary nuclei (LSO and MSO). Generally, double-labeled cells were in expected high-frequency tonotopic areas, but some VNLL and INLL labeling appeared to be in low-frequency representations. To test whether these nuclei provide low frequency-tuned input to the high-frequency IC, we combined retrograde tracing from IC combination-sensitive sites with anterograde tracing from low frequency-tuned sites in the anteroventral cochlear nucleus (AVCN). Only VNLL and INLL contained retrogradely labeled cells near (≤50 μm) anterogradely labeled boutons. These cells likely receive excitatory low-frequency input from AVCN. Results suggest that combination-sensitive facilitation arises through convergence of high-frequency glycinergic inputs from VNLL, INLL, or MSO and low-frequency glycinergic inputs from VNLL or INLL. This work establishes an anatomical basis for spectrotemporal integration in the auditory midbrain and a functional role for monaural nuclei of the lateral lemniscus.

A novel coding mechanism for social vocalizations in the lateral amygdala

The amygdala plays a central role in evaluating the significance of acoustic signals and coordinating the appropriate behavioral responses. To understand how amygdalar responses modulate auditory processing and drive emotional expression, we assessed how neurons respond to and encode information that is carried within complex acoustic stimuli. We characterized responses of single neurons in the lateral nucleus of the amygdala to social vocalizations and synthetic acoustic stimuli in awake big brown bats. Neurons typically responded to most of the social vocalizations presented (mean = nine of 11 vocalizations) but differentially modulated both firing rate and response duration. Surprisingly, response duration provided substantially more information about vocalizations than did spike rate. In most neurons, variation in response duration depended, in part, on persistent excitatory discharge that extended beyond stimulus duration. Information in persistent firing duration was significantly greater than in spike rate, and the majority of neurons displayed more information in persistent firing, which was more likely to be observed in response to aggressive vocalizations (64%) than appeasement vocalizations (25%), suggesting that persistent firing may relate to the behavioral context of vocalizations. These findings suggest that the amygdala uses a novel coding strategy for discriminating among vocalizations and underscore the importance of persistent firing in the general functioning of the amygdala.

The magnitudes of hyperpolarization-activated and low-voltage-activated potassium currents co-vary in neurons of the ventral cochlear nucleus

In the ventral cochlear nucleus (VCN), neurons have hyperpolarization-activated conductances, which in some cells are enormous, that contribute to the ability of neurons to convey acoustic information in the timing of their firing by decreasing the input resistance and speeding-up voltage changes. Comparisons of the electrophysiological properties of neurons in the VCN of mutant mice that lack the hyperpolarization-activated cyclic nucleotide-gated channel α subunit 1 (HCN1(-/-)) (Nolan et al. 2003) with wild-type controls (HCN1(+/+)) and with outbred ICR mice reveal that octopus, T stellate, and bushy cells maintain their electrophysiological distinctions in all strains. Hyperpolarization-activated (I(h)) currents were smaller and slower, input resistances were higher, and membrane time constants were longer in HCN1(-/-) than in HCN1(+/+) in octopus, bushy, and T stellate cells. There were significant differences in the average magnitudes of I(h), input resistances, and time constants between HCN1(+/+) and ICR mice, but the resting potentials did not differ between strains. I(h) is opposed by a low-voltage-activated potassium (I(KL)) current in bushy and octopus cells, whose magnitudes varied widely between neuronal types and between strains. The magnitudes of I(h) and I(KL) were correlated across neuronal types and across mouse strains. Furthermore, these currents balanced one another at the resting potential in individual cells. The magnitude of I(h) and I(KL) is linked in bushy and octopus cells and varies not only between HCN1(-/-) and HCN1(+/+) but also between "wild-type" strains of mice, raising the question to what extent the wild-type strains reflect normal mice.

Neural processing of target distance by echolocating bats: functional roles of the auditory midbrain

Using their biological sonar, bats estimate distance to avoid obstacles and capture moving prey. The primary distance cue is the delay between the bat's emitted echolocation pulse and the return of an echo. The mustached bat's auditory midbrain (inferior colliculus, IC) is crucial to the analysis of pulse-echo delay. IC neurons are selective for certain delays between frequency modulated (FM) elements of the pulse and echo. One role of the IC is to create these "delay-tuned", "FM-FM" response properties through a series of spectro-temporal integrative interactions. A second major role of the midbrain is to project target distance information to many parts of the brain. Pathways through auditory thalamus undergo radical reorganization to create highly ordered maps of pulse-echo delay in auditory cortex, likely contributing to perceptual features of target distance analysis. FM-FM neurons in IC also project strongly to pre-motor centers including the pretectum and the pontine nuclei. These pathways may contribute to rapid adjustments in flight, body position, and sonar vocalizations that occur as a bat closes in on a target.

Substrates of auditory frequency integration in a nucleus of the lateral lemniscus

In the intermediate nucleus of the lateral lemniscus (INLL), some neurons display a form of spectral integration in which excitatory responses to sounds at their best frequency are inhibited by sounds within a frequency band at least one octave lower. Previous work showed that this response property depends on low-frequency-tuned glycinergic input. To identify all sources of inputs to these INLL neurons, and in particular the low-frequency glycinergic input, we combined retrograde tracing with immunohistochemistry for the neurotransmitter glycine. We deposited a retrograde tracer at recording sites displaying either high best frequencies (>75 kHz) in conjunction with combination-sensitive inhibition, or at sites displaying low best frequencies (23-30 kHz). Most retrogradely labeled cells were located in the ipsilateral medial nucleus of the trapezoid body (MNTB) and contralateral anteroventral cochlear nucleus. Consistent labeling, but in fewer numbers, was observed in the ipsilateral lateral nucleus of the trapezoid body (LNTB), contralateral posteroventral cochlear nucleus, and a few other brainstem nuclei. When tracer deposits were combined with glycine immunohistochemistry, most double-labeled cells were observed in the ipsilateral MNTB (84%), with fewer in LNTB (13%). After tracer deposits at combination-sensitive recording sites, a striking result was that MNTB labeling occurred in both medial and lateral regions. This labeling appeared to overlap the MNTB labeling that resulted from tracer deposits in low-frequency recording sites of INLL. These findings suggest that MNTB is the most likely source of low-frequency glycinergic input to INLL neurons with high best frequencies and combination-sensitive inhibition. This work establishes an anatomical basis for frequency integration in the auditory brainstem.

Facilitatory mechanisms shape selectivity for the rate and direction of FM sweeps in the inferior colliculus of the pallid bat

The inferior colliculus (IC) of the pallid bat has a large percentage of neurons that respond selectively to the rate and direction of the bat's echolocation pulse, a downward FM sweep. Three underlying mechanisms have been previously described. Here we describe a fourth mechanism, facilitation, that shapes selectivity for both sweep rate and direction. The neurons studied are termed FM specialists, because they do not respond to tones. Most were selective for the downward sweep direction, and this preference was expressed even when presented with narrowband, 1 kHz sweeps that crossed only a fraction of their excitatory receptive fields. This selectivity was also expressed in response to two tones delayed in time, termed two-tone facilitation (TTF). Direction-selective neurons showed a greatly facilitated response when a higher-frequency tone preceded a lower-frequency tone, simulating conditions in a downward sweep. The degree of temporal asymmetry in facilitation accurately predicted direction selectivity. When the spectral difference between the two tones was increased, the best delay also increased and could be used to predict a neuron's preferred sweep rate. To determine whether TTF alone created rate and direction selectivity, low- and high-frequency inhibitory sidebands, which can also shape selectivity, were eliminated from sweeps. In most cases, selectivity persisted. These results support the idea of spectral delay lines that produce an overlap and summation of excitatory inputs only when a dynamic stimulus traverses a receptive field in one direction at a specific velocity.

Glycinergic inhibition creates a form of auditory spectral integration in nuclei of the lateral lemniscus

For analyses of complex sounds, many neurons integrate information across different spectral elements via suppressive effects that are distant from the neurons' excitatory tuning. In the mustached bat, suppression evoked by sounds within the first sonar harmonic (23-30 kHz) or in the subsonar band (<23 kHz) alters responsiveness to the higher best frequencies of many neurons. This study examined features and mechanisms associated with low-frequency (LF) suppression among neurons of the lateral lemniscal nuclei (NLL). We obtained extracellular recordings from neurons in the intermediate and ventral nuclei of the lateral lemniscus, observing different forms of LF suppression related to the two above-cited frequency bands. To understand the mechanisms underlying this suppression in NLL neurons, we examined the roles of glycinergic and GABAergic input through local microiontophoretic application of strychnine, an antagonist to glycine receptors (GlyRs), or bicuculline, an antagonist to gamma-aminobutyric acid type A receptors (GABA(A)Rs). With blockade of GABA(A)Rs, neurons showed an increase in firing rate to best frequency (BF) and/or LF tones but retained LF suppression of BF sounds. For neurons that displayed LF suppression tuned to 23-30 kHz, the suppression was eliminated or nearly eliminated by GlyR blockade. In contrast, GABA(A)R blockade did not eliminate nor had any consistent effect on suppression tuned to these frequencies. We conclude that LF suppression tuned in the 23- to 30-kHz range results from neuronal inhibition within the NLL via glycinergic inputs. For neurons displaying suppression tuned <23 kHz, neither GlyR nor GABAR blockade altered LF suppression. We conclude that such suppression originates at a lower auditory level, perhaps a result of cochlear mechanisms. These findings demonstrate that neuronal interactions within NLL create a particular form of LF suppression that contributes to the analysis of complex acoustic signals.