Brain-wide inputs to the non-lemniscal inferior colliculus in mice

Population coding of time-varying sounds in the nonlemniscal inferior colliculus

The inferior colliculus (IC) of the midbrain is important for complex sound processing, such as discriminating conspecific vocalizations and human speech. The IC's nonlemniscal, dorsal "shell" region is likely important for this process, as neurons in these layers project to higher-order thalamic nuclei that subsequently funnel acoustic signals to the amygdala and nonprimary auditory cortices, forebrain circuits important for vocalization coding in a variety of mammals, including humans. However, the extent to which shell IC neurons transmit acoustic features necessary to discern vocalizations is less clear, owing to the technical difficulty of recording from neurons in the IC's superficial layers via traditional approaches. Here, we use two-photon Ca2+ imaging in mice of either sex to test how shell IC neuron populations encode the rate and depth of amplitude modulation, important sound cues for speech perception. Most shell IC neurons were broadly tuned, with a low neurometric discrimination of amplitude modulation rate; only a subset was highly selective to specific modulation rates. Nevertheless, neural network classifier trained on fluorescence data from shell IC neuron populations accurately classified amplitude modulation rate, and decoding accuracy was only marginally reduced when highly tuned neurons were omitted from training data. Rather, classifier accuracy increased monotonically with the modulation depth of the training data, such that classifiers trained on full-depth modulated sounds had median decoding errors of ∼0.2 octaves. Thus, shell IC neurons may transmit time-varying signals via a population code, with perhaps limited reliance on the discriminative capacity of any individual neuron.NEW & NOTEWORTHY The IC's shell layers originate a "nonlemniscal" pathway important for perceiving vocalization sounds. However, prior studies suggest that individual shell IC neurons are broadly tuned and have high response thresholds, implying a limited reliability of efferent signals. Using Ca2+ imaging, we show that amplitude modulation is accurately represented in the population activity of shell IC neurons. Thus, downstream targets can read out sounds' temporal envelopes from distributed rate codes transmitted by populations of broadly tuned neurons.

Research progress of the inferior colliculus: from Neuron, neural circuit to auditory disease

The auditory midbrain, also known as the inferior colliculus (IC), serves as a crucial hub in the auditory pathway. Comprising diverse cell types, the IC plays a pivotal role in various auditory functions, including sound localization, auditory plasticity, sound detection, and sound-induced behaviors. Notably, the IC is implicated in several auditory central disorders, such as tinnitus, age-related hearing loss, autism and Fragile X syndrome. Accurate classification of IC neurons is vital for comprehending both normal and dysfunctional aspects of IC function. Various parameters, including dendritic morphology, neurotransmitter synthesis, potassium currents, biomarkers, and axonal targets, have been employed to identify distinct neuron types within the IC. However, the challenge persists in effectively classifying IC neurons into functional categories due to the limited clustering capabilities of most parameters. Recent studies utilizing advanced neuroscience technologies have begun to shed light on biomarker-based approaches in the IC, providing insights into specific cellular properties and offering a potential avenue for understanding IC functions. This review focuses on recent advancements in IC research, spanning from neurons and neural circuits to aspects related to auditory diseases.

Listening to your partner: serotonin increases male responsiveness to female vocal signals in mice

The context surrounding vocal communication can have a strong influence on how vocal signals are perceived. The serotonergic system is well-positioned for modulating the perception of communication signals according to context, because serotonergic neurons are responsive to social context, influence social behavior, and innervate auditory regions. Animals like lab mice can be excellent models for exploring how serotonin affects the primary neural systems involved in vocal perception, including within central auditory regions like the inferior colliculus (IC). Within the IC, serotonergic activity reflects not only the presence of a conspecific, but also the valence of a given social interaction. To assess whether serotonin can influence the perception of vocal signals in male mice, we manipulated serotonin systemically with an injection of its precursor 5-HTP, and locally in the IC with an infusion of fenfluramine, a serotonin reuptake blocker. Mice then participated in a behavioral assay in which males suppress their ultrasonic vocalizations (USVs) in response to the playback of female broadband vocalizations (BBVs), used in defensive aggression by females when interacting with males. Both 5-HTP and fenfluramine increased the suppression of USVs during BBV playback relative to controls. 5-HTP additionally decreased the baseline production of a specific type of USV and male investigation, but neither drug treatment strongly affected male digging or grooming. These findings show that serotonin modifies behavioral responses to vocal signals in mice, in part by acting in auditory brain regions, and suggest that mouse vocal behavior can serve as a useful model for exploring the mechanisms of context in human communication.

Population coding of time-varying sounds in the non-lemniscal Inferior Colliculus

The inferior colliculus (IC) of the midbrain is important for complex sound processing, such as discriminating conspecific vocalizations and human speech. The IC's non-lemniscal, dorsal "shell" region is likely important for this process, as neurons in these layers project to higher-order thalamic nuclei that subsequently funnel acoustic signals to the amygdala and non-primary auditory cortices; forebrain circuits important for vocalization coding in a variety of mammals, including humans. However, the extent to which shell IC neurons transmit acoustic features necessary to discern vocalizations is less clear, owing to the technical difficulty of recording from neurons in the IC's superficial layers via traditional approaches. Here we use 2-photon Ca2+ imaging in mice of either sex to test how shell IC neuron populations encode the rate and depth of amplitude modulation, important sound cues for speech perception. Most shell IC neurons were broadly tuned, with a low neurometric discrimination of amplitude modulation rate; only a subset were highly selective to specific modulation rates. Nevertheless, neural network classifier trained on fluorescence data from shell IC neuron populations accurately classified amplitude modulation rate, and decoding accuracy was only marginally reduced when highly tuned neurons were omitted from training data. Rather, classifier accuracy increased monotonically with the modulation depth of the training data, such that classifiers trained on full-depth modulated sounds had median decoding errors of ~0.2 octaves. Thus, shell IC neurons may transmit time-varying signals via a population code, with perhaps limited reliance on the discriminative capacity of any individual neuron.

Differential projections from the cochlear nucleus to the inferior colliculus in the mouse

The cochlear nucleus (CN) is often regarded as the gateway to the central auditory system because it initiates all ascending pathways. The CN consists of dorsal and ventral divisions (DCN and VCN, respectively), and whereas the DCN functions in the analysis of spectral cues, circuitry in VCN is part of the pathway focused on processing binaural information necessary for sound localization in horizontal plane. Both structures project to the inferior colliculus (IC), which serves as a hub for the auditory system because pathways ascending to the forebrain and descending from the cerebral cortex converge there to integrate auditory, motor, and other sensory information. DCN and VCN terminations in the IC are thought to overlap but given the differences in VCN and DCN architecture, neuronal properties, and functions in behavior, we aimed to investigate the pattern of CN connections in the IC in more detail. This study used electrophysiological recordings to establish the frequency sensitivity at the site of the anterograde dye injection for the VCN and DCN of the CBA/CaH mouse. We examined their contralateral projections that terminate in the IC. The VCN projections form a topographic sheet in the central nucleus (CNIC). The DCN projections form a tripartite set of laminar sheets; the lamina in the CNIC extends into the dorsal cortex (DC), whereas the sheets to the lateral cortex (LC) and ventrolateral cortex (VLC) are obliquely angled away. These fields in the IC are topographic with low frequencies situated dorsally and progressively higher frequencies lying more ventrally and/or laterally; the laminae nestle into the underlying higher frequency fields. The DCN projections are complementary to the somatosensory modules of layer II of the LC but both auditory and spinal trigeminal terminations converge in the VLC. While there remains much to be learned about these circuits, these new data on auditory circuits can be considered in the context of multimodal networks that facilitate auditory stream segregation, signal processing, and species survival.

Perceptual warping exposes categorical representations for speech in human brainstem responses

The brain transforms continuous acoustic events into discrete category representations to downsample the speech signal for our perceptual-cognitive systems. Such phonetic categories are highly malleable, and their percepts can change depending on surrounding stimulus context. Previous work suggests these acoustic-phonetic mapping and perceptual warping of speech emerge in the brain no earlier than auditory cortex. Here, we examined whether these auditory-category phenomena inherent to speech perception occur even earlier in the human brain, at the level of auditory brainstem. We recorded speech-evoked frequency following responses (FFRs) during a task designed to induce more/less warping of listeners' perceptual categories depending on stimulus presentation order of a speech continuum (random, forward, backward directions). We used a novel clustered stimulus paradigm to rapidly record the high trial counts needed for FFRs concurrent with active behavioral tasks. We found serial stimulus order caused perceptual shifts (hysteresis) near listeners' category boundary confirming identical speech tokens are perceived differentially depending on stimulus context. Critically, we further show neural FFRs during active (but not passive) listening are enhanced for prototypical vs. category-ambiguous tokens and are biased in the direction of listeners' phonetic label even for acoustically-identical speech stimuli. These findings were not observed in the stimulus acoustics nor model FFR responses generated via a computational model of cochlear and auditory nerve transduction, confirming a central origin to the effects. Our data reveal FFRs carry category-level information and suggest top-down processing actively shapes the neural encoding and categorization of speech at subcortical levels. These findings suggest the acoustic-phonetic mapping and perceptual warping in speech perception occur surprisingly early along the auditory neuroaxis, which might aid understanding by reducing ambiguity inherent to the speech signal.