Single-unit responses in the inferior colliculus of decerebrate cats. I. Classification based on frequency response maps

Hearing loss-related altered neuronal activity in the inferior colliculus

Hearing loss is well known to cause plastic changes in the central auditory system and pathological changes such as tinnitus and hyperacusis. Impairment of inner ear functions is the main cause of hearing loss. In aged individuals, not only inner ear dysfunction but also senescence of the central nervous system is the cause of malfunction of the auditory system. In most cases of hearing loss, the activity of the auditory nerve is reduced, but that of the successive auditory centers is increased in a compensatory way. It has been reported that activity changes occur in the inferior colliculus (IC), a critical nexus of the auditory pathway. The IC integrates the inputs from the brainstem and drives the higher auditory centers. Since abnormal activity in the IC is likely to affect auditory perception, it is crucial to elucidate the neuronal mechanism to induce the activity changes of IC neurons with hearing loss. This review outlines recent findings on hearing-loss-induced plastic changes in the IC and brainstem auditory neuronal circuits and discusses what neuronal mechanisms underlie hearing-loss-induced changes in the activity of IC neurons. Considering the different causes of hearing loss, we discuss age-related hearing loss separately from other forms of hearing loss (non-age-related hearing loss). In general, the main plastic change of IC neurons caused by both age-related and non-age-related hearing loss is increased central gain. However, plastic changes in the IC caused by age-related hearing loss seem to be more complex than those caused by non-age-related hearing loss.

Identifying neuron types and circuit mechanisms in the auditory midbrain

The inferior colliculus (IC) is a critical computational hub in the central auditory pathway. From its position in the midbrain, the IC receives nearly all the ascending output from the lower auditory brainstem and provides the main source of auditory information to the thalamocortical system. In addition to being a crossroads for auditory circuits, the IC is rich with local circuits and contains more than five times as many neurons as the nuclei of the lower auditory brainstem combined. These results hint at the enormous computational power of the IC, and indeed, systems-level studies have identified numerous important transformations in sound coding that occur in the IC. However, despite decades of effort, the cellular mechanisms underlying IC computations and how these computations change following hearing loss have remained largely impenetrable. In this review, we argue that this challenge persists due to the surprisingly difficult problem of identifying the neuron types and circuit motifs that comprise the IC. After summarizing the extensive evidence pointing to a diversity of neuron types in the IC, we highlight the successes of recent efforts to parse this complexity using molecular markers to define neuron types. We conclude by arguing that the discovery of molecularly identifiable neuron types ushers in a new era for IC research marked by molecularly targeted recordings and manipulations. We propose that the ability to reproducibly investigate IC circuits at the neuronal level will lead to rapid advances in understanding the fundamental mechanisms driving IC computations and how these mechanisms shift following hearing loss.

Head-related transfer functions of rabbits within the front horizontal plane

The head-related transfer function (HRTF) describes the direction-dependent acoustic filtering by the head that occurs between a source signal in free-field space and the signal at the tympanic membrane. HRTFs contain information on sound source location via interaural differences of their magnitude or phase spectra and via the shapes of their magnitude spectra. The present study characterized HRTFs for source locations in the front horizontal plane for nine rabbits, which are a species commonly used in studies of the central auditory system. HRTF magnitude spectra shared several features across individuals, including a broad spectral peak at 2.6kHz that increased gain by 12 to 23dB depending on source azimuth; and a notch at 7.6kHz and peak at 9.8kHz visible for most azimuths. Overall, frequencies above 4kHz were amplified for sources ipsilateral to the ear and progressively attenuated for frontal and contralateral azimuths. The slope of the magnitude spectrum between 3 and 5kHz was found to be an unambiguous monaural cue for source azimuths ipsilateral to the ear. Average interaural level difference (ILD) between 5 and 16kHz varied monotonically with azimuth over ±31dB despite a relatively small head size. Interaural time differences (ITDs) at 0.5kHz and 1.5kHz also varied monotonically with azimuth over ±358 μs and ±260 μs, respectively. Remeasurement of HRTFs after pinna removal revealed that the large pinnae of rabbits were responsible for all spectral peaks and notches in magnitude spectra and were the main contribution to high-frequency ILDs (5-16kHz), whereas the rest of the head was the main contribution to ITDs and low-frequency ILDs (0.2-1.5kHz). Lastly, inter-individual differences in magnitude spectra were found to be small enough that deviations of individual HRTFs from an average HRTF were comparable in size to measurement error. Therefore, the average HRTF may be acceptable for use in neural or behavioral studies of rabbits implementing virtual acoustic space when measurement of individualized HRTFs is not possible.

Head-related transfer functions of rabbits within the front horizontal plane

The head-related transfer function (HRTF) is the direction-dependent acoustic filtering by the head that occurs between a source signal in free-field space and the signal at the tympanic membrane. HRTFs contain information on sound source location via interaural differences of their magnitude or phase spectra and via the shapes of their magnitude spectra. The present study characterized HRTFs for source locations in the front horizontal plane for nine rabbits, which are a species commonly used in studies of the central auditory system. HRTF magnitude spectra shared several features across individuals, including a broad spectral peak at 2.6 kHz that increased gain by 12 to 23 dB depending on source azimuth; and a notch at 7.6 kHz and peak at 9.8 kHz visible for most azimuths. Overall, frequencies above 4 kHz were amplified for sources ipsilateral to the ear and progressively attenuated for frontal and contralateral azimuths. The slope of the magnitude spectrum between 3 and 5 kHz was found to be an unambiguous monaural cue for source azimuths ipsilateral to the ear. Average interaural level difference (ILD) between 5 and 16 kHz varied monotonically with azimuth over ±31 dB despite a relatively small head size. Interaural time differences (ITDs) at 0.5 kHz and 1.5 kHz also varied monotonically with azimuth over ±358 μs and ±260 μs, respectively. Remeasurement of HRTFs after pinna removal revealed that the large pinnae of rabbits were responsible for all spectral peaks and notches in magnitude spectra and were the main contribution to high-frequency ILDs, whereas the rest of the head was the main contribution to ITDs and low-frequency ILDs. Lastly, inter-individual differences in magnitude spectra were found to be small enough that deviations of individual HRTFs from an average HRTF were comparable in size to measurement error. Therefore, the average HRTF may be acceptable for use in neural or behavioral studies of rabbits implementing virtual acoustic space when measurement of individualized HRTFs is not possible.

Gradients of response latencies and temporal precision of auditory neurons extend across the whole inferior colliculus

Neural responses to acoustic stimulation have long been studied throughout the auditory system to understand how sound information is coded for perception. Within the inferior colliculus (IC), a majority of the studies have focused predominantly on characterizing neural responses within the central region (ICC), as it is viewed as part of the lemniscal system mainly responsible for auditory perception. In contrast, the responses of outer cortices (ICO) have largely been unexplored, though they also function in auditory perception tasks. Therefore, we sought to expand on previous work by completing a three-dimensional (3-D) functional mapping study of the whole IC. We analyzed responses to different pure tone and broadband noise stimuli across all IC subregions and correlated those responses with over 2,000 recording locations across the IC. Our study revealed there are well-organized trends for temporal response parameters across the full IC that do not show a clear distinction at the ICC and ICO border. These gradients span from slow, imprecise responses in the caudal-medial IC to fast, precise responses in the rostral-lateral IC, regardless of subregion, including the fastest responses located in the ICO. These trends were consistent at various acoustic stimulation levels. Weaker spatial trends could be found for response duration and spontaneous activity. Apart from tonotopic organization, spatial trends were not apparent for spectral response properties. Overall, these detailed acoustic response maps across the whole IC provide new insights into the organization and function of the IC.NEW & NOTEWORTHY Study of the inferior colliculus (IC) has largely focused on the central nucleus, with little exploration of the outer cortices. Here, we systematically assessed the acoustic response properties from over 2,000 locations in different subregions of the IC. The results revealed spatial trends in temporal response patterns that span all subregions. Furthermore, two populations of temporal response types emerged for neurons in the outer cortices that may contribute to their functional roles in auditory tasks.

Neural coding of dichotic pitches in auditory midbrain

Dichotic pitches such as the Huggins pitch (HP) and the binaural edge pitch (BEP) are perceptual illusions whereby binaural noise that exhibits abrupt changes in interaural phase differences (IPDs) across frequency creates a tonelike pitch percept when presented to both ears, even though it does not produce a pitch when presented monaurally. At the perceptual and cortical levels, dichotic pitches behave as if an actual tone had been presented to the ears, yet investigations of neural correlates of dichotic pitch in single-unit responses at subcortical levels are lacking. We tested for cues to HP and BEP in the responses of binaural neurons in the auditory midbrain of anesthetized cats by varying the expected pitch frequency around each neuron's best frequency (BF). Neuronal firing rates showed specific features (peaks, troughs, or edges) when the pitch frequency crossed the BF, and the type of feature was consistent with a well-established model of binaural processing comprising frequency tuning, internal delays, and firing rates sensitive to interaural correlation. A Jeffress-like neural population model in which the behavior of individual neurons was governed by the cross-correlation model and the neurons were independently distributed along BF and best IPD predicted trends in human psychophysical HP detection but only when the model incorporated physiological BF and best IPD distributions. These results demonstrate the existence of a rate-place code for HP and BEP in the auditory midbrain and provide a firm physiological basis for models of dichotic pitches.NEW & NOTEWORTHY Dichotic pitches are perceptual illusions created centrally through binaural interactions that offer an opportunity to test theories of pitch and binaural hearing. Here we show that binaural neurons in auditory midbrain encode the frequency of two salient types of dichotic pitches via specific features in the pattern of firing rates along the tonotopic axis. This is the first combined single-unit and modeling study of responses of auditory neurons to stimuli evoking a dichotic pitch.

Hierarchical differences in the encoding of sound and choice in the subcortical auditory system

Detection of sounds is a fundamental function of the auditory system. Although studies of auditory cortex have gained substantial insight into detection performance using behaving animals, previous subcortical studies have mostly taken place under anesthesia, in passively listening animals, or have not measured performance at threshold. These limitations preclude direct comparisons between neuronal responses and behavior. To address this, we simultaneously measured auditory detection performance and single-unit activity in the inferior colliculus (IC) and cochlear nucleus (CN) in macaques. The spontaneous activity and response variability of CN neurons were higher than those observed for IC neurons. Signal detection theoretic methods revealed that the magnitude of responses of IC neurons provided more reliable estimates of psychometric threshold and slope compared with the responses of single CN neurons. However, pooling small populations of CN neurons provided reliable estimates of psychometric threshold and slope, suggesting sufficient information in CN population activity. Trial-by-trial correlations between spike count and behavioral response emerged 50-75 ms after sound onset for most IC neurons, but for few neurons in the CN. These results highlight hierarchical differences between neurometric-psychometric correlations in CN and IC and have important implications for how subcortical information could be decoded.NEW & NOTEWORTHY The cerebral cortex is widely recognized to play a role in sensory processing and decision-making. Accounts of the neural basis of auditory perception and its dysfunction are based on this idea. However, significantly less attention has been paid to midbrain and brainstem structures in this regard. Here, we find that subcortical auditory neurons represent stimulus information sufficient for detection and predict behavioral choice on a trial-by-trial basis.

High Spectral and Temporal Acuity in Primary Auditory Cortex of Awake Cats

Most accounts of single- and multi-unit responses in auditory cortex under anesthetized conditions have emphasized V-shaped frequency tuning curves and low-pass sensitivity to rates of repeated sounds. In contrast, single-unit recordings in awake marmosets also show I-shaped and O-shaped response areas having restricted tuning to frequency and (for O units) sound level. That preparation also demonstrates synchrony to moderate click rates and representation of higher click rates by spike rates of non-synchronized tonic responses, neither of which are commonly seen in anesthetized conditions. The spectral and temporal representation observed in the marmoset might reflect special adaptations of that species, might be due to single- rather than multi-unit recording, or might indicate characteristics of awake-versus-anesthetized recording conditions. We studied spectral and temporal representation in the primary auditory cortex of alert cats. We observed V-, I-, and O-shaped response areas like those demonstrated in awake marmosets. Neurons could synchronize to click trains at rates about an octave higher than is usually seen with anesthesia. Representations of click rates by rates of non-synchronized tonic responses exhibited dynamic ranges that covered the entire range of tested click rates. The observation of these spectral and temporal representations in cats demonstrates that they are not unique to primates and, indeed, might be widespread among mammalian species. Moreover, we observed no significant difference in stimulus representation between single- and multi-unit recordings. It appears that the principal factor that has hindered observations of high spectral and temporal acuity in the auditory cortex has been the use of general anesthesia.

Multiple sounds degrade the frequency representation in monkey inferior colliculus

How we distinguish multiple simultaneous stimuli is uncertain, particularly given that such stimuli sometimes recruit largely overlapping populations of neurons. One commonly proposed hypothesis is that the sharpness of tuning curves might change to limit the number of stimuli driving any given neuron when multiple stimuli are present. To test this hypothesis, we recorded the activity of neurons in the inferior colliculus while monkeys made saccades to either one or two simultaneous sounds differing in frequency and spatial location. Although monkeys easily distinguished simultaneous sounds (~90% correct performance), the frequency selectivity of inferior colliculus neurons on dual‐sound trials did not improve in any obvious way. Frequency selectivity was degraded on dual‐sound trials compared to single‐sound trials: neural response functions broadened and frequency accounted for less of the variance in firing rate. These changes in neural firing led a maximum‐likelihood decoder to perform worse on dual‐sound trials than on single‐sound trials. These results fail to support the hypothesis that changes in frequency response functions serve to reduce the overlap in the representation of simultaneous sounds. Instead, these results suggest that alternative possibilities, such as recent evidence of alternations in firing rate between the rates corresponding to each of the two stimuli, offer a more promising approach.

Sensitivity of neural responses in the inferior colliculus to statistical features of sound textures

Previous psychophysical studies have identified a hierarchy of time-averaged statistics which determine the identity of natural sound textures. However, it is unclear whether the neurons in the inferior colliculus (IC) are sensitive to each of these statistical features in the natural sound textures. We used 13 representative sound textures spanning the space of 3 statistics extracted from over 200 natural textures. The synthetic textures were generated by incorporating the statistical features in a step-by-step manner, in which a particular statistical feature was changed while the other statistical features remain unchanged. The extracellular activity in response to the synthetic texture stimuli was recorded in the IC of anesthetized rats. Analysis of the transient and sustained multiunit activity after each transition of statistical feature showed that the IC units were sensitive to the changes of all types of statistics, although to a varying extent. For example, we found that more neurons were sensitive to the changes in variance than that in the modulation correlations. Our results suggest that the sensitivity of the statistical features in the subcortical levels contributes to the identification and discrimination of natural sound textures.

Three psychophysical metrics of auditory temporal integration in macaques

The relationship between sound duration and detection threshold has long been thought to reflect temporal integration. Reports of species differences in this relationship are equivocal: some meta-analyses report no species differences, whereas others report substantial differences, particularly between humans and their close phylogenetic relatives, macaques. This renders translational work in macaques problematic. To reevaluate this difference, tone detection performance was measured in macaques using a go/no-go reaction time (RT) task at various tone durations and in the presence of broadband noise (BBN). Detection thresholds, RTs, and the dynamic range (DR) of the psychometric function decreased as the tone duration increased. The threshold by duration trends suggest macaques integrate at a similar rate to humans. The RT trends also resemble human data and are the first reported in animals. Whereas the BBN did not affect how the threshold or RT changed with the duration, it substantially reduced the DR at short durations. A probabilistic Poisson model replicated the effects of duration on threshold and DR and required integration from multiple simulated auditory nerve fibers to explain the performance at shorter durations. These data suggest that, contrary to previous studies, macaques are uniquely well-suited to model human temporal integration and form the baseline for future neurophysiological studies.

Excitatory cholecystokinin neurons of the midbrain integrate diverse temporal responses and drive auditory thalamic subdomains

Our ability to identify sounds and understand communication signals depends upon our brains’ capacity to combine information about diverse sound features, including temporal patterns. The central nucleus of the inferior colliculus (ICC) performs an initial stage of this integration, but a circuit-based understanding of these processes has been hampered by difficulties in separating clearly defined functional cell types. Here we identify and characterize a major excitatory projection neuron of the ICC. These neurons show uniform intrinsic firing patterns and tuning to frequency, but strikingly diverse temporal responses to sound. Our results suggest that diversity in temporal coding is represented even within a single cell class and is likely primarily driven by differences in circuit connectivity.

The central nucleus of the inferior colliculus (ICC) integrates information about different features of sound and then distributes this information to thalamocortical circuits. However, the lack of clear definitions of circuit elements in the ICC has limited our understanding of the nature of these circuit transformations. Here, we combine virus-based genetic access with electrophysiological and optogenetic approaches to identify a large family of excitatory, cholecystokinin-expressing thalamic projection neurons in the ICC of the Mongolian gerbil. We show that these neurons form a distinct cell type, displaying uniform morphology and intrinsic firing features, and provide powerful, spatially restricted excitation exclusively to the ventral auditory thalamus. In vivo, these neurons consistently exhibit V-shaped receptive field properties but strikingly diverse temporal responses to sound. Our results indicate that temporal response diversity is maintained within this population of otherwise uniform cells in the ICC and then relayed to cortex through spatially restricted thalamic subdomains.

Hearing loss and brain plasticity: the hyperactivity phenomenon

Many aging adults experience some form of hearing problems that may arise from auditory peripheral damage. However, it has been increasingly acknowledged that hearing loss is not only a dysfunction of the auditory periphery but also results from changes within the entire auditory system, from periphery to cortex. Damage to the auditory periphery is associated with an increase in neural activity at various stages throughout the auditory pathway. Here, we review neurophysiological evidence of hyperactivity, auditory perceptual difficulties that may result from hyperactivity, and outline open conceptual and methodological questions related to the study of hyperactivity. We suggest that hyperactivity alters all aspects of hearing-including spectral, temporal, spatial hearing-and, in turn, impairs speech comprehension when background sound is present. By focusing on the perceptual consequences of hyperactivity and the potential challenges of investigating hyperactivity in humans, we hope to bring animal and human electrophysiologists closer together to better understand hearing problems in older adulthood.

Foreground stimuli and task engagement enhance neuronal adaptation to background noise in the inferior colliculus of macaques

Auditory neuronal responses are modified by background noise. Inferior colliculus (IC) neuronal responses adapt to the most frequent sound level within an acoustic scene (adaptation to stimulus statistics), a mechanism that may preserve neuronal and behavioral thresholds for signal detection. However, it is still unclear whether the presence of foreground stimuli and/or task involvement can modify neuronal adaptation. To investigate how task engagement interacts with this mechanism, we compared the response of IC neurons to background noise, which caused adaptation to stimulus statistics, while macaque monkeys performed a masked tone detection task (task-driven condition) with responses recorded when the same background noise was presented alone (passive listening condition). In the task-dependent condition, monkeys performed a Go/No-Go task while 50-ms tones were embedded within an adaptation-inducing continuous background noise whose levels changed every 50 ms and were drawn from a probability distribution. The adaptation to noise stimulus statistics in IC neuronal responses was significantly enhanced in the task-driven condition compared with the passive listening condition, showing that foreground stimuli and/or task-engagement can modify IC neuronal responses. Additionally, the response of IC neurons to noise was significantly affected by the preceding sensory information (history effect) regardless of task involvement. These studies show that dynamic range adaptation in IC preserves behavioral and neurometric thresholds irrespective of noise type and a dependence of neuronal activity on task-related factors at subcortical levels of processing.NEW & NOTEWORTHY Auditory neuronal responses are influenced by maskers and distractors. However, it is still unclear whether the neuronal sensitivity to the masker stimulus is influenced by task-dependent factors. Our study represents one of the first attempts to investigate how task involvement influences the neural representation of background sounds in the subcortical, midbrain auditory neurons of behaving animals.

Amplitude modulation transfer functions reveal opposing populations within both the inferior colliculus and medial geniculate body

Based on single-unit recordings of modulation transfer functions (MTFs) in the inferior colliculus (IC) and the medial geniculate body (MGB) of the unanesthetized rabbit, we identified two opposing populations: band-enhanced (BE) and band-suppressed (BS) neurons. In response to amplitude-modulated (AM) sounds, firing rates of BE and BS neurons were enhanced and suppressed, respectively, relative to their responses to an unmodulated noise with a one-octave bandwidth. We also identified a third population, designated hybrid neurons, whose firing rates were enhanced by some modulation frequencies and suppressed by others. Our finding suggests that perception of AM may be based on the co-occurrence of enhancement and suppression of responses of the opposing populations of neurons. Because AM carries an important part of the content of speech, progress in understanding auditory processing of AM sounds should lead to progress in understanding speech perception. Each of the BE, BS, and hybrid types of MTFs comprised approximately one-third of the total sample. Modulation envelopes having short duty cycles of 20-50% and raised-sine envelopes accentuated the degree of enhancement and suppression and sharpened tuning of the MTFs. With sinusoidal envelopes, peak modulation frequencies were centered around 32-64 Hz among IC BE neurons, whereas the MGB peak frequencies skewed toward lower frequencies, with a median of 16 Hz. We also tested an auditory-brainstem model and found that a simple circuit containing fast excitatory synapses and slow inhibitory synapses was able to reproduce salient features of the BE- and BS-type MTFs of IC neurons.NEW & NOTEWORTHY Opposing populations of neurons have been identified in the mammalian auditory midbrain and thalamus. In response to amplitude-modulated sounds, responses of one population (band-enhanced) increased whereas responses of another (band-suppressed) decreased relative to their responses to an unmodulated sound. These opposing auditory populations are analogous to the ON and OFF populations of the visual system and may improve transfer of information carried by the temporal envelopes of complex sounds such as speech.

Robust Rate-Place Coding of Resolved Components in Harmonic and Inharmonic Complex Tones in Auditory Midbrain

Harmonic complex tones (HCTs) commonly occurring in speech and music evoke a strong pitch at their fundamental frequency (F0), especially when they contain harmonics individually resolved by the cochlea. When all frequency components of an HCT are shifted by the same amount, the pitch of the resulting inharmonic tone (IHCT) can also shift, although the envelope repetition rate is unchanged. A rate-place code, whereby resolved harmonics are represented by local maxima in firing rates along the tonotopic axis, has been characterized in the auditory nerve and primary auditory cortex, but little is known about intermediate processing stages. We recorded single-neuron responses to HCT and IHCT with varying F0 and sound level in the inferior colliculus (IC) of unanesthetized rabbits of both sexes. Many neurons showed peaks in firing rate when a low-numbered harmonic aligned with the neuron's characteristic frequency, demonstrating "rate-place" coding. The IC rate-place code was most prevalent for F0 > 800 Hz, was only moderately dependent on sound level over a 40 dB range, and was not sensitive to stimulus harmonicity. A spectral receptive-field model incorporating broadband inhibition better predicted the neural responses than a purely excitatory model, suggesting an enhancement of the rate-place representation by inhibition. Some IC neurons showed facilitation in response to HCT relative to pure tones, similar to cortical "harmonic template neurons" (Feng and Wang, 2017), but to a lesser degree. Our findings shed light on the transformation of rate-place coding of resolved harmonics along the auditory pathway.SIGNIFICANCE STATEMENT Harmonic complex tones are ubiquitous in speech and music and produce strong pitch percepts when they contain frequency components that are individually resolved by the cochlea. Here, we characterize a "rate-place" code for resolved harmonics in the auditory midbrain that is more robust across sound levels than the peripheral rate-place code and insensitive to the harmonic relationships among frequency components. We use a computational model to show that inhibition may play an important role in shaping the rate-place code. Our study fills a major gap in understanding the transformations in neural representations of resolved harmonics along the auditory pathway.

Population rate-coding predicts correctly that human sound localization depends on sound intensity

Human sound localization is an important computation performed by the brain. Models of sound localization commonly assume that sound lateralization from interaural time differences is level invariant. Here we observe that two prevalent theories of sound localization make opposing predictions. The labelled-line model encodes location through tuned representations of spatial location and predicts that perceived direction is level invariant. In contrast, the hemispheric-difference model encodes location through spike-rate and predicts that perceived direction becomes medially biased at low sound levels. Here, behavioral experiments find that softer sounds are perceived closer to midline than louder sounds, favoring rate-coding models of human sound localization. Analogously, visual depth perception, which is based on interocular disparity, depends on the contrast of the target. The similar results in hearing and vision suggest that the brain may use a canonical computation of location: encoding perceived location through population spike rate relative to baseline.

Being able to localize sounds helps us make sense of the world around us. The brain works out sound direction by comparing the times of when sound reaches the left versus the right ear. This cue is known as interaural time difference, or ITD for short. But how exactly the brain decodes this information is still unknown.

The brain contains nerve cells that each show maximum activity in response to one particular ITD. One idea is that these nerve cells are arranged in the brain like a map from left to right, and that the brain then uses this map to estimate sound direction. This is known as the Jeffress model, after the scientist who first proposed it. There is some evidence that birds and alligators actually use a system like this to localize sounds, but no such map of nerve cells has yet been identified in mammals. An alternative possibility is that the brain compares activity across groups of ITD-sensitive nerve cells. One of the oldest and simplest ways to measure this is to compare nerve activity in the left and right hemispheres of the brain. This readout is known as the hemispheric difference model.

By analyzing data from published studies, Ihlefeld, Alamatsaz, and Shapley discovered that these two models make opposing predictions about the effects of volume. The Jeffress model predicts that the volume of a sound will not affect a person’s ability to localize it. By contrast, the hemispheric difference model predicts that very soft sounds will lead to systematic errors, so that for the same ITD, softer sounds are perceived closer towards the front than louder sounds. To investigate this further, Ihlefeld, Alamatsaz, and Shapley asked healthy volunteers to localize sounds of different volumes. The volunteers tended to mis-localize quieter sounds, believing them to be closer to the body’s midline than they actually were, which is inconsistent with the predictions of the Jeffress model.

These new findings also reveal key parallels to processing in the visual system. Visual areas of the brain estimate how far away an object is by comparing the input that reaches the two eyes. But these estimates are also systematically less accurate for low-contrast stimuli than for high-contrast ones, just as sound localization is less accurate for softer sounds than for louder ones. The idea that the brain uses the same basic strategy to localize both sights and sounds generates a number of predictions for future studies to test.

Differential Inhibitory Configurations Segregate Frequency Selectivity in the Mouse Inferior Colliculus

Receptive fields and tuning curves of sensory neurons represent the neural substrates that allow animals to efficiently detect and distinguish external stimuli. They are progressively refined to create diverse sensitivity and selectivity for neurons along ascending central pathways. However, the neural circuitry mechanisms have not been directly determined for such fundamental qualities in relation to sensory neurons' functional organizations, because of the technical difficulty of correlating neurons' input and output. Here, we obtained spike outputs and synaptic inputs from the same neurons within characteristically defined neural ensembles, to determine the synaptic mechanisms driving their diverse frequency selectivity in the mouse inferior colliculus. We find that the synaptic strength and timing of excitatory and inhibitory inputs are configured differently and independently within individual neurons' receptive fields, which segregate sensitive and selective neurons and endow neural populations with broad receptive fields and sharp frequency tuning. By computationally modeling spike outputs from integrating synaptic inputs and comparing them with real spike responses of the same neurons, we show that space-clamping errors did not qualitatively affect the estimation of spike responses derived from synaptic currents in in vivo voltage-clamp recordings. These data suggest that heterogeneous inhibitory circuits coexist locally for a parallel but differentiated representation of incoming signals.SIGNIFICANCE STATEMENT Sensitivity and selectivity are functional qualities of sensory systems to facilitate animals' survival. There is little direct evidence for the synaptic basis of neurons' functional variance within neural ensembles. Here we adopted a novel framework to fill such a long-standing gap by uniting population activities with single cells' spike outputs and their synaptic inputs. Furthermore, the effects of space-clamping errors on subcortical synaptic currents were evaluated in vivo, by comparing recorded spike responses and simulated spike outputs from computationally integrating synaptic inputs. Our study illustrated that the synaptic strength and timing of inhibition relative to excitation can be configured differently for neurons within a defined neural ensemble, to segregate their selectivity. It provides new insights into coexisting heterogeneous local circuits.

A temporal integration mechanism enhances frequency selectivity of broadband inputs to inferior colliculus

Accurately resolving frequency components in sounds is essential for sound recognition, yet there is little direct evidence for how frequency selectivity is preserved or newly created across auditory structures. We demonstrate that prepotentials (PPs) with physiological properties resembling presynaptic potentials from broadly tuned brainstem inputs can be recorded concurrently with postsynaptic action potentials in inferior colliculus (IC). These putative brainstem inputs (PBIs) are broadly tuned and exhibit delayed and spectrally interleaved excitation and inhibition not present in the simultaneously recorded IC neurons (ICNs). A sharpening of tuning is accomplished locally at the expense of spike-timing precision through nonlinear temporal integration of broadband inputs. A neuron model replicates the finding and demonstrates that temporal integration alone can degrade timing precision while enhancing frequency tuning through interference of spectrally in- and out-of-phase inputs. These findings suggest that, in contrast to current models that require local inhibition, frequency selectivity can be sharpened through temporal integration, thus supporting an alternative computational strategy to quickly refine frequency selectivity.

Robust and Intensity-Dependent Synaptic Inhibition Underlies the Generation of Non-monotonic Neurons in the Mouse Inferior Colliculus

Intensity and frequency are the two main properties of sound. The non-monotonic neurons in the auditory system are thought to represent sound intensity. The central nucleus of the inferior colliculus (ICC), as an important information integration nucleus of the auditory system, is also involved in the processing of intensity encoding. Although previous researchers have hinted at the importance of inhibitory effects on the formation of non-monotonic neurons, the specific underlying synaptic mechanisms in the ICC are still unclear. Therefore, we applied the in vivo whole-cell voltage-clamp technique to record the excitatory and inhibitory postsynaptic currents (EPSCs and IPSCs) in the ICC neurons, and compared the effects of excitation and inhibition on the membrane potential outputs. We found that non-monotonic neuron responses could not only be inherited from the lower nucleus but also be created in the ICC. By integrating with a relatively weak IPSC, approximately 35% of the monotonic excitatory inputs remained in the ICC. In the remaining cases, monotonic excitatory inputs were reshaped into non-monotonic outputs by the dominating inhibition at high intensity, which also enhanced the non-monotonic nature of the non-monotonic excitatory inputs.

A novel class of inferior colliculus principal neurons labeled in vasoactive intestinal peptide-Cre mice

Located in the midbrain, the inferior colliculus (IC) is the hub of the central auditory system. Although the IC plays important roles in speech processing, sound localization, and other auditory computations, the organization of the IC microcircuitry remains largely unknown. Using a multifaceted approach in mice, we have identified vasoactive intestinal peptide (VIP) neurons as a novel class of IC principal neurons. VIP neurons are glutamatergic stellate cells with sustained firing patterns. Their extensive axons project to long-range targets including the auditory thalamus, auditory brainstem, superior colliculus, and periaqueductal gray. Using optogenetic circuit mapping, we found that VIP neurons integrate input from the contralateral IC and the dorsal cochlear nucleus. The dorsal cochlear nucleus also drove feedforward inhibition to VIP neurons, indicating that inhibitory circuits within the IC shape the temporal integration of ascending inputs. Thus, VIP neurons are well-positioned to influence auditory computations in a number of brain regions.

Cochlear Synaptopathy Changes Sound-Evoked Activity Without Changing Spontaneous Discharge in the Mouse Inferior Colliculus

Tinnitus and hyperacusis are life-disrupting perceptual abnormalities that are often preceded by acoustic overexposure. Animal models of overexposure have suggested a link between these phenomena and neural hyperactivity, i.e., elevated spontaneous rates (SRs) and sound-evoked responses. Prior work has focused on changes in central auditory responses, with less attention paid to the exact nature of the associated cochlear damage. The demonstration that acoustic overexposure can cause cochlear neuropathy without permanent threshold elevation suggests cochlear neuropathy per se may be a key elicitor of neural hyperactivity. We addressed this hypothesis by recording responses in the mouse inferior colliculus (IC) following a bilateral, neuropathic noise exposure. One to three weeks post-exposure, mean SRs were unchanged in mice recorded while awake, or under anesthesia. SRs were also unaffected by more intense, or unilateral exposures. These results suggest that neither neuropathy nor hair cell loss are sufficient to raise SRs in the IC, at least in 7-week-old mice, 1-3 weeks post exposure. However, it is not clear whether our mice had tinnitus. Tone-evoked rate-level functions at the CF were steeper following exposure, specifically in the region of maximal neuropathy. Furthermore, suppression driven by off-CF tones and by ipsilateral noise were reduced. Both changes were especially pronounced in neurons of awake mice. This neural hypersensitivity may manifest as behavioral hypersensitivity to sound - prior work reports that this same exposure causes elevated acoustic startle. Together, these results indicate that neuropathy may initiate a compensatory response in the central auditory system leading to the genesis of hyperacusis.

Neuronal adaptation to sound statistics in the inferior colliculus of behaving macaques does not reduce the effectiveness of the masking noise

The detectability of target sounds embedded within noisy backgrounds is affected by the regularities that summarize acoustic sceneries. Previous studies suggested that the dynamic range of neurons in the inferior colliculus (IC) of anesthetized guinea pigs shifts toward the mean sound pressure level in irregular acoustic environments. Yet, it is unclear how this neuronal adaptation processes may influence the effectiveness of sounds as a masker, both behaviorally and in terms of neuronal encoding. To answer this question, we measured the neural response of IC neurons while macaque monkeys performed a Go/No-Go tone detection task. Macaques detected a 50-ms tone that was either simultaneously gated with a burst of noise or embedded within a continuous noise background, whose levels were randomly sampled (every 50 ms) from a probability distribution. The mean of the distribution matched the level of the gated burst of noise. Psychometric and IC neurometric thresholds to tones did not differ between the two masking conditions. However, the neuronal firing rate versus level function was significantly affected by the temporal characteristics of the noise masker. Simultaneously gated noise caused higher baseline responses and greater dynamic range compression compared with noise distribution. The slopes of psychometric and neurometric functions were significantly shallower for higher variance distributions, suggesting that neuronal sensitivity might change with the variability of the sound. Our results suggest that the adaptive response of IC neurons to sound regularities does not affect the effectiveness of the noise-masking signal, which remains invariant to surrounding noise amplitudes. NEW & NOTEWORTHY Auditory neurons adapt to the statistics of sound levels in the acoustic scene. However, it is still unclear to what extent such adaptation influences the effectiveness of the stimulus as a masker. Our study represents the first attempt to investigate how the adaptation to the statistics of masking stimuli may be related to the effectiveness of masking, and to the single-unit encoding of the midbrain auditory neurons in behaving animals.

Decoding sound level in the marmoset primary auditory cortex

Neurons that respond favorably to a particular sound level have been observed throughout the central auditory system, becoming steadily more common at higher processing areas. One theory about the role of these level-tuned or nonmonotonic neurons is the level-invariant encoding of sounds. To investigate this theory, we simulated various subpopulations of neurons by drawing from real primary auditory cortex (A1) neuron responses and surveyed their performance in forming different sound level representations. Pure nonmonotonic subpopulations did not provide the best level-invariant decoding; instead, mixtures of monotonic and nonmonotonic neurons provided the most accurate decoding. For level-fidelity decoding, the inclusion of nonmonotonic neurons slightly improved or did not change decoding accuracy until they constituted a high proportion. These results indicate that nonmonotonic neurons fill an encoding role complementary to, rather than alternate to, monotonic neurons.NEW & NOTEWORTHY Neurons with nonmonotonic rate-level functions are unique to the central auditory system. These level-tuned neurons have been proposed to account for invariant sound perception across sound levels. Through systematic simulations based on real neuron responses, this study shows that neuron populations perform sound encoding optimally when containing both monotonic and nonmonotonic neurons. The results indicate that instead of working independently, nonmonotonic neurons complement the function of monotonic neurons in different sound-encoding contexts.

Identified GABAergic and Glutamatergic Neurons in the Mouse Inferior Colliculus Share Similar Response Properties

GABAergic neurons in the inferior colliculus (IC) play a critical role in auditory information processing, yet their responses to sound are unknown. Here, we used optogenetic methods to characterize the response properties of GABAergic and presumed glutamatergic neurons to sound in the IC. We found that responses to pure tones of both inhibitory and excitatory classes of neurons were similar in their thresholds, response latencies, rate-level functions, and frequency tuning, but GABAergic neurons may have higher spontaneous firing rates. In contrast to their responses to pure tones, the inhibitory and excitatory neurons differed in their ability to follow amplitude modulations. The responses of both cell classes were affected by their location regardless of the cell type, especially in terms of their frequency tuning. These results show that the synaptic domain, a unique organization of local neural circuits in the IC, may interact with all types of neurons to produce their ultimate response to sound.SIGNIFICANCE STATEMENT Although the inferior colliculus (IC) in the auditory midbrain is composed of different types of neurons, little is known about how these specific types of neurons respond to sound. Here, for the first time, we characterized the response properties of GABAergic and glutamatergic neurons in the IC. Both classes of neurons had diverse response properties to tones but were overall similar, except for the spontaneous activity and their ability to follow amplitude-modulated sound. Both classes of neurons may compose a basic local circuit that is replicated throughout the IC. Within each local circuit, the inputs to the local circuit may have a greater influence in determining the response properties to sound than the specific neuron types.

Representation of Multidimensional Stimuli: Quantifying the Most Informative Stimulus Dimension from Neural Responses

A common way to assess the function of sensory neurons is to measure the number of spikes produced by individual neurons while systematically varying a given dimension of the stimulus. Such measured tuning curves can then be used to quantify the accuracy of the neural representation of the stimulus dimension under study, which can in turn be related to behavioral performance. However, tuning curves often change shape when other dimensions of the stimulus are varied, reflecting the simultaneous sensitivity of neurons to multiple stimulus features. Here we illustrate how one-dimensional information analyses are misleading in this context, and propose a framework derived from Fisher information that allows the quantification of information carried by neurons in multidimensional stimulus spaces. We use this method to probe the representation of sound localization in auditory neurons of chinchillas and guinea pigs of both sexes, and show how heterogeneous tuning properties contribute to a representation of sound source position that is robust to changes in sound level.SIGNIFICANCE STATEMENT Sensory neurons' responses are typically modulated simultaneously by numerous stimulus properties, which can result in an overestimation of neural acuity with existing one-dimensional neural information transmission measures. To overcome this limitation, we develop new, compact expressions of Fisher information-derived measures that bound the robust encoding of separate stimulus dimensions in the context of multidimensional stimuli. We apply this method to the problem of the representation of sound source location in the face of changes in sound source level by neurons of the auditory midbrain.

Background firing in the auditory midbrain of the frog

Statistical characteristics of background firing in the midbrain auditory units of grass frog (Rana t. temporaria) located in torus semicircular (TS) were investigated. Only about 5% of the cells demonstrated prominent spontaneous firing. For such units the following characteristics were obtained: the distribution of interpulse intervals, the autocorrelation functions (ACF) for the real firing process and for the process with shuffled intervals, the hazard function (HF) and the joint distribution of adjacent interpulse intervals. The burstiness of firing was also estimated. In the absolute majority of the cells, the background firing demonstrated considerable deviation from the renewal process. There was weak but significant positive correlation between adjacent interpulse intervals. The burstiness of firing in the midbrain auditory units was moderate but higher than reported for medullary auditory neurons. The value of burstiness did not decrease after interval shuffling. Along with the reduction in excitability (generalized refractoriness) in many neurons observed post-spike facilitation effects were observed. Comparing background activity in medullary and midbrain nucleus suggests that there is an increase in complexity of the information processing along the auditory pathway.

Midbrain Synchrony to Envelope Structure Supports Behavioral Sensitivity to Single-Formant Vowel-Like Sounds in Noise

Vowels make a strong contribution to speech perception under natural conditions. Vowels are encoded in the auditory nerve primarily through neural synchrony to temporal fine structure and to envelope fluctuations rather than through average discharge rate. Neural synchrony is thought to contribute less to vowel coding in central auditory nuclei, consistent with more limited synchronization to fine structure and the emergence of average-rate coding of envelope fluctuations. However, this hypothesis is largely unexplored, especially in background noise. The present study examined coding mechanisms at the level of the midbrain that support behavioral sensitivity to simple vowel-like sounds using neurophysiological recordings and matched behavioral experiments in the budgerigar. Stimuli were harmonic tone complexes with energy concentrated at one spectral peak, or formant frequency, presented in quiet and in noise. Behavioral thresholds for formant-frequency discrimination decreased with increasing amplitude of stimulus envelope fluctuations, increased in noise, and were similar between budgerigars and humans. Multiunit recordings in awake birds showed that the midbrain encodes vowel-like sounds both through response synchrony to envelope structure and through average rate. Whereas neural discrimination thresholds based on either coding scheme were sufficient to support behavioral thresholds in quiet, only synchrony-based neural thresholds could account for behavioral thresholds in background noise. These results reveal an incomplete transformation to average-rate coding of vowel-like sounds in the midbrain. Model simulations suggest that this transformation emerges due to modulation tuning, which is shared between birds and mammals. Furthermore, the results underscore the behavioral relevance of envelope synchrony in the midbrain for detection of small differences in vowel formant frequency under real-world listening conditions.

L-type calcium channels refine the neural population code of sound level

The coding of sound level by ensembles of neurons improves the accuracy with which listeners identify how loud a sound is. In the auditory system, the rate at which neurons fire in response to changes in sound level is shaped by local networks. Voltage-gated conductances alter local output by regulating neuronal firing, but their role in modulating responses to sound level is unclear. We tested the effects of L-type calcium channels (Ca_L: Ca_V1.1-1.4) on sound-level coding in the central nucleus of the inferior colliculus (ICC) in the auditory midbrain. We characterized the contribution of Ca_L to the total calcium current in brain slices and then examined its effects on rate-level functions (RLFs) in vivo using single-unit recordings in awake mice. Ca_L is a high-threshold current and comprises ∼50% of the total calcium current in ICC neurons. In vivo, Ca_L activates at sound levels that evoke high firing rates. In RLFs that increase monotonically with sound level, Ca_L boosts spike rates at high sound levels and increases the maximum firing rate achieved. In different populations of RLFs that change nonmonotonically with sound level, Ca_L either suppresses or enhances firing at sound levels that evoke maximum firing. Ca_L multiplies the gain of monotonic RLFs with dynamic range and divides the gain of nonmonotonic RLFs with the width of the RLF. These results suggest that a single broad class of calcium channels activates enhancing and suppressing local circuits to regulate the sensitivity of neuronal populations to sound level.

ON and OFF inhibition as mechanisms for forward masking in the inferior colliculus: a modeling study

Masking effects of a preceding stimulus on the detection or perception of a signal have been found in several sensory systems in mammals, including humans and rodents. In the auditory system, it has been hypothesized that a central "OFF-inhibitory" mechanism, which is generated by neurons that respond after a sound is terminated, may contribute to the observed psychophysics. The present study constructed a systems model for the inferior colliculus that includes major ascending monaural and binaural auditory pathways. The fundamental characteristics of several neuron types along the pathways were captured by Hodgkin-Huxley models with specific membrane and synaptic properties. OFF responses were reproduced with a model of the superior paraolivary nucleus containing a hyperpolarization-activated h current and a T-type calcium current. When the gap between the end of the masker and the onset of the signal was large, e.g., >5 ms, OFF inhibition generated strong suppressive effects on the signal response. For smaller gaps, an additional inhibitory source, which was modeled as ON inhibition from the contralateral dorsal nucleus of the lateral lemniscus, showed the potential of explaining the psychophysics. Meanwhile, the effect of a forward masker on the binaural sensitivity to a low-frequency signal was examined, which was consistent with previous psychophysical findings related to sound localization.

Neural correlates of behavioral amplitude modulation sensitivity in the budgerigar midbrain

Amplitude modulation (AM) is a crucial feature of many communication signals, including speech. Whereas average discharge rates in the auditory midbrain correlate with behavioral AM sensitivity in rabbits, the neural bases of AM sensitivity in species with human-like behavioral acuity are unexplored. Here, we used parallel behavioral and neurophysiological experiments to explore the neural (midbrain) bases of AM perception in an avian speech mimic, the budgerigar (Melopsittacus undulatus). Behavioral AM sensitivity was quantified using operant conditioning procedures. Neural AM sensitivity was studied using chronically implanted microelectrodes in awake, unrestrained birds. Average discharge rates of multiunit recording sites in the budgerigar midbrain were insufficient to explain behavioral sensitivity to modulation frequencies <100 Hz for both tone- and noise-carrier stimuli, even with optimal pooling of information across recording sites. Neural envelope synchrony, in contrast, could explain behavioral performance for both carrier types across the full range of modulation frequencies studied (16-512 Hz). The results suggest that envelope synchrony in the budgerigar midbrain may underlie behavioral sensitivity to AM. Behavioral AM sensitivity based on synchrony in the budgerigar, which contrasts with rate-correlated behavioral performance in rabbits, raises the possibility that envelope synchrony, rather than average discharge rate, might also underlie AM perception in other species with sensitive AM detection abilities, including humans. These results highlight the importance of synchrony coding of envelope structure in the inferior colliculus. Furthermore, they underscore potential benefits of devices (e.g., midbrain implants) that evoke robust neural synchrony.

Responses to Social Vocalizations in the Dorsal Cochlear Nucleus of Mice

Identifying sounds is critical for an animal to make appropriate behavioral responses to environmental stimuli, including vocalizations from conspecifics. Identification of vocalizations may be supported by neuronal selectivity in the auditory pathway. The first place in the ascending auditory pathway where neuronal selectivity to vocalizations has been found is in the inferior colliculus (IC), but very few brainstem nuclei have been evaluated. Here, we tested whether selectivity to vocalizations is present in the dorsal cochlear nucleus (DCN). We recorded extracellular neural responses in the DCN of mice and found that fusiform cells responded in a heterogeneous and selective manner to mouse ultrasonic vocalizations. Most fusiform cells responded to vocalizations that contained spectral energy at much higher frequencies than the characteristic frequencies of the cells. To understand this mismatch of stimulus properties and frequency tuning of the cells, we developed a dynamic, nonlinear model of the cochlea that simulates cochlear distortion products on the basilar membrane. We preprocessed the vocalization stimuli through this model and compared responses to these distorted vocalizations with responses to the original vocalizations. We found that fusiform cells in the DCN respond in a heterogeneous manner to vocalizations, and that these neurons can use distortion products as a mechanism for encoding ultrasonic vocalizations. In addition, the selective neuronal responses were dependent on the presence of inhibitory sidebands that modulated the response depending on the temporal structure of the distortion product. These findings suggest that important processing of complex sounds occurs at a very early stage of central auditory processing and is not strictly a function of the cortex.

Dynamic representation of spectral edges in guinea pig primary auditory cortex

The central representation of a given acoustic motif is thought to be strongly context dependent, i.e., to rely on the spectrotemporal past and present of the acoustic mixture in which it is embedded. The present study investigated the cortical representation of spectral edges (i.e., where stimulus energy changes abruptly over frequency) and its dependence on stimulus duration and depth of the spectral contrast in guinea pig. We devised a stimulus ensemble composed of random tone pips with or without an attenuated frequency band (AFB) of variable depth. Additionally, the multitone ensemble with AFB was interleaved with periods of silence or with multitone ensembles without AFB. We have shown that the representation of the frequencies near but outside the AFB is greatly enhanced, whereas the representation of frequencies near and inside the AFB is strongly suppressed. These cortical changes depend on the depth of the AFB: although they are maximal for the largest depth of the AFB, they are also statistically significant for depths as small as 10 dB. Finally, the cortical changes are quick, occurring within a few seconds of stimulus ensemble presentation with AFB, and are very labile, disappearing within a few seconds after the presentation without AFB. Overall, this study demonstrates that the representation of spectral edges is dynamically enhanced in the auditory centers. These central changes may have important functional implications, particularly in noisy environments where they could contribute to preserving the central representation of spectral edges.

Effects of unilateral acoustic trauma on tinnitus-related spontaneous activity in the inferior colliculus

This study describes the long-term effects of sound-induced cochlear trauma on spontaneous discharge rates in the central nucleus of the inferior colliculus (ICC). As in previous studies, single-unit recordings in Sprague-Dawley rats revealed pervasive increases in spontaneous discharge rates. Based on differences in their sources of input, it was hypothesized that physiologically defined neural populations of the auditory midbrain would reveal the brainstem sources that dictate ICC hyperactivity. Abnormal spontaneous activity was restricted to target neurons of the ventral cochlear nucleus. Nearly identical patterns of hyperactivity were observed in the contralateral and ipsilateral ICC. The elevation in spontaneous activity extended to frequencies well below and above the region of maximum threshold shift. This lack of frequency organization suggests that ICC hyperactivity may be influenced by regions of the brainstem that are not tonotopically organized. Sound-induced hyperactivity is often observed in animals with behavioral signs of tinnitus. Prior to electrophysiological recording, rats were screened for tinnitus by measuring gap pre-pulse inhibition of the acoustic startle reflex (GPIASR). Rats with positive phenotypes did not exhibit unique patterns of ICC hyperactivity. This ambiguity raises concerns regarding animal behavioral models of tinnitus. If our screening procedures were valid, ICC hyperactivity is observed in animals without behavioral indications of the disorder. Alternatively, if the perception of tinnitus is strictly linked to ongoing ICC hyperactivity, our current behavioral approach failed to provide a reliable assessment of tinnitus state.

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.

The organization and physiology of the auditory thalamus and its role in processing acoustic features important for speech perception

The auditory thalamus, or medial geniculate body (MGB), is the primary sensory input to auditory cortex. Therefore, it plays a critical role in the complex auditory processing necessary for robust speech perception. This review will describe the functional organization of the thalamus as it relates to processing acoustic features important for speech perception, focusing on thalamic nuclei that relate to auditory representations of language sounds. The MGB can be divided into three main subdivisions, the ventral, dorsal, and medial subdivisions, each with different connectivity, auditory response properties, neuronal properties, and synaptic properties. Together, the MGB subdivisions actively and dynamically shape complex auditory processing and form ongoing communication loops with auditory cortex and subcortical structures.

Frequency response areas in the inferior colliculus: nonlinearity and binaural interaction

The tuning, binaural properties, and encoding characteristics of neurons in the central nucleus of the inferior colliculus (CNIC) were investigated to shed light on nonlinearities in the responses of these neurons. Results were analyzed for three types of neurons (I, O, and V) in the CNIC of decerebrate cats. Rate responses to binaural stimuli were characterized using a 1st- plus 2nd-order spectral integration model. Parameters of the model were derived using broadband stimuli with random spectral shapes (RSS). This method revealed four characteristics of CNIC neurons: (1) Tuning curves derived from broadband stimuli have fixed (i. e., level tolerant) bandwidths across a 50-60 dB range of sound levels; (2) 1st-order contralateral weights (particularly for type I and O neurons) were usually larger in magnitude than corresponding ipsilateral weights; (3) contralateral weights were more important than ipsilateral weights when using the model to predict responses to untrained noise stimuli; and (4) 2nd-order weight functions demonstrate frequency selectivity different from that of 1st-order weight functions. Furthermore, while the inclusion of 2nd-order terms in the model usually improved response predictions related to untrained RSS stimuli, they had limited impact on predictions related to other forms of filtered broadband noise [e. g., virtual-space stimuli (VS)]. The accuracy of the predictions varied considerably by response type. Predictions were most accurate for I neurons, and less accurate for O and V neurons, except at the lowest stimulus levels. These differences in prediction performance support the idea that type I, O, and V neurons encode different aspects of the stimulus: while type I neurons are most capable of producing linear representations of spectral shape, type O and V neurons may encode spectral features or temporal stimulus properties in a manner not easily explained with the low-order model. Supported by NIH grant DC00115.

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.

Generation of intensity selectivity by differential synaptic tuning: fast-saturating excitation but slow-saturating inhibition

Intensity defines one fundamental aspect of sensory information and is specifically represented in each sensory modality. Interestingly, only in the central auditory system are intensity-selective neurons evolved. These neurons are characterized by nonmonotonic response-level functions. The synaptic circuitry mechanisms underlying the generation of intensity selectivity from nonselective auditory nerve inputs remain largely unclear. Here, we performed in vivo whole-cell recordings from pyramidal neurons in the rat dorsal cochlear nucleus (DCN), where intensity selectivity first emerges along the auditory neuraxis. Our results revealed that intensity-selective cells received fast-saturating excitation but slow-saturating inhibition with intensity increments, whereas in intensity-nonselective cells excitation and inhibition were similarly slow-saturating. The differential intensity tuning profiles of the monotonic excitation and inhibition qualitatively determined the intensity selectivity of output responses. In addition, the selectivity was further strengthened by significantly lower excitation/inhibition ratios at high-intensity levels compared with intensity-nonselective neurons. Our results demonstrate that intensity selectivity in the DCN is generated by extracting the difference between tuning profiles of nonselective excitatory and inhibitory inputs, which we propose can be achieved through a differential circuit mediated by feedforward inhibition.

Inhibition shapes selectivity to vocalizations in the inferior colliculus of awake mice

The inferior colliculus (IC) is a major center for integration of auditory information as it receives ascending projections from a variety of brainstem nuclei as well as descending projections from the thalamus and auditory cortex. The ascending projections are both excitatory and inhibitory and their convergence at the IC results in a microcircuitry that is important for shaping responses to simple, binaural, and modulated sounds in the IC. Here, we examined the role inhibition plays in shaping selectivity to vocalizations in the IC of awake, normal-hearing adult mice (CBA/CaJ strain). Neurons in the IC of mice show selectivity in their responses to vocalizations, and we hypothesized that this selectivity is created by inhibitory microcircuitry in the IC. We compared single unit responses in the IC to pure tones and a variety of ultrasonic mouse vocalizations before and after iontophoretic application of GABA(A) receptor (GABA(A)R) and glycine receptor (GlyR) antagonists. The most pronounced effects of blocking GABA(A)R and GlyR on IC neurons were to increase spike rates and broaden excitatory frequency tuning curves in response to pure tone stimuli, and to decrease selectivity to vocalizations. Thus, inhibition plays an important role in creating selectivity to vocalizations in the IC.

Monopolar intracochlear pulse trains selectively activate the inferior colliculus

Previous cochlear implant studies using isolated electrical stimulus pulses in animal models have reported that intracochlear monopolar stimulus configurations elicit broad extents of neuronal activation within the central auditory system-much broader than the activation patterns produced by bipolar electrode pairs or acoustic tones. However, psychophysical and speech reception studies that use sustained pulse trains do not show clear performance differences for monopolar versus bipolar configurations. To test whether monopolar intracochlear stimulation can produce selective activation of the inferior colliculus, we measured activation widths along the tonotopic axis of the inferior colliculus for acoustic tones and 1,000-pulse/s electrical pulse trains in guinea pigs and cats. Electrical pulse trains were presented using an array of 6-12 stimulating electrodes distributed longitudinally on a space-filling silicone carrier positioned in the scala tympani of the cochlea. We found that for monopolar, bipolar, and acoustic stimuli, activation widths were significantly narrower for sustained responses than for the transient response to the stimulus onset. Furthermore, monopolar and bipolar stimuli elicited similar activation widths when compared at stimulus levels that produced similar peak spike rates. Surprisingly, we found that in guinea pigs, monopolar and bipolar stimuli produced narrower sustained activation than 60 dB sound pressure level acoustic tones when compared at stimulus levels that produced similar peak spike rates. Therefore, we conclude that intracochlear electrical stimulation using monopolar pulse trains can produce activation patterns that are at least as selective as bipolar or acoustic stimulation.

Spectrotemporal sound preferences of neighboring inferior colliculus neurons: implications for local circuitry and processing

How do local circuits in the inferior colliculus (IC) process and transform spectral and temporal sound information? Using a four-tetrode array we examined the functional properties of the IC and metrics of its micro circuitry by recording neural activity from neighboring single neurons in the cat. Spectral and temporal response preferences were compared for neurons found on the same and adjacent tetrodes (ATs), as well as across distant recording sites. We found that neighboring neurons had similar preferences while neurons recorded across distant sites were less similar. Best frequency (BF) was the most correlated parameter between neighboring neurons and BF differences exhibited unique clustering at ~0.3 octave intervals, indicative of the frequency band lamina. Other spectral and temporal parameters of the receptive fields were more similar for neighboring neurons than for those at distant sites and the receptive field similarity was larger for neurons with small differences in BF. Furthermore, correlated firing was stronger for neighboring neuron pairs and increased with proximity and decreasing BF difference. Thus, although response selectivities are quite diverse in the IC, spectral, and temporal preference within a local microcircuit are functionally quite similar. This suggests a scheme where local circuits are organized into zones that are specialized for processing distinct spectrotemporal cues.

Morphological and physiological characteristics of laminar cells in the central nucleus of the inferior colliculus

The central nucleus of the inferior colliculus (IC) is organized into a series of fibro-dendritic laminae, orthogonal to the tonotopic progression. Many neurons have their dendrites confined to one lamina while others have dendrites that cross over a number of laminae. Here, we have used juxtacellular labeling in urethane anesthetized guinea pigs to visualize the cells with biocytin and have analyzed their response properties, in order to try and link their structure and function. Out of a sample of 38 filled cells, 15 had dendrites confined within the fibro-dendritic laminae and in 13 we were also able to reconstruct their local axonal tree. Based on dendritic morphology they were subdivided into flat or less flat; small, medium, or large; elongated or disk-shaped cells. Two of the elongated cells had many dendritic spines while the other cells had few or none. Twelve of the cells had their local axonal tree restricted to the same lamina as their dendrites while one cell had its dendrites in a separate lamina from the axon. The axonal plexus was more extensive (width 0.7–1.4 mm) within the lamina than the dendrites (width generally 0.07–0.53 mm). The intrinsic axons were largely confined to a single lamina within the central nucleus, but at least half the cells also had output axons with two heading for the commissure and five heading into the brachium. We were able to identify similarities in the physiological response profiles of small groups of our filled cells but none appeared to represent a homogeneous morphological cell type. The only common feature of our sample was one of exclusion in that the onset response, a response commonly recorded from IC cells, was never seen in laminar cells, but was in cells with a stellate morphology. Thus cells with laminar dendrites have a wide variety of physiological responses and morphological subtypes, but over 90% have an extensive local axonal tree within their local lamina.

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.

Three-dimensional brain reconstruction of in vivo electrode tracks for neuroscience and neural prosthetic applications

The brain is a densely interconnected network that relies on populations of neurons within and across multiple nuclei to code for features leading to perception and action. However, the neurophysiology field is still dominated by the characterization of individual neurons, rather than simultaneous recordings across multiple regions, without consistent spatial reconstruction of their locations for comparisons across studies. There are sophisticated histological and imaging techniques for performing brain reconstructions. However, what is needed is a method that is relatively easy and inexpensive to implement in a typical neurophysiology lab and provides consistent identification of electrode locations to make it widely used for pooling data across studies and research groups. This paper presents our initial development of such an approach for reconstructing electrode tracks and site locations within the guinea pig inferior colliculus (IC) to identify its functional organization for frequency coding relevant for a new auditory midbrain implant (AMI). Encouragingly, the spatial error associated with different individuals reconstructing electrode tracks for the same midbrain was less than 65 μm, corresponding to an error of ~1.5% relative to the entire IC structure (~4–5 mm diameter sphere). Furthermore, the reconstructed frequency laminae of the IC were consistently aligned across three sampled midbrains, demonstrating the ability to use our method to combine location data across animals. Hopefully, through further improvements in our reconstruction method, it can be used as a standard protocol across neurophysiology labs to characterize neural data not only within the IC but also within other brain regions to help bridge the gap between cellular activity and network function. Clinically, correlating function with location within and across multiple brain regions can guide optimal placement of electrodes for the growing field of neural prosthetics.