Corticofugal modulation of duration-tuned neurons in the midbrain auditory nucleus in bats

Organization of orbitofrontal-auditory pathways in the Mongolian gerbil

Sound perception is highly malleable, rapidly adjusting to the acoustic environment and behavioral demands. This flexibility is the result of ongoing changes in auditory cortical activity driven by fluctuations in attention, arousal, or prior expectations. Recent work suggests that the orbitofrontal cortex (OFC) may mediate some of these rapid changes, but the anatomical connections between the OFC and the auditory system are not well characterized. Here, we used virally mediated fluorescent tracers to map the projection from OFC to the auditory midbrain, thalamus, and cortex in a classic animal model for auditory research, the Mongolian gerbil (Meriones unguiculatus). We observed no connectivity between the OFC and the auditory midbrain, and an extremely sparse connection between the dorsolateral OFC and higher order auditory thalamic regions. In contrast, we observed a robust connection between the ventral and medial subdivisions of the OFC and the auditory cortex, with a clear bias for secondary auditory cortical regions. OFC axon terminals were found in all auditory cortical lamina but were significantly more concentrated in the infragranular layers. Tissue-clearing and lightsheet microscopy further revealed that auditory cortical-projecting OFC neurons send extensive axon collaterals throughout the brain, targeting both sensory and non-sensory regions involved in learning, decision-making, and memory. These findings provide a more detailed map of orbitofrontal-auditory connections and shed light on the possible role of the OFC in supporting auditory cognition.

Descending projections to the auditory midbrain: evolutionary considerations

The mammalian inferior colliculus (IC) is massively innervated by multiple descending projection systems. In addition to a large projection from the auditory cortex (AC) primarily targeting the non-lemniscal portions of the IC, there are less well-characterized projections from non-auditory regions of the cortex, amygdala, posterior thalamus and the brachium of the IC. By comparison, the frog auditory midbrain, known as the torus semicircularis, is a large auditory integration center that also receives descending input, but primarily from the posterior thalamus and without a projection from a putative cortical homolog: the dorsal pallium. Although descending projections have been implicated in many types of behaviors, a unified understanding of their function has not yet emerged. Here, we take a comparative approach to understanding the various top-down modulators of the IC to gain insights into their functions. One key question that we identify is whether thalamotectal projections in mammals and amphibians are homologous and whether they interact with evolutionarily more newly derived projections from the cerebral cortex. We also consider the behavioral significance of these descending pathways, given anurans' ability to navigate complex acoustic landscapes without the benefit of a corticocollicular projection. Finally, we suggest experimental approaches to answer these questions.

Corticofugal Modulation of Temporal and Rate Representations in the Inferior Colliculus of the Awake Marmoset

Temporal processing is crucial for auditory perception and cognition, especially for communication sounds. Previous studies have shown that the auditory cortex and the thalamus use temporal and rate representations to encode slowly and rapidly changing time-varying sounds. However, how the primate inferior colliculus (IC) encodes time-varying sounds at the millisecond scale remains unclear. In this study, we investigated the temporal processing by IC neurons in awake marmosets to Gaussian click trains with varying interclick intervals (2-100 ms). Strikingly, we found that 28% of IC neurons exhibited rate representation with nonsynchronized responses, which is in sharp contrast to the current view that the IC only uses a temporal representation to encode time-varying signals. Moreover, IC neurons with rate representation exhibited response properties distinct from those with temporal representation. We further demonstrated that reversible inactivation of the primary auditory cortex modulated 17% of the stimulus-synchronized responses and 21% of the nonsynchronized responses of IC neurons, revealing that cortico-colliculus projections play a role, but not a crucial one, in temporal processing in the IC. This study has significantly advanced our understanding of temporal processing in the IC of awake animals and provides new insights into temporal processing from the midbrain to the cortex.

Evidence for Layer-Specific Connectional Heterogeneity in the Mouse Auditory Corticocollicular System

The auditory cortex (AC) sends long-range projections to virtually all subcortical auditory structures. One of the largest and most complex of these-the projection between AC and inferior colliculus (IC; the corticocollicular pathway)-originates from layer 5 and deep layer 6. Though previous work has shown that these two corticocollicular projection systems have different physiological properties and network connectivities, their functional organization is poorly understood. Here, using a combination of traditional and viral tracers combined with in vivo imaging in both sexes of the mouse, we observed that layer 5 and layer 6 corticocollicular neurons differ in their areas of origin and termination patterns. Layer 5 corticocollicular neurons are concentrated in primary AC, while layer 6 corticocollicular neurons emanate from broad auditory and limbic areas in the temporal cortex. In addition, layer 5 sends dense projections of both small and large (>1 µm2 area) terminals to all regions of nonlemniscal IC, while layer 6 sends small terminals to the most superficial 50-100 µm of the IC. These findings suggest that layer 5 and 6 corticocollicular projections are optimized to play distinct roles in corticofugal modulation. Layer 5 neurons provide strong, rapid, and unimodal feedback to the nonlemniscal IC, while layer 6 neurons provide heteromodal and limbic modulation diffusely to the nonlemniscal IC. Such organizational diversity in the corticocollicular pathway may help to explain the heterogeneous effects of corticocollicular manipulations and, given similar diversity in corticothalamic pathways, may be a general principle in top-down modulation.SIGNIFICANCE STATEMENT We demonstrate that a major descending system in the brain is actually two systems. That is, the auditory corticocollicular projection, which exerts considerable influence over the midbrain, comprises two projections: one from layer 5 and the other from layer 6. The layer 6 projection is diffusely organized, receives multisensory inputs, and ends in small terminals; while the layer 5 projection is derived from a circumscribed auditory cortical area and ends in large terminals. These data suggest that the varied effects of cortical manipulations on the midbrain may be related to effects on two disparate systems. These findings have broader implications because other descending systems derive from two layers. Therefore, a duplex organization may be a common motif in descending control.

Top-Down Inference in the Auditory System: Potential Roles for Corticofugal Projections

It has become widely accepted that humans use contextual information to infer the meaning of ambiguous acoustic signals. In speech, for example, high-level semantic, syntactic, or lexical information shape our understanding of a phoneme buried in noise. Most current theories to explain this phenomenon rely on hierarchical predictive coding models involving a set of Bayesian priors emanating from high-level brain regions (e.g., prefrontal cortex) that are used to influence processing at lower-levels of the cortical sensory hierarchy (e.g., auditory cortex). As such, virtually all proposed models to explain top-down facilitation are focused on intracortical connections, and consequently, subcortical nuclei have scarcely been discussed in this context. However, subcortical auditory nuclei receive massive, heterogeneous, and cascading descending projections at every level of the sensory hierarchy, and activation of these systems has been shown to improve speech recognition. It is not yet clear whether or how top-down modulation to resolve ambiguous sounds calls upon these corticofugal projections. Here, we review the literature on top-down modulation in the auditory system, primarily focused on humans and cortical imaging/recording methods, and attempt to relate these findings to a growing animal literature, which has primarily been focused on corticofugal projections. We argue that corticofugal pathways contain the requisite circuitry to implement predictive coding mechanisms to facilitate perception of complex sounds and that top-down modulation at early (i.e., subcortical) stages of processing complement modulation at later (i.e., cortical) stages of processing. Finally, we suggest experimental approaches for future studies on this topic.

Cortical Stimulation Induces Excitatory Postsynaptic Potentials of Inferior Colliculus Neurons in a Frequency-Specific Manner

Corticofugal modulation of auditory responses in subcortical nuclei has been extensively studied whereas corticofugal synaptic transmission must still be characterized. This study examined postsynaptic potentials of the corticocollicular system, i.e., the projections from the primary auditory cortex (AI) to the central nucleus of the inferior colliculus (ICc) of the midbrain, in anesthetized C57 mice. We used focal electrical stimulation at the microampere level to activate the AI (ES_AI) and in vivo whole-cell current-clamp to record the membrane potentials of ICc neurons. Following the whole-cell patch-clamp recording of 88 ICc neurons, 42 ICc neurons showed ES_AI-evoked changes in the membrane potentials. We found that the ES_AI induced inhibitory postsynaptic potentials in 6 out of 42 ICc neurons but only when the stimulus current was 96 μA or higher. In the remaining 36 ICc neurons, excitatory postsynaptic potentials (EPSPs) were induced at a much lower stimulus current. The 36 ICc neurons exhibiting EPSPs were categorized into physiologically matched neurons (n = 12) when the characteristic frequencies of the stimulated AI and recorded ICc neurons were similar (≤1 kHz) and unmatched neurons (n = 24) when they were different (>1 kHz). Compared to unmatched neurons, matched neurons exhibited a significantly lower threshold of evoking noticeable EPSP, greater EPSP amplitude, and shorter EPSP latency. Our data allow us to propose that corticocollicular synaptic transmission is primarily excitatory and that synaptic efficacy is dependent on the relationship of the frequency tunings between AI and ICc neurons.

Auditory processing in the zebra finch midbrain: single unit responses and effect of rearing experience

In birds the auditory system plays a key role in providing the sensory input used to discriminate between conspecific and heterospecific vocal signals. In those species that are known to learn their vocalizations, for example, songbirds, it is generally considered that this ability arises and is manifest in the forebrain, although there is no a priori reason why brainstem components of the auditory system could not also play an important part. To test this assumption, we used groups of normal reared and cross-fostered zebra finches that had previously been shown in behavioural experiments to reduce their preference for conspecific songs subsequent to cross fostering experience with Bengalese finches, a related species with a distinctly different song. The question we asked, therefore, is whether this experiential change also changes the bias in favour of conspecific song displayed by auditory midbrain units of normally raised zebra finches. By recording the responses of single units in MLd to a variety of zebra finch and Bengalese finch songs in both normally reared and cross-fostered zebra finches, we provide a positive answer to this question. That is, the difference in response to conspecific and heterospecific songs seen in normal reared zebra finches is reduced following cross-fostering. In birds the virtual absence of mammalian-like cortical projections upon auditory brainstem nuclei argues against the interpretation that MLd units change, as observed in the present experiments, as a result of top-down influences on sensory processing. Instead, it appears that MLd units can be influenced significantly by sensory inputs arising directly from a change in auditory experience during development.

Auditory cortex shapes sound responses in the inferior colliculus

The extensive feedback from the auditory cortex (AC) to the inferior colliculus (IC) supports critical aspects of auditory behavior but has not been extensively characterized. Previous studies demonstrated that activity in IC is altered by focal electrical stimulation and pharmacological inactivation of AC, but these methods lack the ability to selectively manipulate projection neurons. We measured the effects of selective optogenetic modulation of cortico-collicular feedback projections on IC sound responses in mice. Activation of feedback increased spontaneous activity and decreased stimulus selectivity in IC, whereas suppression had no effect. To further understand how microcircuits in AC may control collicular activity, we optogenetically modulated the activity of different cortical neuronal subtypes, specifically parvalbumin-positive (PV) and somatostatin-positive (SST) inhibitory interneurons. We found that modulating the activity of either type of interneuron did not affect IC sound-evoked activity. Combined, our results identify that activation of excitatory projections, but not inhibition-driven changes in cortical activity, affects collicular sound responses.

Optimizing optogenetic stimulation protocols in auditory corticofugal neurons based on closed-loop spike feedback

OBJECTIVE

Optogenetics provides a means to probe functional connections between brain areas. By activating a set of presynaptic neurons and recording the activity from a downstream brain area, one can establish the sign and strength of a feedforward connection. One challenge is that there are virtually limitless patterns that can be used to stimulate a presynaptic brain area. Functional influences on downstream brain areas can depend not just on whether presynaptic neurons were activated, but how they were activated. Corticofugal axons from the auditory cortex (ACtx) heavily innervate the auditory tectum, the inferior colliculus (IC). Here, we sought to determine whether different modes of corticocollicular activation could titrate the strength of feedforward modulation of sound processing in IC neurons.

APPROACH

We used multi-channel electrophysiology and optogenetics to record from multiple regions of the IC in awake head-fixed mice while optogenetically stimulating ACtx neurons expressing Chronos, an ultra-fast channelrhodopsin. To identify cortical activation patterns associated with the strongest effects on IC firing rates, we employed a closed-loop evolutionary optimization procedure that tailored the voltage command signal sent to the laser based on spike feedback from single IC neurons.

MAIN RESULTS

Within minutes, our evolutionary search procedure converged on ACtx stimulation configurations that produced more effective and widespread enhancement of IC unit activity than generic activation parameters. Cortical modulation of midbrain spiking was bi-directional, as the evolutionary search procedure could be programmed to converge on activation patterns that either suppressed or enhanced sound-evoked IC firing rate.

SIGNIFICANCE

This study introduces a closed-loop optimization procedure to probe functional connections between brain areas. Our findings demonstrate that the influence of descending feedback projections on subcortical sensory processing can vary both in sign and degree depending on how cortical neurons are activated in time.

Novel Functions of Feedback in Electrosensory Processing

Environmental signals act as input and are processed across successive stages in the brain to generate a meaningful behavioral output. However, a ubiquitous observation is that descending feedback projections from more central to more peripheral brain areas vastly outnumber ascending feedforward projections. Such projections generally act to modify how sensory neurons respond to afferent signals. Recent studies in the electrosensory system of weakly electric fish have revealed novel functions for feedback pathways in that their transformation of the afferent input generates neural firing rate responses to sensory signals mediating perception and behavior. In this review, we focus on summarizing these novel and recently uncovered functions and put them into context by describing the more "classical" functions of feedback in the electrosensory system. We further highlight the parallels between the electrosensory system and other systems as well as outline interesting future directions.

Thalamocortical and Intracortical Inputs Differentiate Layer-Specific Mouse Auditory Corticocollicular Neurons

Long-range descending projections from the auditory cortex play key roles in shaping response properties in the inferior colliculus. The auditory corticocollicular projection is massive and heterogeneous, with axons emanating from cortical layers 5 and 6, and plays a key role in directing plastic changes in the inferior colliculus. However, little is known about the cortical and thalamic networks within which corticocollicular neurons are embedded. Here, laser scanning photostimulation glutamate uncaging and photoactivation of channelrhodopsin-2 were used to probe the local and long-range network differences between preidentified layer 5 and layer 6 auditory corticocollicular neurons from male and female mice in vitro Layer 5 corticocollicular neurons were found to vertically integrate supragranular excitatory and inhibitory input to a substantially greater degree than their layer 6 counterparts. In addition, all layer 5 corticocollicular neurons received direct and large thalamic inputs from channelrhodopsin-2-labeled thalamocortical fibers, whereas such inputs were less common in layer 6 corticocollicular neurons. Finally, a new low-calcium/synaptic blockade approach to separate direct from indirect inputs using laser photostimulation was validated. These data demonstrate that layer 5 and 6 corticocollicular neurons receive distinct sets of cortical and thalamic inputs, supporting the hypothesis that they have divergent roles in modulating the inferior colliculus. Furthermore, the direct connection between the auditory thalamus and layer 5 corticocollicular neurons reveals a novel and rapid link connecting ascending and descending pathways.SIGNIFICANCE STATEMENT Descending projections from the cortex play a critical role in shaping the response properties of sensory neurons. The projection from the auditory cortex to the inferior colliculus is a massive, yet poorly understood, pathway emanating from two distinct cortical layers. Here we show, using a range of optical techniques, that mouse auditory corticocollicular neurons from different layers are embedded into different cortical and thalamic networks. Specifically, we observed that layer 5 corticocollicular neurons integrate information across cortical lamina and receive direct thalamic input. The latter connection provides a hyperdirect link between acoustic sensation and descending control, thus demonstrating a novel mechanism for rapid "online" modulation of sensory perception.

Optogenetic auditory fMRI reveals the effects of visual cortical inputs on auditory midbrain response

Sensory cortices contain extensive descending (corticofugal) pathways, yet their impact on brainstem processing - particularly across sensory systems - remains poorly understood. In the auditory system, the inferior colliculus (IC) in the midbrain receives cross-modal inputs from the visual cortex (VC). However, the influences from VC on auditory midbrain processing are unclear. To investigate whether and how visual cortical inputs affect IC auditory responses, the present study combines auditory blood-oxygenation-level-dependent (BOLD) functional MRI (fMRI) with cell-type specific optogenetic manipulation of visual cortex. The results show that predominant optogenetic excitation of the excitatory pyramidal neurons in the infragranular layers of the primary VC enhances the noise-evoked BOLD fMRI responses within the IC. This finding reveals that inputs from VC influence and facilitate basic sound processing in the auditory midbrain. Such combined optogenetic and auditory fMRI approach can shed light on the large-scale modulatory effects of corticofugal pathways and guide detailed electrophysiological studies in the future.

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

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

Anatomical characterization of subcortical descending projections to the inferior colliculus in mouse

Descending projections from the thalamus and related structures to the midbrain are evolutionarily highly conserved. However, the basic organization of this auditory thalamotectal pathway has not yet been characterized. The purpose of this study was to obtain a better understanding of the anatomical and neurochemical features of this pathway. Analysis of the distributions of retrogradely labeled cells after focal injections of retrograde tracer into the inferior colliculus (IC) of the mouse revealed that most of the subcortical descending projections originated in the brachium of the IC and the paralaminar portions of the auditory thalamus. In addition, the vast majority of thalamotectal cells were found to be negative for the calcium-binding proteins calbindin, parvalbumin, or calretinin. Using two different strains of GAD-GFP mice, as well as immunostaining for GABA, we found that a subset of neurons in the brachium of the IC is GABAergic, suggesting that part of this descending pathway is inhibitory. Finally, dual retrograde injections into the IC and amygdala plus corpus striatum as well into the IC and auditory cortex did not reveal any double labeling. These data suggest that the thalamocollicular pathway comprises a unique population of thalamic neurons that do not contain typical calcium-binding proteins and do not project to other paralaminar thalamic forebrain targets, and that a previously undescribed descending GABAergic pathway emanates from the brachium of the IC. J. Comp. Neurol. 525:885-900, 2017. © 2016 Wiley Periodicals, Inc.

The effects of repetitive transcranial magnetic stimulation in an animal model of tinnitus

Tinnitus (phantom auditory perception associated with hearing loss) can seriously affect wellbeing. Its neural substrate is unknown however it has been linked with abnormal activity in auditory pathways. Though no cure currently exists, repetitive transcranial magnetic stimulation (rTMS) has been shown to reduce tinnitus in some patients, possibly via induction of cortical plasticity involving brain derived neurotrophic factor (BDNF). We examined whether low intensity rTMS (LI-rTMS) alleviates signs of tinnitus in a guinea pig model and whether this involves changes in BDNF expression and hyperactivity in inferior colliculus. Acoustic trauma was used to evoke hearing loss, central hyperactivity and tinnitus. When animals developed tinnitus, treatment commenced (10 sessions of 10 minutes 1 Hz LI-rTMS or sham over auditory cortex over 14 days). After treatment ceased animals were tested for tinnitus, underwent single-neuron recordings in inferior colliculus to assess hyperactivity and samples from cortex and inferior colliculus were taken for BDNF ELISA. Analysis revealed a significant reduction of tinnitus after LI-rTMS compared to sham, without a statistical significant effect on BDNF levels or hyperactivity. This suggests that LI-rTMS alleviates behavioural signs of tinnitus by a mechanism independent of inferior colliculus hyperactivity and BDNF levels and opens novel therapeutic avenues for tinnitus treatment.

Descending projections from the inferior colliculus to the dorsal cochlear nucleus are excitatory

Ascending projections of the dorsal cochlear nucleus (DCN) target primarily the contralateral inferior colliculus (IC). In turn, the IC sends bilateral descending projections back to the DCN. We sought to determine the nature of these descending axons in order to infer circuit mechanisms of signal processing at one of the earliest stages of the central auditory pathway. An anterograde tracer was injected in the IC of CBA/Ca mice to reveal terminal characteristics of the descending axons. Retrograde tracer deposits were made in the DCN of CBA/Ca and transgenic GAD67-EGFP mice to investigate the cells giving rise to these projections. A multiunit best frequency was determined for each injection site. Brains were processed by using standard histologic methods for visualization and examined by fluorescent, brightfield, and electron microscopy. Descending projections from the IC were inferred to be excitatory because the cell bodies of retrogradely labeled neurons did not colabel with EGFP expression in neurons of GAD67-EGFP mice. Furthermore, additional experiments yielded no glycinergic or cholinergic positive cells in the IC, and descending projections to the DCN were colabeled with antibodies against VGluT2, a glutamate transporter. Anterogradely labeled endings in the DCN formed asymmetric postsynaptic densities, a feature of excitatory neurotransmission. These descending projections to the DCN from the IC were topographic and suggest a feedback pathway that could underlie a frequency-specific enhancement of some acoustic signals and suppression of others. The involvement of this IC-DCN circuit is especially noteworthy when considering the gating of ascending signal streams for auditory processing. J. Comp. Neurol. 525:773-793, 2017. © 2016 Wiley Periodicals, Inc.

Plastic Change in the Auditory Minimum Threshold Induced by Intercollicular Effects in Mice

In the auditory pathway, the commissure of the inferior colliculus (IC) interconnects the two ICs on both sides of the dorsal midbrain. This interconnection could mediate an interaction between the two ICs during sound signal processing. The intercollicular effects evoked by focal electric stimulation for 30 min could inhibit or facilitate auditory responses and induce plastic changes in the response minimum threshold (MT) of IC neurons. Changes in MT are dependent on the best frequency (BF) and MT difference. The MT shift is larger in IC neurons with BF differences ≤2 kHz than in those with BF differences >2 kHz. Moreover, MTs that shift toward electrically stimulated IC neurons increase with the increasing MT difference between the two ICs. The shift in MT lasts for a certain period of time and then returns to previous levels within ~150 min. The collicular interactions are either reciprocal or unilateral under alternate stimulating and recording conditions in both ICs. Our results suggest that intercollicular effects may be involved in the acoustic experience-dependent plasticity of the MT of IC neurons.

Corticofugal modulation of peripheral auditory responses

The auditory efferent system originates in the auditory cortex and projects to the medial geniculate body (MGB), inferior colliculus (IC), cochlear nucleus (CN) and superior olivary complex (SOC) reaching the cochlea through olivocochlear (OC) fibers. This unique neuronal network is organized in several afferent-efferent feedback loops including: the (i) colliculo-thalamic-cortico-collicular; (ii) cortico-(collicular)-OC; and (iii) cortico-(collicular)-CN pathways. Recent experiments demonstrate that blocking ongoing auditory-cortex activity with pharmacological and physical methods modulates the amplitude of cochlear potentials. In addition, auditory-cortex microstimulation independently modulates cochlear sensitivity and the strength of the OC reflex. In this mini-review, anatomical and physiological evidence supporting the presence of a functional efferent network from the auditory cortex to the cochlear receptor is presented. Special emphasis is given to the corticofugal effects on initial auditory processing, that is, on CN, auditory nerve and cochlear responses. A working model of three parallel pathways from the auditory cortex to the cochlea and auditory nerve is proposed.

Auditory midbrain processing is differentially modulated by auditory and visual cortices: An auditory fMRI study

The cortex contains extensive descending projections, yet the impact of cortical input on brainstem processing remains poorly understood. In the central auditory system, the auditory cortex contains direct and indirect pathways (via brainstem cholinergic cells) to nuclei of the auditory midbrain, called the inferior colliculus (IC). While these projections modulate auditory processing throughout the IC, single neuron recordings have samples from only a small fraction of cells during stimulation of the corticofugal pathway. Furthermore, assessments of cortical feedback have not been extended to sensory modalities other than audition. To address these issues, we devised blood-oxygen-level-dependent (BOLD) functional magnetic resonance imaging (fMRI) paradigms to measure the sound-evoked responses throughout the rat IC and investigated the effects of bilateral ablation of either auditory or visual cortices. Auditory cortex ablation increased the gain of IC responses to noise stimuli (primarily in the central nucleus of the IC) and decreased response selectivity to forward species-specific vocalizations (versus temporally reversed ones, most prominently in the external cortex of the IC). In contrast, visual cortex ablation decreased the gain and induced a much smaller effect on response selectivity. The results suggest that auditory cortical projections normally exert a large-scale and net suppressive influence on specific IC subnuclei, while visual cortical projections provide a facilitatory influence. Meanwhile, auditory cortical projections enhance the midbrain response selectivity to species-specific vocalizations. We also probed the role of the indirect cholinergic projections in the auditory system in the descending modulation process by pharmacologically blocking muscarinic cholinergic receptors. This manipulation did not affect the gain of IC responses but significantly reduced the response selectivity to vocalizations. The results imply that auditory cortical gain modulation is mediated primarily through direct projections and they point to future investigations of the differential roles of the direct and indirect projections in corticofugal modulation. In summary, our imaging findings demonstrate the large-scale descending influences, from both the auditory and visual cortices, on sound processing in different IC subdivisions. They can guide future studies on the coordinated activity across multiple regions of the auditory network, and its dysfunctions.

Sound-by-sound thalamic stimulation modulates midbrain auditory excitability and relative binaural sensitivity in frogs

Descending circuitry can modulate auditory processing, biasing sensitivity to particular stimulus parameters and locations. Using awake in vivo single unit recordings, this study tested whether electrical stimulation of the thalamus modulates auditory excitability and relative binaural sensitivity in neurons of the amphibian midbrain. In addition, by using electrical stimuli that were either longer than the acoustic stimuli (i.e., seconds) or presented on a sound-by-sound basis (ms), experiments addressed whether the form of modulation depended on the temporal structure of the electrical stimulus. Following long duration electrical stimulation (3-10 s of 20 Hz square pulses), excitability (spikes/acoustic stimulus) to free-field noise stimuli decreased by 32%, but returned over 600 s. In contrast, sound-by-sound electrical stimulation using a single 2 ms duration electrical pulse 25 ms before each noise stimulus caused faster and varied forms of modulation: modulation lasted <2 s and, in different cells, excitability either decreased, increased or shifted in latency. Within cells, the modulatory effect of sound-by-sound electrical stimulation varied between different acoustic stimuli, including for different male calls, suggesting modulation is specific to certain stimulus attributes. For binaural units, modulation depended on the ear of input, as sound-by-sound electrical stimulation preceding dichotic acoustic stimulation caused asymmetric modulatory effects: sensitivity shifted for sounds at only one ear, or by different relative amounts for both ears. This caused a change in the relative difference in binaural sensitivity. Thus, sound-by-sound electrical stimulation revealed fast and ear-specific (i.e., lateralized) auditory modulation that is potentially suited to shifts in auditory attention during sound segregation in the auditory scene.

Frequency-specific corticofugal modulation of the dorsal cochlear nucleus in mice

The primary auditory cortex (AI) modulates the sound information processing in the lemniscal subcortical nuclei, including the anteroventral cochlear nucleus (AVCN), in a frequency-specific manner. The dorsal cochlear nucleus (DCN) is a non-lemniscal subcortical nucleus but it is tonotopically organized like the AVCN. However, it remains unclear how the AI modulates the sound information processing in the DCN. This study examined the impact of focal electrical stimulation of AI on the auditory responses of the DCN neurons in mice. We found that the electrical stimulation induced significant changes in the best frequency (BF) of DCN neurons. The changes in the BFs were highly specific to the BF differences between the stimulated AI neurons and the recorded DCN neurons. The DCN BFs shifted higher when the AI BFs were higher than the DCN BFs and the DCN BFs shifted lower when the AI BFs were lower than the DCN BFs. The DCN BFs showed no change when the AI and DCN BFs were similar. Moreover, the BF shifts were linearly correlated to the BF differences. Thus, our data suggest that corticofugal modulation of the DCN is also highly specific to frequency information, similar to the corticofugal modulation of the AVCN. The frequency-specificity of corticofugal modulation does not appear limited to the lemniscal ascending pathway.

The auditory corticocollicular system: molecular and circuit-level considerations

We live in a world imbued with a rich mixture of complex sounds. Successful acoustic communication requires the ability to extract meaning from those sounds, even when degraded. One strategy used by the auditory system is to harness high-level contextual cues to modulate the perception of incoming sounds. An ideal substrate for this process is the massive set of top-down projections emanating from virtually every level of the auditory system. In this review, we provide a molecular and circuit-level description of one of the largest of these pathways: the auditory corticocollicular pathway. While its functional role remains to be fully elucidated, activation of this projection system can rapidly and profoundly change the tuning of neurons in the inferior colliculus. Several specific issues are reviewed. First, we describe the complex heterogeneous anatomical organization of the corticocollicular pathway, with particular emphasis on the topography of the pathway. We also review the laminar origin of the corticocollicular projection and discuss known physiological and morphological differences between subsets of corticocollicular cells. Finally, we discuss recent findings about the molecular micro-organization of the inferior colliculus and how it interfaces with corticocollicular termination patterns. Given the assortment of molecular tools now available to the investigator, it is hoped that his review will help guide future research on the role of this pathway in normal hearing.

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.

Cortical modulation of auditory processing in the midbrain

In addition to their ascending pathways that originate at the receptor cells, all sensory systems are characterized by extensive descending projections. Although the size of these connections often outweighs those that carry information in the ascending auditory pathway, we still have a relatively poor understanding of the role they play in sensory processing. In the auditory system one of the main corticofugal projections links layer V pyramidal neurons with the inferior colliculus (IC) in the midbrain. All auditory cortical fields contribute to this projection, with the primary areas providing the largest outputs to the IC. In addition to medium and large pyramidal cells in layer V, a variety of cell types in layer VI make a small contribution to the ipsilateral corticocollicular projection. Cortical neurons innervate the three IC subdivisions bilaterally, although the contralateral projection is relatively small. The dorsal and lateral cortices of the IC are the principal targets of corticocollicular axons, but input to the central nucleus has also been described in some studies and is distinctive in its laminar topographic organization. Focal electrical stimulation and inactivation studies have shown that the auditory cortex can modify almost every aspect of the response properties of IC neurons, including their sensitivity to sound frequency, intensity, and location. Along with other descending pathways in the auditory system, the corticocollicular projection appears to continually modulate the processing of acoustical signals at subcortical levels. In particular, there is growing evidence that these circuits play a critical role in the plasticity of neural processing that underlies the effects of learning and experience on auditory perception by enabling changes in cortical response properties to spread to subcortical nuclei.

Duration tuning across vertebrates

Signal duration is important for identifying sound sources and determining signal meaning. Duration-tuned neurons (DTNs) respond preferentially to a range of stimulus durations and maximally to a best duration (BD). Duration-tuned neurons are found in the auditory midbrain of many vertebrates, although studied most extensively in bats. Studies of DTNs across vertebrates have identified cells with BDs and temporal response bandwidths that mirror the range of species-specific vocalizations. Neural tuning to stimulus duration appears to be universal among hearing vertebrates. Herein, we test the hypothesis that neural mechanisms underlying duration selectivity may be similar across vertebrates. We instantiated theoretical mechanisms of duration tuning in computational models to systematically explore the roles of excitatory and inhibitory receptor strengths, input latencies, and membrane time constant on duration tuning response profiles. We demonstrate that models of duration tuning with similar neural circuitry can be tuned with species-specific parameters to reproduce the responses of in vivo DTNs from the auditory midbrain. To relate and validate model output to in vivo responses, we collected electrophysiological data from the inferior colliculus of the awake big brown bat, Eptesicus fuscus, and present similar in vivo data from the published literature on DTNs in rats, mice, and frogs. Our results support the hypothesis that neural mechanisms of duration tuning may be shared across vertebrates despite species-specific differences in duration selectivity. Finally, we discuss how the underlying mechanisms of duration selectivity relate to other auditory feature detectors arising from the interaction of neural excitation and inhibition.

Auditory cortex stimulation to suppress tinnitus: mechanisms and strategies

Brain stimulation is an important method used to modulate neural activity and suppress tinnitus. Several auditory and non-auditory brain regions have been targeted for stimulation. This paper reviews recent progress on auditory cortex (AC) stimulation to suppress tinnitus and its underlying neural mechanisms and stimulation strategies. At the same time, the author provides his opinions and hypotheses on both animal and human models. The author also proposes a medial geniculate body (MGB)-thalamic reticular nucleus (TRN)-Gating mechanism to reflect tinnitus-related neural information coming from upstream and downstream projection structures. The upstream structures include the lower auditory brainstem and midbrain structures. The downstream structures include the AC and certain limbic centers. Both upstream and downstream information is involved in a dynamic gating mechanism in the MGB together with the TRN. When abnormal gating occurs at the thalamic level, the spilled-out information interacts with the AC to generate tinnitus. The tinnitus signals at the MGB-TRN-Gating may be modulated by different forms of stimulations including brain stimulation. Each stimulation acts as a gain modulator to control the level of tinnitus signals at the MGB-TRN-Gate. This hypothesis may explain why different types of stimulation can induce tinnitus suppression. Depending on the tinnitus etiology, MGB-TRN-Gating may be different in levels and dynamics, which cause variability in tinnitus suppression induced by different gain controllers. This may explain why the induced suppression of tinnitus by one type of stimulation varies across individual patients.

Sex differences in auditory subcortical function

OBJECTIVE

Sex differences have been demonstrated in the peripheral auditory system as well as in higher-level cognitive processing. Here, we aimed to determine if the subcortical response to a complex auditory stimulus is encoded differently between the sexes.

METHODS

Using electrophysiological techniques, we assessed the auditory brainstem response to a synthesized stop-consonant speech syllable [da] in 76 native-English speaking, young adults (38 female). Timing and frequency components of the response were compared between males and females to determine which aspects of the response are affected by sex.

RESULTS

A dissimilarity between males and females was seen in the neural response to the components of the speech stimulus that change rapidly over time; but not in the slower changing, lower frequency information in the stimulus. We demonstrate that, in agreement with the click-evoked brainstem response, females have earlier peaks relative to males in the subcomponents of the response representing the onset of the speech sound. In contrast, the response peaks comprising the frequency-following response, which encode the fundamental frequency (F(0)) of the stimulus, as well as the spectral amplitude of the response to the F(0), is not affected by sex. Notably, the higher-frequency elements of the speech syllable are encoded differently between males and females, with females having greater representation of spectrotemporal information for frequencies above the F(0).

CONCLUSIONS

Our results provide a baseline for interpreting the higher incidence of language impairment (e.g. dyslexia, autism, specific language impairment) in males, and the subcortical deficits associated with these disorders.

SIGNIFICANCE

These results parallel the subcortical encoding patterns that are documented for good and poor readers in that poor readers differ from good readers on encoding fast but not slow components of speech. This parallel may thus help to explain the higher incidence of reading impairment in males compared to females.

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

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

Neural circuits underlying adaptation and learning in the perception of auditory space

Sound localization mechanisms are particularly plastic during development, when the monaural and binaural acoustic cues that form the basis for spatial hearing change in value as the body grows. Recent studies have shown that the mature brain retains a surprising capacity to relearn to localize sound in the presence of substantially altered auditory spatial cues. In addition to the long-lasting changes that result from learning, behavioral and electrophysiological studies have demonstrated that auditory spatial processing can undergo rapid adjustments in response to changes in the statistics of recent stimulation, which help to maintain sensitivity over the range where most stimulus values occur. Through a combination of recording studies and methods for selectively manipulating the activity of specific neuronal populations, progress is now being made in identifying the cortical and subcortical circuits in the brain that are responsible for the dynamic coding of auditory spatial information.

The descending corticocollicular pathway mediates learning-induced auditory plasticity

Descending projections from sensory areas of the cerebral cortex are among the largest pathways in the brain, suggesting that they are important for subcortical processing. Although corticofugal inputs have been shown to modulate neuronal responses in the thalamus and midbrain, the behavioral importance of these changes remains unknown. In the auditory system, one of the major descending pathways is from cortical layer V pyramidal cells to the inferior colliculus in the midbrain. We examined the role of these neurons in experience-dependent recalibration of sound localization in adult ferrets by selectively killing the neurons using chromophore-targeted laser photolysis. When provided with appropriate training, animals normally relearn to localize sound accurately after altering the spatial cues available by reversibly occluding one ear. However, this ability was lost after eliminating corticocollicular neurons, whereas normal sound-localization accuracy was unaffected. The integrity of this descending pathway is therefore critical for learning-induced localization plasticity.

Specific and nonspecific plasticity of the primary auditory cortex elicited by thalamic auditory neurons

The ventral and medial divisions of the medial geniculate body (MGBv and MGBm) respectively are the lemniscal and nonlemniscal thalamic auditory nuclei. Lemniscal neurons are narrowly frequency tuned and provide highly specific frequency information to the primary auditory cortex (AI), whereas nonlemniscal neurons are broadly frequency tuned and project widely to auditory cortical areas including AI. The MGBv and MGBm are presumably different not only in auditory signal processing, but also in eliciting cortical plastic changes. We electrically stimulated MGBv or MGBm neurons and found the following: (1) electric stimulation of narrowly frequency-tuned MGBv neurons evoked the shift of the frequency-tuning curves of AI neurons toward the tuning curves of the stimulated MGBv neurons. This shift was the same as that in the central nucleus of the inferior colliculus and AI elicited by focal electric stimulation of AI or auditory fear conditioning. The widths of the tuning curves of the AI neurons stayed the same or slightly increased. (2) Electric stimulation of broad frequency-tuned MGBm neurons augmented the auditory responses of AI neurons and broadened their frequency-tuning curves which did not shift. These cortical changes evoked by MGBv or MGBm neurons slowly disappeared over 45-60 min after the onset of the electric stimulation. Our findings indicate that lemniscal and nonlemniscal nuclei are indeed different in eliciting cortical plastic changes: the MGBv evokes tone-specific plasticity in AI for adjusting auditory signal processing in the frequency domain, whereas the MGBm evokes nonspecific plasticity in AI for increasing the sensitivity of cortical neurons.

Temporal filtering by prefrontal neurons in duration discrimination

Neural imaging studies have revealed that the prefrontal cortex (PFC) participates in time perception. However, actual functional roles remain unclear. We trained two monkeys to perform a duration-discrimination task, in which two visual cues were presented consecutively for different durations ranging from 0.2 to 2.0 s. The subjects were required to choose the longer cue. We recorded single-neuron activity from the PFC while the subjects were performing the task. Responsive neurons for the first cue period were extracted and classified through a cluster analysis of firing rate curves. The neuronal activity was categorized as phasic, ramping and sustained patterns. Among them, the phasic activity was the most prevailing. Peak time of the phasic activity was broadly distributed about 0.8 s after cue onset, leading to a natural assumption that the phasic activity was related to cognitive processes. The phasic activity with constant delay after cue onset might function to filter current cue duration with the peak time. The broad distribution of the peak time would indicate that various filtering durations had been prepared for estimating C1 duration. The most frequent peak time was close to the time separating cue durations into long and short. The activity with this peak time might have had a role of filtering in attempted duration discrimination. Our results suggest that the PFC contributes to duration discrimination with temporal filtering in the cue period.

Corticofugal modulation of the paradoxical latency shifts of inferior collicular neurons

The central auditory system creates various types of neurons tuned to different acoustic parameters other than a specific frequency. The response latency of auditory neurons typically shortens with an increase in stimulus intensity. However, approximately 10% of collicular neurons of the little brown bat show a "paradoxical latency-shift (PLS)": long latencies to intense sounds but short latencies to weak sounds. These neurons presumably are involved in the processing of target distance information carried by a pair of an intense biosonar pulse and its weak echo. Our current studies show that collicular PLS neurons of the big brown bat are modulated by the corticofugal (descending) system. Electric stimulation of cortical auditory neurons evoked two types of changes in the PLS neurons, depending on the relationship in the best frequency (BF) between the stimulated cortical and recorded collicular neurons. When the BF was matched between them, the cortical stimulation did not shift the BFs of the collicular neurons and shortened their response latencies at intense sounds so that the PLS became smaller. When the BF was unmatched, however, the cortical stimulation shifted the BFs of the collicular neurons and lengthened their response latencies at intense sounds, so that the PLS became larger. Cortical electric stimulation also modulated the response latencies of non-PLS neurons. It produced an inhibitory frequency tuning curve or curves. Our findings indicate that corticofugal feedback is involved in shaping the spectrotemporal patterns of responses of subcortical auditory neurons presumably through inhibition.

Functional architecture and spike timing properties of corticofugal projections from rat ventral temporal cortex

Sensory association and parahippocampal cortex in the ventral temporal lobe plays an important role in sensory object recognition and control of top-down attention. Although layer V neurons located in high-order cortical structures project to multiple cortical and subcortical regions, the architecture and functional organization of this large axonal network are poorly understood. Using a large in vitro slice preparation, we examined the functional organization and spike timing properties of the descending layer V axonal network. We found that most, if not all, layer V neurons in this region can form multiple axonal pathways that project to many brain structures, both proximal and remote. The conduction velocities of different axonal pathways are highly diverse and can vary up to more than threefold. Nevertheless for those axonal projections on the ipsilateral side, the speeds of axonal conduction appear to be tuned to their length. As such, spike delivery becomes nearly isochronic along these pathways regardless of projection distance. In contrast, axons projecting to the contralateral hemisphere are significantly slower and do not participate in this lateralized isochronicity. These structural and functional features of layer V network from the ventral temporal lobe may play an important role in top-down control of sensory cue processing and attention.

Role of corticofugal feedback in hearing

The auditory system consists of the ascending and descending (corticofugal) systems. The corticofugal system forms multiple feedback loops. Repetitive acoustic or auditory cortical electric stimulation activates the cortical neural net and the corticofugal system and evokes cortical plastic changes as well as subcortical plastic changes. These changes are short-term and are specific to the properties of the acoustic stimulus or electrically stimulated cortical neurons. These plastic changes are modulated by the neuromodulatory system. When the acoustic stimulus becomes behaviorally relevant to the animal through auditory fear conditioning or when the cortical electric stimulation is paired with an electric stimulation of the cholinergic basal forebrain, the cortical plastic changes become larger and long-term, whereas the subcortical changes stay short-term, although they also become larger. Acetylcholine plays an essential role in augmenting the plastic changes and in producing long-term cortical changes. The corticofugal system has multiple functions. One of the most important functions is the improvement and adjustment (reorganization) of subcortical auditory signal processing for cortical signal processing.

Modulation of the receptive fields of midbrain neurons elicited by thalamic electrical stimulation through corticofugal feedback

The ascending and descending projections of the central auditory system form multiple tonotopic loops. This study specifically examines the tonotopic pathway from the auditory thalamus to the auditory cortex and then to the auditory midbrain in mice. We observed the changes of receptive fields in the central nucleus of the inferior colliculus of the midbrain evoked by focal electrical stimulation of the ventral division of the medial geniculate body of the thalamus. The receptive field of an auditory neuron was characterized by five parameters: the best frequency, minimum threshold, bandwidth, size of receptive field, and average spike number. We found that focal thalamic stimulation changed the parametric values characterizing the recorded collicular receptive fields toward those characterizing the stimulated thalamic receptive fields. Cortical inactivation with muscimol prevented the development of the collicular plasticity induced by focal thalamic stimulation. Our data suggest that the intact colliculo-thalamo-cortico-collicular loops are important for the coordination of sound-guided plasticity in the central auditory system.

Projections from auditory cortex contact ascending pathways that originate in the superior olive and inferior colliculus

The superior olivary complex (SOC) and inferior colliculus (IC) are targets of cortical projections as well as sources of major ascending auditory pathways. This study examines whether the cortical projections contact cells in the SOC or IC that project to higher levels. First, we placed an anterograde tracer into the auditory cortex to label cortico-olivary axons and a retrograde tracer into the IC to label olivocollicular cells in guinea pigs. Cortical axons contacted many labeled cells in the ipsilateral SOC and fewer labeled cells in the contralateral SOC. Contacted cells projected to the ipsilateral or contralateral IC. In a second experiment, we labeled corticocollicular axons with an anterograde tracer and injected retrograde tracers into the medial geniculate (MG) to label colliculogeniculate cells. In the IC ipsilateral to the cortical injection, many cortical axons contacted colliculogeniculate cells in the dorsal cortex and external cortex of the IC. The contacted cells projected to the ipsilateral MG or, less often, to the contralateral MG. The results indicate that cortical projections are likely to contact cells in the SOC and IC that project to higher centers. This suggests that auditory cortex can modulate the ascending auditory pathways at multiple levels of the brainstem.

Corticofugal modulation of multi-parametric auditory selectivity in the midbrain of the big brown bat

Corticofugal modulation of sub-cortical auditory selectivity has been shown previously in mammals for frequency, amplitude, time, and direction domains in separate studies. As such, these studies do not show if multi-parametric corticofugal modulation can be mediated through the same sub-cortical neuron. Here we specifically studied corticofugal modulation of best frequency (BF), best amplitude (BA), and best azimuth (BAZ) at the same neuron in the inferior colliculus of the big brown bat, Eptesicus fuscus, using focal electrical stimulation in the auditory cortex. Among 53 corticofugally inhibited collicular neurons examined, cortical electrical stimulation produced a shift of all three measurements (i.e., BF, BA, and BAZ) toward the value of stimulated cortical neuron in 13 (24.5%) neurons, two measurements (i.e., BF and BAZ or BA and BAZ) in 19 (36%) neurons, and one measurement in 16 (30%) neurons. Cortical electrical stimulation did not shift any of these measurements in the remaining five (9.5%) neurons. Corticofugally induced collicular BF shift was symmetrical, whereas the shift in collicular BA or BAZ was asymmetrical. The amount of shift in each measurement was significantly correlated with each measurement difference between recorded collicular and stimulated cortical neurons. However, shifts of three measurements were not correlated with each other. Furthermore, average measurement difference between collicular and cortical neurons was larger for collicular neurons with measurement shifts than for those without shifts. These data indicate that multi-parametric corticofugal modulation can be mediated through the same subcortical neuron based on the difference in auditory selectivity between subcortical and cortical neurons.

Some investigations into non-passive listening

Our knowledge of the function of the auditory nervous system is based upon a wealth of data obtained, for the most part, in anaesthetised animals. More recently, it has been generally acknowledged that factors such as attention profoundly modulate the activity of sensory systems and this can take place at many levels of processing. Imaging studies, in particular, have revealed the greater activation of auditory areas and areas outside of sensory processing areas when attending to a stimulus. We present here a brief review of the consequences of such non-passive listening and go on to describe some of the experiments we are conducting to investigate them. In imaging studies, using fMRI, we can demonstrate the activation of attention networks that are non-specific to the sensory modality as well as greater and different activation of the areas of the supra-temporal plane that includes primary and secondary auditory areas. The profuse descending connections of the auditory system seem likely to be part of the mechanisms subserving attention to sound. These are generally thought to be largely inactivated by anaesthesia. However, we have been able to demonstrate that even in an anaesthetised preparation, removing the descending control from the cortex leads to quite profound changes in the temporal patterns of activation by sounds in thalamus and inferior colliculus. Some of these effects seem to be specific to the ear of stimulation and affect interaural processing. To bridge these observations we are developing an awake behaving preparation involving freely moving animals in which it will be possible to investigate the effects of consciousness (by contrasting awake and anaesthetized), passive and active listening.

Corticofugal modulation of acoustically induced Fos expression in the rat auditory pathway

To investigate the corticofugal modulation of acoustic information ascending through the auditory pathway of the rat, immunohistochemical techniques were used to study the functional expression of Fos protein in neurons. With auditory stimulation at different frequencies, Fos expression in the medial geniculate body (MGB), inferior colliculus (IC), superior olivary complex, and cochlear nucleus was examined, and the extent of Fos expression on the two sides was compared. Strikingly, we found densely Fos-labeled neurons in all divisions of the MGB after both presentation of an auditory stimulus and administration of a gamma-aminobutyric acid type A (GABA(A)) antagonist (bicuculline methobromide; BIM) to the auditory cortex. The location of Fos-labeled neurons in the ventral division (MGv) after acoustic stimulation at different frequencies was in agreement with the known tonotopic organization. That no Fos-labeled neurons were found in the MGv with acoustic stimuli alone suggests that the transmission of ascending thalamocortical information is critically governed by corticofugal modulation. The dorsal (DCIC) and external cortices (ECIC) of the IC ipsilateral to the BIM-injected cortex showed a significantly higher number of Fos-labeled neurons than the contralateral IC. However, no difference in the number of Fos-labeled neurons was found between the central nucleus of the IC on either side, indicating that direct corticofugal modulation occurs only in the ECIC and DCIC. Further investigations are needed to assess the functional implications of the morphological differences observed between the descending corticofugal projections to the thalamus and the IC.

The ferret auditory cortex: descending projections to the inferior colliculus

Descending corticofugal projections are thought to play a critical role in shaping the responses of subcortical neurons. Here, we examine the origins and targets of ferret auditory corticocollicular projections. We show that the ectosylvian gyrus (EG), where the auditory cortex is located, can be subdivided into middle, anterior, and posterior regions according to the pattern of cytochrome oxidase staining and immunoreactivity for the neurofilament antibody SMI32. Injection of retrograde tracers in the inferior colliculus (IC) labeled large layer V pyramidal cells throughout the EG and adjacent sulci. Each region of the EG has a different pattern of descending projections. Neurons in the primary auditory fields in the middle EG project to the lateral nucleus (LN) of the ipsilateral IC and bilaterally to the dorsal cortex and dorsal part of the central nucleus (CN). The projection to these dorsomedial regions of the IC is predominantly ipsilateral and topographically organized. The secondary cortical fields in the posterior EG target the same midbrain areas but exclude the CN of the IC. A smaller projection to the ipsilateral LN also arises from the anterior EG, which is the only region of auditory cortex to target tegmental areas surrounding the IC, including the superior colliculus, periaqueductal gray, intercollicular tegmentum, and cuneiform nucleus. This pattern of corticocollicular connectivity is consistent with regional differences in physiological properties and provides another basis for subdividing ferret auditory cortex into functionally distinct areas.

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

Duration selectivity of auditory neurons plays an important role in sound recognition. Previous studies show that GABA-mediated duration selectivity of neurons in the central nucleus of the inferior colliculus (IC) of many animal species behave as band-, short-, long- and all-pass filters to sound duration. The present study examines the organization of duration selectivity of IC neurons of the big brown bat, Eptesicus fuscus, in relation to graded spatial distribution of GABA(A) receptors, which are mostly distributed in the dorsomedial region of the IC but are sparsely distributed in the ventrolateral region. Duration selectivity of IC neuron is studied before and during iontophoretic application of GABA and its antagonist, bicuculline. Bicuculline application decreases and GABA application increases duration selectivity of IC neurons. Bicuculline application produces more pronounced broadening of the duration tuning curves of neurons at upper IC than at deeper IC but the opposite is observed during GABA application. The best duration of IC neurons progressively lengthens and duration selectivity decreases with recording depth both before and during drug application. As such, low best frequency neurons at upper IC have shorter best duration and sharper duration selectivity than high best frequency neurons in the deeper IC have. These data suggest that duration selectivity of IC neurons systematically varies with GABA(A) receptor distribution gradient within the IC.

Stimulus-dependent auditory tuning results in synchronous population coding of vocalizations in the songbird midbrain

Physiological studies in vocal animals such as songbirds indicate that vocalizations drive auditory neurons particularly well. But the neural mechanisms whereby vocalizations are encoded differently from other sounds in the auditory system are unknown. We used spectrotemporal receptive fields (STRFs) to study the neural encoding of song versus the encoding of a generic sound, modulation-limited noise, by single neurons and the neuronal population in the zebra finch auditory midbrain. The noise was designed to match song in frequency, spectrotemporal modulation boundaries, and power. STRF calculations were balanced between the two stimulus types by forcing a common stimulus subspace. We found that 91% of midbrain neurons showed significant differences in spectral and temporal tuning properties when birds heard song and when birds heard modulation-limited noise. During the processing of noise, spectrotemporal tuning was highly variable across cells. During song processing, the tuning of individual cells became more similar; frequency tuning bandwidth increased, best temporal modulation frequency increased, and spike timing became more precise. The outcome was a population response to song that encoded rapidly changing sounds with power and precision, resulting in a faithful neural representation of the temporal pattern of a song. Modeling responses to song using the tuning to modulation-limited noise showed that the population response would not encode song as precisely or robustly. We conclude that stimulus-dependent changes in auditory tuning during song processing facilitate the high-fidelity encoding of the temporal pattern of a song.

Gentamicin abolishes all cochlear effects of electrical stimulation of the inferior colliculus

Electrical stimulation of the inferior colliculus (IC) has been shown to result in suppression of cochlear output, due to activation of the medial olivocochlear system. This auditory efferent system originates in the brainstem and terminates on the outer hair cells in the cochlea. Recently, excitatory effects of IC stimulation have also been reported, both on cochlear gross potentials and on primary auditory afferents. It has been hypothesized that this excitation is due to co-activation of the lateral olivocochlear system, which synapses on the primary auditory afferent fibres contacting the inner hair cells. If stimulation of the IC leads to the activation of both the medial and lateral olivocochlear system, resulting in a mixture of inhibitory and excitatory effects in the cochlea, then removal of the inhibitory effects, by blocking the medial system, should lead to more pronounced excitatory effects out in the periphery. To investigate this hypothesis, we recorded the effect of IC stimulation on cochlear gross potentials as well as on single auditory primary afferents in guinea pigs following block of the medial olivocochlear system with gentamicin. We found that administration of gentamicin, whether intraperitoneally or by intracochlear perfusion, blocked all effects of IC stimulation, whether inhibitory or excitatory. These data strongly suggest that all effects observed after IC stimulation, both inhibitory as well as excitatory, are due to the activation of the medial olivocochlear system.

Corticofugal modulation of directional sensitivity in the midbrain of the big brown bat, Eptesicus fuscus

In our recent study of corticofugal modulation of collicular amplitude sensitivity of the big brown bat, Eptesicus fuscus, we suggested that the corticofugal modulation is based upon the best frequency (BF) differences and the relative amplitude sensitivity difference between collicular (IC) and cortical (AC) neurons but not the absolute amplitude sensitivity of IC and AC neurons. To show that corticofugal modulation is systematic and multiparametric, we studied corticofugal modulation of directional sensitivity in 89 corticofugally inhibited IC neurons in the same bat species under free field stimulation conditions. A neuron's directional sensitivity was expressed with the azimuthal range (AR) at 50% below the maximum of each directional sensitivity curve and the best azimuth (BAZ) at which the neuron discharged maximally. Cortical electrical stimulation did not affect the directional sensitivity of 40 (45%) neurons with BF(IC-AC) differences of 7.3+/-4.4kHz but sharpened the directional sensitivity of other 49 (55%) neurons with BF(IC-AC) differences of 2.3+/-1.8kHz. Corticofugal modulation sharpened directional sensitivity curves of IC neurons by decreasing the AR and shifting collicular BAZ toward cortical BAZ. The decrease in AR and the shift in BAZ increased significantly with AR(IC-AC) and BAZ(IC-AC) differences but not with absolute AR and BAZ of IC and AC neurons or BF(IC-AC) differences. Corticofual modulation also shifted collicular BF toward cortical BF. The shift in BF increased significantly with BF(IC-AC) differences but not with the BF of IC and AC neurons or BAZ shift. Consonant with our previous study, these data indicate that corticofugal modulation of collicular directional sensitivity is based on topographic projections between the IC and the AC and the difference in directional sensitivity but not the absolute directional sensitivity of IC or AC neurons.

Corticofugal feedback for auditory midbrain plasticity elicited by tones and electrical stimulation of basal forebrain in mice

The auditory cortex (AC) is the major origin of descending auditory projections and is one of the targets of the cholinergic basal forebrain, nucleus basalis (NB). In the big brown bat, cortical activation evokes frequency-specific plasticity in the inferior colliculus and the NB augments this collicular plasticity. To examine whether cortical descending function and NB contributions to collicular plasticity are different between the bat and mouse and to extend the findings in the bat, we induced plasticity in the central nucleus of the mouse inferior colliculus by a tone paired with electrical stimulation of the NB (hereafter referred to as tone-ES(NB)). We show here that tone-ES(NB) shifted collicular best frequencies (BFs) towards the frequency of the tone paired with ES(NB) when collicular BFs were different from tone frequency. The shift in collicular BF was linearly correlated to the difference between collicular BFs and tone frequencies. The changes in collicular BFs after tone-ES(NB) were similar to those found in the big brown bat. Compared with cortical plasticity evoked by tone-ES(NB), the pattern of collicular BF shifts was identical but the shifting range of collicular BFs was narrower. A GABA(A) agonist (muscimol) or a muscarinic acetylcholine receptor antagonist (atropine) applied to the AC completely abolished the collicular plasticity evoked by tone-ES(NB). Therefore, our findings strongly suggest that the AC plays a critical role in experience-dependent auditory plasticity through descending projections.

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

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

Unilateral and bilateral projections from cortical cells to the inferior colliculus in guinea pigs

Auditory cortex projects directly and bilaterally to the inferior colliculus (IC). We used multiple fluorescent retrograde tracers to determine whether individual cortical cells project to both the left and right IC. Injection of different tracers into each IC labeled many cells in a sheet that extended throughout much of temporal cortex in both hemispheres. Most cells contained a single tracer, with the majority of these labeled from the ipsilateral IC. Numerous double-labeled cells were observed throughout the same areas of temporal cortex. The double-labeled cells form a small percentage of the cortical cells that project to the ipsilateral IC (6.1% on average) and a much larger percentage of the cells that project to the contralateral IC (46.4% on average). Unilaterally projecting cells are well positioned to have effects limited to one IC, whereas bilaterally projecting cells are likely to have a broader influence and may coordinate activity on the two sides of the midbrain.