Current Research in Neurobiology

The role of subcortical structures in binaural integration is of great interest for auditory processing. The inferior colliculus (IC) is the main auditory midbrain center where ascending and descending auditory projections converge, which was suggested to encode auditory information via a push-pull mechanism between the two ICs. However, the origin of this push-pull mechanism in the brain and how it interacts with other upstream/downstream subcortical areas is still a matter of great debate. Here, we harness functional MRI (fMRI) in combination with IC lesions in the rat to dissect the push-pull interaction from a pathway-wide perspective. We find evidence for the push-pull mechanism in IC through negative/positive fMRI signals in the ipsilateral/contralateral ICs upon monaural stimulation. By unilaterally lesioning the corresponding contralateral IC, we demonstrate the necessity of collicular integrity and intercollicular interactions for the push-pull interaction. Using binaural stimulation and IC lesions, we show that the push-pull interaction is exerted also in binaural processing. Finally, we demonstrate that, at least at the population level revealed by fMRI, the main push-pull interactions occur first at the IC level, and not earlier, and that the outcome of the push-pull "calculation" is relayed downstream to MGB. This dissection of the push-pull interaction sheds light into subcortical auditory function.

The emergent coherent population activity from thousands of stochastic neurons in the brain is believed to constitute a key neuronal mechanism for salient processing of external stimuli and its link to internal states like attention and perception. In the sensory cortex, functional cell assemblies are formed by recurrent excitation and inhibitory influences. The stochastic dynamics of each cell involved is largely orchestrated by presynaptic CAV2.1 voltage-gated calcium channels (VGCCs). Cav2.1 VGCCs initiate the release of neurotransmitters from the presynaptic compartment and are therefore able to add variability into synaptic transmission which can be partly explained by their mobile organization around docked vesicles. To investigate the relevance of Cav2.1 channel surface mobility for the input processing in the primary auditory cortex (A1) in vivo, we make use of a new optogenetic system which allows us to acutely cross-link Cav2.1 VGCCs via a photo-cross-linkable cryptochrome mutant, CRY2olig. In order to map neuronal activity across all cortical layers of the A1, we performed laminar current-source density (CSD) recordings with varying auditory stimulus sets in transgenic mice with a citrine tag on the N-terminus of the VGCCs. Clustering VGCCs suppresses overall sensory-evoked population activity, particularly when stimuli lead to a highly synchronized distribution of synaptic inputs. Our findings reveal the importance of membrane dynamics of presynaptic calcium channels for sensory encoding by dynamically adjusting network activity across a wide range of synaptic input strength. Statement of SignificanceVoltage Gated Calcium Channel (VGCC) mobility plays an important role in neuronal firing dynamics. Failure of these channels to function or be regulated has been linked to migraine and ataxia. We here link the microscopic process of VGCC mobility to the mesoscopic population dynamics as a mechanism to regulate and appropriately amplify synaptic inputs of different strengths to the mouse primary auditory cortex. We also demonstrate a novel and effective technique with which VGCC function can be further explored in meso- or macroscopic scales and with behaving subjects. We believe that this is of importance to the broader scientific community in aspects of non-linear scaling in the brain, potential translational applications, and basic research on cortical mechanisms of physiological function.

The brains of black 6 mice (Mus musculus) and Sebas short-tailed bats (Carollia perspicillata) weigh roughly the same and share mammalian neocortical laminar architecture. Bats have highly developed sonar calls and social communication and are an excellent neuroethological animal model for auditory research. Mice are olfactory and somatosensory specialists, used frequently in auditory neuroscience for their advantage of standardization and wide genetic toolkit. This study presents an analytical approach to overcome the challenge of inter-species comparison with existing data. In both data sets, we recorded with linear multichannel electrodes down the depth of the primary auditory cortex (A1) while presenting repetitive stimuli trains at ~5 and ~40 Hz to awake bats and mice. We found that while there are similarities between cortical response profiles in both, there was a better signal to noise ratio in bats under these conditions, which allowed for a clearer following response to stimuli trains. Model fit analysis supported this, illustrating that bats had stronger response amplitude suppression to consecutive stimuli. Additionally, continuous wavelet transform revealed that bats had significantly stronger power and phase coherence during stimulus response and mice had stronger power in the background. Better signal to noise ratio and lower intertrial phase variability in bats could represent specialization for faster and more accurate temporal processing at lower metabolic costs. Our findings demonstrate a potentially different general auditory processing principle; investigating such differences may increase our understanding of how the ecological need of a species shapes the development and function of its nervous system.

So far, our understanding on the role of the auditory cortex (ACX) in processing visual information has been limited to infragranular layers of the ACX, which have been shown to respond to visual stimulation. Here, we investigate the neurons in supragranular layers of the mouse ACX using 2-photon calcium imaging. Contrary to previous reports, here we show that more than 20% of responding neurons in layer2/3 of the ACX respond to full-field visual stimulation. These responses occur by both excitation and hyperpolarization. The primary ACX (A1) has a greater proportion of visual responses by hyperpolarization compared to excitation likely driven by inhibitory neurons of the infragranular layers of the ACX rather than local layer 2/3 inhibitory neurons. Further, we found that more than 60% of neurons in the layer 2/3 of A1 are multisensory in nature. We also show the presence of multisensory neurons in close proximity to exclusive auditory neurons and that there is a reduction in the noise correlations of the recorded neurons during multisensory presentation. This is evidence in favour of deep and intricate visual influence over auditory processing. The results have strong implications for decoding visual influences over the early auditory cortical regions. Significance statementTo understand, what features of our visual world are processed in the auditory cortex (ACX), understanding response properties of auditory cortical neurons to visual stimuli is important. Here, we show the presence of visual and multisensory responses in the supragranular layers of the ACX. Hyperpolarization to visual stimulation is more commonly observed in the primary ACX. Multisensory stimulation results in suppression of responses compared to unisensory stimulation and an overall decrease in noise correlation in the primary ACX. The close-knit architecture of these neurons with auditory specific neurons suggests the influence of non-auditory stimuli on the auditory processing.

Loud noise-exposure generates tinnitus in both humans and animals. Macroscopic studies show that noise exposure affects the auditory cortex; however, cellular mechanisms of tinnitus generation are unclear. Here we compare membrane properties of layer 5 (L5) pyramidal cells (PCs) of the primary auditory cortex (A1) from control and noise-exposed mice. PCs were previously classified in type A or type B based on connectivity and firing properties. Our analysis based on a logistic regression model predicted that afterhyperpolatization and afterdepolarization following the injection of inward and outward current are enough to predict cell type and these features are preserved after noise trauma. One week after a noise-exposure (4-18kHz, 90dB, 1.5 hr, followed by 1.5hr silence) no passive membrane properties of type A or B PCs were altered but principal component analysis showed greater separation between control/noise-exposure recordings for type A neurons. When comparing individual firing properties, noise exposure differentially affected type A and B PC firing frequency in response to depolarizing current steps. Specifically, type A PCs decreased both initial and steady state firing frequency and type B PCs significantly increased steady state firing frequency following noise exposure. These results show that loud noise can cause distinct effects on type A and B L5 auditory cortex PCs one week following noise exposure. As the type A PC electrophysiological profile is correlated to corticofugal L5 neurons, and type B PCs correlate to contralateral projecting PCs these alterations could partially explain the reorganization of the auditory cortex observed in tinnitus patients.

The wide diversity of cortical inhibitory neuron types populating the cortex allows the assembly of diverse microcircuits and endows these circuits with different computational properties. Thus, characterizing neuronal diversity is fundamental to describe the building blocks of cortical microcircuits and probe their function. To this purpose, the mouse has emerged as a powerful tool to genetically label and manipulate specific inhibitory cell-types in the mammalian brain. Among these cell-types, the parvalbumin-expressing interneuron type (PV-INs) is perhaps the most characterized. Several mouse lines have been generated to target PV-INs. Among these mouse lines, the PV-IRES-Cre lines is the most widely used and demonstrated a high specificity and efficiency in targeting PV-INs in different cortical areas. However, a characterization of the performance across cortical regions is still missing. Here we show that the PV-IRES-Cre mouse line labels only a fraction of parvalbumin immunoreactive neurons in perirhinal cortex and other association areas. Our results point to a yet uncharacterized diversity within the PV-INs and emphasize the need to characterize these tools in specific cortical areas.

Gene delivery via adeno-associated viruses (AAVs) is a valuable tool for understanding the organization and function of auditory neuronal circuitry. In this study, we explored the use of AAVs injected into the inner ear to transfect the lateral olivocochlear (LOC) pathway, a poorly understood ipsilateral component of the auditory efferent feedback system. AAV serotypes exhibit a wide range of properties, including cell type selectivity and the capacity for retrograde transport, but none of the AAV serotypes used for inner ear delivery have been reported to successfully transfect the LOC pathway. Here, we show that unilateral inner ear delivery of AAV2-retro robustly labeled the LOC pathway. However, the ability of AAV2-retro to diffuse via the cochlear aqueduct from the perilymph into the cerebrospinal fluid resulted in off-target transfection in the brain and the labeling of LOC projections in the contralateral cochlea. The extent of contralateral transfection of LOC neurons depended on the age of the animal and the amount of virus delivered to the inner ear. These findings highlight the importance of considering the dosage and properties of AAV serotype when interpreting experimental results.

Evasion from imminent threats and prey attack are opposite behavioral choices critical to survival. Curiously, the lateral periaqueductal gray (LPAG) has been implicated in driving both responses. The LPAG responds to social threats and prey hunting while also drives predatory attacks and active defense. However, the LPAG neural mechanisms mediating these behaviors remain poorly defined. Here, we investigate how the LPAG mediates the choices of predatory hunting and evasion from a social threat. Pharmacogenetic inhibition in Fos DD-Cre mice of neurons responsive specifically to insect predation (IP) or social defeat (SD) revealed that distinct neuronal populations in the LPAG drive the prey hunting and evasion from social threats. We show that the LPAG provides massive glutamatergic projection to the lateral hypothalamic area (LHA). Optogenetic inhibition of the LPAG-LHA pathway impaired IP but did not alter escape/attack ratio during SD. We also found that pharmacogenetic inhibition of LHAGABA neurons impaired IP, but did not change evasion during SD. The results suggest that the LPAG control over evasion to a social attack may be regarded as a stereotyped response depending probably on glutamatergic descending projections. On the other hand, the LPAG control over predatory behavior involves an ascending glutamatergic pathway to the LHA that likely influences LHAGABA neurons driving predatory attack and prey consumption. The LPAG-LHA path supposedly provides an emotional drive for prey hunting and, of relevance, may conceivably have more widespread control on the motivational drive to seek other appetitive rewards.

The functional organization of sensory cortices is modulated by both extrinsic events and intrinsic states. The present study aimed to assess the mouse auditory cortex (AC) responses under varying conditions of stimulus complexity and brain state. Using wide-field calcium imaging, our results suggest a complete outline of topographic maps of frequency and FM rate as well as highly responsive regions to mouse ultrasonic vocalizations (USVs) in both awake and anesthetized states. Three new regions responsive to high-frequency tones and four new gradients responsive to frequency modulations (FMs) were identified. These maps are highly replicable across weeks and between animals. In awake versus anesthetized states, cortical responsiveness to pure tones was stronger, and regions that preferentially responded to slow rate FMs were smaller. In both states, fast FM regions showed the greatest contribution to the processing of USVs. Finally, our modeling of how best tone frequency or FM rate changes as a function of distance along a topographic gradient resulted in a sigmoid function. Together, our findings provide a better understanding of mouse AC functional organization and how this organization is modulated by changes in stimulus complexity and brain state. The function of newly identified regions in higher-order auditory/vocal processing and animal behavior should be considered in future research.

The ability to communicate through vocalization plays a key role in the survival of animals across all vertebrate groups. While avian reptiles have received much attention relating to their stunning sound repertoire, non-avian reptiles have been wrongfully assumed to have less elaborate vocalization types and little is known about the biomechanics of sound production and their underlying neural pathways. We investigated alarm calls of Gekko gecko using audio and cineradiographic recordings of their alarm calls. Acoustic analysis revealed three distinct call types: a sinusoidal call type (type 1), a train-like call type, characterized by distinct pulse trains (type 3), and an intermediary type, which showed both sinusoidal and pulse train components (type 2). Kinematic analysis of cineradiographic recordings showed that laryngeal movements differ significantly between respiratory and vocal behavior: during respiration, animals repeatedly moved their jaws to partially open their mouths, which was accompanied by small glottal movements. During vocalization, the glottis was pulled back, contrasting with what has previously been reported. In-vitro retrograde tracing of the nerve innervating the laryngeal constrictor and dilator muscles revealed round to fusiform motoneurons in the hindbrain-spinal cord transition ipsilateral to the labeled nerve. Taken together, our observations provide insight into the alarm calls generated by G. gecko, the biomechanics of this sound generation and the underlying organization of motoneurons involved in the generation of vocalizations. Our observations suggest that G. gecko may be an excellent non-avian reptile model organism for enhancing our understanding of the evolution of vertebrate vocalization. Summary StatementInvestigation of Gekko gecko alarm calls revealed distinct call types, during which the larynx is being pulled back by muscles innervated by motoneurons located in the hindbrain.

To better understand the cortical connections and organization of visual areas in galagos, we examined the interconnections among cortical visual fields and their relationships with posterior parietal cortex (PPC) and temporal regions associated with dorsal and ventral streams of visual processing. In five galagos, two to four distinguishable tracers were injected into different visual areas, allowing direct comparison of connection patterns within the same cases. To reveal distributions of labeled neurons for each injection, labeled cells were plotted from serial brain sections cut parallel to the flattened cortical surface and summed across sections to generate surface reconstructions. Alternate sections processed for cytoarchitectonic features were used to identify cortical borders, especially those of V1 and middle temporal visual area (MT). Overall, our results support the conclusion that regions of V2 and V3 represent the contralateral visual hemifield in parallel with V1 and with each other. However, dorsal V3, representing the lower visual hemifield, includes at least one discontinuity where representations of the upper visual field extend to the dorsal border of V2. This portion of V3 appears to belong to the dorsomedial visual area (DM), which extends rostrally from V2 into PPC. The dorsal part of the DL-V4 region receives projections from other parts of dorsolateral visual area (DL), central V1, V2 and V3, inferotemporal (IT) cortex, the MT complex, and PPC regions surrounding the intraparietal sulcus (IPS). More central portions of DL-V4 receive inputs from central representations of V1, V2, and V3, as well as from PPC regions lateral to the IPS, the MT complex, and upper IT cortex. The ventral part of DL receives projections from central V2, caudal PPC adjoining DM and ventral PPC, and from IT cortex. These patterns indicate that the DL-V4 region serves as a major node linking dorsal and ventral streams and likely includes more than one functionally distinct visual area. In addition, areas MT and DM show strong reciprocal connections with PPC, while the connections of IT cortex indicate that much of this region is visual in nature having strong connections with higher order visual areas and it is composed of multiple functionally specialized visual domains. Key pointsO_LIOrganization of the visual cortex in galagos is much like that in New World and Old Word monkeys. C_LIO_LIPatterns of cortical connections of early visual areas V1 and V2 support the view that dorsal V3 has a gap in the representation of the lower visual field that is occupied by the proposed dorsomedial visual area (DM). C_LIO_LIVisual areas DM and middle temporal visual area (MT) provide the major visual inputs to posterior parietal cortex (PPC) of the dorsal stream of visual processing for actions. C_LI

In the rodent whisker system, it is well assumed that VPM and POm encode stimulations of the contralateral whisker pad. However, during tactile exploration whiskers are usually stimulated bilaterally. Accordingly, the integration of tactile information from the two sides of the body seems to be fundamental in the processing of these events. Here, to investigate whether POm could be able to codify these bilateral dynamics, whisker-evoked responses in this thalamic nucleus were examined by in vivo extracellular recordings in anesthetized rats using contralateral and ipsilateral stimuli. Strikingly, we found that POm is also able to respond to tactile stimulation of ipsilateral whiskers. Our findings reveal the implication of POm in the representation of bilateral tactile events by integrating simultaneous signals arising from both whisker pads and demonstrate the implication of the higher-order sensory thalamus in the encoding of bilateral sensory events. This can have important implications in bilateral perceptual function.

The discovery and development of electrocochleography (ECochG) in animal models has been fundamental for its implementation in clinical audiology and neurotology. In our laboratory, the use of round-window ECochG recordings in chinchillas has allowed a better understanding of auditory efferent functioning. In previous works, we gave evidence of the corticofugal modulation of auditory-nerve and cochlear responses during visual attention and working memory. However, whether these cognitive top-down mechanisms to the most peripheral structures of the auditory pathway are also active during audiovisual crossmodal stimulation is unknown. Here, we introduce a new technique, wireless ECochG to record compound-action potentials of the auditory nerve (CAP), cochlear microphonics (CM), and round-window noise (RWN) in awake chinchillas during a paradigm of crossmodal (visual and auditory) stimulation. A total of four chinchillas were successfully recorded during the experimental protocol. There were non-significant differences in CAP, CM, and RWN amplitudes in response to auditory stimulation alone (clicks and tones) as compared to audio-visual crossmodal stimulation. These results suggest that cognition, such as attention or working memory, is needed for the corticofugal modulation of auditory-nerve and cochlear responses. In addition, we introduce the use of wireless ECochG in animal models as a useful tool for translational research.

Mastication is a vital function that relies on precise synchronization among multiple brainstem regions, known as being part of a central pattern generator (CPG). Movements can be triggered either by stimulating a sensory-motor region called the cortical masticatory area (CMA), well-documented in various species but not yet formally identified in mice, or by stimulating the oro-facial sensory primary afferents which fibers form the trigeminal tract (Vtr). However, its unclear whether these different inputs activate distinct components of the CPG or converge on the same. This study aims at mapping brainstem areas activated by cortical and sensory inputs using immunohistochemistry against the cellular activity marker, c-Fos and Ca2+-imaging, respectively. Optogenetic stimulation of the cortical masticatory area (CMA) in awake, head-fixed mice reliably induced rhythmic jaw movements (RJMs) and increased c-Fos expression in multiple brainstem regions, with strongest activation in the peritrigeminal area (PeriV) and parvocellular reticular formation (PCRt) ventral to the trigeminal motor nucleus (NVmt). In contrast, in vitro electrical stimulation of trigeminal sensory afferents (Vtr) predominantly activated neurons and astrocytes in the main sensory nucleus (NVsnpr), the dorsal area of PeriV, adjacent to it, and PCRt. The areas containing the highest numbers of activated cells differed clearly according to the type of inputs and overlapped only in the PCRt, ventral and slightly medial to the trigeminal motor nucleus and the most dorso-medial part of PeriV. These findings demonstrate that cortical and sensory inputs take part in distinct components of the brainstem masticatory circuitry, with PCRt emerging as a point of convergence and provide new insights into the components of the CPG of mastication.

Neural oscillations are at the core of important computations in the mammalian brain. Interactions between oscillatory activities in different frequency bands, such as delta (1-4 Hz), theta (4-8 Hz), or gamma (>30 Hz), are a powerful mechanism for binding fundamentally distinct spatiotemporal scales of neural processing. Phase-amplitude coupling (PAC) is one such plausible and well-described interaction, but much is yet to be uncovered regarding how PAC dynamics contribute to sensory representations. In particular, although PAC appears to have a major role in audition, the characteristics of coupling profiles in sensory and integration (i.e. frontal) cortical areas remain obscure. Here, we address this question by studying PAC dynamics in the frontal-auditory field (FAF; an auditory area in the bat frontal cortex) and the auditory cortex (AC) of the bat Carollia perspicillata. By means of simultaneous electrophysiological recordings in frontal and auditory cortices examining local-field potentials (LFPs), we show that the amplitude of gamma-band activity couples with the phase of low-frequency LFPs in both structures. Our results demonstrate that the coupling in FAF occurs most prominently in delta/high-gamma frequencies (1-4/75-100 Hz), whereas in the AC the coupling is strongest in the theta/low-gamma (2-8/25-55 Hz) range. We argue that distinct PAC profiles may represent different mechanisms for neuronal processing in frontal and auditory cortices, and might complement oscillatory interactions for sensory processing in the frontal-auditory cortex network.

ObjectiveTo test the effect of Isoflurane on synaptic transmission of cortico-cortical and thalamocortical projections to the auditory cortex, and investigate how it modulates cortical sensory information processing to produce unconsciousness. MethodsUsing murine auditory thalamocortical brain slices, afferent pathways from the medial geniculate body (MGB) and layer 1 of the proximal cortex were stimulated to evoke excitatory postsynaptic potentials (eEPSPs) in cortical neurons. Whole-cell recordings were made from pyramidal and fast-spiking neurons in layer 2/3 and layer 5. eEPSPs were evaluated along with intrinsic membrane properties in response to stimulation of both pathways with and without isoflurane. ResultsIsoflurane administration resulted in significant eEPSP amplitude reduction following stimulation of both thalamic and cortical pathways, in layer 2/3 (p=0.015, p<0.001) and layer 5 (p<0.001, p<0.001) pyramidal neurons; while it only significantly reduced eEPSP amplitude in fast-spiking interneurons with cortical stimulation (p<0.001). Overall, isoflurane preferentially suppressed synaptic responses to cortico-cortical stimulation compared to thalamocortical (p=0.0002). Under isoflurane, cortico-cortical compared to thalamocortical stimulation evoked eEPSPs with reduced 10-90% rise time in both layer 2/3 and 5 pyramidal neurons, and shorter latency layer 5 neurons. Paired pulse ratio was not changed by isoflurane application, although an interesting loss of depression trend appear in layer 5 pyramidal neurons stimulated by cortical activation. Additional intrinsic neuronal measurements revealed that isoflurane reduced spike threshold significantly in both layer 2/3 and layer 5 neurons, reduced spike latency in layer 2/3 neurons, and input resistance in layer 5 neurons. However, these intrinsic neuronal changes were not seen in fast-spiking interneurons. All isoflurane induced changes were reversible during wash out. ConclusionsApplication of 1% isoflurane to brain slices significantly reduced the amplitudes of eEPSPs and modulated intrinsic neuronal properties. The effects on eEPSP amplitude were greater for cortical stimulation compared to thalamic stimulation. Isoflurane modulated intrinsic neuronal firing properties in pyramidal neurons, but not in fast-spiking interneurons.

Both tinnitus and hyperacusis, likely triggered by hearing loss, can be attributed to maladaptive plasticity in auditory perception. However, owing to their co-occurrence, disentangling their neural mechanisms proves difficult. We hypothesized that the neural correlates of tinnitus are associated with neural activities triggered by low-intensity tones, while hyperacusis is linked to responses to moderate- and high-intensity tones. To test these hypotheses, we conducted behavioral and electrophysiological experiments in rats 2 to 8 days after traumatic tone exposure. In the behavioral experiments, prepulse and gap inhibition tended to exhibit different frequency characteristics (although not reaching sufficient statistical levels), suggesting that exposure to traumatic tones resulted in hyperacusis and tinnitus symptoms at different frequency ranges. When examining the auditory cortex at the thalamocortical recipient layer, we observed that tinnitus symptoms correlated with a disorganized tonotopic map, typically characterized by responses to low-intensity tones. Neural correlates of hyperacusis were found in the cortical recruitment function at the multi-unit activity (MUA) level, but not at the local field potential (LFP) level, in response to moderate- and high-intensity tones. This shift from LFP to MUA was associated with a loss of monotonicity, suggesting a crucial role for inhibitory synapses. Thus, in acute symptoms of traumatic tone exposure, our experiments successfully disentangled the neural correlates of tinnitus and hyperacusis at the thalamocortical recipient layer of the auditory cortex. They also suggested that tinnitus is linked to central noise, whereas hyperacusis is associated with aberrant gain control. Further interactions between animal experiments and clinical studies will offer insights into neural mechanisms, diagnosis and treatments of tinnitus and hyperacusis, specifically in terms of long-term plasticity of chronic symptoms.

We have recently reported that the central nucleus of the inferior colliculus (the auditory midbrain) is innervated by glutamatergic pyramidal cells originating not only in auditory cortex (AC) but also in multiple non-auditory regions of the cerebral cortex. Using optogenetics and electrical stimulation, we investigated the functional properties of these descending connections in vivo in anaesthetised rats. A retrograde virus encoding green fluorescent protein (GFP) and channelrhodopsin (ChR2) injected into the central nucleus of the inferior colliculus (ICC), labelled discrete groups of cells in multiple areas of the cerebral cortex. Light stimulation of AC and M1 caused local activation of cortical neurones and increased the firing rate of neurones in ICc indicating a direct excitatory input from AC and M1 to ICC. Electrical stimulation of M1, secondary motor, somatosensory and prefrontal cortical regions evoked short, fixed latency firing events in ICC as well as longer latency, longer duration increases in firing activity. The short latency events were singular spikes of consistent shape and size likely resulting from monosynaptic excitation of individual ICC units. The longer latency responses comprised multiple units and spikes occurred with significant temporal jitter suggesting polysynaptic activation of local circuits within the ICC. The probability of the monosynaptic event, the magnitude of the polysynaptic response, and the area of ICC affected were dependent on the stimulus current. Our data are consistent with cortical regions exerting an important excitatory direct and indirect regulation of ICc neurones. Significance statementWe have recently described inputs from motor, somatosensory, and executive cortices to the inferior colliculus (IC, auditory midbrain). Here we provide functional evidence for such connections. Optogenetics, using a retrograde virus encoding channelrhodopsin injected into IC revealed a direct excitatory influence of neurones in auditory and motor cortices on firing in IC. Electrical stimulation of discrete cortical regions revealed that multiple non-auditory cortical regions have a direct monosynaptic excitatory influence on neurones in the IC which, in turn, activates local circuits increasing the firing probability of multiple neurones in the IC. This is the first evidence for circuitry by which auditory processing can be influenced at an early stage by activity in the sensory, motor and executive domains.

The basolateral amygdala is a non-layered cortical structure playing a role in various cognitive processes. Despite many studies focusing on local information processing within the circuits of the basolateral amygdala, the characteristics of excitatory principal neurons (PNs) are still not fully revealed. Here, we combined neuroanatomical, electrophysiological, and tracing techniques to determine the single-cell features, dendritic and axonal projections of PNs within the lateral (LA) and basal amygdala (BA). Using a mouse reporter line, we found that cholecystokinin (CCK) expression defines two spatially and functionally segregated groups of PNs both in the LA and BA. PNs in CCK-positive (CCK+) areas of the LA (LAa) had small somata and short dendrites which matched their single-cell electrophysiological properties. PNs in CCK-negative (CCK-) areas of the LA (LAp) and all BA had similarly ramified dendrites and single-cell features with some differences. Importantly, the dendritic arbors of PNs were restricted to the subnuclei defined by CCK expression, which corresponded to various extra-amygdalar afferents indicating specific inputs on distinct PN groups. Axonal arborization patterns of PNs within the basolateral amygdala and surrounding areas showed consistency to their soma location. For instance, BA PNs that projected to the medial prefrontal cortex but not to the lateral nucleus of the central amygdala were present in CCK+ areas. In contrast, those BA PNs that projected to the lateral part of the central nucleus were found in the subnucleus lacking CCK. Our study revealed that the basolateral amygdala is composed of functionally different subnuclei with specific inputs and outputs. This structural arrangement may empower the LA and BA to flexibly channel processed information toward their downstream regions, which can be a key requirement for diverse amygdala functions in cognitive operation.

Studies of human perception have shown early cortical signals for primary information encoding, and later signals for higher order processing. An important late signal is the cortical event-related desynchronization (ERD) in the alpha (8-12Hz) and beta (12-30Hz) frequency band, which has been linked to human perceptual awareness. Detailed mechanistic investigation of the ERD would be greatly facilitated by availability of a suitable animal model. We conducted local field potential recordings in the mouse frontal association cortex (FrA), thalamic intralaminar centrolateral nucleus (Cl), primary auditory cortex (A1), and primary visual cortex (V1) during two auditory tasks. Fully audible brief 50 ms stimuli with both tasks produced early broadband gamma (30-100Hz) frequency activity at 0-250ms, followed by a late cortical alpha/beta ERD 250 - 750 ms after stimulus onset. The ERD was statistically significant in FrA and A1, but not in V1. Interestingly, a significant ERD was also observed in thalamic Cl. The magnitude of the ERD at full stimulus intensity, and the slope of the relationship between stimulus intensity versus ERD magnitude, were both largest in FrA, and smaller in Cl and A1. Conversely, for early broadband gamma activity the magnitude at full intensity and slopes were largest in A1, smaller in Cl and smaller still in FrA. These findings suggest that mice, like humans, process perceptual signals in hierarchically organized corticothalamic networks, and strongly support mice as a promising platform for further investigation of the ERD to better understand the origin and function of this robust yet understudied electrophysiological phenomenon. Significance StatementAuditory-induced alpha/beta event-related desynchronizations (ERDs), decreases in cortical activity between 8 and 30Hz following auditory stimulus presentation, are thought to represent systems underlying higher perception and cognition. However, fundamental mechanistic studies are difficult in humans, the dominant organism for studying this phenomenon. In this study we assess mice as a potential alternative model organism. Our results demonstrate that mice exhibit auditory-induced alpha/beta ERDs, that this response is also present in subcortical regions of the mouse brain, and that the cortical ERD is largest and most strongly related to auditory stimulus amplitude in association cortex rather than in primary auditory cortex. These results support the efficacy of mice as an ideal model organism for further examination of alpha/beta ERDs.