Connections of cat auditory cortex: III. Corticocortical system

Quantifying myelin density in the feline auditory cortex

The cerebral cortex comprises many distinct regions that differ in structure, function, and patterns of connectivity. Current approaches to parcellating these regions often take advantage of functional neuroimaging approaches that can identify regions involved in a particular process with reasonable spatial resolution. However, neuroanatomical biomarkers are also very useful in identifying distinct cortical regions either in addition to, or in place of functional measures. For example, differences in myelin density are thought to relate to functional differences between regions, are sensitive to individual patterns of experience, and have been shown to vary across functional hierarchies in a predictable manner. Accordingly, the current study provides quantitative stereological estimates of myelin density for each of the 13 regions that make up the feline auditory cortex. We demonstrate that significant differences can be observed between auditory cortical regions, with the highest myelin density observed in the regions that comprise the auditory core (i.e., the primary auditory cortex and anterior auditory field). Moreover, our myeloarchitectonic map suggests that myelin density varies in a hierarchical fashion that conforms to the traditional model of spatial organization in auditory cortex. Taken together, these results establish myelin as a useful biomarker for parcellating auditory cortical regions, and provide detailed estimates against which other, less invasive methods of quantifying cortical myelination may be compared.

Gray matter volume of the feline cerebral cortex and structural plasticity following perinatal deafness

In response to sensory deprivation, the brain adapts according to contemporary demands to efficiently navigate a modified perceptual environment. This reorganization may result in improved processing of the remaining senses-a phenomenon referred to as compensatory crossmodal plasticity. One approach to explore this neuroplasticity is to consider the macrostructural changes in neural tissue that mirror this functional optimization. The current study is the first of its kind to measure MRI-derived gray matter (GM) volumes of control felines (n=30), while additionally identifying volumetric differences in response to perinatal deafness (30 ototoxically-deafened cats). To accomplish this purpose, regional and morphometric methods were performed in parallel. The regional analysis evaluated volumetric alterations of global GM, as well as the volumes of 146 regions of interest (ROIs) and 12 functional subgroupings of these ROIs. Results revealed whole-brain GM preservation; however, somatosensory and visual cortices exhibited an overall increase in volume. On a smaller scale, this analysis uncovered two auditory ROIs (second auditory cortex, A2, and ventral auditory field, VAF) that decreased in volume alongside two visual regions (anteromedial lateral suprasylvian area, AMLS and splenial visual area, SVA) that increased-all localized within the right hemisphere. Comparatively, the findings of tensor-based morphometry (TBM) generally aligned with those of the ROI-based method, as this voxel-wise approach demonstrated clusters of expansion coincident with visual- and somatosensory-related loci; although, it failed to detect any GM reductions following deafness. As distinct differences were identified in each analysis, the current study highlights the importance of employing multiple methods when exploring MRI volumetry. Overall, this study proposes that volumetric alterations within sensory loci allude to a redistribution of cortical space arising from modified perceptual demands following auditory deprivation.

Mouse auditory cortex sub-fields receive neuronal projections from MGB subdivisions independently

Mouse auditory cortex is composed of six sub-fields: primary auditory field (AI), secondary auditory field (AII), anterior auditory field (AAF), insular auditory field (IAF), ultrasonic field (UF) and dorsoposterior field (DP). Previous studies have examined thalamo-cortical connections in the mice auditory system and learned that AI, AAF, and IAF receive inputs from the ventral division of the medial geniculate body (MGB). However, the functional and thalamo-cortical connections between nonprimary auditory cortex (AII, UF, and DP) is unclear. In this study, we examined the locations of neurons projecting to these three cortical sub-fields in the MGB, and addressed the question whether these cortical sub-fields receive inputs from different subsets of MGB neurons or common. To examine the distributions of projecting neurons in the MGB, retrograde tracers were injected into the AII, UF, DP, after identifying these areas by the method of Optical Imaging. Our results indicated that neuron cells which in ventral part of dorsal MGB (MGd) and that of ventral MGB (MGv) projecting to UF and AII with less overlap. And DP only received neuron projecting from MGd. Interestingly, these three cortical areas received input from distinct part of MGd and MGv in an independent manner. Based on our foundings these three auditory cortical sub-fields in mice may independently process auditory information.

Information flow in the rat thalamo-cortical system: spontaneous vs. stimulus-evoked activities

The interaction between the thalamus and sensory cortex plays critical roles in sensory processing. Previous studies have revealed pathway-specific synaptic properties of thalamo-cortical connections. However, few studies to date have investigated how each pathway routes moment-to-moment information. Here, we simultaneously recorded neural activity in the auditory thalamus (or ventral division of the medial geniculate body; MGv) and primary auditory cortex (A1) with a laminar resolution in anesthetized rats. Transfer entropy (TE) was used as an information theoretic measure to operationalize “information flow”. Our analyses confirmed that communication between the thalamus and cortex was strengthened during presentation of auditory stimuli. In the resting state, thalamo-cortical communications almost disappeared, whereas intracortical communications were strengthened. The predominant source of information was the MGv at the onset of stimulus presentation and layer 5 during spontaneous activity. In turn, MGv was the major recipient of information from layer 6. TE suggested that a small but significant population of MGv-to-A1 pairs was “information-bearing,” whereas A1-to-MGv pairs typically exhibiting small effects played modulatory roles. These results highlight the capability of TE analyses to unlock novel avenues for bridging the gap between well-established anatomical knowledge of canonical microcircuits and physiological correlates via the concept of dynamic information flow.

Plasticity of Multidimensional Receptive Fields in Core Rat Auditory Cortex Directed by Sound Statistics

Sensory cortical neurons can nonlinearly integrate a wide range of inputs. The outcome of this nonlinear process can be approximated by more than one receptive field component or filter to characterize the ensuing stimulus preference. The functional properties of multidimensional filters are, however, not well understood. Here we estimated two spectrotemporal receptive fields (STRFs) per neuron using maximally informative dimension analysis. We compared their temporal and spectral modulation properties and determined the stimulus information captured by the two STRFs in core rat auditory cortical fields, primary auditory cortex (A1) and ventral auditory field (VAF). The first STRF is the dominant filter and acts as a sound feature detector in both fields. The second STRF is less feature specific, preferred lower modulations, and had less spike information compared to the first STRF. The information jointly captured by the two STRFs was larger than that captured by the sum of the individual STRFs, reflecting nonlinear interactions of two filters. This information gain was larger in A1. We next determined how the acoustic environment affects the structure and relationship of these two STRFs. Rats were exposed to moderate levels of spectrotemporally modulated noise during development. Noise exposure strongly altered the spectrotemporal preference of the first STRF in both cortical fields. The interaction between the two STRFs was reduced by noise exposure in A1 but not in VAF. The results reveal new functional distinctions between A1 and VAF indicating that (i) A1 has stronger interactions of the two STRFs than VAF, (ii) noise exposure diminishes modulation parameter representation contained in the noise more strongly for the first STRF in both fields, and (iii) plasticity induced by noise exposure can affect the strength of filter interactions in A1. Taken together, ascertaining two STRFs per neuron enhances the understanding of cortical information processing and plasticity effects in core auditory cortex.

Mismatch-negativity (MMN) in animal models: Homology of human MMN?

Mismatch negativity (MMN) has long been considered to be one of the deviance-detecting neural characteristics. Animal models exhibit similar neural activities, called MMN-like responses; however, there has been considerable debate on whether MMN-like responses are homologous to MMN in humans. Herein, we reviewed several studies that compared the electrophysiological, pharmacological, and functional properties of MMN-like responses and adaptation-exhibiting middle-latency responses (MLRs) in animals with those in humans. Accumulating evidence suggests that there are clear differences between MMN-like responses and MLRs, in particular that MMN-like responses can be distinguished from mere effects of adaptation, i.e., stimulus-specific adaptation. Finally, we discuss a new direction for research on MMN-like responses by introducing our recent work, which demonstrated that MMN-like responses represent empirical salience of deviant stimuli, suggesting a new functional role of MMN beyond simple deviance detection.

Reciprocal connectivity between secondary auditory cortical field and amygdala in mice

Recent studies have examined the feedback pathway from the amygdala to the auditory cortex in conjunction with the feedforward pathway from the auditory cortex to the amygdala. However, these connections have not been fully characterized. Here, to visualize the comprehensive connectivity between the auditory cortex and amygdala, we injected cholera toxin subunit b (CTB), a bidirectional tracer, into multiple subfields in the mouse auditory cortex after identifying the location of these subfields using flavoprotein fluorescence imaging. After injecting CTB into the secondary auditory field (A2), we found densely innervated CTB-positive axon terminals that were mainly located in the lateral amygdala (La), and slight innervations in other divisions such as the basal amygdala. Moreover, we found a large number of retrogradely-stained CTB-positive neurons in La after injecting CTB into A2. When injecting CTB into the primary auditory cortex (A1), a small number of CTB-positive neurons and axons were visualized in the amygdala. Finally, we found a near complete absence of connections between the other auditory cortical fields and the amygdala. These data suggest that reciprocal connections between A2 and La are main conduits for communication between the auditory cortex and amygdala in mice.

Direct Relay Pathways from Lemniscal Auditory Thalamus to Secondary Auditory Field in Mice

Tonotopy is an essential functional organization in the mammalian auditory cortex, and its source in the primary auditory cortex (A1) is the incoming frequency-related topographical projections from the ventral division of the medial geniculate body (MGv). However, circuits that relay this functional organization to higher-order regions such as the secondary auditory field (A2) have yet to be identified. Here, we discovered a new pathway that projects directly from MGv to A2 in mice. Tonotopy was established in A2 even when primary fields including A1 were removed, which indicates that tonotopy in A2 can be established solely by thalamic input. Moreover, the structural nature of differing thalamocortical connections was consistent with the functional organization of the target regions in the auditory cortex. Retrograde tracing revealed that the region of MGv input to a local area in A2 was broader than the region of MGv input to A1. Consistent with this anatomy, two-photon calcium imaging revealed that neuronal responses in the thalamocortical recipient layer of A2 showed wider bandwidth and greater heterogeneity of the best frequency distribution than those of A1. The current study demonstrates a new thalamocortical pathway that relays frequency information to A2 on the basis of the MGv compartmentalization.

Layer-specific representation of long-lasting sustained activity in the rat auditory cortex

In the auditory system, distinct and reproducible transient activities responding to the onset of sound have long been the focus when characterizing the auditory cortex, i.e., tonotopic maps, subregions, and layer-specific representation. There is limited information on sustained activities because the rapid adaptation impairs reproducibility and the signal-to-noise ratio. We recently overcame this problem by focusing on neural synchrony and machine learning demonstrated that band-specific power and the phase locking value (PLV) represent sound information in a tonotopic and region-specific manner. Here, we attempted to reveal the layer-specific representation of sustained activities. A microelectrode array recorded sustained activities from layers 2/3, 4, and 5/6 of the rat auditory cortex. We characterized band-specific power and PLV patterns and applied sparse logistic regression (SLR) to discriminate (1) between the sound-induced and spontaneous activities and (2) five test frequencies from the sound-induced activities in each layer. SLR achieved the highest discrimination performance in high-gamma activities in layers 4 and 5/6, higher than in layer 2/3, indicating poor sound representation in layer 2/3. Moreover, the recording sites that contributed to the discrimination in layers 4 and 5/6 had a characteristic frequency similar to the test frequency and were often located in the belt area, indicating tonotopic and region-specific representation. These results indicate that information processing of sustained activities may depend on high-gamma oscillators, i.e., cortical inhibitory interneurons, and reflects layer-specific thalamocortical and corticocortical neural circuits in the auditory system, which may contribute to associative information processing beyond sound frequency in auditory perception.

In vivo transcranial flavoprotein autofluorescence imaging of tonotopic map reorganization in the mouse auditory cortex with impaired auditory periphery

Ototoxic-drug-induced hearing disturbances in the auditory periphery are associated with tonotopic map reorganization and neural activity modulation, as well as changes in neural correlates in the central auditory pathway, including the auditory cortex (AC). Previous studies have reported that peripheral auditory impairment induces AC plasticity that involves changes in the balance of excitatory vs. inhibitory synapses, within existing and newly forming patterns of connectivity. Although we know that such plastic changes modulate sound-evoked neural responses and the organization of tonotopic maps in the primary AC (A1), little is known about the effects of peripheral impairment on other frequency-organized AC subfields, such as the anterior auditory field (AAF) and the secondary auditory cortex (A2). Therefore, to examine ototoxic-drug-induced spatiotemporal effects on AC subfields, we measured sound-evoked neural activity in mice before and after the administration of kanamycin sulfate (1 mg/g body weight) and bumetanide (0.05 mg/g body weight), using in vivo transcranial flavoprotein autofluorescence imaging over a 4-week period. At first, ototoxic treatment gradually reduced responses driven by tone bursts with lower- (≤8 kHz) and middle- (e.g., 16 kHz) range frequencies in all AC subfields. Subsequently, response intensities in the A1 recovered to more than 78% of the pre-drug condition; however, in the AAF and A2, they remained significantly lower and were unchanged over 3 weeks. Furthermore, after drug administration, the best frequency (BF) areas of the lower (4 and 8 kHz) and higher (25 and 32 kHz) ranges in all subfields were reduced and shifted to those of a middle range (centered around 16 kHz) during the 3 weeks following drug administration. Our results also indicated that, compared with A1, BF distributions in the AAF and A2 were sharper around 16 kHz 3 weeks after drug administration. These results indicate that the ototoxic-damage-induced tonotopic map reorganizations that occurred in each of the three AC subfields were similar, but that there were subfield-dependent differences in the extent of response intensities and in the activated areas that were responsive to tone bursts with specific frequencies. Thus, by examining cortical reorganization induced by ototoxic drugs, we may contribute to the understanding of how this reorganization can be caused by peripheral damage.

A Missing Connection: A Review of the Macrostructural Anatomy and Tractography of the Acoustic Radiation

The auditory system of mammals is dedicated to encoding, elaborating and transporting acoustic information from the auditory nerve to the auditory cortex. The acoustic radiation (AR) constitutes the thalamo-cortical projection of this system, conveying the auditory signals from the medial geniculate nucleus (MGN) of the thalamus to the transverse temporal gyrus on the superior temporal lobe. While representing one of the major sensory pathways of the primate brain, the currently available anatomical information of this white matter bundle is quite limited in humans, thus constituting a notable omission in clinical and general studies on auditory processing and language perception. Tracing procedures in humans have restricted applications, and the in vivo reconstruction of this bundle using diffusion tractography techniques remains challenging. Hence, a more accurate and reliable reconstruction of the AR is necessary for understanding the neurobiological substrates supporting audition and language processing mechanisms in both health and disease. This review aims to unite available information on the macroscopic anatomy and topography of the AR in humans and non-human primates. Particular attention is brought to the anatomical characteristics that make this bundle difficult to reconstruct using non-invasive techniques, such as diffusion-based tractography. Open questions in the field and possible future research directions are discussed.

Pathophysiology of Subjective Tinnitus: Triggers and Maintenance

Tinnitus is the conscious perception of a sound without a corresponding external acoustic stimulus, usually described as a phantom perception. One of the major challenges for tinnitus research is to understand the pathophysiological mechanisms triggering and maintaining the symptoms, especially for subjective chronic tinnitus. Our objective was to synthesize the published literature in order to provide a comprehensive update on theoretical and experimental advances and to identify further research and clinical directions. We performed literature searches in three electronic databases, complemented by scanning reference lists from relevant reviews in our included records, citation searching of the included articles using Web of Science, and manual searching of the last 6 months of principal otology journals. One-hundred and thirty-two records were included in the review and the information related to peripheral and central mechanisms of tinnitus pathophysiology was collected in order to update on theories and models. A narrative synthesis examined the main themes arising from this information. Tinnitus pathophysiology is complex and multifactorial, involving the auditory and non-auditory systems. Recent theories assume the necessary involvement of extra-auditory brain regions for tinnitus to reach consciousness. Tinnitus engages multiple active dynamic and overlapping networks. We conclude that advancing knowledge concerning the origin and maintenance of specific tinnitus subtypes origin and maintenance mechanisms is of paramount importance for identifying adequate treatment.

Effects of neonatal deafness on resting-state functional network connectivity

Normal brain development depends on early sensory experience. Behavioral consequences of brain maturation in the absence of sensory input early in life are well documented. For example, experiments with mature, neonatally deaf human or animal subjects have revealed improved peripheral visual motion detection and spatial localization abilities. Such supranormal behavioral abilities in the nondeprived sensory modality are evidence of compensatory plasticity occurring in deprived brain regions at some point or throughout development. Sensory deprived brain regions may simply become unused neural real-estate resulting in a loss of function. Compensatory plasticity and loss of function are likely reflected in the differences in correlations between brain networks in deaf compared with hearing subjects. To address this, we used resting-state functional magnetic resonance imaging (fMRI) in lightly anesthetized hearing and neonatally deafened cats. Group independent component analysis (ICA) was used to identify 20 spatially distinct brain networks across all animals including auditory, visual, somatosensory, cingulate, insular, cerebellar, and subcortical networks. The resulting group ICA components were back-reconstructed to individual animal brains. The maximum correlations between the time-courses associated with each spatial component were computed using functional network connectivity (FNC). While no significant differences in the delay to peak correlations were identified between hearing and deaf cats, we observed 10 (of 190) significant differences in the amplitudes of between-network correlations. Six of the significant differences involved auditory-related networks and four involved visual, cingulate, or somatosensory networks. The results are discussed in context of known behavioral, electrophysiological, and anatomical differences following neonatal deafness. Furthermore, these results identify novel targets for future investigations at the neuronal level.

Cortical and thalamic connectivity to the second auditory cortex of the cat is resilient to the onset of deafness

It has been well established that following sensory loss, cortical areas that would normally be involved in perceiving stimuli in the absent modality are recruited to subserve the remaining senses. Despite this compensatory functional reorganization, there is little evidence to date for any substantial change in the patterns of anatomical connectivity between sensory cortices. However, while many auditory areas are contracted in the deaf, the second auditory cortex (A2) of the cat undergoes a volumetric expansion following hearing loss, suggesting this cortical area may demonstrate a region-specific pattern of structural reorganization. To address this hypothesis, and to complement existing literature on connectivity within auditory cortex, we injected a retrograde neuronal tracer across the breadth and cortical thickness of A2 to provide the first comprehensive quantification of projections from cortical and thalamic auditory and non-auditory regions to the second auditory cortex, and to determine how these patterns are affected by the onset of deafness. Neural projections arising from auditory, visual, somatomotor, and limbic cortices, as well as thalamic nuclei, were compared across normal hearing, early-deaf, and late-deaf animals. The results demonstrate that, despite previously identified changes in A2 volume, the pattern of projections into this cortical region are unaffected by the onset of hearing loss. These results fail to support the idea that crossmodal plasticity reflects changes in the pattern of projections between cortical regions and provides evidence that the pattern of connectivity that supports normal hearing is retained in the deaf brain.

Synaptic distribution and plasticity in primary auditory cortex (A1) exhibits laminar and cell-specific changes in the deaf

The processing sequence through primary auditory cortex (A1) is impaired by deafness as evidenced by reduced neuronal activation in A1 of cochlear-implanted deaf cats. Such a loss of neuronal excitation should be manifest as changes in excitatory synaptic number and/or size, for which the post-synaptic correlate is the dendritic spine. Therefore, the present study sought evidence for this functional disruption using Golgi-Cox/light microscopic techniques that examined spine-bearing neurons and their dendritic spine features across all laminae in A1 of early-deaf (ototoxic lesion <1 month; raised into adulthood >16 months) and hearing cats. Surprisingly, in the early-deaf significant increases in spine density and size were observed in the supragranular layers, while significant reductions in spine density were observed for spiny non-pyramidal, but not pyramidal, neurons in the granular layer. No changes in dendritic spine density consistent with loss of excitatory inputs were seen for infragranular neurons. These results indicate that long-term early-deafness induces plastic changes in the excitatory circuitry of A1 that are laminar and cell-specific. An additional finding was that, unlike the expected abundance of stellate neurons that characterize the granular layer of other primary sensory cortices, pyramidal neurons predominate within layer 4 of A1. Collectively, these observations are important for understanding how neuronal connectional configurations contribute to region-specific processing capabilities in normal brains as well as those with altered sensory experiences.

Auditory thalamic circuits and GABAA receptor function: Putative mechanisms in tinnitus pathology

Tinnitus is defined as a phantom sound (ringing in the ears), and can significantly reduce the quality of life for those who suffer its effects. Ten to fifteen percent of the general adult population report symptoms of tinnitus with 1-2% reporting that tinnitus negatively impacts their quality of life. Noise exposure is the most common cause of tinnitus and the military environment presents many challenging high-noise situations. Military noise levels can be so intense that standard hearing protection is not adequate. Recent studies suggest a role for inhibitory neurotransmitter dysfunction in response to noise-induced peripheral deafferentation as a key element in the pathology of tinnitus. The auditory thalamus, or medial geniculate body (MGB), is an obligate auditory brain center in a unique position to gate the percept of sound as it projects to auditory cortex and to limbic structures. Both areas are thought to be involved in those individuals most impacted by tinnitus. For MGB, opposing hypotheses have posited either a tinnitus-related pathologic decrease or pathologic increase in GABAergic inhibition. In sensory thalamus, GABA mediates fast synaptic inhibition via synaptic GABAA receptors (GABAARs) as well as a persistent tonic inhibition via high-affinity extrasynaptic GABAARs and slow synaptic inhibition via GABABRs. Down-regulation of inhibitory neurotransmission, related to partial peripheral deafferentation, is consistently presented as partially underpinning neuronal hyperactivity seen in animal models of tinnitus. This maladaptive plasticity/Gain Control Theory of tinnitus pathology (see Auerbach et al., 2014; Richardson et al., 2012) is characterized by reduced inhibition associated with increased spontaneous and abnormal neuronal activity, including bursting and increased synchrony throughout much of the central auditory pathway. A competing hypothesis suggests that maladaptive oscillations between the MGB and auditory cortex, thalamocortical dysrhythmia, predict tinnitus pathology (De Ridder et al., 2015). These unusual oscillations/rhythms reflect net increased tonic inhibition in a subset of thalamocortical projection neurons resulting in abnormal bursting. Hyperpolarizing de-inactivation of T-type Ca2+ channels switches thalamocortical projection neurons into burst mode. Thalamocortical dysrhythmia originating in sensory thalamus has been postulated to underpin neuropathies including tinnitus and chronic pain. Here we review the relationship between noise-induced tinnitus and altered inhibition in the MGB.

Is territorial expansion a mechanism for crossmodal plasticity?

Crossmodal plasticity is the phenomenon whereby, following sensory damage or deprivation, the lost sensory function of a brain region is replaced by one of the remaining senses. One of several proposed mechanisms for this phenomenon involves the expansion of a more active brain region at the expense of another whose sensory inputs have been damaged or lost. This territorial expansion hypothesis was examined in the present study. The cat ectosylvian visual area (AEV) borders the auditory field of the anterior ectosylvian sulcus (FAES), which becomes visually reorganized in the early deaf. If this crossmodal effect in the FAES is due to the expansion of the adjoining AEV into the territory of the FAES after hearing loss, then the reorganized FAES should exhibit connectional features characteristic of the AEV. However, tracer injections revealed significantly different patterns of cortical connectivity between the AEV and the early deaf FAES, and substantial cytoarchitectonic and behavioral distinctions occur as well. Therefore, the crossmodal reorganization of the FAES cannot be mechanistically attributed to the expansion of the adjoining cortical territory of the AEV and an overwhelming number of recent studies now support unmasking of existing connections as the operative mechanism underlying crossmodal plasticity.

Reconsidering Tonotopic Maps in the Auditory Cortex and Lemniscal Auditory Thalamus in Mice

The auditory thalamus and auditory cortex (AC) are pivotal structures in the central auditory system. However, the thalamocortical mechanisms of processing sounds are largely unknown. Investigation of this process benefits greatly from the use of mice because the mouse is a powerful animal model in which various experimental techniques, especially genetic tools, can be applied. However, the use of mice has been limited in auditory research, and thus even basic anatomical knowledge of the mouse central auditory system has not been sufficiently collected. Recently, optical imaging combined with morphological analyses has enabled the elucidation of detailed anatomical properties of the mouse auditory system. These techniques have uncovered fine AC maps with multiple frequency-organized regions, each of which receives point-to-point thalamocortical projections from different origins inside the lemniscal auditory thalamus, the ventral division of the medial geniculate body (MGv). This precise anatomy now provides a platform for physiological research. In this mini review article, we summarize these recent achievements that will facilitate physiological investigations in the mouse auditory system.

Predictive Ensemble Decoding of Acoustical Features Explains Context-Dependent Receptive Fields

A primary goal of auditory neuroscience is to identify the sound features extracted and represented by auditory neurons. Linear encoding models, which describe neural responses as a function of the stimulus, have been primarily used for this purpose. Here, we provide theoretical arguments and experimental evidence in support of an alternative approach, based on decoding the stimulus from the neural response. We used a Bayesian normative approach to predict the responses of neurons detecting relevant auditory features, despite ambiguities and noise. We compared the model predictions to recordings from the primary auditory cortex of ferrets and found that: (1) the decoding filters of auditory neurons resemble the filters learned from the statistics of speech sounds; (2) the decoding model captures the dynamics of responses better than a linear encoding model of similar complexity; and (3) the decoding model accounts for the accuracy with which the stimulus is represented in neural activity, whereas linear encoding model performs very poorly. Most importantly, our model predicts that neuronal responses are fundamentally shaped by "explaining away," a divisive competition between alternative interpretations of the auditory scene.

SIGNIFICANCE STATEMENT

Neural responses in the auditory cortex are dynamic, nonlinear, and hard to predict. Traditionally, encoding models have been used to describe neural responses as a function of the stimulus. However, in addition to external stimulation, neural activity is strongly modulated by the responses of other neurons in the network. We hypothesized that auditory neurons aim to collectively decode their stimulus. In particular, a stimulus feature that is decoded (or explained away) by one neuron is not explained by another. We demonstrated that this novel Bayesian decoding model is better at capturing the dynamic responses of cortical neurons in ferrets. Whereas the linear encoding model poorly reflects selectivity of neurons, the decoding model can account for the strong nonlinearities observed in neural data.

A Postsynaptic Role for Short-Term Neuronal Facilitation in Dendritic Spines

Synaptic plasticity is a fundamental component of information processing in the brain. Presynaptic facilitation in response to repetitive stimuli, often referred to as paired-pulse facilitation (PPF), is a dominant form of short-term synaptic plasticity. Recently, an additional cellular mechanism for short-term facilitation, short-term postsynaptic plasticity (STPP), has been proposed. While a dendritic mechanism was described in hippocampus, its expression has not yet been demonstrated at the levels of the spine. Furthermore, it is unknown whether the mechanism can be expressed in other brain regions, such as sensory cortex. Here, we demonstrated that a postsynaptic response can be facilitated by prior spine excitation in both hippocampal and cortical neurons, using 3D digital holography and two-photon calcium imaging. The coordinated action of pre- and post-synaptic plasticity may provide a more thorough account of information processing in the brain.

Microelectrode mapping of tonotopic, laminar, and field-specific organization of thalamo-cortical pathway in rat

The rat has long been considered an important model system for studying neural mechanisms of auditory perception and learning, and particularly mechanisms involving auditory thalamo-cortical processing. However, the functional topography of the auditory thalamus, or medial geniculate body (MGB) has not yet been fully characterized in the rat, and the anatomically-defined features of field-specific, layer-specific and tonotopic thalamo-cortical projections have never been confirmed electrophysiologically. In the present study, we have established a novel technique for recording simultaneously from a surface microelectrode array on the auditory cortex, and a depth electrode array across auditory cortical layers and within the MGB, and characterized the rat MGB and thalamo-cortical projections under isoflurane anesthesia. We revealed that the ventral division of the MGB (MGv) exhibited a low-high-low CF gradient and long-short-long latency gradient along the dorsolateral-to-ventromedial axis, suggesting that the rat MGv is divided into two subdivisions. We also demonstrated that microstimulation in the MGv elicited cortical activation in layer-specific, region-specific and tonotopically organized manners. To our knowledge, the present study has provided the first and most compelling electrophysiological confirmation of the anatomical organization of the primary thalamo-cortical pathway in the rat, setting the groundwork for further investigation.

Species-dependent role of crossmodal connectivity among the primary sensory cortices

When a major sense is lost, crossmodal plasticity substitutes functional processing from the remaining, intact senses. Recent studies of deafness-induced crossmodal plasticity in different subregions of auditory cortex indicate that the phenomenon is largely based on the "unmasking" of existing inputs. However, there is not yet a consensus on the sources or effects of crossmodal inputs to primary sensory cortical areas. In the present review, a rigorous re-examination of the experimental literature indicates that connections between different primary sensory cortices consistently occur in rodents, while primary-to-primary projections are absent/inconsistent in non-rodents such as cats and monkeys. These observations suggest that crossmodal plasticity that involves primary sensory areas are likely to exhibit species-specific distinctions.

Functional congruity in local auditory cortical microcircuits

Functional columns of primary auditory cortex (AI) are arranged in layers, each composed of highly connected fine-scale networks. The basic response properties and interactions within these local subnetworks have only begun to be assessed. We examined the functional diversity of neurons within the laminar microarchitecture of cat AI to determine the relationship of spectrotemporal processing between neighboring neurons. Neuronal activity was recorded across the cortical layers while presenting a dynamically modulated broadband noise. Spectrotemporal receptive fields (STRFs) and their nonlinear input/output functions (nonlinearities) were constructed for each neuron and compared for pairs of neurons simultaneously recorded at the same contact site. Properties of these local neuron pairs showed greater similarity than non-paired neurons within the same column for all considered parameters including firing rate, envelope-phase precision, preferred spectral and temporal modulation frequency, as well as for the threshold and transition of the response nonlinearity. This higher functional similarity of paired versus non-paired neurons was most apparent in infragranular neuron pairs, and less for local supragranular and granular pairs. The functional similarity of local paired neurons for firing rate, best temporal modulation frequency and two nonlinearity aspects was laminar dependent, with infragranular local pair-wise differences larger than for granular or supragranular layers. Synchronous spiking events between pairs of neurons revealed that simultaneous 'Bicellular' spikes, in addition to carrying higher stimulus information than non-synchronized spikes, encoded faster modulation frequencies. Bicellular functional differences to the best matched of the paired neurons could be substantial. Bicellular nonlinearities showed that synchronous spikes act to transmit stimulus information with higher fidelity and precision than non-synchronous spikes of the individual neurons, thus, likely enhancing stimulus feature selectivity in their target neurons. Overall, the well-correlated and temporally precise processing within local subnetworks of cat AI showed laminar-dependent functional diversity in spectrotemporal processing, despite high intra-columnar congruity in frequency preference.

Cortical and thalamic connectivity of the auditory anterior ectosylvian cortex of early-deaf cats: Implications for neural mechanisms of crossmodal plasticity

Early hearing loss leads to crossmodal plasticity in regions of the cerebrum that are dominated by acoustical processing in hearing subjects. Until recently, little has been known of the connectional basis of this phenomenon. One region whose crossmodal properties are well-established is the auditory field of the anterior ectosylvian sulcus (FAES) in the cat, where neurons are normally responsive to acoustic stimulation and its deactivation leads to the behavioral loss of accurate orienting toward auditory stimuli. However, in early-deaf cats, visual responsiveness predominates in the FAES and its deactivation blocks accurate orienting behavior toward visual stimuli. For such crossmodal reorganization to occur, it has been presumed that novel inputs or increased projections from non-auditory cortical areas must be generated, or that existing non-auditory connections were 'unmasked.' These possibilities were tested using tracer injections into the FAES of adult cats deafened early in life (and hearing controls), followed by light microscopy to localize retrogradely labeled neurons. Surprisingly, the distribution of cortical and thalamic afferents to the FAES was very similar among early-deaf and hearing animals. No new visual projection sources were identified and visual cortical connections to the FAES were comparable in projection proportions. These results support an alternate theory for the connectional basis for cross-modal plasticity that involves enhanced local branching of existing projection terminals that originate in non-auditory as well as auditory cortices.

Exploring functions for the non-lemniscal auditory thalamus

The functions of the medial geniculate body (MGB) in normal hearing still remain somewhat enigmatic, in part due to the relatively unexplored properties of the non-lemniscal MGB nuclei. Indeed, the canonical view of the thalamus as a simple relay for transmitting ascending information to the cortex belies a role in higher-order forebrain processes. However, recent anatomical and physiological findings now suggest important information and affective processing roles for the non-primary auditory thalamic nuclei. The non-lemniscal nuclei send and receive feedforward and feedback projections among a wide constellation of midbrain, cortical, and limbic-related sites, which support potential conduits for auditory information flow to higher auditory cortical areas, mediators for transitioning among arousal states, and synchronizers of activity across expansive cortical territories. Considered here is a perspective on the putative and unresolved functional roles of the non-lemniscal nuclei of the MGB.

Cortico-Cortical Connectivity Within Ferret Auditory Cortex

Despite numerous studies of auditory cortical processing in the ferret (Mustela putorius), very little is known about the connections between the different regions of the auditory cortex that have been characterized cytoarchitectonically and physiologically. We examined the distribution of retrograde and anterograde labeling after injecting tracers into one or more regions of ferret auditory cortex. Injections of different tracers at frequency‐matched locations in the core areas, the primary auditory cortex (A1) and anterior auditory field (AAF), of the same animal revealed the presence of reciprocal connections with overlapping projections to and from discrete regions within the posterior pseudosylvian and suprasylvian fields (PPF and PSF), suggesting that these connections are frequency specific. In contrast, projections from the primary areas to the anterior dorsal field (ADF) on the anterior ectosylvian gyrus were scattered and non‐overlapping, consistent with the non‐tonotopic organization of this field. The relative strength of the projections originating in each of the primary fields differed, with A1 predominantly targeting the posterior bank fields PPF and PSF, which in turn project to the ventral posterior field, whereas AAF projects more heavily to the ADF, which then projects to the anteroventral field and the pseudosylvian sulcal cortex. These findings suggest that parallel anterior and posterior processing networks may exist, although the connections between different areas often overlap and interactions were present at all levels. J. Comp. Neurol. 523:2187–2210, 2015. © 2015 Wiley Periodicals, Inc.

Dissociable influences of primary auditory cortex and the posterior auditory field on neuronal responses in the dorsal zone of auditory cortex

Current models of hierarchical processing in auditory cortex have been based principally on anatomical connectivity while functional interactions between individual regions have remained largely unexplored. Previous cortical deactivation studies in the cat have addressed functional reciprocal connectivity between primary auditory cortex (A1) and other hierarchically lower level fields. The present study sought to assess the functional contribution of inputs along multiple stages of the current hierarchical model to a higher order area, the dorsal zone (DZ) of auditory cortex, in the anaesthetized cat. Cryoloops were placed over A1 and posterior auditory field (PAF). Multiunit neuronal responses to noise burst and tonal stimuli were recorded in DZ during cortical deactivation of each field individually and in concert. Deactivation of A1 suppressed peak neuronal responses in DZ regardless of stimulus and resulted in increased minimum thresholds and reduced absolute bandwidths for tone frequency receptive fields in DZ. PAF deactivation had less robust effects on DZ firing rates and receptive fields compared with A1 deactivation, and combined A1/PAF cooling was largely driven by the effects of A1 deactivation at the population level. These results provide physiological support for the current anatomically based model of both serial and parallel processing schemes in auditory cortical hierarchical organization.

Spectral and Temporal Acoustic Features Modulate Response Irregularities within Primary Auditory Cortex Columns

Assemblies of vertically connected neurons in the cerebral cortex form information processing units (columns) that participate in the distribution and segregation of sensory signals. Despite well-accepted models of columnar architecture, functional mechanisms of inter-laminar communication remain poorly understood. Hence, the purpose of the present investigation was to examine the effects of sensory information features on columnar response properties. Using acute recording techniques, extracellular response activity was collected from the right hemisphere of eight mature cats (felis catus). Recordings were conducted with multichannel electrodes that permitted the simultaneous acquisition of neuronal activity within primary auditory cortex columns. Neuronal responses to simple (pure tones), complex (noise burst and frequency modulated sweeps), and ecologically relevant (con-specific vocalizations) acoustic signals were measured. Collectively, the present investigation demonstrates that despite consistencies in neuronal tuning (characteristic frequency), irregularities in discharge activity between neurons of individual A1 columns increase as a function of spectral (signal complexity) and temporal (duration) acoustic variations.

Synaptic Basis for Cross-modal Plasticity: Enhanced Supragranular Dendritic Spine Density in Anterior Ectosylvian Auditory Cortex of the Early Deaf Cat

In the cat, the auditory field of the anterior ectosylvian sulcus (FAES) is sensitive to auditory cues and its deactivation leads to orienting deficits toward acoustic, but not visual, stimuli. However, in early deaf cats, FAES activity shifts to the visual modality and its deactivation blocks orienting toward visual stimuli. Thus, as in other auditory cortices, hearing loss leads to cross-modal plasticity in the FAES. However, the synaptic basis for cross-modal plasticity is unknown. Therefore, the present study examined the effect of early deafness on the density, distribution, and size of dendritic spines in the FAES. Young cats were ototoxically deafened and raised until adulthood when they (and hearing controls) were euthanized, the cortex stained using Golgi-Cox, and FAES neurons examined using light microscopy. FAES dendritic spine density averaged 0.85 spines/μm in hearing animals, but was significantly higher (0.95 spines/μm) in the early deaf. Size distributions and increased spine density were evident specifically on apical dendrites of supragranular neurons. In separate tracer experiments, cross-modal cortical projections were shown to terminate predominantly within the supragranular layers of the FAES. This distributional correspondence between projection terminals and dendritic spine changes indicates that cross-modal plasticity is synaptically based within the supragranular layers of the early deaf FAES.

Frequency Selectivity of Voxel-by-Voxel Functional Connectivity in Human Auditory Cortex

While functional connectivity in the human cortex has been increasingly studied, its relationship to cortical representation of sensory features has not been documented as much. We used functional magnetic resonance imaging to demonstrate that voxel-by-voxel intrinsic functional connectivity (FC) is selective to frequency preference of voxels in the human auditory cortex. Thus, FC was significantly higher for voxels with similar frequency tuning than for voxels with dissimilar tuning functions. Frequency-selective FC, measured via the correlation of residual hemodynamic activity, was not explained by generic FC that is dependent on spatial distance over the cortex. This pattern remained even when FC was computed using residual activity taken from resting epochs. Further analysis showed that voxels in the core fields in the right hemisphere have a higher frequency selectivity in within-area FC than their counterpart in the left hemisphere, or than in the noncore-fields in the same hemisphere. Frequency-selective FC is consistent with previous findings of topographically organized FC in the human visual and motor cortices. The high degree of frequency selectivity in the right core area is in line with findings and theoretical proposals regarding the asymmetry of human auditory cortex for spectral processing.

Anatomy of hierarchy: feedforward and feedback pathways in macaque visual cortex

The laminar location of the cell bodies and terminals of interareal connections determines the hierarchical structural organization of the cortex and has been intensively studied. However, we still have only a rudimentary understanding of the connectional principles of feedforward (FF) and feedback (FB) pathways. Quantitative analysis of retrograde tracers was used to extend the notion that the laminar distribution of neurons interconnecting visual areas provides an index of hierarchical distance (percentage of supragranular labeled neurons [SLN]). We show that: 1) SLN values constrain models of cortical hierarchy, revealing previously unsuspected areal relations; 2) SLN reflects the operation of a combinatorial distance rule acting differentially on sets of connections between areas; 3) Supragranular layers contain highly segregated bottom-up and top-down streams, both of which exhibit point-to-point connectivity. This contrasts with the infragranular layers, which contain diffuse bottom-up and top-down streams; 4) Cell filling of the parent neurons of FF and FB pathways provides further evidence of compartmentalization; 5) FF pathways have higher weights, cross fewer hierarchical levels, and are less numerous than FB pathways. Taken together, the present results suggest that cortical hierarchies are built from supra- and infragranular counterstreams. This compartmentalized dual counterstream organization allows point-to-point connectivity in both bottom-up and top-down directions.

Optogenetic activation of an inhibitory network enhances feedforward functional connectivity in auditory cortex

The mammalian neocortex is a highly interconnected network of different types of neurons organized into both layers and columns. Overlaid on this structural organization is a pattern of functional connectivity that can be rapidly and flexibly altered during behavior. Parvalbumin-positive (PV+) inhibitory neurons, which are implicated in cortical oscillations and can change neuronal selectivity, may play a pivotal role in these dynamic changes. We found that optogenetic activation of PV+ neurons in the auditory cortex enhanced feedforward functional connectivity in the putative thalamorecipient circuit and in cortical columnar circuits. In contrast, stimulation of PV+ neurons induced no change in connectivity between sites in the same layers. The activity of PV+ neurons may thus serve as a gating mechanism to enhance feedforward, but not lateral or feedback, information flow in cortical circuits. Functionally, it may preferentially enhance the contribution of bottom-up sensory inputs to perception.

Functional convergence of thalamic and intrinsic projections to cortical layers 4 and 6

Ascending sensory information is conveyed from the thalamus to layers 4 and 6 of sensory cortical areas. Interestingly, receptive field properties of cortical layer 6 neurons are different from those in layer 4. Do such differences reflect distinct inheritance patterns from the thalamus or are they derived instead from local cortical circuits? To distinguish between these possibilities, we utilized in vitro slice preparations containing the thalamocortical pathways in the auditory and somatosensory systems. Responses from neurons in layers 4 and 6 that resided in the same column were recorded using whole-cell patch clamp. Laser-scanning photostimulation via uncaging of glutamate in the thalamus and cortex was used to map the functional topography of thalamocortical and intracortical inputs to each layer. In addition, we assessed the functional divergence of thalamocortical inputs by optical imaging of flavoprotein autofluorescence. We found that the thalamocortical inputs to layers 4 and 6 originated from the same thalamic domain, but the intracortical projections to the same neurons differed dramatically. Our results suggest that the intracortical projections, rather than the thalamic inputs, to each layer contribute more to the differences in their receptive field properties.

Cortical mapping of mismatch negativity with deviance detection property in rat

Mismatch Negativity (MMN) is an N-methyl-d-aspartic acid (NMDA)-mediated, negative deflection in human auditory evoked potentials in response to a cognitively discriminable change. MMN-like responses have been extensively investigated in animal models, but the existence of MMN equivalent is still controversial. In this study, we aimed to investigate how closely the putative MMN (MMNp) in rats exhibited the comparable properties of human MMN. We used a surface microelectrode array with a grid of 10×7 recording sites within an area of 4.5×3.0 mm to densely map evoked potentials in the auditory cortex of anesthetized rats under the oddball paradigm. Firstly, like human MMN, deviant stimuli elicited negative deflections in auditory evoked potentials following the positive middle-latency response, termed P1. Secondly, MMNp exhibited deviance-detecting property, which could not be explained by simple stimulus specific adaptation (SSA). Thirdly, this MMNp occurred focally in the auditory cortex, including both the core and belt regions, while P1 activation focus was obtained in the core region, indicating that both P1 and MMNp are generated in the auditory cortex, yet the sources of these signals do not completely overlap. Fourthly, MMNp significantly decreased after the application of AP5 (D-(-)-2-amino-5-phosphonopentanoic acid), an antagonist at NMDA receptors. In stark contrast, AP5 affected neither P1 amplitude nor SSA of P1. These results provide compelling evidence that the MMNp we have examined in rats is functionally comparable to human MMN. The present work will stimulate translational research into MMN, which may help bridge the gap between electroencephalography (EEG)/magnetoencephalography (MEG) studies in humans and electrophysiological studies in animals.

Effects of core auditory cortex deactivation on neuronal response to simple and complex acoustic signals in the contralateral anterior auditory field

Interhemispheric communication has been implicated in various functions of sensory signal processing and perception. Despite ample evidence demonstrating this phenomenon in the visual and somatosensory systems, to date, limited functional assessment of transcallosal transmission during periods of acoustic signal exposure has hindered our understanding of the role of interhemispheric connections between auditory cortical fields. Consequently, the present investigation examines the impact of core auditory cortical field deactivation on response properties of contralateral anterior auditory field (AAF) neurons in the felis catus. Single-unit responses to simple and complex acoustic signals were measured across AAF before, during, and after individual and combined cooling deactivation of contralateral primary auditory cortex (A1) and AAF neurons. Data analyses revealed that on average: 1) interhemispheric projections from core auditory areas to contralateral AAF neurons are predominantly excitatory, 2) changes in response strength vary based on acoustic features, 3) A1 and AAF projections can modulate AAF activity differently, 4) decreases in response strength are not specific to particular cortical laminae, and 5) contralateral inputs modulate AAF neuronal response thresholds. Collectively, these observations demonstrate that A1 and AAF neurons predominantly modulate AAF response properties via excitatory projections.

Thalamic and cortical pathways supporting auditory processing

The neural processing of auditory information engages pathways that begin initially at the cochlea and that eventually reach forebrain structures. At these higher levels, the computations necessary for extracting auditory source and identity information rely on the neuroanatomical connections between the thalamus and cortex. Here, the general organization of these connections in the medial geniculate body (thalamus) and the auditory cortex is reviewed. In addition, we consider two models organizing the thalamocortical pathways of the non-tonotopic and multimodal auditory nuclei. Overall, the transfer of information to the cortex via the thalamocortical pathways is complemented by the numerous intracortical and corticocortical pathways. Although interrelated, the convergent interactions among thalamocortical, corticocortical, and commissural pathways enable the computations necessary for the emergence of higher auditory perception.

Reorganization of the connectivity of cortical field DZ in congenitally deaf cat

Psychophysics and brain imaging studies in deaf patients have revealed a functional crossmodal reorganization that affects the remaining sensory modalities. Similarly, the congenital deaf cat (CDC) shows supra-normal visual skills that are supported by specific auditory fields (DZ-dorsal zone and P-posterior auditory cortex) but not the primary auditory cortex (A1). To assess the functional reorganization observed in deafness we analyzed the connectivity pattern of the auditory cortex by means of injections of anatomical tracers in DZ and A1 in both congenital deaf and normally hearing cats. A quantitative analysis of the distribution of the projecting neurons revealed the presence of non-auditory inputs to both A1 and DZ of the CDC which were not observed in the hearing cats. Firstly, some visual (areas 19/20) and somatosensory (SIV) areas were projecting toward DZ of the CDC but not in the control. Secondly, A1 of the deaf cat received a weak projection from the visual lateral posterior nuclei (LP). Most of these abnormal projections to A1 and DZ represent only a small fraction of the normal inputs to these areas. In addition, most of the afferents to DZ and A1 appeared normal in terms of areal specificity and strength of projection, with preserved but smeared nucleotopic gradient of A1 in CDCs. In conclusion, while the abnormal projections revealed in the CDC can participate in the crossmodal compensatory mechanisms, the observation of a limited reorganization of the connectivity pattern of the CDC implies that functional reorganization in congenital deafness is further supported also by normal cortico-cortical connectivity.

Cerebral origins of the auditory projection to the superior colliculus of the cat

The superior colliculus (SC) is critical for directing accurate head and eye movements to visual and acoustic targets. In visual cortex, areas involved in orienting of the head and eyes to a visual stimulus have direct projections to the SC. In auditory cortex of the cat, four areas have been identified to be critical for the accurate orienting of the head and body to an acoustic stimulus. These areas include primary auditory cortex (A1), the posterior auditory field (PAF), the dorsal zone of auditory cortex (DZ), and the auditory field of the anterior ectosylvian sulcus (fAES). Therefore, we hypothesized that these four regions of auditory cortex would have direct projections to the SC. To test this hypothesis, deposits of wheat germ agglutinin conjugated to horseradish peroxidase (WGA-HRP) were made into the superficial and deep layers of the SC to label, by means of retrograde transport, the auditory cortical origins of the corticotectal pathway. Bilateral examination of auditory cortex revealed that the vast majority of the labeled cells were located in the hemisphere ipsilateral to the SC injection. In ipsilateral auditory cortex, nearly all the labeled neurons were found in the infragranular layers, predominately in layer V. The largest population of labeled cells was located in the fAES. Few labeled neurons were identified in A1, PAF, or DZ. Thus, in contrast to the visual system, only one of the auditory cortical areas involved in orienting to an acoustic stimulus has a strong direct projection to the SC. Sound localization signals processed in primary (A1) and other non-primary (PAF and DZ) auditory cortices may be transmitted to the SC via a multi-synaptic corticotectal network.