Functional subregions in primary auditory cortex defined by thalamocortical terminal arbors: an electrophysiological and anterograde labeling study

Lemniscal Corticothalamic Feedback in Auditory Scene Analysis

Sound information is transmitted from the ear to central auditory stations of the brain via several nuclei. In addition to these ascending pathways there exist descending projections that can influence the information processing at each of these nuclei. A major descending pathway in the auditory system is the feedback projection from layer VI of the primary auditory cortex (A1) to the ventral division of medial geniculate body (MGBv) in the thalamus. The corticothalamic axons have small glutamatergic terminals that can modulate thalamic processing and thalamocortical information transmission. Corticothalamic neurons also provide input to GABAergic neurons of the thalamic reticular nucleus (TRN) that receives collaterals from the ascending thalamic axons. The balance of corticothalamic and TRN inputs has been shown to refine frequency tuning, firing patterns, and gating of MGBv neurons. Therefore, the thalamus is not merely a relay stage in the chain of auditory nuclei but does participate in complex aspects of sound processing that include top-down modulations. In this review, we aim (i) to examine how lemniscal corticothalamic feedback modulates responses in MGBv neurons, and (ii) to explore how the feedback contributes to auditory scene analysis, particularly on frequency and harmonic perception. Finally, we will discuss potential implications of the role of corticothalamic feedback in music and speech perception, where precise spectral and temporal processing is essential.

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.

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

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

Neural integration and enhancement from the inferior colliculus up to different layers of auditory cortex

While the cochlear implant has successfully restored hearing to many deaf patients, it cannot benefit those without a functional auditory nerve or an implantable cochlea. As an alternative, the auditory midbrain implant (AMI) has been developed and implanted into deaf patients. Consisting of a single-shank array, the AMI is designed for stimulation along the tonotopic gradient of the inferior colliculus (ICC). Although the AMI can provide frequency cues, it appears to insufficiently transmit temporal cues for speech understanding because repeated stimulation of a single site causes strong suppressive and refractory effects. Applying the electrical stimulation to at least two sites within an isofrequency lamina can circumvent these refractory processes. Moreover, coactivation with short intersite delays (<5 ms) can elicit cortical activation which is enhanced beyond the summation of activity induced by the individual sites. The goal of our study was to further investigate the role of the auditory cortex in this enhancement effect. In guinea pigs, we electrically stimulated two locations within an ICC lamina or along different laminae with varying interpulse intervals (0-10 ms) and recorded activity in different locations and layers of primary auditory cortex (A1). Our findings reveal a neural mechanism that integrates activity only from neurons located within the same ICC lamina for short spiking intervals (<6 ms). This mechanism leads to enhanced activity into layers III-V of A1 that is further magnified in supragranular layers. This integration mechanism may contribute to perceptual coding of different sound features that are relevant for improving AMI performance.

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.

How different are the local field potentials and spiking activities? Insights from multi-electrodes arrays

Simultaneous recording of multiple neurons, or neuron groups, offers new promise for investigating fundamental questions about the neural code. We used arrays of 16 electrodes in the tonotopic, primary, auditory cortex of guinea pigs and we extracted LFP- and spike-based spectro-temporal receptive fields (STRFs). We confirm here that LFP signals provide broadly tuned activity which lacks frequency resolution compared to multiunit signals and, therefore, lead to large redundancy in neural responses even between recording sites far apart. Thanks to the use of multi-electrode arrays which allows simultaneous recordings, we also focused on functional relationships between neuronal discharges (through cross-correlations) and between LFPs (through coherence). Since the LFP is composed of distinct brain rhythms, the LFP results were split into three frequency bands from the slowest to the fastest components of LFPs. For driven as well as spontaneous activity, we show that components >70 Hz in LFPs are much less coherent between recording sites than slower components. In general, coherence between LFPs from two recordings sites is positively correlated with the degree of frequency overlap between the two corresponding STRFs, similar to cross-correlation between multiunit activities. However, coherence is only weakly correlated with cross-correlation in all frequency ranges. Altogether, these results suggest that LFPs reflect global functional connectivity in the thalamocortical auditory system whereas spiking activities reflect more independent local processing.

Functional connectivity and cholinergic modulation in auditory cortex

Although it is known that primary auditory cortex (A1) contributes to the processing and perception of sound, its precise functions and the underlying mechanisms are not well understood. Recent studies point to a remarkably broad spectral range of largely subthreshold inputs to individual neurons in A1--seemingly encompassing, in some cases, the entire audible spectrum--as evidence for potential, and potentially unique, cortical functions. We have proposed a general mechanism for spectral integration by which information converges on neurons in A1 via a combination of thalamocortical pathways and intracortical long-distance, "horizontal", pathways. Here, this proposal is briefly reviewed and updated with results from multiple laboratories. Since spectral integration in A1 is dynamically regulated, we also show how one regulatory mechanism--modulation by the neurotransmitter acetylcholine (ACh)--could act within the hypothesized framework to alter integration in single neurons. The results of these studies promote a cellular understanding of information processing in A1.

The functional asymmetry of auditory cortex is reflected in the organization of local cortical circuits

The primary auditory cortex (A1) is organized tonotopically, with neurons sensitive to high and low frequencies arranged in a rostro-caudal gradient. We used laser scanning photostimulation in acute slices to study the organization of local excitatory connections onto layers 2 and 3 (L2/3) of the mouse A1. Consistent with the organization of other cortical regions, synaptic inputs along the isofrequency axis (orthogonal to the tonotopic axis) arose predominantly within a column. By contrast, we found that local connections along the tonotopic axis differed from those along the isofrequency axis: some input pathways to L3 (but not L2) arose predominantly out-of-column. In vivo cell-attached recordings revealed differences between the sound-responsiveness of neurons in L2 and L3. Our results are consistent with the hypothesis that auditory cortical microcircuitry is specialized to the one-dimensional representation of frequency in the auditory cortex.

Location of cells giving phase-locked responses to pure tones in the primary auditory cortex

Phase-locked responses to pure tones have previously been described in the primary auditory cortex (AI) of the guinea pig. They are interesting because they show that some cells may use a temporal code for representing sounds of 60-300 Hz rather than the rate or place mechanisms used over most of AI. Our previous study had shown that the phase-locked responses were grouped together, but it was not clear whether they were in separate minicolumns or a larger macrocolumn. We now show that the phase-locked cells are arranged in a macrocolumn within AI that forms a subdivision of the isofrequency bands. Phase-locked responses were recorded from 158 multiunits using silicon based multiprobes with four shanks. The phase-locked units gave the strongest response in layers III/IV but phase-locked units were also recorded in layers II, V and VI. The column included cells with characteristic frequencies of 80 Hz-1.3 kHz (0.5-0.8 mm long) and was about 0.5 mm wide. It was located at a constant position at the intersection of the coronal plane 1 mm caudal to bregma and the suture that forms the lateral edge of the parietal bone.

Severe and extensive neonatal hearing loss in cats results in auditory cortex plasticity that differentiates into two regions

We examined the response characteristics of primary auditory cortex (A1) neurons in adult cats partially but extensively deafened by ototoxic drugs 2-8 days after birth. The damage evoked extensive A1 topographic map reorganization as also found by others, but a novel finding was that in the majority of cats with low-frequency edges to the cochlear lesion, the area of reorganization segregated into two areas expressing the same novel frequency inputs but differentiated by neuronal sensitivity and responsiveness. Immediately adjacent to normal A1 is an approximately 1.2-mm-wide area of reorganization in which sensitivity and responsiveness to sound are similar to that in normal A1 in the same animals and in unlesioned adult animals. Extending further into deprived A1 is a more extensive area of reorganization where neurons have poorer sensitivity and responsiveness to new inputs. These two areas did not differ in response-area bandwidth and response latency. We interpret these novel changes as the cortical consequences of severe receptor organ lesions extending to low-frequency cochlear regions. We speculate that the two areas of A1 reorganization may reflect differences in the transcortical spatial distribution of thalamo-cortical and horizontal intracortical connections. Qualitatively similar changes in response properties have been seen after retinal lesions producing large areas of visual cortical reorganization, suggesting they might be a general consequence of receptor lesions that deprive large regions of cortex of normal input. These effects may have perceptual implications for the use of cochlear implants in patients with residual low-frequency hearing.

GABA shapes a systematic map of binaural sensitivity in the auditory cortex

A consistent organizational feature of auditory cortex is a clustered representation of binaural properties. Here we address two questions. What is the intrinsic organization of binaural clusters and to what extent does intracortical processing contribute to binaural representation. We address these issues in the auditory cortex of the pallid bat. The pallid bat listens to prey-generated noise transients to localize and hunt terrestrial prey. As in other species studied, binaural clusters are present in the auditory cortex of the pallid bat. One cluster contains neurons that require binaural stimulation to be maximally excited, and are commonly termed predominantly binaural (PB) neurons. These neurons do not respond to monaural stimulation of either ear but show a peaked sensitivity to interaural intensity differences (IID) centered near 0 dB IID. We show that the peak IID varies systematically within this cluster. The peak IID is also correlated with the best frequency (BF) of neurons within this cluster. In addition, the IID selectivity of PB neurons is shaped by intracortical GABAergic input. Iontophoresis of GABA(A) receptor antagonists on PB neurons converts a majority of them to binaurally inhibited (EI) neurons that respond best to sounds favoring the contralateral ear. These data indicate that the cortex does not simply inherit binaural properties from lower levels but instead sharpens them locally through intracortical inhibition. The IID selectivity of the PB cluster indicates that the pallid bat cortex contains an increased representation of the frontal space that may underlie increased localization accuracy in this region.

Fast propagating waves within the rodent auditory cortex

Central processing of acoustic signals is assumed to take place in a stereotypical spatial and temporal pattern involving different fields of auditory cortex. So far, cortical propagating waves representing such patterns have mainly been demonstrated by optical imaging, repeatedly in the visual and somatosensory cortex. In this study, the surface of rat auditory cortex was mapped by recording local field potentials (LFPs) in response to a broadband acoustic stimulus. From the peak amplitudes of LFPs, cortical activation maps were constructed over 4 cortical auditory fields. Whereas response onset had same latencies across primary auditory field (A1), anterior auditory field (AAF), and ventral auditory field and longer latencies in posterior auditory field, activation maps revealed a reproducible wavelike pattern of activity propagating for ∼45 ms poststimulus through all cortical fields. The movement observed started with 2 waves within the primary auditory fields A1 and AAF moving from ventral to dorsal followed by a motion from rostral to caudal, passing continuously through higher-order fields. The pattern of propagating waves was well reproducible and showed only minor changes if different anesthetics were used. The results question the classical "hierarchical" model of cortical areas and demonstrate that the different fields process incoming information as a functional unit.

Regional and laminar distribution of the vesicular glutamate transporter, VGluT2, in the macaque monkey auditory cortex

The auditory cortex of primates contains 13 areas distributed among 3 hierarchically connected regions: core, belt, and parabelt. Thalamocortical inputs arise in parallel from four divisions of the medial geniculate complex (MGC), which have regionally distinct projection patterns. These inputs terminate in layers IIIb and/or IV, and are assumed to be glutamatergic, although this has not been verified. In the present study, immunoreactivity (-ir) for the vesicular glutamate transporter, VGluT2, was used to estimate the regional and laminar distribution of the glutamatergic thalamocortical projection in the macaque auditory cortex. Coronal sections containing auditory cortex were processed for VGluT2 and other markers concentrated in the thalamorecipient layers: cytochrome oxidase, acetylcholinesterase, and parvalbumin. Marker expression was studied with wide field and confocal microscopy. The main findings were: (1) VGluT2-ir was highest in the core, intermediate in the belt, and sparse in the parabelt; (2) VGluT2-ir was concentrated in the neuropil of layers IIIb/IV in the core and layer IIIb in the belt; (3) VGluT2-ir matched regional and laminar expression of the other chemoarchitectonic markers. The results indicate that the glutamatergic thalamic projection to auditory cortex, as indexed by VGluT2-ir, varies along the core-belt-parabelt axis in a manner that matches the gradients of other markers. These chemoarchitectonic features are likely to subserve regional differences in neuronal activity between regions of auditory cortex.

Connections of cat auditory cortex: I. Thalamocortical system

Despite the functional importance of the medial geniculate body (MGB) in normal hearing, many aspects of its projections to auditory cortex are unknown. We analyzed the MGB projections to 13 auditory areas in the cat using two retrograde tracers to investigate thalamocortical nuclear origins, topography, convergence, and divergence. MGB divisions and auditory cortex areas were defined independently of the connectional results using architectonic, histochemical, and immunocytochemical criteria. Each auditory cortex area received a unique pattern of input from several MGB nuclei, and these patterns of input identify four groups of cortical areas distinguished by their putative functional affiliations: tonotopic, nontonotopic, multisensory, and limbic. Each family of areas received projections from a functionally related set of MGB nuclei; some nuclei project to only a few areas (e.g., the MGB ventral division to tonotopic areas), and others project to all areas (e.g., the medial division input to every auditory cortical area and to other regions). Projections to tonotopic areas had fewer nuclear origins than those to multisensory or limbic-affiliated fields. All projections were organized topographically, even those from nontonotopic nuclei. The few divergent neurons (mean: 2%) are consistent with a model of multiple segregated streams ascending to auditory cortex. The expanded cortical representation of MGB auditory, multisensory, and limbic affiliated streams appears to be a primary facet of forebrain auditory function. The emergence of several auditory cortex representations of characteristic frequency may be a functional multiplication of the more limited maps in the MGB. This expansion suggests emergent cortical roles consistent with the divergence of thalamocortical connections.

Branched projections in the auditory thalamocortical and corticocortical systems

Branched axons (BAs) projecting to different areas of the brain can create multiple feature-specific maps or synchronize processing in remote targets. We examined the organization of BAs in the cat auditory forebrain using two sensitive retrograde tracers. In one set of experiments (n=4), the tracers were injected into different frequency-matched loci in the primary auditory area (AI) and the anterior auditory field (AAF). In the other set (n=4), we injected primary, non-primary, or limbic cortical areas. After mapped injections, percentages of double-labeled cells (PDLs) in the medial geniculate body (MGB) ranged from 1.4% (ventral division) to 2.8% (rostral pole). In both ipsilateral and contralateral areas AI and AAF, the average PDLs were <1%. In the unmapped cases, the MGB PDLs ranged from 0.6% (ventral division) after insular cortex injections to 6.7% (dorsal division) after temporal cortex injections. Cortical PDLs ranged from 0.1% (ipsilateral AI injections) to 3.7% in the second auditory cortical area (AII) (contralateral AII injections). PDLs within the smaller (minority) projection population were significantly higher than those in the overall population. About 2% of auditory forebrain projection cells have BAs and such cells are organized differently than those in the subcortical auditory system, where BAs can be far more numerous. Forebrain branched projections follow different organizational rules than their unbranched counterparts. Finally, the relatively larger proportion of visual and somatic sensory forebrain BAs suggests modality specific rules for BA organization.

Multiple topographically organized projections connect the central nucleus of the inferior colliculus to the ventral division of the medial geniculate nucleus in the gerbil, Meriones unguiculatus

The ventral division of the medial geniculate nucleus (MGv) receives almost all of its ascending input from the ipsilateral central nucleus of the inferior colliculus (CNIC). In a previous study (Cant and Benson [2006] J. Comp. Neurol. 495:511-528), we made injections of biotinylated dextran amine into the CNIC of the gerbil and demonstrated that it can be divided into two parts. One part (zone 1) receives almost all of its ascending input from the cochlear nuclei, the nuclei of the lateral lemniscus, and the main nuclei of the superior olivary complex; the other part (zone 2) receives inputs from the cochlear nuclei and nuclei of the lateral lemniscus but few or no inputs from the main olivary nuclei. Here we show that these two parts of the CNIC project differentially to the MGv. Axons labeled anterogradely by injections in zone 1 project throughout the rostral two-thirds of the MGv, whereas axons from zone 2 project to the caudal third of the MGv. Throughout much of their extent, the terminal fields do not appear to overlap, although both parts of the CNIC project to medial and dorsal parts of the MGv, and there may be overlap in the most ventral part as well. The results indicate that two parallel pathways arising in the CNIC remain largely separate in the medial geniculate nucleus of the gerbil. It seems most likely that the neurons in the two terminal zones in the MGv perform different functions in audition.

Cholinergic modulation incorporated with a tone presentation induces frequency-specific threshold decreases in the auditory cortex of the mouse

Learning-induced or experience-dependent auditory cortical plasticity has often been characterized by frequency-specificity. Studies have revealed the critical role of the cholinergic basal forebrain and acoustic guidance. Cholinergic facilitation of specific thalamocortical inputs potentially determines such frequency-specificity but this issue requires further clarification. To examine the cholinergic effects on thalamocortical circuitry of specific frequency channels, we recorded the responses of cortical neurons while pairing basal forebrain activation or acetylcholine (ACh) microiontophoresis with tone presentations at 10 dB below the neuronal response threshold. We found that both basal forebrain activation and acetylcholine microiontophoresis paired with a tone induced a significant decrease in response threshold of the recorded cortical neurons to the frequency of the paired tone, and that this threshold decrease could be eliminated by atropine microiontophoresis. Our data suggest that cortical acetylcholine specifically facilitates thalamocortical circuitry tuned to the frequency of a presented tone; it is the first, fundamental step towards frequency-specific cortical plasticity evoked by auditory learning and experience.

Multiparametric Auditory Receptive Field Organization Across Five Cortical Fields in the Albino Rat

The auditory cortex of the rat is becoming an increasingly popular model system for studies of experience-dependent receptive field plasticity. However, the relative position of various fields within the auditory core and the receptive field organization within each field have yet to be fully described in the normative case. In this study, the macro- and micro-organizational features of the auditory cortex were studied in pentobarbital-anesthetized adult rats with a combination of physiological and anatomical methods. Dense microelectrode mapping procedures were used to identify the relative position of five tonotopically organized fields within the auditory core: primary auditory cortex (AI), the posterior auditory field (PAF), the anterior auditory field (AAF), the ventral auditory field (VAF), and the suprarhinal auditory field (SRAF). AI and AAF both featured short-latency, sharply tuned responses with predominantly monotonic intensity-response functions. SRAF and PAF were both characterized by longer-latency, broadly tuned responses. VAF directly abutted the ventral boundary of AI but was almost exclusively composed of low-threshold nonmonotonic intensity-tuned responses. Dual injection of retrograde tracers into AI and VAF was used to demonstrate that the sources of thalamic input from the medial geniculate body to each area were essentially nonoverlapping. An analysis of receptive field parameters beyond characteristic frequency revealed independent spatially ordered representations for features related to spectral tuning, intensity tuning, and onset response properties in AI, AAF, VAF, and SRAF. These data demonstrate that despite its greatly reduced physical scale, the rat auditory cortex features a surprising degree of organizational complexity and detail.

Parallel thalamocortical pathways for echolocation and passive sound localization in a gleaning bat, Antrozous pallidus

We present evidence for parallel auditory thalamocortical pathways that serve two different behaviors. The pallid bat listens for prey-generated noise (5-35 kHz) to localize prey, while reserving echolocation [downward frequency-modulated (FM) sweeps, 60-30 kHz] for obstacle avoidance. Its auditory cortex contains a tonotopic map representing frequencies from 6 to 70 kHz. The high-frequency (BF > 30 kHz) representation is dominated by FM sweep-selective neurons, whereas most neurons tuned to lower frequencies prefer noise. Retrograde tracer injections into these physiologically distinct cortical regions revealed that the high-frequency region receives input from the suprageniculate (SG) nucleus, but not the ventral division of the medial geniculate body (MGBv), in all experiments (n = 9). In contrast, the low-frequency region receives tonotopically organized input from the MGBv in all experiments (n = 16). Labeling in the SG was observed in only two of these experiments. Both cortical regions also receive sparse inputs from medial (MGBm) and parts of the dorsal division (MGBd) outside the SG. These results show that the low- and high-frequency regions of a single tonotopic map receive dominant inputs from different thalamic divisions. Within the low-frequency region, most neurons are binaurally inhibited, and an orderly map of interaural intensity difference (IID) sensitivity is present. We show that the input to the IID map arises from topographically organized projections from the MGBv. As observed in other species, a frequency-dependent organization is observed in the lateromedial direction in the MGBv. These data demonstrate that MGBv-to-auditory cortex connections are organized with respect to both frequency and binaural selectivity.

Antidromic Activation Reveals Tonotopically Organized Projections From Primary Auditory Cortex to the Central Nucleus of the Inferior Colliculus in Guinea Pig

The inferior colliculus (IC) is highly modulated by descending projections from higher auditory and nonauditory centers. Traditionally, corticofugal fibers were believed to project mainly to the extralemniscal IC regions. However, there is some anatomical evidence suggesting that a substantial number of fibers from the primary auditory cortex (A1) project into the IC central nucleus (ICC) and appear to be tonotopically organized. In this study, we used antidromic stimulation combined with other electrophysiological techniques to further investigate the spatial organization of descending fibers from A1 to the ICC in ketamine-anesthetized guinea pigs. Based on our findings, corticofugal fibers originate predominantly from layer V of A1, are amply scattered throughout the ICC and only project to ICC neurons with a similar best frequency (BF). This strict tonotopic pattern suggests that these corticofugal projections are involved with modulating spectral features of sound. Along the isofrequency dimension of the ICC, there appears to be some differences in projection patterns that depend on BF region and possibly isofrequency location within A1 and may be indicative of different descending coding strategies. Furthermore, the success of the antidromic stimulation method in our study demonstrates that it can be used to investigate some of the functional properties associated with corticofugal projections to the ICC as well as to other regions (e.g., medial geniculate body, cochlear nucleus). Such a method can address some of the limitations with current anatomical techniques for studying the auditory corticofugal system.

Development of functional organization of the pallid bat auditory cortex

The primary auditory cortex is characterized by a tonotopic map and a clustered organization of binaural properties. The factors involved in the development of overlain representation of these two properties are unclear. We addressed this issue in the auditory cortex of the pallid bat. The adult pallid bat cortex contains a systematic relationship between best frequency (BF) and binaural properties. Most neurons with BF<30 kHz are binaurally inhibited (EO/I), while most neurons with BF>30 kHz are monaural (EO). As in other species, binaural properties are clustered. The EO/I cluster contains a systematic map of interaural intensity difference (IID) sensitivity. We asked if these properties are present at the time the bat acquires its full audible range (postnatal day [P] 15). Tonotopy, relationship between BF and binaural properties, and the map of IID sensitivity are adult-like at P15. However, binaural facilitation is only observed in pups older than P25. Frequency selectivity shows a BF-dependent sharpening during development. Thus, overlain representation of binaural properties and tonotopy in the pallid bat cortex is remarkably adult-like at an age when the full audible range is first present, suggesting an experience-independent development of overlapping feature maps.

A spiking neuron model of cortical correlates of sensorineural hearing loss: Spontaneous firing, synchrony, and tinnitus

Hearing loss due to peripheral damage is associated with cochlear hair cell damage or loss and some retrograde degeneration of auditory nerve fibers. Surviving auditory nerve fibers in the impaired region exhibit elevated and broadened frequency tuning, and the cochleotopic representation of broadband stimuli such as speech is distorted. In impaired cortical regions, increased tuning to frequencies near the edge of the hearing loss coupled with increased spontaneous and synchronous firing is observed. Tinnitus, an auditory percept in the absence of sensory input, may arise under these circumstances as a result of plastic reorganization in the auditory cortex. We present a spiking neuron model of auditory cortex that captures several key features of cortical organization. A key assumption in the model is that in response to reduced afferent excitatory input in the damaged region, a compensatory change in the connection strengths of lateral excitatory and inhibitory connections occurs. These changes allow the model to capture some of the cortical correlates of sensorineural hearing loss, including changes in spontaneous firing and synchrony; these phenomena may explain central tinnitus. This model may also be useful for evaluating procedures designed to segregate synchronous activity underlying tinnitus and for evaluating adaptive hearing devices that compensate for selective hearing loss.

Representation of lateralization and tonotopy in primary versus secondary human auditory cortex

Functional MRI was performed to investigate differences in the basic functional organization of the primary and secondary auditory cortex regarding preferred stimulus lateralization and frequency. A modified sparse acquisition scheme was used to spatially map the characteristics of the auditory cortex at the level of individual voxels. In the regions of Heschl's gyrus and sulcus that correspond with the primary auditory cortex, activation was systematically strongest in response to contralateral stimulation. Contrarily, in the surrounding secondary active regions including the planum polare and the planum temporale, large-scale preferences with respect to stimulus lateralization were absent. Regarding optimal stimulus frequency, low- to high-frequency spatial gradients were discernable along the Heschl's gyrus and sulcus in anterolateral to posteromedial direction, especially in the right hemisphere, consistent with the presence of a tonotopic organization in these primary areas. However, in the surrounding activated secondary areas frequency preferences were erratic. Lateralization preferences did not depend on stimulus frequency, and frequency preferences did not depend on stimulus lateralization. While the primary auditory cortex is topographically organized with respect to physical stimulus properties (i.e., lateralization and frequency), such organizational principles are no longer obvious in secondary and higher areas. This suggests a neural re-encoding of sound signals in the transition from primary to secondary areas, possibly in relation to auditory scene analysis and the processing of auditory objects.

Auditory cortical responses to electrical stimulation of the inferior colliculus: implications for an auditory midbrain implant

The success and limitations of cochlear implants (CIs) along with recent advances in deep brain stimulation and neural engineering have motivated the development of a central auditory prosthesis. In this study, we investigated the effects of electrical stimulation of the inferior colliculus central nucleus (ICC) on primary auditory cortex (A1) activity to determine the potential benefits of an auditory midbrain implant (AMI). We recorded multiunit activity in A1 of ketamine-anesthetized guinea pigs in response to single-pulse (200 micros/phase) monopolar stimulation of the ICC using multisite silicon-substrate probes. We then compared measures of threshold, dynamic range, and tonotopic spread of activation for ICC stimulation with that of published data for CI stimulation. Our results showed that compared with cochlear stimulation, ICC stimulation achieved: 1) thresholds about 8 dB lower; 2) dynamic ranges > or = 4 dB greater; and 3) more localized, frequency-specific activation, even though frequency specificity was partially lost at higher stimulus levels for low-frequency ICC regions. Our results also showed that stimulation of rostral ICC regions elicited lower thresholds but with greater activation spread along the tonotopic gradient of A1 than did stimulation of more caudal regions. These results suggest that an AMI may improve frequency and level coding with lower energy requirements compared with CIs. However, a trade-off between lower perceptual thresholds and better frequency discrimination may exist that depends on location of stimulation along the caudorostral dimension of the ICC. Overall, this study provides the foundation for future AMI research and development.

Spectral integration in auditory cortex: mechanisms and modulation

Auditory cortex contributes to the processing and perception of spectrotemporally complex stimuli. However, the mechanisms by which this is accomplished are not well understood. In this review, we examine evidence that single cortical neurons receive input covering much of the audible spectrum. We then propose an anatomical framework by which spectral information converges on single neurons in primary auditory cortex, via a combination of thalamocortical and intracortical "horizontal" pathways. By its nature, the framework confers sensitivity to specific, spectrotemporally complex stimuli. Finally, to address how spectral integration can be regulated, we show how one neuromodulator, acetylcholine, could act within the hypothesized framework to alter integration in single neurons. The results of these studies promote a cellular understanding of information processing in auditory cortex.

Cortical Intrinsic Circuits Can Support Activity Propagation through an Isofrequency Strip of the Guinea Pig Primary Auditory Cortex

A pure tone evokes propagating activities in a strip of the primary auditory cortex (AI), an isofrequency strip (IS). A fundamental issue concerns the roles that thalamocortical input and intracortical connectivity play in generating the activities. Here we addressed this issue in guinea pigs using in vivo and in vitro real-time optical imaging techniques. As reported previously, tone-evoked activity propagated dorsoventrally along a strip (an IS) in AI. We found that an electrical pulse applied focally within the strip, triggered activity propagation with a spatiotemporal pattern highly similar to tone-evoked activation. The propagation velocity of electrically evoked activity was significantly slower than that of tone-evoked activity, but was comparable to the velocity of lateral activity propagation in cortical slices, suggesting that the electrically evoked activity propagation in vivo is mediated by intracortical circuits. To test this notion, we lesioned the auditory thalamus chemically; in such animals, electrically evoked activity in AI was not affected, although tone-evoked activity was abolished. Further, in slices of the AI, the extent of electrically evoked activity propagation in layer II/III was significantly larger in coronal slices than in horizontal slices. Together, our results suggest that intracortical connectivity in AI enables a focally evoked activity to propagate throughout an IS.

Auditory thalamic organization: Cellular slabs, dendritic arbors and tectothalamic axons underlying the frequency map

A model of auditory thalamic organization is presented incorporating cellular laminae, oriented dendritic arbors and tectothalamic axons as a basis for the tonotopic map at this level of the central auditory system. The heart of this model is the laminar organization of neuronal somata in the ventral division of the medial geniculate body (MGV) of the rabbit, visible in routine Nissl stains. Microelectrode studies have demonstrated a step-wise ascending progression of best frequencies perpendicular to the cell layers. The dendritic arbors of MGV neurons are aligned parallel to the cellular laminae and dendritic tree width along the frequency axis corresponds closely with the frequency steps seen in microelectrode studies. In the laminated subdivision, tectothalamic axons terminate in the form of bands closely aligned with the laminae and dendritic arbors of thalamic relay neurons. The bands of tectothalamic axons extend in the anterior-posterior (A-P) plane forming a dorsal-ventral series of stacked frequency slabs. In the pars ovoidea region, the homologous spiraling of somata, dendritic fields and tectothalamic axons appear to represent a low-frequency area in this species. At least two types of tectothalamic terminals were found within the bands: large boutons frequently arranged in a glomerular pattern and smaller boutons arising from fine caliber axons. We propose that the rabbit is an ideal model to investigate the structural-functional basis of functional maps in the mammalian auditory forebrain.

Time course of embryonic midbrain and thalamic auditory connection development in mice as revealed by carbocyanine dye tracing

Central auditory connections develop in mice before the onset of hearing, around postnatal day 7. Two previous studies have investigated the development of auditory nuclei projections and lateral lemniscal nuclear projections in embryonic rats, respectively. Here, we provide detail for the first time of the initiation and progression of projections from the inferior colliculus (IC) to the medial geniculate body (MGB) and from the MGB to the auditory cortex (AC). Overall, the developmental progression of projections follows that of terminal mitoses in various nuclei, suggesting the consistent use of a developmental timetable at a given nucleus, independent of that of other nuclei. Our data further suggest that neurons project specifically and reciprocally from the MGB to the AC as early as embryonic day 14.5. These projections develop approximately a day before the reciprocal connections between the MGB and IC and before development of projections from the auditory nuclei to the IC. The development of IC projections is prolonged and progresses from rostral to caudal areas. Brainstem nuclear projections to the IC arrive first from the lateral lemniscus nuclei then the superior olive and finally the cochlear nuclei. Overall, the auditory connection development strongly suggests that most of the overall specificity of nuclear connections is set up at least 2 weeks before the onset of sound-mediated cochlea responses in mice and, thus, is likely governed predominantly by molecular genetic clues.

Modular Functional Organization of Cat Anterior Auditory Field

Two tonotopic areas, the primary auditory cortex (AI) and the anterior auditory field (AAF), are the primary cortical fields in the cat auditory system. They receive largely independent, concurrent thalamocortical projections from the different thalamic divisions despite their hierarchical equivalency. The parallel streams of thalamic inputs to AAF and AI suggest that AAF neurons may differ from AI neurons in physiological properties. Although a modular functional organization in cat AI has been well documented, little is known about the internal organization of AAF beyond tonotopy. We studied how basic receptive field parameters (RFPs) are spatially organized in AAF with single- and multiunit recording techniques. A distorted tonotopicity with an underrepresentation in midfrequencies (1 and 5 kHz) and an overrepresentation in the high-frequency range was found. Spectral bandwidth (Q-values) and response threshold were significantly correlated with characteristic frequency (CF). To understand whether AAF has a modular organization of RFPs, CF dependencies were eliminated by a nonparametric, local regression model, and the residuals (difference between the model and observed values) were evaluated. In a given isofrequency domain, clusters of low or high residual RFP values were interleaved for threshold, spectral bandwidth, and latency, suggesting a modular organization. However, RFP modules in AAF were not expressed as robustly as in AI. A comparison of RFPs between AAF and AI shows that AAF neurons were more broadly tuned and had shorter latencies than AI neurons. These physiological field differences are consistent with anatomical evidence of largely independent, concurrent thalamocortical projections in AI and AAF, which strongly suggest field-specific processing.

Tonotopic and heterotopic projection systems in physiologically defined auditory cortex

Combined physiological and connectional studies show significant non-topographic extrinsic projections to frequency-specific domains in the cat auditory cortex. These frequency-mismatched loci in the thalamus, ipsilateral cortex, and commissural system complement the predicted topographic and tonotopic projections. Two tonotopic areas, the primary auditory cortex (AI) and the anterior auditory field (AAF), were electrophysiologically characterized by their frequency organization. Next, either cholera toxin beta subunit or cholera toxin beta subunit gold conjugate was injected into frequency-matched locations in each area to reveal the projection pattern from the thalamus and cortex. Most retrograde labeling was found at tonotopically appropriate locations within a 1 mm-wide strip in the thalamus and a 2-3 mm-wide expanse of cortex (approximately 85%). However, approximately 13-30% of the neurons originated from frequency-mismatched locations far from their predicted positions in thalamic nuclei and cortical areas, respectively. We propose that these heterotopic projections satisfy at least three criteria that may be necessary to support the magnitude and character of plastic changes in physiological studies. First, they are found in the thalamus, ipsilateral and commissural cortex; since this reorganization could arise from any of these routes and may involve each, such projections ought to occur in them. Second, they originate from nuclei and areas with or without tonotopy; it is likely that plasticity is not exclusively shaped by spectral influences and not limited to cochleotopic regions. Finally, the projections are appropriate in magnitude and sign to plausibly support such rearrangements; given the rapidity of some aspects of plastic changes, they should be mediated by substantial existing connections. Alternative roles for these heterotopic projections are also considered.

Cortical mechanisms in hearing

Current understanding of neural processing in the auditory cortex has been shaped by a variety of experimental approaches in animals and humans. It remains a daunting challenge to reconcile data as diverse as synaptic properties recorded in a rodent brain slice and functional images of auditory cortex in a behaving human. Nevertheless, the gaps are narrowing through a renewed focus on humans and other primates, a continuing interest in evidence for functional pathways, a broader application of modern imaging techniques, a growing awareness of cortical sensitivity to dynamic features of sounds, and an improved understanding of auditory cortical circuitry.