Sharpening of frequency tuning by inhibition in the central auditory system: tribute to Yasuji Katsuki

Opposite effects of rapid auditory stimulation on tetanized and non-tetanized tone of adjacent frequency: Mismatch negativity study

Our study describes the effects of sensory tetanization on neurophysiological and behavioral measures in humans linking cellular studies of long-term potentiation with high-level brain processes. Rapid (every 75ms) presentation of pure tone (1020 Hz, 50ms) for 2 minutes was preceded and followed by oddball blocks that contained the same stimulus presented as deviant (probability of 5-10%) interspersed with standard (80-90%) and deviant tones (5-10%) of adjacent frequencies (1000 and 980Hz, respectively). Mismatch negativity (MMN) component in response to tetanized tone (1020Hz), while being similar to MMN for non-tetanized tone before tetanization, became larger than that after tetanization, pointing to the increase in cortical differentiation of these tones. However, this differentiation was partially due to the MMN decrease after tetanization for tones adjacent to tetanized frequency, suggesting the influence of lateral inhibition to this effect. Although MMN correlated with tone discriminability in a psychophysical task, the behavioral improvement after tetanization was not statistically detectable. To conclude, short-term auditory tetanization affects cortical representation of tones that are not limited to the tetanized stimuli.

Spectrally non-overlapping background noise disturbs echolocation via acoustic masking in the CF-FM bat, Hipposideros pratti

The environment noise may disturb animal behavior and echolocation via three potential mechanisms: acoustic masking, reduced attention and noise avoidance. Compared with the mechanisms of reduced attention and noise avoidance, acoustic masking is thought to occur only when the signal and background noise overlap spectrally and temporally. In this study, we investigated the effects of spectrally non-overlapping noise on echolocation pulses and electrophysiological responses of a constant frequency-frequency modulation (CF-FM) bat, Hipposideros pratti. We found that H. pratti called at higher intensities while keeping the CFs of their echolocation pulses consistent. Electrophysiological tests indicated that the noise could decrease auditory sensitivity and sharp intensity tuning, suggesting that spectrally non-overlapping noise imparts an acoustic masking effect. Because anthropogenic noises are usually concentrated at low frequencies and are spectrally non-overlapping with the bat's echolocation pulses, our results provide further evidence of negative consequences of anthropogenic noise. On this basis, we sound a warning against noise in the foraging habitats of echolocating bats.

A lateral inhibition mechanism explains the dissociation between mismatch negativity and behavioral pitch discrimination

Although mismatch negativity (MMN), a change-specific component of auditory event-related potential, is considered to be an index of sound discrimination accuracy, the amplitude of the MMN responses elicited by pitch height deviations in musicians and tone language speakers with superior pitch discrimination is usually not enhanced compared to that elicited in individuals with inferior pitch discrimination. We hypothesized that superior pitch discrimination is accompanied by enhanced lateral inhibition, a critical neural mechanism that sharpens the tuning curves of the auditory neurons in the tonotopy. Forty Mandarin-speaking healthy adults completed an auditory EEG experiment in which MMN was elicited by pitch height deviations in both pure and harmonic tones. Their behavioral pitch discrimination was indexed by the difference limens measured using pure and harmonic tones. Behavioral pitch discrimination correlated significantly with the MMN elicited by pure tones, but not by harmonic tones; this could be due to lateral inhibition strongly influencing the MMN elicited by harmonic tones but having less effect on the MMN elicited by pure tones. As lateral inhibition is a neural mechanism for attenuating the amplitude of MMN, our results support the notion that an enhanced lateral inhibition mechanism underlies superior pitch discrimination.

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

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

Inhibitory mechanisms shaping delay-tuned combination-sensitivity in the auditory cortex and thalamus of the mustached bat

Delay-tuned auditory neurons of the mustached bat show facilitative responses to a combination of signal elements of a biosonar pulse-echo pair with a specific echo delay. The subcollicular nuclei produce latency-constant phasic on-responding neurons, and the inferior colliculus produces delay-tuned combination-sensitive neurons, designated "FM-FM" neurons. The combination-sensitivity is a facilitated response to the coincidence of the excitatory rebound following glycinergic inhibition to the pulse (1st harmonic) and the short-latency response to the echo (2nd-4th harmonics). The facilitative response of thalamic FM-FM neurons is mediated by glutamate receptors (NMDA and non-NMDA receptors). Different from collicular FM-FM neurons, thalamic ones respond more selectively to pulse-echo pairs than individual signal elements. A number of differences in response properties between collicular and thalamic or cortical FM-FM neurons have been reported. However, differences between thalamic and cortical FM-FM neurons have remained to be studied. Here, we report that GABAergic inhibition controls the duration of burst of spikes of facilitative responses of thalamic FM-FM neurons and sharpens the delay tuning of cortical ones. That is, intra-cortical inhibition sharpens the delay tuning of cortical FM-FM neurons that is potentially broad because of divergent/convergent thalamo-cortical projections. Compared with thalamic neurons, cortical ones tend to show sharper delay tuning, longer response duration, and larger facilitation index. However, those differences are statistically insignificant.

Sensitivity to Vocalization Pitch in the Caudal Auditory Cortex of the Marmoset: Comparison of Core and Belt Areas

Based on anatomical connectivity and basic response characteristics, primate auditory cortex is divided into a central core surrounded by belt and parabelt regions. The encoding of pitch, a prototypical element of sound identity, has been studied in primary auditory cortex (A1) but little is known about how it is encoded and represented beyond A1. The caudal auditory belt and parabelt cortical fields process spatial information but also contain information on non-spatial aspects of sounds. In this study, we examined neuronal responses in these areas to pitch-varied marmoset vocalizations, to derive the consequent representation of pitch in these regions and the potential underlying mechanisms, to compare to the encoding and representation of pitch of the same sounds in A1. With respect to response patterns to the vocalizations, neurons in caudal medial belt (CM) showed similar short-latency and short-duration response patterns to A1, but caudal lateral belt (CL) neurons at the same hierarchical level and caudal parabelt (CPB) neurons at a higher hierarchical level showed delayed or much delayed response onset and prolonged response durations. With respect to encoding of pitch, neurons in all cortical fields showed sensitivity to variations in the vocalization pitch either through modulation of spike-count or of first spike-latency. The utility of the encoding mechanism differed between fields: pitch sensitivity was reliably represented by spike-count variations in A1 and CM, while first spike-latency variation was better for encoding pitch in CL and CPB. In summary, our data show that (a) the traditionally-defined belt area CM is functionally very similar to A1 with respect to the representation and encoding of complex naturalistic sounds, (b) the CL belt area, at the same hierarchical level as CM, and the CPB area, at a higher hierarchical level, have very different response patterns and appear to use different pitch-encoding mechanisms, and (c) caudal auditory fields, proposed to be specialized for encoding spatial location, can also contain robust representations of sound identity.

Behaviorally relevant frequency selectivity in single- and double-on neurons in the inferior colliculus of the Pratt's roundleaf bat, Hipposideros pratti

Frequency analysis is a fundamental function of the auditory system, and it is essential to study the auditory response properties using behavior-related sounds. Our previous study has shown that the inferior collicular (IC) neurons of CF-FM (constant frequency-frequency modulation) bats could be classified into single-on (SO) and double-on (DO) neurons under CF-FM stimulation. Here, we employed Pratt's roundleaf bats, Hipposideros pratti, to investigate the frequency selectivity of SO and DO neurons in response to CF and behavior-related CF-FM sounds using in vivo extracellular recordings. The results demonstrated that the bandwidths (BWs) of iso-frequency tuning curves had no significant differences between the SO and the DO neurons when stimulated by CF sounds. However, the SO neurons had significant narrower BWs than DO neurons when stimulated with CF-FM sounds. In vivo intracellular recordings showed that both SO and DO neurons had significantly shorter post-spike hyperpolarization latency and excitatory duration in response to CF-FM in comparison to CF stimuli, suggesting that the FM component had an inhibitory effect on the responses to the CF component. These results suggested that SO neurons had higher frequency selectivity than DO neurons under behavior-related CF-FM stimulation, making them suitable for detecting frequency changes during echolocation.

Synaptic convergence of afferent inputs in primary infrared-sensitive nucleus (LTTD) neurons of rattlesnakes (Crotalinae) as the origin for sensory contrast enhancement

Pitvipers have a specialized sensory system in the upper jaw to detect infrared (IR) radiation. The bilateral pit organs resemble simple pinhole cameras that map IR objects onto the sensory epithelium as blurred representations of the environment. Trigeminal afferents transmit information about changing temperature patterns as neuronal spike discharge in a topographic manner to the hindbrain nucleus of the lateral descending trigeminal tract (LTTD). A presumed, yet so far unknown neuronal connectivity within this central nucleus exerts a synaptic computation that constrains the relatively large receptive field of primary afferent fibers. Here, we used intracellular recordings of LTTD neurons in isolated rattlesnake brains to decipher the spatio-temporal pattern of excitatory and inhibitory responses following electrical stimulation of single and multiple peripheral pit organ-innervating nerve branches. The responses of individual neurons consisted of complex spike sequences that derived from spatially and temporally specific interactions between excitatory and inhibitory synaptic inputs from the same as well as from adjacent peripheral nerve terminal areas. This pattern complies with a central excitation that is flanked by a delayed lateral inhibition, thereby enhancing the contrast of IR sensory input, functionally reminiscent of the computations for contrast enhancement in the peripheral visual system.

Rate, not selectivity, determines neuronal population coding accuracy in auditory cortex

The notion that neurons with higher selectivity carry more information about external sensory inputs is widely accepted in neuroscience. High-selectivity neurons respond to a narrow range of sensory inputs, and thus would be considered highly informative by rejecting a large proportion of possible inputs. In auditory cortex, neuronal responses are less selective immediately after the onset of a sound and then become highly selective in the following sustained response epoch. These 2 temporal response epochs have thus been interpreted to encode first the presence and then the content of a sound input. Contrary to predictions from that prevailing theory, however, we found that the neural population conveys similar information about sound input across the 2 epochs in spite of the neuronal selectivity differences. The amount of information encoded turns out to be almost completely dependent upon the total number of population spikes in the read-out window for this system. Moreover, inhomogeneous Poisson spiking behavior is sufficient to account for this property. These results imply a novel principle of sensory encoding that is potentially shared widely among multiple sensory systems.

Modulatory Effects of Attention on Lateral Inhibition in the Human Auditory Cortex

Reduced neural processing of a tone is observed when it is presented after a sound whose spectral range closely frames the frequency of the tone. This observation might be explained by the mechanism of lateral inhibition (LI) due to inhibitory interneurons in the auditory system. So far, several characteristics of bottom up influences on LI have been identified, while the influence of top-down processes such as directed attention on LI has not been investigated. Hence, the study at hand aims at investigating the modulatory effects of focused attention on LI in the human auditory cortex. In the magnetoencephalograph, we present two types of masking sounds (white noise vs. withe noise passing through a notch filter centered at a specific frequency), followed by a test tone with a frequency corresponding to the center-frequency of the notch filter. Simultaneously, subjects were presented with visual input on a screen. To modulate the focus of attention, subjects were instructed to concentrate either on the auditory input or the visual stimuli. More specific, on one half of the trials, subjects were instructed to detect small deviations in loudness in the masking sounds while on the other half of the trials subjects were asked to detect target stimuli on the screen. The results revealed a reduction in neural activation due to LI, which was larger during auditory compared to visual focused attention. Attentional modulations of LI were observed in two post-N1m time intervals. These findings underline the robustness of reduced neural activation due to LI in the auditory cortex and point towards the important role of attention on the modulation of this mechanism in more evaluative processing stages.

Encoding of frequency-modulation (FM) rates in human auditory cortex

Frequency-modulated sounds play an important role in our daily social life. However, it currently remains unclear whether frequency modulation rates affect neural activity in the human auditory cortex. In the present study, using magnetoencephalography, we investigated the auditory evoked N1m and sustained field responses elicited by temporally repeated and superimposed frequency-modulated sweeps that were matched in the spectral domain, but differed in frequency modulation rates (1, 4, 16, and 64 octaves per sec). The results obtained demonstrated that the higher rate frequency-modulated sweeps elicited the smaller N1m and the larger sustained field responses. Frequency modulation rate had a significant impact on the human brain responses, thereby providing a key for disentangling a series of natural frequency-modulated sounds such as speech and music.

Synaptic interactions and inhibitory regulation in auditory cortex

This Special Issue focuses on the auditory-evoked mismatch negativity (MMN), an electrophysiological index of change, and its reduction in schizophrenia. The following brief review is an attempt to complement the behavioral and clinical contributions to the Special Issue by providing basic information on synaptic interactions and processing in auditory cortex. A key observation in previous studies is that the MMN involves activation of cortical N-methyl-D-aspartate (NMDA) receptors. Yet, NMDA receptor activation is regulated by a number of synaptic events, which also may contribute to the MMN reduction in schizophrenia. Accordingly, this review will focus on synaptic interactions, notably inhibitory regulation of NMDA receptor-mediated activity, in auditory cortex.

Effect of echolocation behavior-related constant frequency-frequency modulation sound on the frequency tuning of inferior collicular neurons in Hipposideros armiger

In constant frequency-frequency modulation (CF-FM) bats, the CF-FM echolocation signals include both CF and FM components, yet the role of such complex acoustic signals in frequency resolution by bats remains unknown. Using CF and CF-FM echolocation signals as acoustic stimuli, the responses of inferior collicular (IC) neurons of Hipposideros armiger were obtained by extracellular recordings. We tested the effect of preceding CF or CF-FM sounds on the shape of the frequency tuning curves (FTCs) of IC neurons. Results showed that both CF-FM and CF sounds reduced the number of FTCs with tailed lower-frequency-side of IC neurons. However, more IC neurons experienced such conversion after adding CF-FM sound compared with CF sound. We also found that the Q 20 value of the FTC of IC neurons experienced the largest increase with the addition of CF-FM sound. Moreover, only CF-FM sound could cause an increase in the slope of the neurons' FTCs, and such increase occurred mainly in the lower-frequency edge. These results suggested that CF-FM sound could increase the accuracy of frequency analysis of echo and cut-off low-frequency elements from the habitat of bats more than CF sound.

Layer specific sharpening of frequency tuning by selective attention in primary auditory cortex

Recent electrophysiological and neuroimaging studies provide converging evidence that attending to sounds increases the response selectivity of neuronal ensembles even at the first cortical stage of auditory stimulus processing in primary auditory cortex (A1). This is achieved by enhancement of responses in the regions that process attended frequency content, and by suppression of responses in the surrounding regions. The goals of our study were to define the extent to which A1 neuronal ensembles are involved in this process, determine its effect on the frequency tuning of A1 neuronal ensembles, and examine the involvement of the different cortical layers. To accomplish these, we analyzed laminar profiles of synaptic activity and action potentials recorded in A1 of macaques performing a rhythmic intermodal selective attention task. We found that the frequency tuning of neuronal ensembles was sharpened due to both increased gain at the preferentially processed or best frequency and increased response suppression at all other frequencies when auditory stimuli were attended. Our results suggest that these effects are due to a frequency-specific counterphase entrainment of ongoing delta oscillations, which predictively orchestrates opposite sign excitability changes across all of A1. This results in a net suppressive effect due to the large proportion of neuronal ensembles that do not specifically process the attended frequency content. Furthermore, analysis of laminar activation profiles revealed that although attention-related suppressive effects predominate the responses of supragranular neuronal ensembles, response enhancement is dominant in the granular and infragranular layers, providing evidence for layer-specific cortical operations in attentive stimulus processing.

A 6 μW per channel analog biomimetic cochlear implant processor filterbank architecture with across channels AGC

A new analog cochlear implant processor filterbank architecture of increased biofidelity, enhanced across-channel contrast and very low power consumption has been designed and prototyped. Each channel implements a biomimetic, asymmetric bandpass-like One-Zero-Gammatone-Filter (OZGF) transfer function, using class-AB log-domain techniques. Each channel's quality factor and suppression are controlled by means of a new low power Automatic Gain Control (AGC) scheme which is coupled across the neighboring channels and emulates lateral inhibition (LI) phenomena in the auditory system. Detailed measurements from a five-channel silicon IC prototype fabricated in a 0.35 μm AMS technology confirm the operation of the coupled AGC scheme and its ability to enhance contrast among channel outputs. The prototype is characterized by an input dynamic range of 92 dB while consuming only 28 μW of power in total ( ∼ 6 μW per channel) under a 1.8 V power supply. The architecture is well-suited for fully-implantable cochlear implants.

Modulatory effects of spectral energy contrasts on lateral inhibition in the human auditory cortex: an MEG study

We investigated the modulation of lateral inhibition in the human auditory cortex by means of magnetoencephalography (MEG). In the first experiment, five acoustic masking stimuli (MS), consisting of noise passing through a digital notch filter which was centered at 1 kHz, were presented. The spectral energy contrasts of four MS were modified systematically by either amplifying or attenuating the edge-frequency bands around the notch (EFB) by 30 dB. Additionally, the width of EFB amplification/attenuation was varied (3/8 or 7/8 octave on each side of the notch). N1m and auditory steady state responses (ASSR), evoked by a test stimulus with a carrier frequency of 1 kHz, were evaluated. A consistent dependence of N1m responses upon the preceding MS was observed. The minimal N1m source strength was found in the narrowest amplified EFB condition, representing pronounced lateral inhibition of neurons with characteristic frequencies corresponding to the center frequency of the notch (NOTCH CF) in secondary auditory cortical areas. We tested in a second experiment whether an even narrower bandwidth of EFB amplification would result in further enhanced lateral inhibition of the NOTCH CF. Here three MS were presented, two of which were modified by amplifying 1/8 or 1/24 octave EFB width around the notch. We found that N1m responses were again significantly smaller in both amplified EFB conditions as compared to the NFN condition. To our knowledge, this is the first study demonstrating that the energy and width of the EFB around the notch modulate lateral inhibition in human secondary auditory cortical areas. Because it is assumed that chronic tinnitus is caused by a lack of lateral inhibition, these new insights could be used as a tool for further improvement of tinnitus treatments focusing on the lateral inhibition of neurons corresponding to the tinnitus frequency, such as the tailor-made notched music training.

Cortical inhibition reduces information redundancy at presentation of communication sounds in the primary auditory cortex

In all sensory modalities, intracortical inhibition shapes the functional properties of cortical neurons but also influences the responses to natural stimuli. Studies performed in various species have revealed that auditory cortex neurons respond to conspecific vocalizations by temporal spike patterns displaying a high trial-to-trial reliability, which might result from precise timing between excitation and inhibition. Studying the guinea pig auditory cortex, we show that partial blockage of GABAA receptors by gabazine (GBZ) application (10 μm, a concentration that promotes expansion of cortical receptive fields) increased the evoked firing rate and the spike-timing reliability during presentation of communication sounds (conspecific and heterospecific vocalizations), whereas GABAB receptor antagonists [10 μm saclofen; 10-50 μm CGP55845 (p-3-aminopropyl-p-diethoxymethyl phosphoric acid)] had nonsignificant effects. Computing mutual information (MI) from the responses to vocalizations using either the evoked firing rate or the temporal spike patterns revealed that GBZ application increased the MI derived from the activity of single cortical site but did not change the MI derived from population activity. In addition, quantification of information redundancy showed that GBZ significantly increased redundancy at the population level. This result suggests that a potential role of intracortical inhibition is to reduce information redundancy during the processing of natural stimuli.

Enhanced representation of spectral contrasts in the primary auditory cortex

The role of early auditory processing may be to extract some elementary features from an acoustic mixture in order to organize the auditory scene. To accomplish this task, the central auditory system may rely on the fact that sensory objects are often composed of spectral edges, i.e., regions where the stimulus energy changes abruptly over frequency. The processing of acoustic stimuli may benefit from a mechanism enhancing the internal representation of spectral edges. While the visual system is thought to rely heavily on this mechanism (enhancing spatial edges), it is still unclear whether a related process plays a significant role in audition. We investigated the cortical representation of spectral edges, using acoustic stimuli composed of multi-tone pips whose time-averaged spectral envelope contained suppressed or enhanced regions. Importantly, the stimuli were designed such that neural responses properties could be assessed as a function of stimulus frequency during stimulus presentation. Our results suggest that the representation of acoustic spectral edges is enhanced in the auditory cortex, and that this enhancement is sensitive to the characteristics of the spectral contrast profile, such as depth, sharpness and width. Spectral edges are maximally enhanced for sharp contrast and large depth. Cortical activity was also suppressed at frequencies within the suppressed region. To note, the suppression of firing was larger at frequencies nearby the lower edge of the suppressed region than at the upper edge. Overall, the present study gives critical insights into the processing of spectral contrasts in the auditory system.

Neural interactions in unilateral colliculus and between bilateral colliculi modulate auditory signal processing

In the auditory pathway, the inferior colliculus (IC) is a major center for temporal and spectral integration of auditory information. There are widespread neural interactions in unilateral (one) IC and between bilateral (two) ICs that could modulate auditory signal processing such as the amplitude and frequency selectivity of IC neurons. These neural interactions are either inhibitory or excitatory, and are mostly mediated by γ-aminobutyric acid (GABA) and glutamate, respectively. However, the majority of interactions are inhibitory while excitatory interactions are in the minority. Such unbalanced properties between excitatory and inhibitory projections have an important role in the formation of unilateral auditory dominance and sound location, and the neural interaction in one IC and between two ICs provide an adjustable and plastic modulation pattern for auditory signal processing.

Functional magnetic resonance imaging of sound pressure level encoding in the rat central auditory system

Intensity is an important physical property of a sound wave and is customarily reported as sound pressure level (SPL). Invasive techniques such as electrical recordings, which typically examine one brain region at a time, have been used to study neuronal encoding of SPL throughout the central auditory system. Non-invasive functional magnetic resonance imaging (fMRI) with large field of view can simultaneously examine multiple auditory structures. We applied fMRI to measure the hemodynamic responses in the rat brain during sound stimulation at seven SPLs over a 72 dB range. This study used a sparse temporal sampling paradigm to reduce the adverse effects of scanner noise. Hemodynamic responses were measured from the central nucleus of the inferior colliculus (CIC), external cortex of the inferior colliculus (ECIC), lateral lemniscus (LL), medial geniculate body (MGB), and auditory cortex (AC). BOLD signal changes generally increase significantly (p<0.001) with SPL and the dependence is monotonic in CIC, ECIC, and LL. The ECIC has higher BOLD signal change than CIC and LL at high SPLs. The difference between BOLD signal changes at high and low SPLs is less in the MGB and AC. This suggests that the SPL dependences of the LL and IC are different from those in the MGB and AC and the SPL dependence of the CIC is different from that of the ECIC. These observations are likely related to earlier observations that neurons with firing rates that increase monotonically with SPL are dominant in the CIC, ECIC, and LL while non-monotonic neurons are dominant in the MGB and AC. Further, the IC's SPL dependence measured in this study is very similar to that measured in our earlier study using the continuous imaging method. Therefore, sparse temporal sampling may not be a prerequisite in auditory fMRI studies of the IC.

Human pitch detectors are tuned on a fine scale, but are perceptually accessed on a coarse scale

Single neurons in auditory cortex display highly selective spectrotemporal properties: their receptive fields modulate over small fractions of an octave and integrate across temporal windows of 100-200 ms. We investigated how these characteristics impact auditory behavior. Human observers were asked to detect a specific sound frequency masked by broadband noise; we adopted an experimental design which required the engagement of frequency-selective mechanisms to perform above chance. We then applied psychophysical reverse correlation to derive spectrotemporal perceptual filters for the assigned task. We were able to expose signatures of neuronal-like spectrotemporal tuning on a scale of 1/10 octave and 50-100 ms, but detailed modeling of our results showed that observers were not able to rely on the explicit output of these channels. Instead, human observers pooled from a large bank of highly selective channels via a weighting envelope poorly tuned for frequency (on a scale of 1.5 octave) with sluggish temporal dynamics, followed by a highly nonlinear max-like operation. We conclude that human detection of specific frequencies embedded within complex sounds suffers from a high degree of intrinsic spectrotemporal uncertainty, resulting in low efficiency values (<1 %) for this perceptual ability. Signatures of the underlying neural circuitry can be exposed, but there does not appear to be a direct line for accessing individual neural channels on a fine scale.

Music-induced cortical plasticity and lateral inhibition in the human auditory cortex as foundations for tonal tinnitus treatment

Over the past 15 years, we have studied plasticity in the human auditory cortex by means of magnetoencephalography (MEG). Two main topics nurtured our curiosity: the effects of musical training on plasticity in the auditory system, and the effects of lateral inhibition. One of our plasticity studies found that listening to notched music for 3 h inhibited the neuronal activity in the auditory cortex that corresponded to the center-frequency of the notch, suggesting suppression of neural activity by lateral inhibition. Subsequent research on this topic found that suppression was notably dependent upon the notch width employed, that the lower notch-edge induced stronger attenuation of neural activity than the higher notch-edge, and that auditory focused attention strengthened the inhibitory networks. Crucially, the overall effects of lateral inhibition on human auditory cortical activity were stronger than the habituation effects. Based on these results we developed a novel treatment strategy for tonal tinnitus-tailor-made notched music training (TMNMT). By notching the music energy spectrum around the individual tinnitus frequency, we intended to attract lateral inhibition to auditory neurons involved in tinnitus perception. So far, the training strategy has been evaluated in two studies. The results of the initial long-term controlled study (12 months) supported the validity of the treatment concept: subjective tinnitus loudness and annoyance were significantly reduced after TMNMT but not when notching spared the tinnitus frequencies. Correspondingly, tinnitus-related auditory evoked fields (AEFs) were significantly reduced after training. The subsequent short-term (5 days) training study indicated that training was more effective in the case of tinnitus frequencies ≤ 8 kHz compared to tinnitus frequencies >8 kHz, and that training should be employed over a long-term in order to induce more persistent effects. Further development and evaluation of TMNMT therapy are planned. A goal is to transfer this novel, completely non-invasive and low-cost treatment approach for tonal tinnitus into routine clinical practice.

Is the effect of tinnitus on auditory steady-state response amplitude mediated by attention?

Objectives: Auditory steady-state response (ASSR) amplitude enhancement effects have been reported in tinnitus patients. As ASSR amplitude is also enhanced by attention, the effect of tinnitus on ASSR amplitude could be interpreted as an effect of attention mediated by tinnitus. As N1 attention effects are significantly larger than those on the ASSR, if the effect of tinnitus on ASSR amplitude were due to attention, there should be similar amplitude enhancement effects in tinnitus for the N1 component of the auditory-evoked response. Methods: MEG recordings which were previously examined for the ASSR (Diesch et al., 2010a) were analyzed with respect to the N1m component. Like the ASSR previously, the N1m was analyzed in the source domain (source space projection). Stimuli were amplitude-modulated (AM) tones with one of three carrier frequencies matching the tinnitus frequency or a surrogate frequency 1½ octave above the audiometric edge frequency in controls, the audiometric edge frequency, and a frequency below the audiometric edge. Single AM-tones were presented in a single condition and superpositions of three AM-tones differing in carrier and modulation frequency in a composite condition. Results: In the earlier ASSR study (Diesch et al., 2010a), the ASSR amplitude in tinnitus patients, but not in controls, was significantly larger in the (surrogate) tinnitus condition than in the edge condition. Patients showed less evidence than controls of reciprocal inhibition of component ASSR responses in the composite condition. In the present study, N1m amplitudes elicited by stimuli located at the audiometric edge and at the (surrogate) tinnitus frequency were smaller than N1m amplitudes elicited by sub-edge tones both in patients and controls. The relationship of the N1m response in the composite condition to the N1m response in the single condition indicated that reciprocal inhibition among component N1m responses was reduced in patients compared against controls. Conclusions: In the present study, no evidence was found for an N1-amplitude enhancement effect in tinnitus. Compared to controls, reciprocal inhibition is reduced in tinnitus patients. Thus, as there is no effect on N1m that could potentially be attributed to attention, it seems unlikely that the enhancement effect of tinnitus on ASSR amplitude could be accounted for in terms of attention induced by tinnitus.

Intensity-invariant coding in the auditory system

The auditory system faithfully represents sufficient details from sound sources such that downstream cognitive processes are capable of acting upon this information effectively even in the face of signal uncertainty, degradation or interference. This robust sound source representation leads to an invariance in perception vital for animals to interact effectively with their environment. Due to unique nonlinearities in the cochlea, sound representations early in the auditory system exhibit a large amount of variability as a function of stimulus intensity. In other words, changes in stimulus intensity, such as for sound sources at differing distances, create a unique challenge for the auditory system to encode sounds invariantly across the intensity dimension. This challenge and some strategies available to sensory systems to eliminate intensity as an encoding variable are discussed, with a special emphasis upon sound encoding.

Dual mechanism of neuronal ensemble inhibition in primary auditory cortex

Inhibition plays an essential role in shaping and refining the brain's representation of sensory stimulus attributes. In primary auditory cortex (A1), so-called "sideband" inhibition helps to sharpen the tuning of local neuronal responses. Several distinct types of anatomical circuitry could underlie sideband inhibition, including direct thalamocortical (TC) afferents, as well as indirect intracortical mechanisms. The goal of the present study was to characterize sideband inhibition in A1 and to determine its mechanism by analyzing laminar profiles of neuronal ensemble activity. Our results indicate that both lemniscal and nonlemniscal TC afferents play a role in inhibitory responses via feedforward inhibition and oscillatory phase reset, respectively. We propose that the dynamic modulation of excitability in A1 due to the phase reset of ongoing oscillations may alter the tuning of local neuronal ensembles and can be regarded as a flexible overlay on the more obligatory system of lemniscal feedforward type responses.

Layer-specific noradrenergic modulation of inhibition in cortical layer II/III

Norepinephrine (NE) is released in the neocortex after activation of the locus coeruleus of the brain stem in response to novel, salient, or fight-or-flight stimuli. The role of adrenergic modulation in sensory cortices is not completely understood. We investigated the possibility that NE modifies the balance of inhibition acting on 2 different γ-aminobutyric acid (GABA)ergic pathways. Using patch-clamp recordings, we found that the application of NE induces an α(1) adrenergic receptor-mediated decrease of the amplitude of inhibitory postsynaptic currents (IPSCs) evoked by stimulation of layer I (LI-eIPSCs) and a β and α(2) receptor-mediated increase in the amplitude of IPSCs evoked by stimulation of layer II/III (LII/III-eIPSCs). Analysis of minimal stimulation IPSCs, IPSC kinetics, and sensitivity to the GABA(A) receptor subunit-selective enhancer zolpidem corroborated the functional difference between LI- and LII/III-eIPSCs, suggestive of a distal versus somatic origin of LI- and LII/III-eIPSCs, respectively. These findings suggest that NE shifts the balance between distal and somatic inhibition to the advantage of the latter. We speculate that such shift modifies the balance of sensory-specific and emotional information in the integration of neural input to the upper layers of the auditory cortex.

Interaction among the components of multiple auditory steady-state responses: enhancement in tinnitus patients, inhibition in controls

Amplitude and phase of steady-state signals recorded in response to amplitude-modulated (AM) sine tones vary over time, suggesting that the steady-state response (SSR) reflects not only stimulus input but also its interaction with other input streams or internally generated signals. Alterations of the interaction between simultaneous SSRs associated with tinnitus were studied by recording the magnetic field evoked by AM-tones with one of three carrier and one of three modulation frequencies. Single AM-tones were presented in single presentation mode and superpositions of three AM-tones differing in carrier and modulation frequency in multiple presentation mode. Modulation frequency-specific SSR components were recovered by bandpass filtering. Compared with single mode, in multiple mode SSR amplitude was reduced in healthy controls, but increased in tinnitus patients. Thus, while in controls multiple response components seem to reciprocally inhibit one another, in tinnitus reciprocal facilitation seems to predominate. Reciprocal inhibition was unrelated to the phase coherence among SSR components, but was correlated with the frequency of phase slips, indicating that the lateral interaction among SSR components acts in a quasi-paroxysmal manner and manifests itself in terms of a random train of phase reset events. Phase slips were more frequent in patients than controls both in single and multiple mode. Together, these findings indicate that lateral or surround inhibition of single units in auditory cortex is reduced and suggest that in-field inhibition is increased in tinnitus.

Tone-specific and nonspecific plasticity of inferior colliculus elicited by pseudo-conditioning: role of acetylcholine and auditory and somatosensory cortices

Experience-dependent plasticity in the central sensory systems depends on activation of both the sensory and neuromodulatory systems. Sensitization or nonspecific augmentation of central auditory neurons elicited by pseudo-conditioning with unpaired conditioning tonal (CS) and unconditioned electric leg (US) stimuli is quite different from tone-specific plasticity, called best frequency (BF) shifts, of the neurons elicited by auditory fear conditioning with paired CS and US. Therefore the neural circuits eliciting the nonspecific augmentation must be different from that eliciting the BF shifts. We first examined plastic changes in the response properties of collicular neurons of the big brown bat elicited by pseudo-conditioning and found that it elicited prominent nonspecific augmentation-an auditory response increase, a frequency-tuning broadening, and a threshold decreas-and that, in addition, it elicited a small short-lasting BF shift only when the CS frequency was 5 kHz lower than the BF of a recorded neuron. We examined the role of acetylcholine and the auditory and somatosensory cortices in these collicular changes. The development of the nonspecific augmentation was affected little by a muscarinic acetylcholine receptor antagonist applied to the inferior colliculus and by a GABA(A) receptor agonist applied to the auditory or somatosensory cortex. However, these drugs abolished the small short-lasting BF shift as they abolished the large long-lasting cortical and short-lasting collicular BF shifts elicited by the conditioning. These results indicate that, different from the BF shift, the nonspecific augmentation of the inferior colliculus depends on neither the cholinergic neuromodulator nor the auditory and somatosensory cortices.

Retuning of inferior colliculus neurons following spiral ganglion lesions: a single-neuron model of converging inputs

Lesions of spiral ganglion cells, representing a restricted sector of the auditory nerve array, produce immediate changes in the frequency tuning of inferior colliculus (IC) neurons. There is a loss of excitation at the lesion frequencies, yet responses to adjacent frequencies remain intact and new regions of activity appear. This leads to immediate changes in tuning and in tonotopic progression. Similar effects are seen after different methods of peripheral damage and in auditory neurons in other nuclei. The mechanisms that underlie these postlesion changes are unknown, but the acute effects seen in IC strongly suggest the "unmasking" of latent inputs by the removal of inhibition. In this study, we explore computational models of single neurons with a convergence of excitatory and inhibitory inputs from a range of characteristic frequencies (CFs), which can simulate the narrow prelesion tuning of IC neurons, and account for the changes in CF tuning after a lesion. The models can reproduce the data if inputs are aligned relative to one another in a precise order along the dendrites of model IC neurons. Frequency tuning in these neurons approximates that seen physiologically. Removal of inputs representing a narrow range of frequencies leads to unmasking of previously subthreshold excitatory inputs, which causes changes in CF. Conversely, if all of the inputs converge at the same point on the cell body, receptive fields are broad and unmasking rarely results in CF changes. However, if the inhibition is tonic with no stimulus-driven component, then unmasking can still produce changes in CF.

Selective neurophysiologic responses to music in instrumentalists with different listening biographies

To appropriately adapt to constant sensory stimulation, neurons in the auditory system are tuned to various acoustic characteristics, such as center frequencies, frequency modulations, and their combinations, particularly those combinations that carry species-specific communicative functions. The present study asks whether such tunings extend beyond acoustic and communicative functions to auditory self-relevance and expertise. More specifically, we examined the role of the listening biography--an individual's long term experience with a particular type of auditory input--on perceptual-neural plasticity. Two groups of expert instrumentalists (violinists and flutists) listened to matched musical excerpts played on the two instruments (J.S. Bach Partitas for solo violin and flute) while their cerebral hemodynamic responses were measured using fMRI. Our experimental design allowed for a comprehensive investigation of the neurophysiology (cerebral hemodynamic responses as measured by fMRI) of auditory expertise (i.e., when violinists listened to violin music and when flutists listened to flute music) and nonexpertise (i.e., when subjects listened to music played on the other instrument). We found an extensive cerebral network of expertise, which implicates increased sensitivity to musical syntax (BA 44), timbre (auditory association cortex), and sound-motor interactions (precentral gyrus) when listening to music played on the instrument of expertise (the instrument for which subjects had a unique listening biography). These findings highlight auditory self-relevance and expertise as a mechanism of perceptual-neural plasticity, and implicate neural tuning that includes and extends beyond acoustic and communication-relevant structures.

Mapping unpleasantness of sounds to their auditory representation

Certain sounds, for example, the squeal of chalk on a blackboard, are typically perceived as highly unpleasant. This study addressed the question of what aspects of the auditory representation of such sounds are associated with judgments of unpleasantness. Participants rated the perceived unpleasantness of a large number of sounds that included "griding" and other unpleasant sounds. A multivariate partial least-squares (PLS) model was then built to relate the ratings of unpleasantness with an auditory representation derived from a model of processing in the auditory pathway. The "existence region" of unpleasantness in the auditory space of frequency-temporal modulation was determined after the PLS model had been validated by predicting the unpleasantness of novel sounds from the auditory representation. It was observed that the existence region corresponded to spectral frequencies between 2500 and 5500 Hz, and temporal modulations in the range 1-16 Hz.

Linking the response properties of cells in auditory cortex with network architecture: cotuning versus lateral inhibition

The frequency-intensity receptive fields (RF) of neurons in primary auditory cortex (AI) are heterogeneous. Some neurons have V-shaped RFs, whereas others have enclosed ovoid RFs. Moreover, there is a wide range of temporal response profiles ranging from phasic to tonic firing. The mechanisms underlying this diversity of receptive field properties are yet unknown. Here we study the characteristics of thalamocortical (TC) and intracortical connectivity that give rise to the individual cell responses. Using a mouse auditory TC slice preparation, we found that the amplitude of synaptic responses in AI varies non-monotonically with the intensity of the stimulation in the medial geniculate nucleus (MGv). We constructed a network model of MGv and AI that was simulated using either rate model cells or in vitro neurons through an iterative procedure that used the recorded neural responses to reconstruct network activity. We compared the receptive fields and firing profiles obtained with networks configured to have either cotuned excitatory and inhibitory inputs or relatively broad, lateral inhibitory inputs. Each of these networks yielded distinct response properties consistent with those documented in vivo with natural stimuli. The cotuned network produced V-shaped RFs, phasic-tonic firing profiles, and predominantly monotonic rate-level functions. The lateral inhibitory network produced enclosed RFs with narrow frequency tuning, a variety of firing profiles, and robust non-monotonic rate-level functions. We conclude that both types of circuits must be present to account for the wide variety of responses observed in vivo.

Tonotopic Control of Auditory Thalamus Frequency Tuning by Reticular Thalamic Neurons

GABAergic cells of the thalamic reticular nucleus (TRN) can potentially exert strong control over transmission of information through thalamus to the cerebral cortex. Anatomical studies have shown that the reticulo-thalamic connections are spatially organized in the visual, somatosensory, and auditory systems. However, the issue of how inhibitory input from TRN controls the functional properties of thalamic relay cells and whether this control follows topographic rules remains largely unknown. Here we assessed the consequences of increasing or decreasing the activity of small ensembles of TRN neurons on the receptive field properties of medial geniculate (MG) neurons. For each MG cell, the frequency tuning curve and the rate-level function were tested before, during, and after microiontophoretic applications of GABA, or of glutamate, in the auditory sector of the TRN. For 66 MG cells tested during potent pharmacological control of TRN activity, group data did not reveal any significant effects. However, for a population of 20/66 cells (all but 1 recorded in the ventral, tonotopic, division), the breadth of tuning, the frequency selectivity and the acoustic threshold were significantly modified in the directions expected from removing, or reinforcing, a dominant inhibitory input onto MG cells. Such effects occurred only when the distance between the characteristic frequency of the recorded ventral MG cell and that of the TRN cells at the ejection site was <0.25 octaves; they never occurred for larger distances. This relationship indicates that the functional interactions between TRN cells and ventral MG cells rely on precise topographic connections.

Attention improves population-level frequency tuning in human auditory cortex

Attention improves auditory performance in noisy environments by either enhancing the processing of task-relevant stimuli ("gain"), suppressing task-irrelevant information ("sharpening"), or both. In the present study, we investigated the effect of focused auditory attention on the population-level frequency tuning in human auditory cortex by means of magnetoencephalography. Using complex stimuli consisting of a test tone superimposed on different band-eliminated noises during active listening or distracted listening conditions, we observed that focused auditory attention caused not only gain, but also sharpening of frequency tuning in human auditory cortex as reflected by the N1m auditory evoked response. This combination of gain and sharpening in the auditory cortex may contribute to better auditory performance during focused auditory attention.

Spatial Interaction Between Spectral Integration and Frequency Gradient in Primary Auditory Cortex

Primary sensory cortical areas are characterized by orderly and largely independent representations of several receptive field properties. This is expressed in multiple, spatially overlaying parameter distributions, such as orientation preference, spatial frequency, and ocular dominance maps in the primary visual cortex. In the auditory cortex, two main and presumably independent representational parameters are the center frequency and the frequency extent of spectral tuning curves. Here we demonstrate interactions between cortical tonotopic gradient and spectral bandwidth modules in cat primary auditory cortex (AI). First, the spatial representation of spectral integration is not equally expressed across the whole frequency range in AI. Narrow-bandwidth modules are found only in the mid-frequency region (5–20 kHz). Thus spectral integration properties delineate three frequency regions (<5, 5–20, and >20 kHz) in cat AI. Second, the extent of spectral integration covaries with the local tonotopic gradient in the low- and mid-frequency ranges. Regions with a shallow frequency gradient tend to have narrower spectral integration than those with a steep gradient. These relationships between spectral selectivity and frequency gradient constrain forebrain models of thalamo- and corticocortical convergence and connectivity and may reflect the processing of behaviorally relevant stimulus constellations.

Comparison of bandwidths in the inferior colliculus and the auditory nerve. I. Measurement using a spectrally manipulated stimulus

A defining feature of auditory systems across animal divisions is the ability to sort different frequency components of a sound into separate neural frequency channels. Narrowband filtering in the auditory periphery is of obvious advantage for the representation of sound spectrum and manifests itself pervasively in human psychophysical studies as the critical band. Peripheral filtering also alters coding of the temporal waveform, so that temporal responses in the auditory periphery reflect both the stimulus waveform and peripheral filtering. Temporal coding is essential for the measurement of the time delay between waveforms at the two ears-a critical component of sound localization. A number of human psychophysical studies have shown a wider effective critical bandwidth with binaural stimuli than with monaural stimuli, although other studies found no difference. Here we directly compare binaural and monaural bandwidths (BWs) in the anesthetized cat. We measure monaural BW in the auditory nerve (AN) and binaural BW in the inferior colliculus (IC) using spectrally manipulated broadband noise and response metrics that reflect spike timing. The stimulus was a pair of noise tokens that were interaurally in phase for all frequencies below a certain flip frequency (f(flip)) and that had an interaural phase difference of pi above f(flip). The response was measured as a function of f(flip) and, using a separate stimulus protocol, as a function of interaural correlation. We find that both AN and IC filter BW depend on characteristic frequency, but that there is no difference in mean BW between the AN and IC.

Preceding weak noise sharpens the frequency tuning and elevates the response threshold of the mouse inferior collicular neurons through GABAergic inhibition

In acoustic communication, animals must extract biologically relevant signals that are embedded in noisy environment. The present study examines how weak noise may affect the auditory sensitivity of neurons in the central nucleus of the mouse inferior colliculus (IC) which receives convergent excitatory and inhibitory inputs from both lower and higher auditory centers. Specifically, we studied the frequency sensitivity and minimum threshold of IC neurons using a pure tone probe and a weak white noise masker under forward masking paradigm. For most IC neurons, probe-elicited response was decreased by a weak white noise that was presented at a specific gap (i.e. time window). When presented within this time window, weak noise masking sharpened the frequency tuning curve and increased the minimum threshold of IC neurons. The degree of weak noise masking of these two measurements increased with noise duration. Sharpening of the frequency tuning curve and increasing of the minimum threshold of IC neurons during weak noise masking were mostly mediated through GABAergic inhibition. In addition, sharpening of frequency tuning curve by the weak noise masker was more effective at the high than at low frequency limb. These data indicate that in the real world the ambient noise may improve frequency sensitivity of IC neurons through GABAergic inhibition while inevitably decrease the frequency response range and sensitivity of IC neurons.

Asymmetric lateral inhibitory neural activity in the auditory system: a magnetoencephalographic study

BACKGROUND

Decrements of auditory evoked responses elicited by repeatedly presented sounds with similar frequencies have been well investigated by means of electroencephalography and magnetoencephalography (MEG). However the possible inhibitory interactions between different neuronal populations remains poorly understood. In the present study, we investigated the effect of proceeding notch-filtered noises (NFNs) with different frequency spectra on a following test tone using MEG.

RESULTS

Three-second exposure to the NFNs resulted in significantly different N1m responses to a 1000 Hz test tone presented 500 ms after the offset of the NFNs. The NFN with a lower spectral edge closest to the test tone mostly decreased the N1m amplitude.

CONCLUSION

The decrement of the N1m component after exposure to the NFNs could be explained partly in terms of lateral inhibition. The results demonstrated that the amplitude of the N1m was more effectively influenced by inhibitory lateral connections originating from neurons corresponding to lower rather than higher frequencies. We interpret this effect of asymmetric lateral inhibition in the auditory system as an important contribution to reduce the asymmetric neural activity profiles originating from the cochlea.

Hierarchical processing of auditory objects in humans

This work examines the computational architecture used by the brain during the analysis of the spectral envelope of sounds, an important acoustic feature for defining auditory objects. Dynamic causal modelling and Bayesian model selection were used to evaluate a family of 16 network models explaining functional magnetic resonance imaging responses in the right temporal lobe during spectral envelope analysis. The models encode different hypotheses about the effective connectivity between Heschl's Gyrus (HG), containing the primary auditory cortex, planum temporale (PT), and superior temporal sulcus (STS), and the modulation of that coupling during spectral envelope analysis. In particular, we aimed to determine whether information processing during spectral envelope analysis takes place in a serial or parallel fashion. The analysis provides strong support for a serial architecture with connections from HG to PT and from PT to STS and an increase of the HG to PT connection during spectral envelope analysis. The work supports a computational model of auditory object processing, based on the abstraction of spectro-temporal “templates” in the PT before further analysis of the abstracted form in anterior temporal lobe areas.

The past decade has seen a phenomenal rise in applications of functional magnetic resonance imaging for both research and clinical applications. Most of the applications, however, concentrate on finding the regions of the brain that mediate the processing of a cognitive/motor task without determining the interaction between the identified regions. It is, however, the interactions between the different regions that accomplish a given task. In this study, we have examined the interactions between three regions—Heshl's gyrus (HG), planum temporale (PT), and superior temporal sulcus (STS)—that have been implicated in processing the spectral envelope of sounds. The spectral envelope is one of the dimensions of timbre that determine the identity of two sounds that have the same pitch, duration, and intensity. The interaction between the regions is examined using a system-based mathematical modelling technique called dynamic causal modelling (DCM). It is found that flow of information is serial, with HG sending information to PT and then to STS with the connectivity between HG to PT being effectively increased by the extraction of spectral envelope. The study provides evidence for an earlier hypothesis that PT is a computational hub.

Binaral interaction and centrifugal input enhances spatial contrast in olfactory bulb activation

We used paired-pulse odorant stimulation, with a conditioning stimulus delivered either ipsilateral or contralateral to a test stimulus, to unmask the effects of centrifugal feedback on olfactory bulb responses. In reptiles and mammals there are no direct connections between the paired olfactory bulbs, and thus all information transfer between the olfactory bulbs depends on feedback from retrobulbar structures. We measured odor-induced activity in the turtle olfactory bulb using a voltage-sensitive dye and a 464-element photodiode array, which allowed us to monitor the spatial variation in activation of the olfactory bulb. We found that both contralateral and ipsilateral conditioning stimuli evoked long-lasting inhibition of olfactory bulb activation. In contrast to previous studies using local field potential recording to monitor activity at a single site, we found that this inhibition increased contrast in the spatial patterning of activation over the dorsal surface of the olfactory bulb. Inhibition was also increased when different odorants were used as conditioning and test stimuli, suggesting a role for centrifugal feedback in olfactory discrimination. These results highlight the functional importance of centrifugal feedback and information processing in a broadly distributed olfactory network.

Spectral integration in the inferior colliculus of the CBA/CaJ mouse

The inferior colliculus receives a massive convergence of inputs and in the mustached bat, this convergence leads to the creation of neurons in the inferior colliculus that integrate information across multiple frequency bands. These neurons are tuned to multiple frequency bands or are combination-sensitive; responding best to the combination of two signals of different frequency composition. The importance of combination-sensitive neurons in processing echolocation signals is well described, and it has been thought that combination sensitivity is a neural specialization for echolocation behaviors. Combination sensitivity and other response properties indicative of spectral integration have not been thoroughly examined in the inferior colliculus of non-echolocating mammals. In this study we tested the hypothesis that integration across frequencies occurs in the inferior colliculus of mice. We tested excitatory frequency response areas in the inferior colliculus of unanesthetized mice by varying the frequency of a single tone between 6 and 100 kHz. We then tested combination-sensitive responses by holding one tone at the unit's best frequency, and varying the frequency and intensity of a second tone. Thirty-two percent of the neurons were tuned to multiple frequency bands, 16% showed combination-sensitive facilitation and another 12% showed combination-sensitive inhibition. These findings suggests that the neural mechanisms underlying processing of complex sounds in the inferior colliculus share some common features among mammals as different as the bat and the mouse.