Modulation of stimulus-specific adaptation by GABA(A) receptor activation or blockade in the medial geniculate body of the anaesthetized rat

Neural correlates of novelty detection in the primary auditory cortex of behaving monkeys

The neural mechanisms underlying novelty detection are not well understood, especially in relation to behavior. Here, we present single-unit responses from the primary auditory cortex (A1) from two monkeys trained to detect deviant tones amid repetitive ones. Results show that monkeys can detect deviant sounds, and there is a strong correlation between late neuronal responses (250-350 ms after deviant onset) and the monkeys' perceptual decisions. The magnitude and timing of both neuronal and behavioral responses are increased by larger frequency differences between the deviant and standard tones and by increasing the number of standard tones preceding the deviant. This suggests that A1 neurons encode novelty detection in behaving monkeys, influenced by stimulus relevance and expectations. This study provides evidence supporting aspects of predictive coding in the sensory cortex.

Novelty detection in an auditory oddball task on freely moving rats

The relative importance or saliency of sensory inputs depend on the animal's environmental context and the behavioural responses to these same inputs can vary over time. Here we show how freely moving rats, trained to discriminate between deviant tones embedded in a regular pattern of repeating stimuli and different variations of the classic oddball paradigm, can detect deviant tones, and this discriminability resembles the properties that are typical of neuronal adaptation described in previous studies. Moreover, the auditory brainstem response (ABR) latency decreases after training, a finding consistent with the notion that animals develop a type of plasticity to auditory stimuli. Our study suggests the existence of a form of long-term memory that may modulate the level of neuronal adaptation according to its behavioural relevance, and sets the ground for future experiments that will help to disentangle the functional mechanisms that govern behavioural habituation and its relation to neuronal adaptation.

Exploratory evidence for differences in GABAergic regulation of auditory processing in autism spectrum disorder

Altered reactivity and responses to auditory input are core to the diagnosis of autism spectrum disorder (ASD). Preclinical models implicate ϒ-aminobutyric acid (GABA) in this process. However, the link between GABA and auditory processing in humans (with or without ASD) is largely correlational. As part of a study of potential biosignatures of GABA function in ASD to inform future clinical trials, we evaluated the role of GABA in auditory repetition suppression in 66 adults (n = 28 with ASD). Neurophysiological responses (temporal and frequency domains) to repetitive standard tones and novel deviants presented in an oddball paradigm were compared after double-blind, randomized administration of placebo, 15 or 30 mg of arbaclofen (STX209), a GABA type B (GABAB) receptor agonist. We first established that temporal mismatch negativity was comparable between participants with ASD and those with typical development (TD). Next, we showed that temporal and spectral responses to repetitive standards were suppressed relative to responses to deviants in the two groups, but suppression was significantly weaker in individuals with ASD at baseline. Arbaclofen reversed weaker suppression of spectral responses in ASD but disrupted suppression in TD. A post hoc analysis showed that arbaclofen-elicited shift in suppression was correlated with autistic symptomatology measured using the Autism Quotient across the entire group, though not in the smaller sample of the ASD and TD group when examined separately. Thus, our results confirm: GABAergic dysfunction contributes to the neurophysiology of auditory sensory processing alterations in ASD, and can be modulated by targeting GABAB activity. These GABA-dependent sensory differences may be upstream of more complex autistic phenotypes.

Habituation, Adaptation and Prediction Processes in Neurodevelopmental Disorders: A Comprehensive Review

Habituation, the simplest form of learning preserved across species and evolution, is characterized by a response decrease as a stimulus is repeated. This adaptive function has been shown to be altered in some psychiatric and neurodevelopmental disorders such as autism spectrum disorder (ASD), attention-deficit/hyperactivity disorder (ADHD) or schizophrenia. At the brain level, habituation is characterized by a decrease in neural activity as a stimulation is repeated, referred to as neural adaptation. This phenomenon influences the ability to make predictions and to detect change, two processes altered in some neurodevelopmental and psychiatric disorders. In this comprehensive review, the objectives are to characterize habituation, neural adaptation, and prediction throughout typical development and in neurodevelopmental disorders; and to evaluate their implication in symptomatology, specifically in sensitivity to change or need for sameness. A summary of the different approaches to investigate adaptation will be proposed, in which we report the contribution of animal studies as well as electrophysiological studies in humans to understanding of underlying neuronal mechanisms.

Adaptation in auditory processing

Adaptation is an essential feature of auditory neurons, which reduces their responses to unchanging and recurring sounds and allows their response properties to be matched to the constantly changing statistics of sounds that reach the ears. As a consequence, processing in the auditory system highlights novel or unpredictable sounds and produces an efficient representation of the vast range of sounds that animals can perceive by continually adjusting the sensitivity and, to a lesser extent, the tuning properties of neurons to the most commonly encountered stimulus values. Together with attentional modulation, adaptation to sound statistics also helps to generate neural representations of sound that are tolerant to background noise and therefore plays a vital role in auditory scene analysis. In this review, we consider the diverse forms of adaptation that are found in the auditory system in terms of the processing levels at which they arise, the underlying neural mechanisms, and their impact on neural coding and perception. We also ask what the dynamics of adaptation, which can occur over multiple timescales, reveal about the statistical properties of the environment. Finally, we examine how adaptation to sound statistics is influenced by learning and experience and changes as a result of aging and hearing loss.

Cortical and thalamic modulation of auditory gating in the posterior parietal cortex of awake mice

Auditory gating (AG) is an adaptive mechanism for filtering out redundant acoustic stimuli to protect the brain against information overload. AG deficits have been found in many mental illnesses, including schizophrenia (SZ). However, the neural correlates of AG remain poorly understood. Here, we found that the posterior parietal cortex (PPC) shows an intermediate level of AG in auditory thalamocortical circuits, with a laminar profile in which the strongest AG is in the granular layer. Furthermore, AG of the PPC was decreased and increased by optogenetic inactivation of the medial dorsal thalamic nucleus (MD) and auditory cortex (AC), respectively. Optogenetically activating the axons from the MD and AC drove neural activities in the PPC without an obvious AG. These results indicated that AG in the PPC is determined by the integrated signal streams from the MD and AC in a bottom-up manner. We also found that a mouse model of SZ (postnatal administration of noncompetitive N-methyl-d-aspartate receptor antagonist) presented an AG deficit in the PPC, which may be inherited from the dysfunction of MD. Together, our findings reveal a neural circuit underlying the generation of AG in the PPC and its involvement in the AG deficit of SZ.

Predictive encoding of pure tones and FM-sweeps in the human auditory cortex

Expectations substantially influence perception, but the neural mechanisms underlying this influence are not fully understood. A prominent view is that sensory neurons encode prediction error with respect to expectations on upcoming sensory input. Although the encoding of prediction error has been previously demonstrated in the human auditory cortex (AC), previous studies often induced expectations using stimulus repetition, potentially confounding prediction error with neural habituation. These studies also measured AC as a single population, failing to consider possible predictive specializations of different AC fields. Moreover, the few studies that considered prediction error to stimuli other than pure tones yielded conflicting results. Here, we used functional magnetic resonance imaging (fMRI) to systematically investigate prediction error to subjective expectations in auditory cortical fields Te1.0, Te1.1, Te1.2, and Te3, and two types of stimuli: pure tones and frequency modulated (FM) sweeps. Our results show that prediction error is elicited with respect to the participants' expectations independently of stimulus repetition and similarly expressed across auditory fields. Moreover, despite the radically different strategies underlying the decoding of pure tones and FM-sweeps, both stimulus modalities were encoded as prediction error in most fields of AC. Altogether, our results provide unequivocal evidence that predictive coding is the general encoding mechanism in AC.

Adjudicating Between Local and Global Architectures of Predictive Processing in the Subcortical Auditory Pathway

Predictive processing, a leading theoretical framework for sensory processing, suggests that the brain constantly generates predictions on the sensory world and that perception emerges from the comparison between these predictions and the actual sensory input. This requires two distinct neural elements: generative units, which encode the model of the sensory world; and prediction error units, which compare these predictions against the sensory input. Although predictive processing is generally portrayed as a theory of cerebral cortex function, animal and human studies over the last decade have robustly shown the ubiquitous presence of prediction error responses in several nuclei of the auditory, somatosensory, and visual subcortical pathways. In the auditory modality, prediction error is typically elicited using so-called oddball paradigms, where sequences of repeated pure tones with the same pitch are at unpredictable intervals substituted by a tone of deviant frequency. Repeated sounds become predictable promptly and elicit decreasing prediction error; deviant tones break these predictions and elicit large prediction errors. The simplicity of the rules inducing predictability make oddball paradigms agnostic about the origin of the predictions. Here, we introduce two possible models of the organizational topology of the predictive processing auditory network: (1) the global view, that assumes that predictions on the sensory input are generated at high-order levels of the cerebral cortex and transmitted in a cascade of generative models to the subcortical sensory pathways; and (2) the local view, that assumes that independent local models, computed using local information, are used to perform predictions at each processing stage. In the global view information encoding is optimized globally but biases sensory representations along the entire brain according to the subjective views of the observer. The local view results in a diminished coding efficiency, but guarantees in return a robust encoding of the features of sensory input at each processing stage. Although most experimental results to-date are ambiguous in this respect, recent evidence favors the global model.

Abstract rules drive adaptation in the subcortical sensory pathway

The subcortical sensory pathways are the fundamental channels for mapping the outside world to our minds. Sensory pathways efficiently transmit information by adapting neural responses to the local statistics of the sensory input. The long-standing mechanistic explanation for this adaptive behaviour is that neural activity decreases with increasing regularities in the local statistics of the stimuli. An alternative account is that neural coding is directly driven by expectations of the sensory input. Here, we used abstract rules to manipulate expectations independently of local stimulus statistics. The ultra-high-field functional-MRI data show that abstract expectations can drive the response amplitude to tones in the human auditory pathway. These results provide first unambiguous evidence of abstract processing in a subcortical sensory pathway. They indicate that the neural representation of the outside world is altered by our prior beliefs even at initial points of the processing hierarchy.

Mechanisms of GABAergic and cholinergic neurotransmission in auditory thalamus: Impact of aging

Age-related hearing loss is a complex disorder affecting a majority of the elderly population. As people age, speech understanding becomes a challenge especially in complex acoustic settings and negatively impacts the ability to accurately analyze the auditory scene. This is in part due to an inability to focus auditory attention on a particular stimulus source while simultaneously filtering out other sound stimuli. The present review examines the impact of aging on two neurotransmitter systems involved in accurate temporal processing and auditory gating in auditory thalamus (medial geniculate body; MGB), a critical brain region involved in the coding and filtering of auditory information. The inhibitory neurotransmitter GABA and its synaptic receptors (GABA_ARs) are key to maintaining accurate temporal coding of complex sounds, such as speech, throughout the central auditory system. In the MGB, synaptic and extrasynaptic GABA_ARs mediate fast phasic and slow tonic inhibition respectively, which in turn regulate MGB neuron excitability, firing modes, and engage thalamocortical oscillations that shape coding and gating of acoustic content. Acoustic coding properties of MGB neurons are further modulated through activation of tegmental cholinergic afferents that project to MGB to potentially modulate attention and help to disambiguate difficult to understand or novel sounds. Acetylcholine is released onto MGB neurons and presynaptic terminals in MGB activating neuronal nicotinic and muscarinic acetylcholine receptors (nAChRs, mAChRs) at a subset of MGB afferents to optimize top-down and bottom-up information flow. Both GABAergic and cholinergic neurotransmission is significantly altered with aging and this review will detail how age-related changes in these circuits within the MGB may impact coding of acoustic stimuli.

Deviance detection in physiologically identified cell types in the rat auditory cortex

Auditory deviance detection is a function of the auditory system that allows reduction of the processing demand for repetitive stimuli while stressing unpredictable ones, which are potentially more informative. Deviance detection has been extensively studied in humans using the oddball paradigm, which evokes an event-related potential known as mismatch negativity (MMN). The same stimulation paradigms are used in animal studies that aim to elucidate the neuronal mechanisms underlying deviance detection. In order to understand the circuitry responsible for deviance detection in the auditory cortex (AC), it is necessary to determine the properties of excitatory and inhibitory neurons separately. Measuring the spike widths of neurons recorded extracellularly from the anaesthetized rat AC, we classified them as fast spiking or regular spiking units. These two neuron types are generally considered as putative inhibitory or excitatory, respectively. In response to an oddball paradigm, we found that both types of units showed similar amounts of deviance detection overall. When considering each AC field separately, we found that only in A1 fast spiking neurons showed higher deviance detection levels than regular spiking neurons, while in the rest of the fields there was no such distinction. Interpreting these responses in the context of the predictive coding framework, we found that the responses of both types of units reflect mainly prediction error signaling (i.e., genuine deviance detection) rather than repetition suppression.

Cortical Microcircuit Mechanisms of Mismatch Negativity and Its Underlying Subcomponents

In the neocortex, neuronal processing of sensory events is significantly influenced by context. For instance, responses in sensory cortices are suppressed to repetitive or redundant stimuli, a phenomenon termed “stimulus-specific adaptation” (SSA). However, in a context in which that same stimulus is novel, or deviates from expectations, neuronal responses are augmented. This augmentation is termed “deviance detection” (DD). This contextual modulation of neural responses is fundamental for how the brain efficiently processes the sensory world to guide immediate and future behaviors. Notably, context modulation is deficient in some neuropsychiatric disorders such as schizophrenia (SZ), as quantified by reduced “mismatch negativity” (MMN), an electroencephalography waveform reflecting a combination of SSA and DD in sensory cortex. Although the role of NMDA-receptor function and other neuromodulatory systems on MMN is established, the precise microcircuit mechanisms of MMN and its underlying components, SSA and DD, remain unknown. When coupled with animal models, the development of powerful precision neurotechnologies over the past decade carries significant promise for making new progress into understanding the neurobiology of MMN with previously unreachable spatial resolution. Currently, rodent models represent the best tool for mechanistic study due to the vast genetic tools available. While quantifying human-like MMN waveforms in rodents is not straightforward, the “oddball” paradigms used to study it in humans and its underlying subcomponents (SSA/DD) are highly translatable across species. Here we summarize efforts published so far, with a focus on cortically measured SSA and DD in animals to maintain relevance to the classically measured MMN, which has cortical origins. While mechanistic studies that measure and contrast both components are sparse, we synthesize a potential set of microcircuit mechanisms from the existing rodent, primate, and human literature. While MMN and its subcomponents likely reflect several mechanisms across multiple brain regions, understanding fundamental microcircuit mechanisms is an important step to understand MMN as a whole. We hypothesize that SSA reflects adaptations occurring at synapses along the sensory-thalamocortical pathways, while DD depends on both SSA inherited from afferent inputs and resulting disinhibition of non-adapted neurons arising from the distinct physiology and wiring properties of local interneuronal subpopulations and NMDA-receptor function.

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

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

The role of adaptation in generating monotonic rate codes in auditory cortex

In primary auditory cortex, slowly repeated acoustic events are represented temporally by the stimulus-locked activity of single neurons. Single-unit studies in awake marmosets (Callithrix jacchus) have shown that a sub-population of these neurons also monotonically increase or decrease their average discharge rate during stimulus presentation for higher repetition rates. Building on a computational single-neuron model that generates stimulus-locked responses with stimulus evoked excitation followed by strong inhibition, we find that stimulus-evoked short-term depression is sufficient to produce synchronized monotonic positive and negative responses to slowly repeated stimuli. By exploring model robustness and comparing it to other models for adaptation to such stimuli, we conclude that short-term depression best explains our observations in single-unit recordings in awake marmosets. Together, our results show how a simple biophysical mechanism in single neurons can generate complementary neural codes for acoustic stimuli.

Large-Scale Networks for Auditory Sensory Gating in the Awake Mouse

The amplitude of the brain response to a repeated auditory stimulus is diminished as compared to the response to the first tone (T1) for interstimulus intervals (ISI) lasting up to hundreds of milliseconds. This adaptation process, called auditory sensory gating (ASG), is altered in various psychiatric diseases including schizophrenia and is classically studied by focusing on early evoked cortical responses to the second tone (T2) using 500-ms ISI. However, mechanisms underlying ASG are still not well-understood. We investigated ASG in awake mice from the brainstem to cortex at variable ISIs (125–2000 ms) using high-density EEG and intracerebral recordings. While ASG decreases at longer ISIs, it is still present at durations (500–2000 ms) far beyond the time during which brain responses to T1 could still be detected. T1 induces a sequence of specific stable scalp EEG topographies that correspond to the successive activation of distinct neural networks lasting about 350 ms. These brain states remain unaltered if T2 is presented during this period, although T2 is processed by the brain, suggesting that ongoing networks of brain activity are active for longer than early evoked-potentials and are not overwritten by an upcoming new stimulus. Intracerebral recordings demonstrate that ASG is already present at the level of ventral cochlear nucleus (vCN) and inferior colliculus and is amplified across the hierarchy in bottom-up direction. This study uncovers the extended stability of sensory-evoked brain states and long duration of ASG, and sheds light on generators of ASG and possible interactions between bottom-up and top-down mechanisms.

Top-down or bottom up: decreased stimulus salience increases responses to predictable stimuli of auditory thalamic neurons

KEY POINTS

Temporal imprecision leads to deficits in the comprehension of signals in cluttered acoustic environments, and the elderly are shown to use cognitive resources to disambiguate these signals. To mimic ageing in young rats, we delivered sound signals that are temporally degraded, which led to temporally imprecise neural codes. Instead of adaptation to repeated stimuli, with degraded signals, there was a relative increase in firing rates, similar to that seen in aged rats. We interpret this increase with repetition as a repair mechanism for strengthening the internal representations of degraded signals by the higher-order structures.

ABSTRACT

To better understand speech in challenging environments, older adults increasingly use top-down cognitive and contextual resources. The medial geniculate body (MGB) integrates ascending inputs with descending predictions to dynamically gate auditory representations based on salience and context. A previous MGB single-unit study found an increased preference for predictable sinusoidal amplitude modulated (SAM) stimuli in aged rats relative to young rats. The results suggested that the age-degraded/jittered up-stream acoustic code may engender an increased preference for predictable/repeating acoustic signals, possibly reflecting increased use of top-down resources. In the present study, we recorded from units in young-adult MGB, comparing responses to standard SAM with those evoked by less salient SAM (degraded) stimuli. We hypothesized that degrading the SAM stimulus would simulate the degraded ascending acoustic code seen in the elderly, increasing the preference for predictable stimuli. Single units were recorded from clusters of advanceable tetrodes implanted above the MGB of young-adult awake rats. Less salient SAM significantly increased the preference for predictable stimuli, especially at higher modulation frequencies. Rather than adaptation, higher modulation frequencies elicited increased numbers of spikes with each successive trial/repeat of the less salient SAM. These findings are consistent with previous findings obtained in aged rats suggesting that less salient acoustic signals engage the additional use of top-down resources, as reflected by an increased preference for repeating stimuli that enhance the representation of complex environmental/communication sounds.

The Neuronal Basis of Predictive Coding Along the Auditory Pathway: From the Subcortical Roots to Cortical Deviance Detection

In this review, we attempt to integrate the empirical evidence regarding stimulus-specific adaptation (SSA) and mismatch negativity (MMN) under a predictive coding perspective (also known as Bayesian or hierarchical-inference model). We propose a renewed methodology for SSA study, which enables a further decomposition of deviance detection into repetition suppression and prediction error, thanks to the use of two controls previously introduced in MMN research: the many-standards and the cascade sequences. Focusing on data obtained with cellular recordings, we explain how deviance detection and prediction error are generated throughout hierarchical levels of processing, following two vectors of increasing computational complexity and abstraction along the auditory neuraxis: from subcortical toward cortical stations and from lemniscal toward nonlemniscal divisions. Then, we delve into the particular characteristics and contributions of subcortical and cortical structures to this generative mechanism of hierarchical inference, analyzing what is known about the role of neuromodulation and local microcircuitry in the emergence of mismatch signals. Finally, we describe how SSA and MMN are occurring at similar time frame and cortical locations, and both are affected by the manipulation of N-methyl- D-aspartate receptors. We conclude that there is enough empirical evidence to consider SSA and MMN, respectively, as the microscopic and macroscopic manifestations of the same physiological mechanism of deviance detection in the auditory cortex. Hence, the development of a common theoretical framework for SSA and MMN is all the more recommendable for future studies. In this regard, we suggest a shared nomenclature based on the predictive coding interpretation of deviance detection.

Increased cortical reactivity to repeated tones at 8 months in infants with later ASD

Dysregulation of cortical excitation/inhibition (E/I) has been proposed as a neuropathological mechanism underlying core symptoms of autism spectrum disorder (ASD). Determining whether dysregulated E/I could contribute to the emergence of behavioural symptoms of ASD requires evidence from human infants prior to diagnosis. In this prospective longitudinal study, we examine differences in neural responses to auditory repetition in infants later diagnosed with ASD. Eight-month-old infants with (high-risk: n = 116) and without (low-risk: n = 27) an older sibling with ASD were tested in a non-linguistic auditory oddball paradigm. Relative to high-risk infants with typical development (n = 44), infants with later ASD (n = 14) showed reduced repetition suppression of 40-60 Hz evoked gamma and significantly greater 10-20 Hz inter-trial coherence (ITC) for repeated tones. Reduced repetition suppression of cortical gamma and increased phase-locking to repeated tones are consistent with cortical hyper-reactivity, which could in turn reflect disturbed E/I balance. Across the whole high-risk sample, a combined index of cortical reactivity (cortical gamma amplitude and ITC) was dimensionally associated with reduced growth in language skills between 8 months and 3 years, as well as elevated levels of parent-rated social communication symptoms at 3 years. Our data show that cortical 'hyper-reactivity' may precede the onset of behavioural traits of ASD in development, potentially affecting experience-dependent specialisation of the developing brain.

Modeling Neural Adaptation in Auditory Cortex

Neural responses recorded from auditory cortex exhibit adaptation, a stimulus-specific decrease that occurs when the same sound is presented repeatedly. Stimulus-specific adaptation is thought to facilitate perception in noisy environments. Although adaptation is assumed to arise independently from cortex, this has been difficult to validate directly in vivo. In this study, we used a neural network model of auditory cortex with multicompartmental cell modeling to investigate cortical adaptation. We found that repetitive, non-adapted inputs to layer IV neurons in the model elicited frequency-specific decreases in simulated single neuron, population-level and local field potential (LFP) activity, consistent with stimulus-specific cortical adaptation. Simulated recordings of LFPs, generated solely by excitatory post-synaptic inputs and recorded from layers II/III in the model, showed similar waveform morphologies and stimulus probability effects as auditory evoked responses recorded from human cortex. We tested two proposed mechanisms of cortical adaptation, neural fatigue and neural sharpening, by varying the strength and type of inter- and intra-layer synaptic connections (excitatory, inhibitory). Model simulations showed that synaptic depression modeled in excitatory (AMPA) synapses was sufficient to elicit a reduction in neural firing rate, consistent with neural fatigue. However, introduction of lateral inhibition from local layer II/III interneurons resulted in a reduction in the number of responding neurons, but not their firing rates, consistent with neural sharpening. These modeling results demonstrate that adaptation can arise from multiple neural mechanisms in auditory cortex.

A Brain without Brakes: Reduced Inhibition Is Associated with Enhanced but Dysregulated Plasticity in the Aged Rat Auditory Cortex

During early developmental windows known as critical periods (CPs) of plasticity, passive alterations in the quality and quantity of sensory inputs are sufficient to induce profound and long-lasting distortions in cortical sensory representations. With CP closure, those representations are stabilized, a process requiring the maturation of inhibitory networks and the maintenance of sufficient GABAergic tone in the cortex. In humans and rodents, however, cortical inhibition progressively decreases with advancing age, raising the possibility that the regulation of plasticity could be altered in older individuals. Here we tested the hypothesis that aging results in a destabilization of sensory representations and maladaptive dysregulated plasticity in the rat primary auditory cortex (A1). Consistent with this idea, we found that passive tone exposure is sufficient to distort frequency tuning in the A1 of older but not younger adult rats. However, we also found that these passive distortions decayed rapidly, indicating an ongoing instability of A1 tuning in the aging cortex. These changes were associated with a decrease in GABA neurotransmitter concentration and a reduction in parvalbumin and perineuronal net expression in the cortex. Finally, we show that artificially increasing GABA tone in the aging A1 is sufficient to restore representational stability and improve the retention of learning.

GABAergic and cholinergic modulation of repetition suppression in inferior temporal cortex

Neurons in many brain areas of different species reduce their response when a stimulus is repeated. Such adaptation or repetition suppression is prevalent in inferior temporal (IT) cortex. The mechanisms underlying repetition suppression in IT are still poorly understood. Studies in rodents and in-vitro experiments suggest that acetylcholine and GABA can contribute to repetition suppression by interacting with fatigue-related or local adaptation mechanisms. Here, we examined the contribution of cholinergic and GABAergic mechanisms to repetition suppression in macaque IT, using an adaptation paradigm in which familiar images were presented successively with a short interstimulus interval. We found that intracortical local injections of acetylcholine and of the GABA_A receptor antagonist Gabazine both increased repetition suppression in awake macaque IT. The increased repetition suppression was observed for both spiking activity and local field potential power. The latter was present mainly for frequencies below 50 Hz, spectral bands that typically do not show consistent repetition suppression in IT. Although increased with drug application, repetition suppression remained stimulus selective. These findings agree with the hypothesis that repetition suppression of IT neurons mainly results from suppressed input from upstream and other IT neurons but depend less on intrinsic neuronal fatigue.

[Effects of auditory response patterns on stimulus-specific adaptation of inferior colliculus neurons in awake mice]

OBJECTIVE

To explore whether the pattern of neuron's auditory response to a sound stimulus affects the characteristics of stimulus-specific adaptation (SSA) in awake mice.

METHODS

The auditory responses of the neurons in the inferior colliculus to sound stimuli were recorded using microelectrodes in awake mice. The sequence of sound stimuli consisted of random combinations of pure tones of two different frequencies (f1 and f2) with different repetition rates. The auditory responses of the neurons to standard and deviant stimuli were calculated, namely s(f2)/s(f2) and d(f1)/d(f2), respectively. Three indexes of the responses were also calculated, including the firing difference index (FDI), frequency-specific index (SI), and common SSA index(CSI).

RESULTS

The CSI of neurons with a greater FDI was significantly higher than that of neurons with a smaller FDI (P < 0.05). The primary-like neurons showed different characteristics of SSAs in different time periods; SSA was significantly increased in the phase of sustained response compared with that at the onset of response (P < 0.05).

CONCLUSIONS

The auditory response pattern to sound stimuli is also an important factor that affect SSA of inferior colliculus neurons in awake mice.

Neurons along the auditory pathway exhibit a hierarchical organization of prediction error

Perception is characterized by a reciprocal exchange of predictions and prediction error signals between neural regions. However, the relationship between such sensory mismatch responses and hierarchical predictive processing has not yet been demonstrated at the neuronal level in the auditory pathway. We recorded single-neuron activity from different auditory centers in anaesthetized rats and awake mice while animals were played a sequence of sounds, designed to separate the responses due to prediction error from those due to adaptation effects. Here we report that prediction error is organized hierarchically along the central auditory pathway. These prediction error signals are detectable in subcortical regions and increase as the signals move towards auditory cortex, which in turn demonstrates a large-scale mismatch potential. Finally, the predictive activity of single auditory neurons underlies automatic deviance detection at subcortical levels of processing. These results demonstrate that prediction error is a fundamental component of singly auditory neuron responses.

Sensory Stream Adaptation in Chaotic Networks

Implicit expectations induced by predictable stimuli sequences affect neuronal response to upcoming stimuli at both single cell and neural population levels. Temporally regular sensory streams also phase entrain ongoing low frequency brain oscillations but how and why this happens is unknown. Here we investigate how random recurrent neural networks without plasticity respond to stimuli streams containing oddballs. We found the neuronal correlates of sensory stream adaptation emerge if networks generate chaotic oscillations which can be phase entrained by stimulus streams. The resultant activity patterns are close to critical and support history dependent response on long timescales. Because critical network entrainment is a slow process stimulus response adapts gradually over multiple repetitions. Repeated stimuli generate suppressed responses but oddball responses are large and distinct. Oscillatory mismatch responses persist in population activity for long periods after stimulus offset while individual cell mismatch responses are strongly phasic. These effects are weakened in temporally irregular sensory streams. Thus we show that network phase entrainment provides a biologically plausible mechanism for neural oddball detection. Our results do not depend on specific network characteristics, are consistent with experimental studies and may be relevant for multiple pathologies demonstrating altered mismatch processing such as schizophrenia and depression.

The effect of inhibition on stimulus-specific adaptation in the inferior colliculus

The inferior colliculus is a center of convergence for inhibitory and excitatory synaptic inputs that may be activated simultaneously by sound stimulation. Stimulus repetition may generate response habituation by changing the efficacy of neuron's synaptic inputs. Specialized IC neurons reduce their response to repetitive tones, but restore their firing when a different and infrequent tone occurs, a phenomenon known as stimulus specific adaptation. Here, using the microiontophoresis technique, we determined the role of GABA_A-, GABA_B-, and glycinergic receptors in stimulus-specific adaptation (SSA). We found that blockade of postsynaptic GABA_B receptors selectively modulated response adaptation to repetitive sounds, whereas blockade of presynaptic GABA_B receptors exerted a gain control effect on neuron excitability. Adaptation decreased when postsynaptic GABA_B receptors were blocked, but increased if the blockade affected the presynaptic GABA_B receptors. A dual, paradoxical effect was elicited by blockade of glycinergic receptors, i.e., both increase and decrease in adaptation. Moreover, simultaneous co-application of GABA_A, GABA_B, and glycinergic antagonists demonstrated that local GABA- and glycine-mediated inhibition contributes to only about 50% of SSA. Therefore, inhibition via chemical synapses dynamically modulate the strength and dynamics of stimulus-specific adaptation, but does not generate it.

The implicit learning of metrical and non-metrical rhythms in blind and sighted adults

Forming temporal expectancies plays a crucial role in our survival as it allows us to identify the occurrence of temporal deviants that might signal potential dangers. The dynamic attending theory suggests that temporal expectancies are formed more readily for rhythms that imply a beat (i.e., metrical rhythms) compared to those that do not (i.e., nonmetrical rhythms). Moreover, metrical frameworks can be used to detect temporal deviants. Although several studies have demonstrated that congenital or early blindness correlates with modality-specific neural changes that reflect compensatory mechanisms, few have examined whether blind individuals show a learning advantage for auditory rhythms and whether learning can occur unintentionally and without awareness, that is, implicitly. We compared blind to sighted controls in their ability to implicitly learn metrical and nonmetrical auditory rhythms. We reasoned that the loss of sight in blindness might lead to improved sensitivity to rhythms and predicted that the blind learn rhythms more readily than the sighted. We further hypothesized that metrical rhythms are learned more readily than nonmetrical rhythms. Results partially confirmed our predictions; the blind group learned nonmetrical rhythms more readily than the sighted group but the blind group learned metrical rhythms less readily than the sighted group. Only the sighted group learned metrical rhythms more readily than nonmetrical rhythms. The blind group demonstrated awareness of the nonmetrical rhythms while learning was implicit for all other conditions. Findings suggest that improved deviant-sensitivity might have provided the blind group a learning advantage for nonmetrical rhythms. Future research could explore the plastic changes that affect deviance-detection and stimulus-specific adaptation in blindness.

Endocannabinoid Modulation of Stimulus-Specific Adaptation in Inferior Colliculus Neurons of the Rat

Cannabinoid receptors (CBRs) are widely distributed in the brain, including the inferior colliculus (IC). Here, we aim to study whether endocannabinoids influence a specific type of neuronal adaptation, namely, stimulus-specific adaptation (SSA) found in some IC neurons. SSA is important because it has been found as early as the level of the midbrain and therefore it may be a neuronal correlate of early indices of deviance detection. Furthermore, recent studies have demonstrated a direct link between SSA and MMN, that is widely used as an outcome measure in a variety of human neurodegenerative disorders. SSA is considered a form of short-term plasticity, and CBRs have been shown to play a role in short-term neural plasticity. Therefore, it is reasonable to hypothesize that endocannabinoids may play a role in the generation or modulation of SSA. We recorded single units in the IC under an oddball paradigm stimulation. The results demonstrate that cannabinoid agonists lead to a reduction in the neuronal adaptation. This change is due to a differential increase of the neuronal firing rate to the standard tone alone. Furthermore, we show that the effect is mediated by the cannabinoid receptor 1 (CBR1). Thus, cannabinoid agonists down-modulate SSA in IC neurons.

Effect of vagus nerve stimulation on neural adaptation in thalamo-cortical system in rats

Vagus nerve stimulation (VNS) is a therapy on medically refractory epilepsy, and recently reported to improve cognitive function including learning and memory. The thalamo-cortical system may underlie such VNS-induced cognitive improvements. Thus, the present study targeted the auditory cortex and thalamus in rats, and investigated whether and how VNS modulates stimulus-specific adaptation (SSA) of the neural activity in these nuclei. A depth electrode array recorded auditory evoked potentials from the auditory thalamus and cortex under an oddball paradigm either with or without VNS. Consequently, VNS weakened SSA in the cortex, but did not affect that in the thalamus, indicating that VNS has neuromodulatory effects on the cortical inhibitory system and the thalamo-cortical projections, but not on the feedforward projections from the auditory periphery up to thalamus.

Extracellular Recording of Neuronal Activity Combined with Microiontophoretic Application of Neuroactive Substances in Awake Mice

Differences in the activity of neurotransmitters and neuromodulators, and consequently different neural responses, can be found between anesthetized and awake animals. Therefore, methods allowing the manipulation of synaptic systems in awake animals are required in order to determine the contribution of synaptic inputs to neuronal processing unaffected by anesthetics. Here, we present methodology for the construction of electrodes to simultaneously record extracellular neural activity and release multiple neuroactive substances at the vicinity of the recording sites in awake mice. By combining these procedures, we performed microiontophoretic injections of gabazine to selectively block GABAA receptors in neurons of the inferior colliculus of head-restrained mice. Gabazine successfully modified neural response properties such as the frequency response area and stimulus-specific adaptation. Thus, we demonstrate that our methods are suitable for recording single-unit activity and for dissecting the role of specific neurotransmitter receptors in auditory processing. The main limitation of the described procedure is the relatively short recording time (~3 hr), which is determined by the level of habituation of the animal to the recording sessions. On the other hand, multiple recording sessions can be performed in the same animal. The advantage of this technique over other experimental procedures used to manipulate the level of neurotransmission or neuromodulation (such as systemic injections or the use of optogenetic models), is that the drug effect is confined to the local synaptic inputs to the target neuron. In addition, the custom-manufacture of electrodes allows adjustment of specific parameters according to the neural structure and type of neuron of interest (such as the tip resistance for improving the signal-to-noise ratio of the recordings).

Neurons in the inferior colliculus of the rat show stimulus-specific adaptation for frequency, but not for intensity

Electrophysiological and psychophysical responses to a low-intensity probe sound tend to be suppressed by a preceding high-intensity adaptor sound. Nevertheless, rare low-intensity deviant sounds presented among frequent high-intensity standard sounds in an intensity oddball paradigm can elicit an electroencephalographic mismatch negativity (MMN) response. This has been taken to suggest that the MMN is a correlate of true change or “deviance” detection. A key question is where in the ascending auditory pathway true deviance sensitivity first emerges. Here, we addressed this question by measuring low-intensity deviant responses from single units in the inferior colliculus (IC) of anesthetized rats. If the IC exhibits true deviance sensitivity to intensity, IC neurons should show enhanced responses to low-intensity deviant sounds presented among high-intensity standards. Contrary to this prediction, deviant responses were only enhanced when the standards and deviants differed in frequency. The results could be explained with a model assuming that IC neurons integrate over multiple frequency-tuned channels and that adaptation occurs within each channel independently. We used an adaptation paradigm with multiple repeated adaptors to measure the tuning widths of these adaption channels in relation to the neurons’ overall tuning widths.

Repetition suppression and its contextual determinants in predictive coding

This paper presents a review of theoretical and empirical work on repetition suppression in the context of predictive coding. Predictive coding is a neurobiologically plausible scheme explaining how biological systems might perform perceptual inference and learning. From this perspective, repetition suppression is a manifestation of minimising prediction error through adaptive changes in predictions about the content and precision of sensory inputs. Simulations of artificial neural hierarchies provide a principled way of understanding how repetition suppression – at different time scales – can be explained in terms of inference and learning implemented under predictive coding. This formulation of repetition suppression is supported by results of numerous empirical studies of repetition suppression and its contextual determinants.

Specific Early and Late Oddball-Evoked Responses in Excitatory and Inhibitory Neurons of Mouse Auditory Cortex

A major challenge for sensory processing in the brain is considering stimulus context, such as stimulus probability, which may be relevant for survival. Excitatory neurons in auditory cortex, for example, adapt to repetitive tones in a stimulus-specific manner without fully generalizing to a low-probability deviant tone ("oddball") that breaks the preceding regularity. Whether such stimulus-specific adaptation (SSA) also prevails in inhibitory neurons and how it might relate to deviance detection remains elusive. We obtained whole-cell recordings from excitatory neurons and somatostatin- and parvalbumin-positive GABAergic interneurons in layer 2/3 of mouse auditory cortex and measured tone-evoked membrane potential responses. All cell types displayed SSA of fast ("early") subthreshold and suprathreshold responses with oddball tones of a deviant frequency eliciting enlarged responses compared with adapted standards. SSA was especially strong when oddball frequency matched neuronal preference. In addition, we identified a slower "late" response component (200-400 ms after tone onset), most clearly in excitatory and parvalbumin-positive neurons, which also displayed SSA. For excitatory neurons, this late component reflected genuine deviance detection. Moreover, intracellular blockade of NMDA receptors reduced early and late responses in excitatory but not parvalbumin-positive neurons. The late component in excitatory neurons thus shares time course, deviance detection, and pharmacological features with the deviant-evoked event-related potential known as mismatch negativity (MMN) and provides a potential link between neuronal SSA and MMN. In summary, our results suggest a two-phase cortical activation upon oddball stimulation, with oddball tones first reactivating the adapted auditory cortex circuitry and subsequently triggering delayed reverberating network activity. Significance statement: Understanding how the brain encodes sensory context in addition to stimulus feature has been a main focus in neuroscience. Using in vivo targeted whole-cell recordings from excitatory and inhibitory neurons of mouse primary auditory cortex, we report two temporally distinct components of membrane potential responses encoding oddball tones that break stimulus regularity. Both components display stimulus-specific adaptation upon oddball paradigm stimulation in the three recorded cell types. The late response component, in particular, carries signatures of genuine deviance detection. In excitatory but not parvalbumin-positive inhibitory neurons, both early and late components depend on NMDA receptor-signaling. Our work proposes a potential neuronal substrate of a known deviant-evoked event-related potential, which is of fundamental significance in basic and clinical neuroscience.

Neurochemical modulation of repetition suppression and novelty signals in the human brain

The repeated processing of a sensory stimulus, such as a picture or sound, leads to a decrement in response in neurons that fired to the initial presentation. These effects are well known from single cell recordings in the inferior temporal cortex in monkeys, and functional neuroimaging in humans on large-scale neural activity could show similar effects in extrastriate, frontal and medial temporal lobe (MTL) regions. The role of specific neurotransmitters in repeated processing of information is, however, less clear. In the first part of this article, we will introduce the two concepts of repetition suppression and novelty signals, which is followed by a brief overview of pharmacological neuroimaging in humans. We will then summarize human studies suggesting that gamma-aminobutyric-acid (GABA) and acetylcholine (ACh) play an important role in modulating behavioral priming and associated repetition suppression in extrastriate and frontal brain regions. Finally, we review studies on neural novelty signals in the dopaminergic mesolimbic system, and conclude that dopamine (DA) regulates the temporal aspects of novelty processing and closely relates to long-term memory encoding rather than behavioral priming. As such, this review describes differential roles of GABA, ACh and DA in repeated stimulus processing, and further suggests that repetition suppression and neural novelty signals may not be two sides of the same coin but rather independent processes.

Complementary control of sensory adaptation by two types of cortical interneurons

Reliably detecting unexpected sounds is important for environmental awareness and survival. By selectively reducing responses to frequently, but not rarely, occurring sounds, auditory cortical neurons are thought to enhance the brain's ability to detect unexpected events through stimulus-specific adaptation (SSA). The majority of neurons in the primary auditory cortex exhibit SSA, yet little is known about the underlying cortical circuits. We found that two types of cortical interneurons differentially amplify SSA in putative excitatory neurons. Parvalbumin-positive interneurons (PVs) amplify SSA by providing non-specific inhibition: optogenetic suppression of PVs led to an equal increase in responses to frequent and rare tones. In contrast, somatostatin-positive interneurons (SOMs) selectively reduce excitatory responses to frequent tones: suppression of SOMs led to an increase in responses to frequent, but not to rare tones. A mutually coupled excitatory-inhibitory network model accounts for distinct mechanisms by which cortical inhibitory neurons enhance the brain's sensitivity to unexpected sounds.

DOI:http://dx.doi.org/10.7554/eLife.09868.001

In everyday life, we are often exposed to a mix of different sounds. An essential task for our brain is to separate the important sounds from the unimportant ones. For example, stepping out onto a busy street, you may at first be very aware of the noise of traffic. Later, you may start to ignore the din and instead only notice sounds that break the monotony: a honking car horn or maybe a stranger's voice. This is because the neurons in the auditory pathway respond differently to common and rare sounds. In particular, excitatory neurons in the region termed the ‘auditory cortex’ send fewer nerve impulses in response to frequent sounds, but respond vigorously to rare sounds. This phenomenon is called ‘stimulus-specific adaptation’, but it is not known exactly which neurons in this brain region enable this process to occur.

Now, Natan et al. have combined different cutting-edge neuroscience techniques to identify the circuit of brain cells that drives this stimulus specific adaptation. A technique called optogenetics was used to effectively ‘turn off’ each of two kinds of inhibitory neuron in the auditory cortex of mice, by exposing the brain to colored light from a laser.

Natan et al. found that both kinds of inhibitory neuron amplified stimulus-specific adaptation, but via different mechanisms. One of these neuron types, called ‘parvalbumin-positive interneurons’, exerted a general effect on excitatory neurons and suppressed responses to both frequent and rare sounds As the responses to rare sounds started off greater than the responses to frequent sounds, suppressing both by an equal amount actually led to an increase in the relative difference between them. On the other hand, the second kind of inhibitory neuron, called ‘somatostatin-positive interneurons’, only reduced the excitatory neurons' responses to frequent sounds; these neurons had no effect on responses to rare noises.

Future studies will test how specific adaptation in different contexts can help us to behaviorally detect rare sounds while ignoring common ones, and search for the circuits beyond the auditory cortex that support hearing in complex sound environments.

DOI:http://dx.doi.org/10.7554/eLife.09868.002

Sensitivity of rat inferior colliculus neurons to frequency distributions

Stimulus-specific adaptation refers to a neural response reduction to a repeated stimulus that does not generalize to other stimuli. However, stimulus-specific adaptation appears to be influenced by additional factors. For example, the statistical distribution of tone frequencies has recently been shown to dynamically alter stimulus-specific adaptation in human auditory cortex. The present study investigated whether statistical stimulus distributions also affect stimulus-specific adaptation at an earlier stage of the auditory hierarchy. Neural spiking activity and local field potentials were recorded from inferior colliculus neurons of rats while tones were presented in oddball sequences that formed two different statistical contexts. Each sequence consisted of a repeatedly presented tone (standard) and three rare deviants of different magnitudes (small, moderate, large spectral change). The critical manipulation was the relative probability with which large spectral changes occurred. In one context the probability was high (relative to all deviants), while it was low in the other context. We observed larger responses for deviants compared with standards, confirming previous reports of increased response adaptation for frequently presented tones. Importantly, the statistical context in which tones were presented strongly modulated stimulus-specific adaptation. Physically and probabilistically identical stimuli (moderate deviants) in the two statistical contexts elicited different response magnitudes consistent with neural gain changes and thus neural sensitivity adjustments induced by the spectral range of a stimulus distribution. The data show that already at the level of the inferior colliculus stimulus-specific adaptation is dynamically altered by the statistical context in which stimuli occur.

Cholinergic Modulation of Stimulus-Specific Adaptation in the Inferior Colliculus

Neural encoding of an ever-changing acoustic environment is a complex and demanding process that depends on modulation by neuroactive substances. Some neurons of the inferior colliculus (IC) exhibit "stimulus-specific adaptation" (SSA), i.e., a decrease in their response to a repetitive sound, but not to a rare one. Previous studies have demonstrated that acetylcholine (ACh) alters the frequency response areas of auditory neurons and therefore is important in the encoding of spectral information. Here, we address how microiontophoretic application of ACh modulates SSA in the IC of the anesthetized rat. We found that ACh decreased SSA in IC neurons by increasing the response to the repetitive tone. This effect was mainly mediated by muscarinic receptors. The strength of the cholinergic modulation depended on the baseline SSA level, exerting its greatest effect on neurons with intermediate SSA responses across IC subdivisions. Our data demonstrate that the increased availability of ACh exerts transient functional changes in partially adapting IC neurons, enhancing the sensory encoding of the ongoing stimulation. This effect potentially contributes to the propagation of ascending sensory-evoked afferent activity through the thalamus en route to the cortex.

SIGNIFICANCE STATEMENT

Neural encoding of an ever-changing acoustic environment is a complex and demanding task that may depend on the available levels of neuroactive substances. We explored how the cholinergic inputs affect the responses of neurons in the auditory midbrain that exhibit different degrees of stimulus-specific adaptation (SSA), i.e., a specific decrease in their response to a repeated sound that does not generalize to other, rare sounds. This work addresses the role of cholinergic synaptic inputs as well as the contribution of the muscarinic and nicotinic receptors on SSA. This is the first report on the role of neuromodulation on SSA, and the results contribute to our understanding of the cellular bases for processing low- and high-probability sounds.

GABAergic inhibition shapes SAM responses in rat auditory thalamus

Auditory thalamus (medial geniculate body [MGB]) receives ascending inhibitory GABAergic inputs from inferior colliculus (IC) and descending GABAergic projections from the thalamic reticular nucleus (TRN) with both inputs postulated to play a role in shaping temporal responses. Previous studies suggested that enhanced processing of temporally rich stimuli occurs at the level of MGB, with our recent study demonstrating enhanced GABA sensitivity in MGB compared to IC. The present study used sinusoidal amplitude-modulated (SAM) stimuli to generate modulation transfer functions (MTFs), to examine the role of GABAergic inhibition in shaping the response properties of MGB single units in anesthetized rats. Rate MTFs (rMTFs) were parsed into "bandpass (BP)", "mixed (Mixed)", "highpass (HP)" or "atypical" response types, with most units showing the Mixed response type. GABAA receptor blockade with iontophoretic application of the GABAA receptor (GABAAR) antagonist gabazine (GBZ) selectively altered the response properties of most MGB neurons examined. Mixed and HP units showed significant GABAAR-mediated SAM-evoked rate response changes at higher modulation frequencies (fms), which were also altered by N-methyl-d-aspartic acid (NMDA) receptor blockade (2R)-amino-5-phosphonopentanoate (AP5). BP units, and the lower arm of Mixed units responded to GABAAR blockade with increased responses to SAM stimuli at or near the rate best modulation frequency (rBMF). The ability of GABA circuits to shape responses at higher modulation frequencies is an emergent property of MGB units, not observed at lower levels of the auditory pathway and may reflect activation of MGB NMDA receptors (Rabang and Bartlett, 2011; Rabang et al., 2012). Together, GABAARs exert selective rate control over selected fms, generally without changing the units' response type. These results showed that coding of modulated stimuli at the level of auditory thalamus is at least, in part, strongly controlled by GABA neurotransmission, in delicate balance with glutamatergic neurotransmission.

The cortical modulation of stimulus-specific adaptation in the auditory midbrain and thalamus: a potential neuronal correlate for predictive coding

To follow an ever-changing auditory scene, the auditory brain is continuously creating a representation of the past to form expectations about the future. Unexpected events will produce an error in the predictions that should “trigger” the network’s response. Indeed, neurons in the auditory midbrain, thalamus and cortex, respond to rarely occurring sounds while adapting to frequently repeated ones, i.e., they exhibit stimulus specific adaptation (SSA). SSA cannot be explained solely by intrinsic membrane properties, but likely involves the participation of the network. Thus, SSA is envisaged as a high order form of adaptation that requires the influence of cortical areas. However, present research supports the hypothesis that SSA, at least in its simplest form (i.e., to frequency deviants), can be transmitted in a bottom-up manner through the auditory pathway. Here, we briefly review the underlying neuroanatomy of the corticofugal projections before discussing state of the art studies which demonstrate that SSA present in the medial geniculate body (MGB) and inferior colliculus (IC) is not inherited from the cortex but can be modulated by the cortex via the corticofugal pathways. By modulating the gain of neurons in the thalamus and midbrain, the auditory cortex (AC) would refine SSA subcortically, preventing irrelevant information from reaching the cortex.

Statistical context shapes stimulus-specific adaptation in human auditory cortex

Stimulus-specific adaptation is the phenomenon whereby neural response magnitude decreases with repeated stimulation. Inconsistencies between recent nonhuman animal recordings and computational modeling suggest dynamic influences on stimulus-specific adaptation. The present human electroencephalography (EEG) study investigates the potential role of statistical context in dynamically modulating stimulus-specific adaptation by examining the auditory cortex-generated N1 and P2 components. As in previous studies of stimulus-specific adaptation, listeners were presented with oddball sequences in which the presentation of a repeated tone was infrequently interrupted by rare spectral changes taking on three different magnitudes. Critically, the statistical context varied with respect to the probability of small versus large spectral changes within oddball sequences (half of the time a small change was most probable; in the other half a large change was most probable). We observed larger N1 and P2 amplitudes (i.e., release from adaptation) for all spectral changes in the small-change compared with the large-change statistical context. The increase in response magnitude also held for responses to tones presented with high probability, indicating that statistical adaptation can overrule stimulus probability per se in its influence on neural responses. Computational modeling showed that the degree of coadaptation in auditory cortex changed depending on the statistical context, which in turn affected stimulus-specific adaptation. Thus the present data demonstrate that stimulus-specific adaptation in human auditory cortex critically depends on statistical context. Finally, the present results challenge the implicit assumption of stationarity of neural response magnitudes that governs the practice of isolating established deviant-detection responses such as the mismatch negativity.

Stimulus-specific adaptation at the synapse level in vitro

Stimulus-specific adaptation (SSA) is observed in many brain regions in humans and animals. SSA of cortical neurons has been proposed to accumulate through relays in ascending pathways. Here, we examined SSA at the synapse level using whole-cell patch-clamp recordings of primary cultured cortical neurons of the rat. First, we found that cultured neurons had high firing capability with 100-Hz current injection. However, neuron firing started to adapt to repeated electrically activated synaptic inputs at 10 Hz. Next, to activate different dendritic inputs, electrical stimulations were spatially separated. Cultured neurons showed similar SSA properties in the oddball stimulation paradigm compared to those reported in vivo. Single neurons responded preferentially to a deviant stimulus over repeated, standard stimuli considering both synapse-driven spikes and excitatory postsynaptic currents (EPSCs). Compared with two closely placed stimulating electrodes that activated highly overlapping dendritic fields, two separately placed electrodes that activated less overlapping dendritic fields elicited greater SSA. Finally, we used glutamate puffing to directly activate postsynaptic glutamate receptors. Neurons showed SSA to two separately placed puffs repeated at 10 Hz. Compared with EPSCs, GABAa receptor-mediated inhibitory postsynaptic currents showed weaker SSA. Heterogeneity of the synaptic inputs was critical for producing SSA, with glutamate receptor desensitization participating in the process. Our findings suggest that postsynaptic fatigue contributes largely to SSA at low frequencies.