Associative representational plasticity in the auditory cortex: a synthesis of two disciplines

The use of non-invasive brain stimulation in auditory perceptual learning: A review

Auditory perceptual learning is an experience-dependent form of auditory learning that can improve substantially throughout adulthood with practice. A key mechanism associated with perceptual learning is synaptic plasticity. In the last decades, an increasingly better understanding has formed about the neural mechanisms related to auditory perceptual learning. Research in animal models found an association between the functional organization of the primary auditory cortex and frequency discrimination ability. Several studies observed an increase in the area of representation to be associated with improved frequency discrimination. Non-invasive brain stimulation techniques have been related to the promotion of plasticity. Despite its popularity in other fields, non-invasive brain stimulation has not been used much in auditory perceptual learning. The present review has discussed the application of non-invasive brain stimulation methods in auditory perceptual learning by discussing the mechanisms, current evidence and challenges, and future directions.

Learning induces unique transcriptional landscapes in the auditory cortex

Learning can induce neurophysiological plasticity in the auditory cortex at multiple timescales. Lasting changes to auditory cortical function that persist over days, weeks, or even a lifetime, require learning to induce de novo gene expression. Indeed, transcription is the molecular determinant for long-term memories to form with a lasting impact on sound-related behavior. However, auditory cortical genes that support auditory learning, memory, and acquired sound-specific behavior are largely unknown. Using an animal model of adult, male Sprague-Dawley rats, this report is the first to identify genome-wide changes in learning-induced gene expression within the auditory cortex that may underlie long-lasting discriminative memory formation of acoustic frequency cues. Auditory cortical samples were collected from animals in the initial learning phase of a two-tone discrimination sound-reward task known to induce sound-specific neurophysiological and behavioral effects. Bioinformatic analyses on gene enrichment profiles from bulk RNA sequencing identified cholinergic synapse (KEGG rno04725), extra-cellular matrix receptor interaction (KEGG rno04512), and neuroactive receptor interaction (KEGG rno04080) among the top biological pathways are likely to be important for auditory discrimination learning. The findings characterize candidate effectors underlying the early stages of changes in cortical and behavioral function to ultimately support the formation of long-term discriminative auditory memory in the adult brain. The molecules and mechanisms identified are potential therapeutic targets to facilitate experiences that induce long-lasting changes to sound-specific auditory function in adulthood and prime for future gene-targeted investigations.

Learning induces unique transcriptional landscapes in the auditory cortex

Learning can induce neurophysiological plasticity in the auditory cortex at multiple timescales. Lasting changes to auditory cortical function that persist over days, weeks, or even a lifetime, require learning to induce de novo gene expression. Indeed, transcription is the molecular determinant for long-term memories to form with a lasting impact on sound-related behavior. However, auditory cortical genes that support auditory learning, memory, and acquired sound-specific behavior are largely unknown. This report is the first to identify in young adult male rats (Sprague-Dawley) genome-wide changes in learning-induced gene expression within the auditory cortex that may underlie the formation of long-lasting discriminative memory for acoustic frequency cues. Auditory cortical samples were collected from animals in the initial learning phase of a two-tone discrimination sound-reward task known to induce sound-specific neurophysiological and behavioral effects (e.g., Shang et al., 2019). Bioinformatic analyses on gene enrichment profiles from bulk RNA sequencing identified cholinergic synapse (KEGG 04725), extra-cellular matrix receptor interaction (KEGG 04512) , and neuroactive ligand-receptor interaction (KEGG 04080) as top biological pathways for auditory discrimination learning. The findings characterize key candidate effectors underlying changes in cortical function that support the initial formation of long-term discriminative auditory memory in the adult brain. The molecules and mechanisms identified are potential therapeutic targets to facilitate lasting changes to sound-specific auditory function in adulthood and prime for future gene-targeted investigations.

Review of Major Social Determinants of Health in Schizophrenia-Spectrum Psychotic Disorders: III. Biology

BACKGROUND

Social determinants of health (SDoHs) are nonmedical factors that significantly impact health and longevity. We found no published reviews on the biology of SDoHs in schizophrenia-spectrum psychotic disorders (SSPD).

STUDY DESIGN

We present an overview of pathophysiological mechanisms and neurobiological processes plausibly involved in the effects of major SDoHs on clinical outcomes in SSPD.

STUDY RESULTS

This review of the biology of SDoHs focuses on early-life adversities, poverty, social disconnection, discrimination including racism, migration, disadvantaged neighborhoods, and food insecurity. These factors interact with psychological and biological factors to increase the risk and worsen the course and prognosis of schizophrenia. Published studies on the topic are limited by cross-sectional design, variable clinical and biomarker assessments, heterogeneous methods, and a lack of control for confounding variables. Drawing on preclinical and clinical studies, we propose a biological framework to consider the likely pathogenesis. Putative systemic pathophysiological processes include epigenetics, allostatic load, accelerated aging with inflammation (inflammaging), and the microbiome. These processes affect neural structures, brain function, neurochemistry, and neuroplasticity, impacting the development of psychosis, quality of life, cognitive impairment, physical comorbidities, and premature mortality. Our model provides a framework for research that could lead to developing specific strategies for prevention and treatment of the risk factors and biological processes, thereby improving the quality of life and increasing the longevity of people with SSPD.

CONCLUSIONS

Biology of SDoHs in SSPD is an exciting area of research that points to innovative multidisciplinary team science for improving the course and prognosis of these serious psychiatric disorders.

Inhibitory top-down projections from zona incerta mediate neocortical memory

Top-down projections convey a family of signals encoding previous experiences and current aims to the sensory neocortex, where they converge with external bottom-up information to enable perception and memory. Whereas top-down control has been attributed to excitatory pathways, the existence, connectivity, and information content of inhibitory top-down projections remain elusive. Here, we combine synaptic two-photon calcium imaging, circuit mapping, cortex-dependent learning, and chemogenetics in mice to identify GABAergic afferents from the subthalamic zona incerta as a major source of top-down input to the neocortex. Incertocortical transmission undergoes robust plasticity during learning that improves information transfer and mediates behavioral memory. Unlike excitatory pathways, incertocortical afferents form a disinhibitory circuit that encodes learned top-down relevance in a bidirectional manner where the rapid appearance of negative responses serves as the main driver of changes in stimulus representation. Our results therefore reveal the distinctive contribution of long-range (dis)inhibitory afferents to the computational flexibility of neocortical circuits.

Epigenetic mechanisms regulate cue memory underlying discriminative behavior

The burgeoning field of neuroepigenetics has introduced chromatin modification as an important interface between experience and brain function. For example, epigenetic mechanisms like histone acetylation and DNA methylation operate throughout a lifetime to powerfully regulate gene expression in the brain that is required for experiences to be transformed into long-term memories. This review highlights emerging evidence from sensory models of memory that converge on the premise that epigenetic regulation of activity-dependent transcription in the sensory brain facilitates highly precise memory recall. Chromatin modifications may be key for neurophysiological responses to transient sensory cue features experienced in the "here and now" to be recapitulated over the long term. We conclude that the function of epigenetic control of sensory system neuroplasticity is to regulate the amount and type of sensory information retained in long-term memories by regulating neural representations of behaviorally relevant cues that guide behavior. This is of broad importance in the neuroscience field because there are few circumstances in which behavioral acts are devoid of an initiating sensory experience.

Inhibiting Human Aversive Memory by Transcranial Theta-Burst Stimulation to the Primary Sensory Cortex

BACKGROUND

Predicting adverse events from past experience is fundamental for many biological organisms. However, some individuals suffer from maladaptive memories that impair behavioral control and well-being, e.g., after psychological trauma. Inhibiting the formation and maintenance of such memories would have high clinical relevance. Previous preclinical research has focused on systemically administered pharmacological interventions, which cannot be targeted to specific neural circuits in humans. Here, we investigated the potential of noninvasive neural stimulation on the human sensory cortex in inhibiting aversive memory in a laboratory threat conditioning model.

METHODS

We build on an emerging nonhuman literature suggesting that primary sensory cortices may be crucially required for threat memory formation and consolidation. Immediately before conditioning innocuous somatosensory stimuli (conditioned stimuli [CS]) to aversive electric stimulation, healthy human participants received continuous theta-burst transcranial magnetic stimulation (cTBS) to individually localized primary somatosensory cortex in either the CS-contralateral (experimental) or CS-ipsilateral (control) hemisphere. We measured fear-potentiated startle to infer threat memory retention on the next day, as well as skin conductance and pupil size during learning.

RESULTS

After overnight consolidation, threat memory was attenuated in the experimental group compared with the control cTBS group. There was no evidence that this differed between simple and complex CS or that CS identification or initial learning were affected by cTBS.

CONCLUSIONS

Our results suggest that cTBS to the primary sensory cortex inhibits threat memory, likely by an impact on postlearning consolidation. We propose that noninvasive targeted stimulation of the sensory cortex may provide a new avenue for interfering with aversive memories in humans.

The Relationship Between Self-Reported Misophonia Symptoms and Auditory Aversive Generalization Leaning: A Preliminary Report

Misophonia is characterized by excessive aversive reactions to specific "trigger" sounds. Although this disorder is increasingly recognized in the literature, its etiological mechanisms and maintaining factors are currently unclear. Several etiological models propose a role of Pavlovian conditioning, an associative learning process heavily researched in similar fear and anxiety-related disorders. In addition, generalization of learned associations has been noted as a potential causal or contributory factor. Building upon this framework, we hypothesized that Misophonia symptoms arise as a consequence of overgeneralized associative learning, in which aversive responses to a noxious event also occur in response to similar events. Alternatively, heightened discrimination between conditioned threat and safety cues may be present in participants high in Misophonia symptoms, as predicted by associative learning models of Misophonia. This preliminary report (n = 34) examines auditory generalization learning using self-reported behavioral (i.e., valence and arousal ratings) and EEG alpha power reduction. Participants listened to three sine tones differing in pitch, with one pitch (i.e., CS+) paired with an aversive loud white noise blast, prompting aversive Pavlovian generalization learning. We assessed the extent to which overgeneralization versus heightened discrimination learning is associated with self-reported Misophonia symptoms, by comparing aversive responses to the CS+ and other tones similar in pitch. Behaviorally, all participants learned the contingencies between CS+ and noxious noise, with individuals endorsing elevated Misophonia showing heightened aversive sensitivity to all stimuli, regardless of conditioning and independent of hyperacusis status. Across participants, parieto-occipital EEG alpha-band power reduction was most pronounced in response to the CS+ tone, and this difference was greater in those with self-reported Misophonia symptoms. The current preliminary findings do not support the notion that overgeneralization is a feature of self-reported emotional experience in Misophonia, but that heightened sensitivity and discrimination learning may be present at the neural level.

Pattern differentiation and tuning shift in human sensory cortex underlie long-term threat memory

The amygdala-prefrontal-cortex circuit has long occupied the center of the threat system,¹ but new evidence has rapidly amassed to implicate threat processing outside this canonical circuit.^2-4 Through nonhuman research, the sensory cortex has emerged as a critical substrate for long-term threat memory,^5-9 underpinned by sensory cortical pattern separation/completion^10,11 and tuning shift.^12,13 In humans, research has begun to associate the human sensory cortex with long-term threat memory,^14,15 but the lack of mechanistic insights obscures a direct linkage. Toward that end, we assessed human olfactory threat conditioning and long-term (9 days) threat memory, combining affective appraisal, olfactory psychophysics, and functional magnetic resonance imaging (fMRI) over a linear odor-morphing continuum (five levels of binary mixtures of the conditioned stimuli/CS+ and CS- odors). Affective ratings and olfactory perceptual discrimination confirmed (explicit) affective and perceptual learning and memory via conditioning. fMRI representational similarity analysis (RSA) and voxel-based tuning analysis further revealed associative plasticity in the human olfactory (piriform) cortex, including immediate and lasting pattern differentiation between CS and neighboring non-CS and a late onset, lasting tuning shift toward the CS. The two plastic processes were especially salient and lasting in anxious individuals, among whom they were further correlated. These findings thus support an evolutionarily conserved sensory cortical system of long-term threat representation, which can underpin threat perception and memory. Importantly, hyperfunctioning of this sensory mnemonic system of threat in anxiety further implicates a hitherto underappreciated sensory mechanism of anxiety.

Basal Forebrain Cholinergic Neurons: Linking Down Syndrome and Alzheimer's Disease

Down syndrome (DS, trisomy 21) is characterized by intellectual impairment at birth and Alzheimer's disease (AD) pathology in middle age. As individuals with DS age, their cognitive functions decline as they develop AD pathology. The susceptibility to degeneration of a subset of neurons, known as basal forebrain cholinergic neurons (BFCNs), in DS and AD is a critical link between cognitive impairment and neurodegeneration in both disorders. BFCNs are the primary source of cholinergic innervation to the cerebral cortex and hippocampus, as well as the amygdala. They play a critical role in the processing of information related to cognitive function and are directly engaged in regulating circuits of attention and memory throughout the lifespan. Given the importance of BFCNs in attention and memory, it is not surprising that these neurons contribute to dysfunctional neuronal circuitry in DS and are vulnerable in adults with DS and AD, where their degeneration leads to memory loss and disturbance in language. BFCNs are thus a relevant cell target for therapeutics for both DS and AD but, despite some success, efforts in this area have waned. There are gaps in our knowledge of BFCN vulnerability that preclude our ability to effectively design interventions. Here, we review the role of BFCN function and degeneration in AD and DS and identify under-studied aspects of BFCN biology. The current gaps in BFCN relevant imaging studies, therapeutics, and human models limit our insight into the mechanistic vulnerability of BFCNs in individuals with DS and AD.

Emotional Arousal and Valence Jointly Modulate the Auditory Response: A 40-Hz ASSR Study

Emotion is defined as a response to external stimuli and internal mental representations. It has been characterized as a multidimensional concept, primarily comprising two dimensions: valence and arousal. Existing studies have demonstrated that emotional experience exerts a powerful impact on auditory processing in terms of valence. However, it has also been shown that while negative emotion can improve auditory perception in healthy subjects, patients with depression show deficits in auditory perception. We thus speculated that both arousal and valence jointly modulate auditory perception. To examine the emotion-driven effects on the auditory response, we induced positive, negative, and neutral emotional states in healthy subjects and collected auditory steady-state response (ASSR) evoked by a 40-Hz chirp sound. We calculated peak-to-peak amplitude (PPA) and event-related spectral perturbation (ERSP) of evoked ASSRs and observed that the positive emotions significantly enhanced brain responses to auditory stimuli (p < 0.001), but that ASSRs in a negative state were not significantly enhanced compared with the neutral state. Subsequently, regression analysis showed a significant positive multiple linear relationship between the PPA and ratings of two emotional dimensions, indicating that arousal and valence jointly regulated the auditory cortex's synchronous oscillation, rather than the valence in isolation, offering the potential to clarify the conflicting results surrounding the role of negative emotions in auditory responses. Because depression is generally characterized by low arousal and low valence in actual life, whereas the negative emotion evoked under laboratory conditions is always with low valence but high arousal.

Heterogeneous associative plasticity in the auditory cortex induced by fear learning - novel insight into the classical conditioning paradigm

We used two-photon calcium imaging with single-cell and cell-type resolution. Fear conditioning induced heterogeneous tuning shifts at single-cell level in the auditory cortex, with shifts both to CS+ frequency and to the control CS- stimulus frequency. We thus extend the view of simple expansion of CS+ tuned regions. Instead of conventional freezing reactions only, we observe selective orienting responses towards the conditioned stimuli. The orienting responses were often followed by escape behavior.

State Transitions During Discrimination Learning in the Gerbil Auditory Cortex Analyzed by Network Causality Metrics

This work studies the evolution of cortical networks during the transition from escape strategy to avoidance strategy in auditory discrimination learning in Mongolian gerbils trained by the well-established two-way active avoidance learning paradigm. The animals were implanted with electrode arrays centered on the surface of the primary auditory cortex and electrocorticogram (ECoG) recordings were made during performance of an auditory Go/NoGo discrimination task. Our experiments confirm previous results on a sudden behavioral change from the initial naïve state to an avoidance strategy as learning progresses. We employed two causality metrics using Granger Causality (GC) and New Causality (NC) to quantify changes in the causality flow between ECoG channels as the animals switched to avoidance strategy. We found that the number of channel pairs with inverse causal interaction significantly increased after the animal acquired successful discrimination, which indicates structural changes in the cortical networks as a result of learning. A suitable graph-theoretical model is developed to interpret the findings in terms of cortical networks evolving during cognitive state transitions. Structural changes lead to changes in the dynamics of neural populations, which are described as phase transitions in the network graph model with small-world connections. Overall, our findings underscore the importance of functional reorganization in sensory cortical areas as a possible neural contributor to behavioral changes.

A contribution to the debate on tinnitus definition

Tinnitus is generally defined as an auditory perception in the absence of environmental sound stimulation. However, this definition is quite incomplete as it omits an essential aspect, the patient's point of view. This point of view constitutes, first and foremost, a global and unified lived experience, which is not only sensory (localization, loudness, pitch and tone), but also cognitive (thoughts, attentiveness, behaviors) and emotional (discomfort, suffering). This experience can be lived in a very unpleasant way and consequently have a very negative impact on quality of life. This article proposes and justifies a new definition for tinnitus elaborated by a group of French clinicians and researchers, which is more in line with its phenomenology. It also provides a minimum knowledge base, including possibilities for clinical care, hoping to eradicate all misinformation, misconceptions and inappropriate attitudes or practices toward this condition. Here is the short version of our definition: Tinnitus is an auditory sensation without an external sound stimulation or meaning, which can be lived as an unpleasant experience, possibly impacting quality of life.

A critical review of mechanisms of adaptation to trauma: Implications for early interventions for posttraumatic stress disorder

Although many attempts have been made to limit development of posttraumatic stress disorder (PTSD) by early intervention after trauma exposure, these attempts have achieved only modest success. This review critiques the biological and cognitive strategies used for early intervention and outlines the extent to which they have prevented PTSD. The major predictors of PTSD are reviewed, with an emphasis on potential mechanisms that may underpin the transition from acute stress reaction to development of PTSD. This review highlights that there is a wide range of biological and cognitive factors that have been shown to predict PTSD. Despite this, the major attempts at early intervention have focused on strategies that attempt to augment extinction processes or alter appraisals in the acute period. The documented predictors of PTSD indicate that a broader range of potential strategies could be explored to limit PTSD. The evidence that people follow different trajectories of stress response following trauma and there is a wide array of acute predictors of PTSD indicates that a flexible and tailored approach needs to be investigated to evaluate more effective early intervention strategies.

Absence Makes the Mind Grow Fonder: Reconceptualizing Studies of Safety Learning in Translational Research on Anxiety

Overgeneralized fear (OGF), or indiscriminate fear responses to signals of threat and nonthreat, is a well-studied cognitive mechanism in human anxiety. Anxiety-related OGF has been studied primarily through fear-learning paradigms and conceptualized as overly exaggerated learning of cues signaling imminent threat. However, the role of safety learning in OGF has not only received much less empirical attention but has been fundamentally conceptualized as learning about the absence of threat rather than the presence of safety. As a result, the relative contributions of exaggerated fear learning and weakened safety learning to anxiety-related OGF remain poorly understood, as do the potentially unique biological and behavioral underpinnings of safety learning. The present review outlines these gaps by, first, summarizing animal and human research on safety learning related to anxiety and OGF. Second, we outline innovations in methods to tease apart unique biological and behavioral contributions of safety learning to OGF. Lastly, we describe clinical and treatment implications of this framework for translational research relevant to human anxiety.

Functional Subdivisions of Magnocellular Cell Groups in Human Basal Forebrain: Test-Retest Resting-State Study at Ultra-high Field, and Meta-analysis

The heterogeneous neuronal subgroups of the basal forebrain corticopetal system (BFcs) have been shown to modulate cortical functions through their cholinergic, gamma-aminobutyric acid-ergic, and glutamatergic projections to the entire cortex. Although previous studies suggested that the basalo-cortical projection system influences various cognitive functions, particularly via its cholinergic component, these studies only focused on certain parts of the BFcs or nearby structures, leaving aside a more systematic picture of the functional connectivity of BFcs subcompartments. Moreover, these studies lacked the high-spatial resolution and the probability maps needed to identify specific subcompartments. Recent advances in the ultra-high field 7T functional magnetic resonance imaging (fMRI) provided potentially unprecedented spatial resolution of functional MRI images to study the subdivision of the BFcs. In this study, the BF space containing corticopetal cells was divided into 3 functionally distinct subdivisions based on functional connection to cortical regions derived from fMRI. The overall functional connection of each BFcs subdivision was examined with a test-retest study. Finally, a meta-analysis was used to study the related functional topics of each BF subdivision. Our results demonstrate distinct functional connectivity patterns of these subdivisions along the rostrocaudal axis of the BF. All three compartments have shown consistent segregation and overlap at specific target regions including the hippocampus, insula, thalamus, and the cingulate gyrus, suggesting functional integration and separation in BFcs.

Regulation of auditory plasticity during critical periods and following hearing loss

Sensory input has profound effects on neuronal organization and sensory maps in the brain. The mechanisms regulating plasticity of the auditory pathway have been revealed by examining the consequences of altered auditory input during both developmental critical periods—when plasticity facilitates the optimization of neural circuits in concert with the external environment—and in adulthood—when hearing loss is linked to the generation of tinnitus. In this review, we summarize research identifying the molecular, cellular, and circuit-level mechanisms regulating neuronal organization and tonotopic map plasticity during developmental critical periods and in adulthood. These mechanisms are shared in both the juvenile and adult brain and along the length of the auditory pathway, where they serve to regulate disinhibitory networks, synaptic structure and function, as well as structural barriers to plasticity. Regulation of plasticity also involves both neuromodulatory circuits, which link plasticity with learning and attention, as well as ascending and descending auditory circuits, which link the auditory cortex and lower structures. Further work identifying the interplay of molecular and cellular mechanisms associating hearing loss-induced plasticity with tinnitus will continue to advance our understanding of this disorder and lead to new approaches to its treatment.

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During CPs, brain plasticity is enhanced and sensitive to acoustic experience.

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Enhanced plasticity can be reinstated in the adult brain following hearing loss.

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Molecular, cellular, and circuit-level mechanisms regulate CP and adult plasticity.

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Plasticity resulting from hearing loss may contribute to the emergence of tinnitus.

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Modifying plasticity in the adult brain may offer new treatments for tinnitus.

An Emergent Discriminative Learning Is Elicited During Multifrequency Testing

In auditory-conditioned fear learning, the freezing response is independent of the sound frequencies used, but the frequency of the conditioned sound is considered distinct from those of unrelated sounds based on electrophysiological responses in the auditory system. Whether an emergent discriminative learning underlies auditory fear conditioning and which nuclei and pathways are involved in it remain unclear. Using behavioral and electrophysiological assays, we found that the response of medial prefrontal cortex (mPFC) neurons to a conditioned auditory stimulus (CS) was enhanced relative to the response to unrelated frequencies (UFs) after auditory fear conditioning, and mice could distinguish the CS during multifrequency testing, a phenomenon called emergent discriminative learning. After silencing the mPFC with muscimol, emergent discriminative learning was blocked. In addition, the pure tone responses of mPFC neurons were inhibited after injection of lidocaine in the ipsilateral primary auditory cortex (A1), and the emergent discriminative learning was blocked by silencing both sides of A1 with muscimol. This study, therefore, provides evidence for an emergent discriminative learning mediated by mPFC and A1 neurons after auditory fear conditioning.

Emotional learning promotes perceptual predictions by remodeling stimulus representation in visual cortex

Emotions exert powerful effects on perception and memory, notably by modulating activity in sensory cortices so as to capture attention. Here, we examine whether emotional significance acquired by a visual stimulus can also change its cortical representation by linking neuronal populations coding for different memorized versions of the same stimulus, a mechanism that would facilitate recognition across different appearances. Using fMRI, we show that after pairing a given face with threat through conditioning, viewing this face activates the representation of another viewpoint of the same person, which itself was never conditioned, leading to robust repetition-priming across viewpoints in the ventral visual stream (including medial fusiform, lateral occipital, and anterior temporal cortex). We also observed a functional-anatomical segregation for coding view-invariant and view-specific identity information. These results indicate emotional signals may induce plasticity of stimulus representations in visual cortex, serving to generate new sensory predictions about different appearances of threat-associated stimuli.

Primary auditory cortex representation of fear-conditioned musical sounds

Auditory cortex is required for discriminative fear conditioning beyond the classical amygdala microcircuit, but its precise role is unknown. It has previously been suggested that Heschl's gyrus, which includes primary auditory cortex (A1), but also other auditory areas, encodes threat predictions during presentation of conditioned stimuli (CS) consisting of monophones, or frequency sweeps. The latter resemble natural prosody and contain discriminative spectro‐temporal information. Here, we use functional magnetic resonance imaging (fMRI) in humans to address CS encoding in A1 for stimuli that contain only spectral but no temporal discriminative information. Two musical chords (complex) or two monophone tones (simple) were presented in a signaled reinforcement context (reinforced CS+ and nonreinforced CS−), or in a different context without reinforcement (neutral sounds, NS1 and NS2), with an incidental sound detection task. CS/US association encoding was quantified by the increased discriminability of BOLD patterns evoked by CS+/CS−, compared to NS pairs with similar physical stimulus differences and task demands. A1 was defined on a single‐participant level and based on individual anatomy. We find that in A1, discriminability of CS+/CS− was higher than for NS1/NS2. This representation of unconditioned stimulus (US) prediction was of comparable magnitude for both types of sounds. We did not observe such encoding outside A1. Different from frequency sweeps investigated previously, musical chords did not share representations of US prediction with monophone sounds. To summarize, our findings suggest decodable representation of US predictions in A1, for various types of CS, including musical chords that contain no temporal discriminative information.

Noninvasive Bimodal Neuromodulation for the Treatment of Tinnitus: Protocol for a Second Large-Scale Double-Blind Randomized Clinical Trial to Optimize Stimulation Parameters

Background

There is increasing evidence from animal and human studies that bimodal neuromodulation combining sound and electrical somatosensory stimulation of the tongue can induce extensive brain changes and treat tinnitus.

Objective

The main objectives of the proposed clinical study are to confirm the efficacy, safety, and tolerability of treatment demonstrated in a previous large-scale study of bimodal auditory and trigeminal nerve (tongue) stimulation (Treatment Evaluation of Neuromodulation for Tinnitus - Stage A1); evaluate the therapeutic effects of adjusting stimulation parameters over time; and determine the contribution of different features of bimodal stimulation in improving tinnitus outcomes.

Methods

This study will be a prospective, randomized, double-blind, parallel-arm, comparative clinical trial of a 12-week treatment for tinnitus using a Conformité Européenne (CE)–marked device with a pre-post and 12-month follow-up design. Four treatment arms will be investigated, in which each arm consists of two different stimulation settings, with the first setting presented during the first 6 weeks and the second setting presented during the next 6 weeks of treatment. The study will enroll 192 participants, split in a ratio of 80:80:16:16 across the four arms. Participants will be randomized to one of four arms and stratified to minimize baseline variability in four categories: two separate strata for sound level tolerance (using loudness discomfort level as indicators for hyperacusis severity), high tinnitus symptom severity based on the Tinnitus Handicap Inventory (THI), and tinnitus laterality. The primary efficacy endpoints are within-arm changes in THI and Tinnitus Functional Index as well as between-arm changes in THI after 6 weeks of treatment for the full cohort and two subgroups of tinnitus participants (ie, one hyperacusis subgroup and a high tinnitus symptom severity subgroup). Additional efficacy endpoints include within-arm or between-arm changes in THI after 6 or 12 weeks of treatment and in different subgroups of tinnitus participants as well as at posttreatment assessments at 6 weeks, 6 months, and 12 months. Treatment safety, attrition rates, and compliance rates will also be assessed and reported.

Results

This study protocol was approved by the Tallaght University Hospital/St. James’s Hospital Joint Research Ethics Committee in Dublin, Ireland. The first participant was enrolled on March 20, 2018. The data collection and database lock are expected to be completed by February 2020, and the data analysis and manuscript submission are expected to be conducted in autumn of 2020.

Conclusions

The findings of this study will be disseminated to relevant research, clinical, and health services and patient communities through publications in peer-reviewed journals and presentations at scientific and clinical conferences.

Trial Registration

ClinicalTrials.gov NCT03530306; https://clinicaltrials.gov/ct2/show/NCT03530306

International Registered Report Identifier (IRRID)

DERR1-10.2196/13176

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.

Learning-Related Plasticity in Dendrite-Targeting Layer 1 Interneurons

A wealth of data has elucidated the mechanisms by which sensory inputs are encoded in the neocortex, but how these processes are regulated by the behavioral relevance of sensory information is less understood. Here, we focus on neocortical layer 1 (L1), a key location for processing of such top-down information. Using Neuron-Derived Neurotrophic Factor (NDNF) as a selective marker of L1 interneurons (INs) and in vivo 2-photon calcium imaging, electrophysiology, viral tracing, optogenetics, and associative memory, we find that L1 NDNF-INs mediate a prolonged form of inhibition in distal pyramidal neuron dendrites that correlates with the strength of the memory trace. Conversely, inhibition from Martinotti cells remains unchanged after conditioning but in turn tightly controls sensory responses in NDNF-INs. These results define a genetically addressable form of dendritic inhibition that is highly experience dependent and indicate that in addition to disinhibition, salient stimuli are encoded at elevated levels of distal dendritic inhibition.

Video Abstract

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NDNF is a selective marker for neocortical layer 1 interneurons

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NDNF interneurons mediate prolonged inhibition of distal pyramidal neuron dendrites

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Inhibition from Martinotti cells tightly controls NDNF interneuron responses

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Dendritic inhibition by NDNF interneurons is highly experience dependent

Auditory midbrain coding of statistical learning that results from discontinuous sensory stimulation

Detecting regular patterns in the environment, a process known as statistical learning, is essential for survival. Neuronal adaptation is a key mechanism in the detection of patterns that are continuously repeated across short (seconds to minutes) temporal windows. Here, we found in mice that a subcortical structure in the auditory midbrain was sensitive to patterns that were repeated discontinuously, in a temporally sparse manner, across windows of minutes to hours. Using a combination of behavioral, electrophysiological, and molecular approaches, we found changes in neuronal response gain that varied in mechanism with the degree of sound predictability and resulted in changes in frequency coding. Analysis of population activity (structural tuning) revealed an increase in frequency classification accuracy in the context of increased overlap in responses across frequencies. The increase in accuracy and overlap was paralleled at the behavioral level in an increase in generalization in the absence of diminished discrimination. Gain modulation was accompanied by changes in gene and protein expression, indicative of long-term plasticity. Physiological changes were largely independent of corticofugal feedback, and no changes were seen in upstream cochlear nucleus responses, suggesting a key role of the auditory midbrain in sensory gating. Subsequent behavior demonstrated learning of predictable and random patterns and their importance in auditory conditioning. Using longer timescales than previously explored, the combined data show that the auditory midbrain codes statistical learning of temporally sparse patterns, a process that is critical for the detection of relevant stimuli in the constant soundscape that the animal navigates through.

Some things are learned simply because they are there and not because they are relevant at that moment in time. This is particularly true of surrounding sounds, which we process automatically and continuously, detecting their repetitive patterns or singularities. Learning about rewards and punishment is typically attributed to cortical structures in the brain and known to occur over long time windows. Learning of surrounding regularities, on the other hand, is attributed to subcortical structures and has been shown to occur in seconds. The brain can, however, also detect the regularity in sounds that are discontinuously repeated across intervals of minutes and hours. For example, we learn to identify people by the sound of their steps through an unconscious process involving repeated but isolated exposures to the coappearance of sound and person. Here, we show that a subcortical structure, the auditory midbrain, can code such temporally spread regularities. Neurons in the auditory midbrain changed their response pattern in mice that heard a fixed tone whenever they went into one room in the environment they lived in. Learning of temporally spread sound patterns can, therefore, occur in subcortical structures.

Common Sense in Choice: The Effect of Sensory Modality on Neural Value Representations

Although it is well established that the ventromedial prefrontal cortex (vmPFC) represents value using a common currency across categories of rewards, it is unknown whether the vmPFC represents value irrespective of the sensory modality in which alternatives are presented. In the current study, male and female human subjects completed a decision-making task while their neural activity was recorded using functional magnetic resonance imaging. On each trial, subjects chose between a safe alternative and a lottery, which was presented visually or aurally. A univariate conjunction analysis revealed that the anterior portion of the vmPFC tracks subjective value (SV) irrespective of the sensory modality. Using a novel cross-modality multivariate classifier, we were able to decode auditory value based on visual trials and vice versa. In addition, we found that the visual and auditory sensory cortices, which were identified using functional localizers, are also sensitive to the value of stimuli, albeit in a modality-specific manner. Whereas both primary and higher-order auditory cortices represented auditory SV (aSV), only a higher-order visual area represented visual SV (vSV). These findings expand our understanding of the common currency network of the brain and shed a new light on the interplay between sensory and value information processing.

Region- and Layer-Specific Activation of the Higher Order Auditory Cortex Te2 after Remote Retrieval of Fear or Appetitive Memories

The auditory cortex is involved in encoding sounds which have acquired an emotional-motivational charge. However, the neural circuitry engaged by emotional memory processes in the auditory cortex is poorly understood. In this study, we investigated the layers and regions that are recruited in the higher order auditory cortex Te2 by a tone previously paired to either fear or appetitive stimuli in rats. By tracking the protein coded by the immediate early gene zif268, we found that fear memory retrieval engages layers II-III in most regions of Te2. These results were neither due to an enhanced fear state nor to fear-evoked motor responses, as they were absent in animals retrieving an olfactory fear memory. These layers were also activated by appetitive auditory memory retrieval. Strikingly, layer IV was recruited by fear, but not appetitive memories, whereas layer V activity was related to the behavioral responses displayed to the CS. In addition to revealing the layers and regions that are recruited in the Te2 by either fear or appetitive remote memories, our study also shows that the neural circuitry within the Te2 that processes and stores emotional memories varies on the basis of the affective motivational charge of tones.

Value generalization in human avoidance learning

Generalization during aversive decision-making allows us to avoid a broad range of potential threats following experience with a limited set of exemplars. However, over-generalization, resulting in excessive and inappropriate avoidance, has been implicated in a variety of psychological disorders. Here, we use reinforcement learning modelling to dissect out different contributions to the generalization of instrumental avoidance in two groups of human volunteers (N = 26, N = 482). We found that generalization of avoidance could be parsed into perceptual and value-based processes, and further, that value-based generalization could be subdivided into that relating to aversive and neutral feedback - with corresponding circuits including primary sensory cortex, anterior insula, amygdala and ventromedial prefrontal cortex. Further, generalization from aversive, but not neutral, feedback was associated with self-reported anxiety and intrusive thoughts. These results reveal a set of distinct mechanisms that mediate generalization in avoidance learning, and show how specific individual differences within them can yield anxiety.

Extent of Dorsolateral Prefrontal Cortex Plasticity and Its Association With Working Memory in Patients With Alzheimer Disease

IMPORTANCE

The extent of dorsolateral prefrontal cortex (DLPFC) plasticity in Alzheimer disease (AD) and its association with working memory are not known.

OBJECTIVES

To determine whether participants with AD had impaired DLPFC plasticity compared with healthy control participants, to compare working memory between participants with AD and controls, and to determine whether DLPFC plasticity was associated with working memory.

DESIGN, SETTING, AND PARTICIPANTS

This cross-sectional study included 32 participants with AD who were 65 years or older and met diagnostic criteria for dementia due to probable AD with a score of at least 17 on the Mini-Mental State Examination and 16 age-matched control participants. Participants were recruited from a university teaching hospital from May 2013 to October 2016.

MAIN OUTCOMES AND MEASURES

Plasticity of the DLPFC measured as potentiation of cortical-evoked activity using paired associative stimulation (a combination of peripheral nerve electrical stimulation and transcranial magnetic stimulation) combined with electroencephalography. Working memory was assessed with the n-back task (1- and 2-back) and measured using the A' statistic.

RESULTS

Among the 32 participants with AD, 17 were women and 15 were men (mean [SD] age, 76.3 [6.3] years); among the 16 controls, 8 were men and 8 were women (mean [SD] age, 76.4 [5.1] years). Participants with AD had impaired DLPFC plasticity (mean [SD] potentiation, 1.18 [0.25]) compared with controls (mean [SD] potentiation, 1.40 [0.35]; F1,44 = 5.90; P = .02; between-group comparison, Cohen d = 0.77; P = .01). Participants with AD also had impaired performances on the 1-back condition (mean [SD] A' = 0.47 [0.30]) compared with controls (mean [SD] A' = 0.96 [0.01]; Cohen d = 1.86; P < .001), with similar findings for participants with AD on the 2-back condition (mean [SD] A' = 0.29 [0.2]) compared with controls (mean [SD], A' = 0.85 [0.18]; Cohen d = 2.83; P < .001). Plasticity of DLPFC was positively associated with working memory performance on the 1-back A' (parameter estimate B [SE] = 0.32 [0.13]; standardized β = 0.29; P = .02) and 2-back A' (B [SE] = 0.43 [0.15]; β = 0.39; P = .006) across both groups after controlling for age, education, and attention.

CONCLUSIONS AND RELEVANCE

This study demonstrated impaired in vivo DLPFC plasticity in patients with AD. The findings support the use of DLPFC plasticity as a measure of DLPFC function and a potential treatment target to enhance DLPFC function and working memory in patients with AD.

Persistent, generalized hypersensitivity of olfactory bulb interneurons after olfactory fear generalization

Generalization of fear from previously threatening stimuli to novel but related stimuli can be beneficial, but if fear overgeneralizes to inappropriate situations it can produce maladaptive behaviors and contribute to pathological anxiety. Appropriate fear learning can selectively facilitate early sensory processing of threat-predictive stimuli, but it is unknown if fear generalization has similarly generalized neurosensory consequences. We performed in vivo optical neurophysiology to visualize odor-evoked neural activity in populations of periglomerular interneurons in the olfactory bulb 1 day before, 1 day after, and 1 month after each mouse underwent an olfactory fear conditioning paradigm designed to promote generalized fear of odors. Behavioral and neurophysiological changes were assessed in response to a panel of odors that varied in similarity to the threat-predictive odor at each time point. After conditioning, all odors evoked similar levels of freezing behavior, regardless of similarity to the threat-predictive odor. Freezing significantly correlated with large changes in odor-evoked periglomerular cell activity, including a robust, generalized facilitation of the response to all odors, broadened odor tuning, and increased neural responses to lower odor concentrations. These generalized effects occurred within 24 h of a single conditioning session, persisted for at least 1 month, and were detectable even in the first moments of the brain's response to odors. The finding that generalized fear includes altered early sensory processing of not only the threat-predictive stimulus but also novel though categorically-similar stimuli may have important implications for the etiology and treatment of anxiety disorders with sensory sequelae.

Stimulus-invariant auditory cortex threat encoding during fear conditioning with simple and complex sounds

Learning to predict threat depends on amygdala plasticity and does not require auditory cortex (ACX) when threat predictors (conditioned stimuli, CS) are simple sine tones. However, ACX is required in rodents to learn from some naturally occurring CS. Yet, the precise function of ACX, and whether it differs for different CS types, is unknown. Here, we address how ACX encodes threat predictions during human fear conditioning using functional magnetic resonance imaging (fMRI) with multivariate pattern analysis. As in previous rodent work, CS+ and CS- were defined either by direction of frequency modulation (complex) or by frequency of pure tones (simple). In an instructed non-reinforcement context, different sets of simple and complex sounds were always presented without reinforcement (neutral sounds, NS). Threat encoding was measured by separation of fMRI response patterns induced by CS+/CS-, or similar NS1/NS2 pairs. We found that fMRI patterns in Heschl's gyrus encoded threat prediction over and above encoding the physical stimulus features also present in NS, i.e. CS+/CS- could be separated better than NS1/NS2. This was the case both for simple and complex CS. Furthermore, cross-prediction demonstrated that threat representations were similar for simple and complex CS, and thus unlikely to emerge from stimulus-specific top-down, or learning-induced, receptive field plasticity. Searchlight analysis across the entire ACX demonstrated further threat representations in a region including BA22 and BA42. However, in this region, patterns were distinct for simple and complex sounds, and could thus potentially arise from receptive field plasticity. Strikingly, across participants, individual size of Heschl's gyrus predicted strength of fear learning for complex sounds. Overall, our findings suggest that ACX represents threat predictions, and that Heschl's gyrus contains a threat representation that is invariant across physical stimulus categories.

Plasticity in the auditory system

Over the last 30 years a wide range of manipulations of auditory input and experience have been shown to result in plasticity in auditory cortical and subcortical structures. The time course of plasticity ranges from very rapid stimulus-specific adaptation to longer-term changes associated with, for example, partial hearing loss or perceptual learning. Evidence for plasticity as a consequence of these and a range of other manipulations of auditory input and/or its significance is reviewed, with an emphasis on plasticity in adults and in the auditory cortex. The nature of the changes in auditory cortex associated with attention, memory and perceptual learning depend critically on task structure, reward contingencies, and learning strategy. Most forms of auditory system plasticity are adaptive, in that they serve to optimize auditory performance, prompting attempts to harness this plasticity for therapeutic purposes. However, plasticity associated with cochlear trauma and partial hearing loss appears to be maladaptive, and has been linked to tinnitus. Three important forms of human learning-related auditory system plasticity are those associated with language development, musical training, and improvement in performance with a cochlear implant. Almost all forms of plasticity involve changes in synaptic excitatory - inhibitory balance within existing patterns of connectivity. An attractive model applicable to a number of forms of learning-related plasticity is dynamic multiplexing by individual neurons, such that learning involving a particular stimulus attribute reflects a particular subset of the diverse inputs to a given neuron being gated by top-down influences. The plasticity evidence indicates that auditory cortex is a component of complex distributed networks that integrate the representation of auditory stimuli with attention, decision and reward processes.

Differential aversive learning enhances orientation discrimination

A number of recent studies have documented rapid changes in behavioural sensory acuity induced by aversive learning in the olfactory and auditory modalities. The effect of aversive learning on the discrimination of low-level features in the visual system of humans remains unclear. Here, we used a psychophysical staircase procedure to estimate discrimination thresholds for oriented grating stimuli, before and after differential aversive learning. We discovered that when a target grating orientation was conditioned with an aversive loud noise, it subsequently led to an improvement of discrimination acuity in nearly all subjects. However, no such change was observed in a control group conditioned to an orientation shifted by ±90° from the target. Our findings cannot be explained by contextual learning or sensitisation factors. The results converge with those reported in the olfactory modality and provide further evidence that early sensory systems can be rapidly modified by recently experienced reinforcement histories.

CS-specific modifications of auditory evoked potentials in the behaviorally conditioned rat

The current report provides a detailed analysis of the changes in the first two components of the auditory evoked potential (AEP) that accompany associative learning. AEPs were recorded from the primary auditory cortex before and after training sessions. Experimental subjects underwent one (n=5) or two (n=7) days of conditioning in which a tone, serving as a conditioned stimulus (CS), was paired with mild foot shock. Control subjects received one (n=5) or two (n=7) days of exposure to the same stimuli delivered randomly. Only animals receiving paired CS-US training developed a conditioned tachycardia response to the tone. Our analyses demonstrated that both early components of the AEP recorded from the granular layer of the cortex undergo CS-specific associative changes: (1) the first, negative component (occurring ∼21ms following tone onset) was significantly augmented after one and two days of training while maintaining its latency, and (2) the second, positive component (occurring ∼50ms following tone onset) was augmented after two days of training, and showed a significant reduction in latency after one and two days of training. We view these changes as evidence of increased cortical synchronization, thereby lending new insight into the temporal dynamics of neural network activity related to auditory learning.

Perineuronal Nets in the Adult Sensory Cortex Are Necessary for Fear Learning

Lattice-like structures known as perineuronal nets (PNNs) are key components of the extracellular matrix (ECM). Once fully crystallized by adulthood, they are largely stable throughout life. Contrary to previous reports that PNNs inhibit processes involving plasticity, here we report that the dynamic regulation of PNN expression in the adult auditory cortex is vital for fear learning and consolidation in response to pure tones. Specifically, after first confirming the necessity of auditory cortical activity for fear learning and consolidation, we observed that mRNA levels of key proteoglycan components of PNNs were enhanced 4 hr after fear conditioning but were no longer different from the control groups 24 hr later. A similar pattern of regulation was observed in numbers of cells surrounded by PNNs and area occupied by them in the auditory cortex. Finally, the removal of auditory cortex PNNs resulted in a deficit in fear learning and consolidation.

Basal Forebrain Cholinergic-Auditory Cortical Network: Primary Versus Nonprimary Auditory Cortical Areas

Acetylcholine (ACh) release in the cortex is critical for learning, memory, attention, and plasticity. Here, we explore the cholinergic and noncholinergic projections from the basal forebrain (BF) to the auditory cortex using classical retrograde and monosynaptic viral tracers deposited in electrophysiologically identified regions of the auditory cortex. Cholinergic input to both primary (A1) and nonprimary auditory cortical (belt) areas originates in a restricted area in the caudal BF within the globus pallidus (GP) and in the dorsal part of the substantia innominata (SId). On the other hand, we found significant differences in the proportions of cholinergic and noncholinergic projection neurons to primary and nonprimary auditory areas. Inputs to A1 projecting cholinergic neurons were restricted to the GP, caudate-putamen, and the medial part of the medial geniculate body, including the posterior intralaminar thalamic group. In addition to these areas, afferents to belt-projecting cholinergic neurons originated from broader areas, including the ventral secondary auditory cortex, insular cortex, secondary somatosensory cortex, and the central amygdaloid nucleus. These findings support a specific BF projection pattern to auditory cortical areas. Additionally, these findings point to potential functional differences in how ACh release may be regulated in the A1 and auditory belt areas.

Threat intensity widens fear generalization gradients

Research in nonhuman animals reveals threat-sensitive generalization of defensive behavior that favors widespread generalization when threat intensity is high and limited generalization (i.e., specificity) when threat intensity is low. Here, we used Pavlovian fear conditioning to systematically investigate whether threat intensity widens behavioral generalization gradients to stimuli that decreasingly resemble a learned threat cue. Using a between-subjects design, volunteers underwent fear conditioning with a tone paired with either a high-intensity or low-intensity aversive stimulus prior to a test of fear generalization to novel tones. Results showed no effect of threat intensity on initial acquisition of conditioned fear. However, volunteers who underwent fear conditioning with a high-intensity aversive stimulus exhibited widespread generalization of autonomic arousal (skin conductance responses) as compared to volunteers who received a low-intensity aversive stimulus. These results show a transition from normal (selective) to overgeneralized fear as threat intensity increases, and have implications for understanding overgeneralization characteristic of trauma- and stress-related disorders. (PsycINFO Database Record

Neural Substrates of Overgeneralized Conditioned Fear in PTSD

OBJECTIVE

Heightened generalization of fear from an aversively reinforced conditioned stimulus (CS+, a conditioned danger cue) to resembling stimuli is widely accepted as a pathogenic marker of posttraumatic stress disorder (PTSD). Indeed, a distress response to benign stimuli that "resemble" aspects of the trauma is a central feature of the disorder. To date, the link between overgeneralization of conditioned fear and PTSD derives largely from clinical observations, with limited empirical work on the subject. This represents the first effort to examine behavioral and brain indices of generalized conditioned fear in PTSD using systematic methods developed in animals known as generalization gradients: the gradual decline in conditioned responding as the presented stimulus gradually differentiates from CS+.

METHOD

Gradients of conditioned fear generalization were assessed using functional MRI and behavioral measures in U.S. combat veterans who served in Iraq or Afghanistan and had PTSD (N=26), subthreshold PTSD (N=19), or no PTSD (referred to as trauma control subjects) (N=17). Presented stimuli included rings of graded size, with extreme sizes serving as CS+ (paired with shock) and as a nonreinforced conditioned stimulus (CS-, a conditioned safety cue), and with intermediate sizes forming a continuum of similarity between CS+ and CS-. Generalization gradients were assessed as response slopes from CS+, through intermediate ring sizes, to CS-, with less steep slopes indicative of stronger generalization.

RESULTS

Relative to trauma control subjects, PTSD patients showed stronger conditioned generalization, as evidenced by less steep generalization gradients in both behavioral risk ratings and brain responses in the left and right anterior insula, left ventral hippocampus, dorsolateral and dorsomedial prefrontal cortex, and caudate nucleus. Severity of PTSD symptoms across the three study groups was positively correlated with levels of generalization at two such loci: the right anterior insula and left ventral hippocampus.

CONCLUSIONS

The results point to evidence of brain-based markers of overgeneralized fear conditioning related to PTSD. These findings provide further understanding of a central yet understudied symptom of trauma-related psychopathology.

Tinnitus distress is linked to enhanced resting-state functional connectivity from the limbic system to the auditory cortex

The phantom sound of tinnitus is believed to be triggered by aberrant neural activity in the central auditory pathway, but since this debilitating condition is often associated with emotional distress and anxiety, these comorbidities likely arise from maladaptive functional connections to limbic structures such as the amygdala and hippocampus. To test this hypothesis, resting-state functional magnetic resonance imaging (fMRI) was used to identify aberrant effective connectivity of the amygdala and hippocampus in tinnitus patients and to determine the relationship with tinnitus characteristics. Chronic tinnitus patients (n = 26) and age-, sex-, and education-matched healthy controls (n = 23) were included. Both groups were comparable for hearing level. Granger causality analysis utilizing the amygdala and hippocampus as seed regions were used to investigate the directional connectivity and the relationship with tinnitus duration or distress. Relative to healthy controls, tinnitus patients demonstrated abnormal directional connectivity of the amygdala and hippocampus, including primary and association auditory cortex, and other non-auditory areas. Importantly, scores on the Tinnitus Handicap Questionnaires were positively correlated with increased connectivity from the left amygdala to left superior temporal gyrus (r = 0.570, P = 0.005), and from the right amygdala to right superior temporal gyrus (r = 0.487, P = 0.018). Moreover, enhanced effective connectivity from the right hippocampus to left transverse temporal gyrus was correlated with tinnitus duration (r = 0.452, P = 0.030). The results showed that tinnitus distress strongly correlates with enhanced effective connectivity that is directed from the amygdala to the auditory cortex. The longer the phantom sensation, the more likely acute tinnitus becomes permanently encoded by memory traces in the hippocampus. Hum Brain Mapp 38:2384-2397, 2017. © 2017 Wiley Periodicals, Inc.

Associations of Unilateral Whisker and Olfactory Signals Induce Synapse Formation and Memory Cell Recruitment in Bilateral Barrel Cortices: Cellular Mechanism for Unilateral Training Toward Bilateral Memory

Somatosensory signals and operative skills learned by unilateral limbs can be retrieved bilaterally. In terms of cellular mechanism underlying this unilateral learning toward bilateral memory, we hypothesized that associative memory cells in bilateral cortices and synapse innervations between them were produced. In the examination of this hypothesis, we have observed that paired unilateral whisker and odor stimulations led to odorant-induced whisker motions in bilateral sides, which were attenuated by inhibiting the activity of barrel cortices. In the mice that showed bilateral cross-modal responses, the neurons in both sides of barrel cortices became to encode this new odor signal alongside the innate whisker signal. Axon projections and synapse formations from the barrel cortex, which was co-activated with the piriform cortex, toward its contralateral barrel cortex (CBC) were upregulated. Glutamatergic synaptic transmission in bilateral barrel cortices was upregulated and GABAergic synaptic transmission was downregulated. The associative activations of the sensory cortices facilitate new axon projection, glutamatergic synapse formation and GABAergic synapse downregulation, which drive the neurons to be recruited as associative memory cells in the bilateral cortices. Our data reveal the productions of associative memory cells and synapse innervations in bilateral sensory cortices for unilateral training toward bilateral memory.

Coordinated Plasticity between Barrel Cortical Glutamatergic and GABAergic Neurons during Associative Memory

Neural plasticity is associated with memory formation. The coordinated refinement and interaction between cortical glutamatergic and GABAergic neurons remain elusive in associative memory, which we examine in a mouse model of associative learning. In the mice that show odorant-induced whisker motion after pairing whisker and odor stimulations, the barrel cortical glutamatergic and GABAergic neurons are recruited to encode the newly learnt odor signal alongside the innate whisker signal. These glutamatergic neurons are functionally upregulated, and GABAergic neurons are refined in a homeostatic manner. The mutual innervations between these glutamatergic and GABAergic neurons are upregulated. The analyses by high throughput sequencing show that certain microRNAs related to regulating synapses and neurons are involved in this cross-modal reflex. Thus, the coactivation of the sensory cortices through epigenetic processes recruits their glutamatergic and GABAergic neurons to be the associative memory cells as well as drive their coordinated refinements toward the optimal state for the storage of the associated signals.

Associative learning changes cross-modal representations in the gustatory cortex

A growing body of literature has demonstrated that primary sensory cortices are not exclusively unimodal, but can respond to stimuli of different sensory modalities. However, several questions concerning the neural representation of cross-modal stimuli remain open. Indeed, it is poorly understood if cross-modal stimuli evoke unique or overlapping representations in a primary sensory cortex and whether learning can modulate these representations. Here we recorded single unit responses to auditory, visual, somatosensory, and olfactory stimuli in the gustatory cortex (GC) of alert rats before and after associative learning. We found that, in untrained rats, the majority of GC neurons were modulated by a single modality. Upon learning, both prevalence of cross-modal responsive neurons and their breadth of tuning increased, leading to a greater overlap of representations. Altogether, our results show that the gustatory cortex represents cross-modal stimuli according to their sensory identity, and that learning changes the overlap of cross-modal representations.

DOI:http://dx.doi.org/10.7554/eLife.16420.001

Imagine that you are waiting for a cappuccino at your favorite café. You hear the sound of the steamer, and shortly afterwards the barista calls your name and announces that your cappuccino is ready. As they hand it to you, you see the foam sprinkled with cocoa and the aroma of the cappuccino reaches your nose. You can almost taste it. When you finally take your first sip, the taste is hardly a surprise; it is just as your eyes and nose predicted.

How does the brain deal with such a rich and multisensory experience? How does it learn to associate the sight and smell of a cappuccino with its taste? Specialized regions of the brain called associative areas were traditionally thought to perform this task. These areas receive inputs from every sensory system and can link information from these different sources together. According to this view, the job of each individual sensory system is to pass along information relevant to one particular sense.

More recent results, however, challenge this strict division of labor and suggest that individual sensory systems may be able to combine information from multiple senses. Thus the sights, sounds and odors associated with our cappuccino may also activate the area of the brain in charge of processing taste: the gustatory cortex. To investigate this possibility, Vincis and Fontanini set out to determine whether neurons in the gustatory cortex of rats can process stimuli belonging to senses other than taste.

As predicted, neurons in the gustatory cortex did change their firing rates in response to odors, touch, sounds and light. However, more of the gustatory neurons responded to odors and touch than to sounds and light. In addition, of the four stimuli, the rats most easily learned to associate odors and touch with a sugary solution. This is consistent with the fact that rodents rely more upon their whiskers and their sense of smell to find food they do their eyes and ears. Finally, learning to associate a stimulus other than taste with a sugary solution increased the number of neurons in the gustatory cortex that subsequently responded to other senses and changed their response properties.

Further studies are now required to answer three questions. Why can some senses more effectively influence the activity of the gustatory cortex than others? Can gustatory neurons distinguish between different stimuli of the same type – different odors, for example? What are the neural pathways that convey multisensory information to the gustatory cortex? Answering these questions will help us to better understand how sensory systems link information from multiple senses.

DOI:http://dx.doi.org/10.7554/eLife.16420.002

Decomposing fear perception: A combination of psychophysics and neurometric modeling of fear perception

Emotion perception is known to involve multiple operations and waves of analysis, but specific nature of these processes remains poorly understood. Combining psychophysical testing and neurometric analysis of event-related potentials (ERPs) in a fear detection task with parametrically varied fear intensities (N=45), we sought to elucidate key processes in fear perception. Building on psychophysics marking fear perception thresholds, our neurometric model fitting identified several putative operations and stages: four key processes arose in sequence following face presentation - fear-neutral categorization (P1 at 100ms), fear detection (P300 at 320ms), valuation (early subcomponent of the late positive potential/LPP at 400-500ms) and conscious awareness (late subcomponent LPP at 500-600ms). Furthermore, within-subject brain-behavior association suggests that initial emotion categorization was mandatory and detached from behavior whereas valuation and conscious awareness directly impacted behavioral outcome (explaining 17% and 31% of the total variance, respectively). The current study thus reveals the chronometry of fear perception, ascribing psychological meaning to distinct underlying processes. The combination of early categorization and late valuation of fear reconciles conflicting (categorical versus dimensional) emotion accounts, lending support to a hybrid model. Importantly, future research could specifically interrogate these psychological processes in various behaviors and psychopathologies (e.g., anxiety and depression).

Perspectives on fear generalization and its implications for emotional disorders

Although generalization to conditioned stimuli is not a new phenomenon, renewed interest in understanding its biological underpinning has stemmed from its association with a number of anxiety disorders. Generalization as it relates to fear processing is a temporally dynamic process in which animals, including humans, display fear in response to similar yet distinct cues or contexts as the time between training and testing increases. This Review surveys the literature on contextual fear generalization and its relation to several views of memory, including systems consolidation, forgetting, and transformation hypothesis, which differentially implicate roles of the hippocampus and neocortex in memory consolidation and retrieval. We discuss recent evidence on the neurobiological mechanisms contributing to the increase in fear generalization over time and how generalized responding may be modulated by acquisition, consolidation, and retrieval mechanisms. Whereas clinical perspectives of generalization emphasize a lack of fear inhibition to CS^- cues or fear toward intermediate CS cues, the time-dependent nature of generalization and its relation to traditional views on memory consolidation and retrieval are often overlooked. Understanding the time-dependent increase in fear generalization has important implications not only for understanding how generalization contributes to anxiety disorders but also for understanding basic long-term memory function. © 2016 Wiley Periodicals, Inc.

Converging prefrontal pathways support associative and perceptual features of conditioned stimuli

Perceptually similar stimuli often predict vastly different outcomes, requiring the brain to maintain specific associations in the face of potential ambiguity. This could be achieved either through local changes in stimulus representations, or through modulation of functional connections between stimulus-coding and outcome-coding regions. Here we test these competing hypotheses using classical conditioning of perceptually similar odours in the context of human fMRI. Pattern-based analyses of odour-evoked fMRI activity reveal that odour category, identity and value are coded in piriform (PC), orbitofrontal (OFC) and ventromedial prefrontal (vmPFC) cortices, respectively. However, we observe no learning-related reorganization of category or identity representations. Instead, changes in connectivity between vmPFC and OFC are correlated with learning-related changes in value, whereas connectivity changes between vmPFC and PC predict changes in perceived odour similarity. These results demonstrate that dissociable neural pathways support associative and perceptual representations of sensory stimuli.

Animals often need to form specific associations between perceptually similar stimuli and the different outcomes they may predict. Howard et al. find that the human brain accomplishes this via enhanced coupling between stable codes of sensory features and flexible codes of stimulus reward value.

Individual Differences in Anticipatory Somatosensory Cortex Activity for Shock is Positively Related with Trait Anxiety and Multisensory Integration

Anxiety is associated with an exaggerated expectancy of harm, including overestimation of how likely a conditioned stimulus (CS+) predicts a harmful unconditioned stimulus (US). In the current study we tested whether anxiety-associated expectancy of harm increases primary sensory cortex (S1) activity on non-reinforced (i.e., no shock) CS+ trials. Twenty healthy volunteers completed a differential-tone trace conditioning task while undergoing fMRI, with shock delivered to the left hand. We found a positive correlation between trait anxiety and activity in right, but not left, S1 during CS+ versus CS− conditions. Right S1 activity also correlated with individual differences in both primary auditory cortices (A1) and amygdala activity. Lastly, a seed-based functional connectivity analysis demonstrated that trial-wise S1 activity was positively correlated with regions of dorsolateral prefrontal cortex (dlPFC), suggesting that higher-order cognitive processes contribute to the anticipatory sensory reactivity. Our findings indicate that individual differences in trait anxiety relate to anticipatory reactivity for the US during associative learning. This anticipatory reactivity is also integrated along with emotion-related sensory signals into a brain network implicated in fear-conditioned responding.

Histone Deacetylase Inhibition via RGFP966 Releases the Brakes on Sensory Cortical Plasticity and the Specificity of Memory Formation

Research over the past decade indicates a novel role for epigenetic mechanisms in memory formation. Of particular interest is chromatin modification by histone deacetylases (HDACs), which, in general, negatively regulate transcription. HDAC deletion or inhibition facilitates transcription during memory consolidation and enhances long-lasting forms of synaptic plasticity and long-term memory. A key open question remains: How does blocking HDAC activity lead to memory enhancements? To address this question, we tested whether a normal function of HDACs is to gate information processing during memory formation. We used a class I HDAC inhibitor, RGFP966 (C21H19FN4O), to test the role of HDAC inhibition for information processing in an auditory memory model of learning-induced cortical plasticity. HDAC inhibition may act beyond memory enhancement per se to instead regulate information in ways that lead to encoding more vivid sensory details into memory. Indeed, we found that RGFP966 controls memory induction for acoustic details of sound-to-reward learning. Rats treated with RGFP966 while learning to associate sound with reward had stronger memory and additional information encoded into memory for highly specific features of sounds associated with reward. Moreover, behavioral effects occurred with unusually specific plasticity in primary auditory cortex (A1). Class I HDAC inhibition appears to engage A1 plasticity that enables additional acoustic features to become encoded in memory. Thus, epigenetic mechanisms act to regulate sensory cortical plasticity, which offers an information processing mechanism for gating what and how much is encoded to produce exceptionally persistent and vivid memories. Significance statement: Here we provide evidence of an epigenetic mechanism for information processing. The study reveals that a class I HDAC inhibitor (Malvaez et al., 2013; Rumbaugh et al., 2015; RGFP966, chemical formula C21H19FN4O) alters the formation of auditory memory by enabling more acoustic information to become encoded into memory. Moreover, RGFP966 appears to affect cortical plasticity: the primary auditory cortex reorganized in a manner that was unusually "tuned-in" to the specific sound cues and acoustic features that were related to reward and subsequently remembered. We propose that HDACs control "informational capture" at a systems level for what and how much information is encoded by gating sensory cortical plasticity that underlies the sensory richness of newly formed memories.