Neural substrates for tone-conditioned bradycardia demonstrated with 2-deoxyglucose. II. Auditory cortex plasticity

Positive feedback of efferent copy via pontine nucleus facilitates cerebellum-mediated associative learning

The cerebellum is critical for motor coordination and learning. However, the role of feedback circuitry in this brain region has not been fully explored. Here, we characterize a nucleo-ponto-cortical feedback pathway in classical delayed eyeblink conditioning (dEBC) of rats. We find that the efference copy is conveyed from the interposed cerebellar nucleus (Int) to cerebellar cortex through pontine nucleus (PN). Inhibiting or exciting the projection from the Int to the PN can decelerate or speed up acquisition of dEBC, respectively. Importantly, we identify two subpopulations of PN neurons (PN1 and PN2) that convey and integrate the feedback signals with feedforward sensory signals. We also show that the feedforward and feedback pathways via different types of PN neurons contribute to the plastic changes and cooperate synergistically to the learning of dEBC. Our results suggest that this excitatory nucleo-ponto-cortical feedback plays a significant role in modulating associative motor learning in cerebellum.

Generalizable Sample-Efficient Siamese Autoencoder for Tinnitus Diagnosis in Listeners With Subjective Tinnitus

Electroencephalogram (EEG)-based neurofeedback has been widely studied for tinnitus therapy in recent years. Most existing research relies on experts' cognitive prediction, and studies based on machine learning and deep learning are either data-hungry or not well generalizable to new subjects. In this paper, we propose a robust, data-efficient model for distinguishing tinnitus from the healthy state based on EEG-based tinnitus neurofeedback. We propose trend descriptor, a feature extractor with lower fineness, to reduce the effect of electrode noises on EEG signals, and a siamese encoder-decoder network boosted in a supervised manner to learn accurate alignment and to acquire high-quality transferable mappings across subjects and EEG signal channels. Our experiments show the proposed method significantly outperforms state-of-the-art algorithms when analyzing subjects' EEG neurofeedback to 90dB and 100dB sound, achieving an accuracy of 91.67%-94.44% in predicting tinnitus and control subjects in a subject-independent setting. Our ablation studies on mixed subjects and parameters show the method's stability in performance.

The Sensory Neocortex and Associative Memory

Most behaviors in mammals are directly or indirectly guided by prior experience and therefore depend on the ability of our brains to form memories. The ability to form an association between an initially possibly neutral sensory stimulus and its behavioral relevance is essential for our ability to navigate in a changing environment. The formation of a memory is a complex process involving many areas of the brain. In this chapter we review classic and recent work that has shed light on the specific contribution of sensory cortical areas to the formation of associative memories. We discuss synaptic and circuit mechanisms that mediate plastic adaptations of functional properties in individual neurons as well as larger neuronal populations forming topographically organized representations. Furthermore, we describe commonly used behavioral paradigms that are used to study the mechanisms of memory formation. We focus on the auditory modality that is receiving increasing attention for the study of associative memory in rodent model systems. We argue that sensory cortical areas may play an important role for the memory-dependent categorical recognition of previously encountered sensory stimuli.

Telencephalic neural activation following passive avoidance learning in a terrestrial toad

The present study explores passive avoidance learning and its neural basis in toads (Rhinella arenarum). In Experiment 1, two groups of toads learned to move from a lighted compartment into a dark compartment. After responding, animals in the experimental condition were exposed to an 800-mM strongly hypertonic NaCl solution that leads to weight loss. Control animals received exposure to a 300-mM slightly hypertonic NaCl solution that leads to neither weight gain nor loss. After 10 daily acquisition trials, animals in the experimental group showed significantly longer latency to enter the dark compartment. Additionally, 10 daily trials in which both groups received the 300-mM NaCl solution after responding eliminated this group effect. Thus, experimental animals showed gradual acquisition and extinction of a passive avoidance respond. Experiment 2 replicated the gradual acquisition effect, but, after the last trial, animals were sacrificed and neural activation was assessed in five brain regions using AgNOR staining for nucleoli-an index of brain activity. Higher activation in the experimental animals, relative to controls, was observed in the amygdala and striatum. Group differences in two other regions, lateral pallium and septum, were borderline, but nonsignificant, whereas group differences in the medial pallium were nonsignificant. These preliminary results suggest that a striatal-amygdala activation could be a key component of the brain circuit controlling passive avoidance learning in amphibians. The results are discussed in relation to the results of analogous experiments with other vertebrates.

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.

Relational associative learning induces cross-modal plasticity in early visual cortex

Neurobiological theories of memory posit that the neocortex is a storage site of declarative memories, a hallmark of which is the association of two arbitrary neutral stimuli. Early sensory cortices, once assumed uninvolved in memory storage, recently have been implicated in associations between neutral stimuli and reward or punishment. We asked whether links between neutral stimuli also could be formed in early visual or auditory cortices. Rats were presented with a tone paired with a light using a sensory preconditioning paradigm that enabled later evaluation of successful association. Subjects that acquired this association developed enhanced sound evoked potentials in their primary and secondary visual cortices. Laminar recordings localized this potential to cortical Layers 5 and 6. A similar pattern of activation was elicited by microstimulation of primary auditory cortex in the same subjects, consistent with a cortico-cortical substrate of association. Thus, early sensory cortex has the capability to form neutral stimulus associations. This plasticity may constitute a declarative memory trace between sensory cortices.

Medial auditory thalamus is necessary for acquisition and retention of eyeblink conditioning to cochlear nucleus stimulation

Associative learning tasks commonly involve an auditory stimulus, which must be projected through the auditory system to the sites of memory induction for learning to occur. The cochlear nucleus (CN) projection to the pontine nuclei has been posited as the necessary auditory pathway for cerebellar learning, including eyeblink conditioning. However, the medial auditory thalamic nuclei (MATN), consisting of the medial division of the medial geniculate, suprageniculate, and posterior interlaminar nucleus have also been implicated as a critical auditory relay to the pontine nuclei for cerebellum-dependent motor learning. The MATN also conveys auditory information to the amygdala necessary for avoidance and fear conditioning. The current study used CN stimulation to increase activity in the pontine nuclei, relative to a tone stimulus, and possibly provide sufficient input to the cerebellum for acquisition or retention of eyeblink conditioning during MATN inactivation. Primary and secondary effects of CN stimulation and MATN inactivation were examined using 2-deoxy-glucose autoradiography. Stimulation of CN increased activity in the pontine nuclei, however, this increase was not sufficient for cerebellar learning during MATN inactivation. Results of the current experiment provide additional evidence indicating the MATN may be the critical auditory relay for many associative learning tasks.

New perspectives on the auditory cortex: learning and memory

Primary ("early") sensory cortices have been viewed as stimulus analyzers devoid of function in learning, memory, and cognition. However, studies combining sensory neurophysiology and learning protocols have revealed that associative learning systematically modifies the encoding of stimulus dimensions in the primary auditory cortex (A1) to accentuate behaviorally important sounds. This "representational plasticity" (RP) is manifest at different levels. The sensitivity and selectivity of signal tones increase near threshold, tuning above threshold shifts toward the frequency of acoustic signals, and their area of representation can increase within the tonotopic map of A1. The magnitude of area gain encodes the level of behavioral stimulus importance and serves as a substrate of memory strength. RP has the same characteristics as behavioral memory: it is associative, specific, develops rapidly, consolidates, and can last indefinitely. Pairing tone with stimulation of the cholinergic nucleus basalis induces RP and implants specific behavioral memory, while directly increasing the representational area of a tone in A1 produces matching behavioral memory. Thus, RP satisfies key criteria for serving as a substrate of auditory memory. The findings suggest a basis for posttraumatic stress disorder in abnormally augmented cortical representations and emphasize the need for a new model of the cerebral cortex.

Processing of communication sounds: contributions of learning, memory, and experience

Abundant evidence from both field and lab studies has established that conspecific vocalizations (CVs) are of critical ecological significance for a wide variety of species, including humans, non-human primates, rodents, and other mammals and birds. Correspondingly, a number of experiments have demonstrated behavioral processing advantages for CVs, such as in discrimination and memory tasks. Further, a wide range of experiments have described brain regions in many species that appear to be specialized for processing CVs. For example, several neural regions have been described in both mammals and birds wherein greater neural responses are elicited by CVs than by comparison stimuli such as heterospecific vocalizations, nonvocal complex sounds, and artificial stimuli. These observations raise the question of whether these regions reflect domain-specific neural mechanisms dedicated to processing CVs, or alternatively, if these regions reflect domain-general neural mechanisms for representing complex sounds of learned significance. Inasmuch as CVs can be viewed as complex combinations of basic spectrotemporal features, the plausibility of the latter position is supported by a large body of literature describing modulated cortical and subcortical representation of a variety of acoustic features that have been experimentally associated with stimuli of natural behavioral significance (such as food rewards). Herein, we review a relatively small body of existing literature describing the roles of experience, learning, and memory in the emergence of species-typical neural representations of CVs and auditory system plasticity. In both songbirds and mammals, manipulations of auditory experience as well as specific learning paradigms are shown to modulate neural responses evoked by CVs, either in terms of overall firing rate or temporal firing patterns. In some cases, CV-sensitive neural regions gradually acquire representation of non-CV stimuli with which subjects have training and experience. These results parallel literature in humans describing modulation of responses in face-sensitive neural regions through learning and experience. Thus, although many questions remain, the available evidence is consistent with the notion that CVs may acquire distinct neural representation through domain-general mechanisms for representing complex auditory objects that are of learned importance to the animal. This article is part of a Special Issue entitled "Communication Sounds and the Brain: New Directions and Perspectives".

Remodeling sensory cortical maps implants specific behavioral memory

Neural mechanisms underlying the capacity of memory to be rich in sensory detail are largely unknown. A candidate mechanism is learning-induced plasticity that remodels the adult sensory cortex. Here, expansion in the primary auditory cortical (A1) tonotopic map of rats was induced by pairing a 3.66-kHz tone with activation of the nucleus basalis, mimicking the effects of natural associative learning. Remodeling of A1 produced de novo specific behavioral memory, but neither memory nor plasticity was consistently at the frequency of the paired tone, which typically decreased in A1 representation. Rather, there was a specific match between individual subjects' area of expansion and the tone that was strongest in each animal's memory, as determined by post-training frequency generalization gradients. These findings provide the first demonstration of a match between the artificial induction of specific neural representational plasticity and artificial induction of behavioral memory. As such, together with prior and present findings for detection, correlation and mimicry of plasticity with the acquisition of memory, they satisfy a key criterion for neural substrates of memory. This demonstrates that directly remodeling sensory cortical maps is sufficient for the specificity of memory formation.

Impaired development and competitive refinement of the cortical frequency map in tumor necrosis factor-α-deficient mice

Early experience shapes sensory representations in a critical period of heightened plasticity. This adaptive process is thought to involve both Hebbian and homeostatic synaptic plasticity. Although Hebbian plasticity has been investigated as a mechanism for cortical map reorganization, less is known about the contribution of homeostatic plasticity. We investigated the role of homeostatic synaptic plasticity in the development and refinement of frequency representations in the primary auditory cortex using the tumor necrosis factor-α (TNF-α) knockout (KO), a mutant mouse with impaired homeostatic but normal Hebbian plasticity. Our results indicate that these mice develop weaker tonal responses and incomplete frequency representations. Rearing in a single-frequency revealed a normal expansion of cortical representations in KO mice. However, TNF-α KOs lacked homeostatic adjustments of cortical responses following exposure to multiple frequencies. Specifically, while this sensory over-stimulation resulted in competitive refinement of frequency tuning in wild-type controls, it broadened frequency tuning in TNF-α KOs. Our results suggest that homeostatic plasticity plays an important role in gain control and competitive interaction in sensory cortical development.

Differential changes of metabolic brain activity and interregional functional coupling in prefronto-limbic pathways during different stress conditions: functional imaging in freely behaving rodent pups

The trumpet-tailed rat or degu (Octodon degus) is an established model to investigate the consequences of early stress on the development of emotional brain circuits and behavior. The aim of this study was to identify brain circuits, that respond to different stress conditions and to test if acute stress alters functional coupling of brain activity among prefrontal and limbic regions. Using functional imaging (2-Fluoro-deoxyglucose method) in 8-day-old male degu pups the following stress conditions were compared: (A) pups together with parents and siblings (control), (B) separation of the litter from the parents, (C) individual separation from parents and siblings, and (D) individual separation and presentation of maternal calls. Condition (B) significantly downregulated brain activity in the prefrontal cortex, hippocampus, nucleus accumbens (NAcc), and sensory areas compared to controls. Activity decrease was even more pronounced during condition (C), where, in contrast to all other regions, activity in the PAG was increased. Interestingly, brain activity in stress-associated brain regions such as the amygdala and habenula was not affected. In condition (D) maternal vocalizations "reactivated" brain activity in the cingulate and precentral medial cortex, NAcc, and striatum and in sensory areas. In contrast, reduced activity was measured in the prelimbic and infralimbic cortex (IL) and in the hippocampus and amygdala. Correlation analysis revealed complex, region- and situation-specific changes of interregional functional coupling among prefrontal and limbic brain regions during stress exposure. We show here for the first time that early life stress results in a widespread reduction of brain activity in the infant brain and changes interregional functional coupling. Moreover, maternal vocalizations can partly buffer stress-induced decrease in brain activity in some regions and evoked very different functional coupling patterns compared to the three other conditions.

Extinction reveals that primary sensory cortex predicts reinforcement outcome

Primary sensory cortices are traditionally regarded as stimulus analysers. However, studies of associative learning-induced plasticity in the primary auditory cortex (A1) indicate involvement in learning, memory and other cognitive processes. For example, the area of representation of a tone becomes larger for stronger auditory memories and the magnitude of area gain is proportional to the degree that a tone becomes behaviorally important. Here, we used extinction to investigate whether 'behavioral importance' specifically reflects a sound's ability to predict reinforcement (reward or punishment) vs. to predict any significant change in the meaning of a sound. If the former, then extinction should reverse area gains as the signal no longer predicts reinforcement. Rats (n = 11) were trained to bar-press to a signal tone (5.0 kHz) for water-rewards, to induce signal-specific area gains in A1. After subsequent withdrawal of reward, A1 was mapped to determine representational areas. Signal-specific area gains, estimated from a previously established brain-behavior quantitative function, were reversed, supporting the 'reinforcement prediction' hypothesis. Area loss was specific to the signal tone vs. test tones, further indicating that withdrawal of reinforcement, rather than unreinforced tone presentation per se, was responsible for area loss. Importantly, the amount of area loss was correlated with the amount of extinction (r = 0.82, P < 0.01). These findings show that primary sensory cortical representation can encode behavioral importance as a signal's value to predict reinforcement, and that the number of cells tuned to a stimulus can dictate its ability to command behavior.

Plasticity in the Primary Auditory Cortex, Not What You Think it is: Implications for Basic and Clinical Auditory Neuroscience

Standard beliefs that the function of the primary auditory cortex (A1) is the analysis of sound have proven to be incorrect. Its involvement in learning, memory and other complex processes in both animals and humans is now well-established, although often not appreciated. Auditory coding is strongly modifed by associative learning, evident as associative representational plasticity (ARP) in which the representation of an acoustic dimension, like frequency, is re-organized to emphasize a sound that has become behaviorally important. For example, the frequency tuning of a cortical neuron can be shifted to match that of a significant sound and the representational area of sounds that acquire behavioral importance can be increased. ARP depends on the learning strategy used to solve an auditory problem and the increased cortical area confers greater strength of auditory memory. Thus, primary auditory cortex is involved in cognitive processes, transcending its assumed function of auditory stimulus analysis. The implications for basic neuroscience and clinical auditory neuroscience are presented and suggestions for remediation of auditory processing disorders are introduced.

Remodeling the cortex in memory: Increased use of a learning strategy increases the representational area of relevant acoustic cues

Associative learning induces plasticity in the representation of sensory information in sensory cortices. Such high-order associative representational plasticity (HARP) in the primary auditory cortex (A1) is a likely substrate of auditory memory: it is specific, rapidly acquired, long-lasting and consolidates. Because HARP is likely to support the detailed content of memory, it is important to identify the necessary behavioral factors that dictate its induction. Learning strategy is a critical factor for the induction of plasticity (Bieszczad & Weinberger, 2010b). Specifically, use of a strategy that relies on tone onsets induces HARP in A1 in the form of signal-specific decreased threshold and bandwidth. The present study tested the hypothesis that the form and degree of HARP in A1 reflects the amount of use of an "onset strategy". Adult male rats (n=7) were trained in a protocol that increased the use of this strategy from approximately 20% in prior studies to approximately 80%. They developed signal-specific gains in representational area, transcending plasticity in the form of local changes in threshold and bandwidth. Furthermore, the degree of area gain was proportional to the amount of use of the onset strategy. A second complementary experiment demonstrated that use of a learning strategy that specifically did not rely on tone onsets did not produce gains in representational area; but rather produced area loss. Together, the findings indicate that the amount of strategy use is a dominant factor for the induction of learning-induced cortical plasticity along a continuum of both form and degree.

Representational gain in cortical area underlies increase of memory strength

Neuronal plasticity that develops in the cortex during learning is assumed to represent memory content, but the functions of such plasticity are actually unknown. The shift in spectral tuning in primary auditory cortex (A1) to the frequency of a tone signal is a compelling candidate for a substrate of memory because it has all of the cardinal attributes of associative memory: associativity, specificity, rapid induction, consolidation, and long-term retention. Tuning shifts increase the representational area of the signal in A1, as an increasing function of performance level, suggesting that area encodes the magnitude of acquired stimulus significance. The present study addresses the question of the specific function of learning-induced associative representational plasticity. We tested the hypothesis that specific increases in A1 representational area for an auditory signal serve the mnemonic function of enhancing memory strength for that signal. Rats were trained to bar-press for reward contingent on the presence of a signal tone (5.0 kHz), and assessed for memory strength during extinction. The amount of representational area gain for the signal frequency band was significantly positively correlated with resistance to extinction to the signal frequency in two studies that spanned the range of task difficulty. These findings indicate that specific gain in cortical representational area underlies the strength of the behaviorally-relevant contents of memory. Thus, mnemonic functions of cortical plasticity are determinable.

Metabolic mapping of rat forebrain and midbrain during delay and trace eyeblink conditioning

While the essential neural circuitry for delay eyeblink conditioning has been largely identified, much of the neural circuitry for trace conditioning has yet to be determined. The major difference between delay and trace conditioning is a time gap between the presentation of the conditioned stimulus (CS) and the unconditioned stimulus (US) during trace conditioning. It is this time gap, which accounts for the additional memory component and may require extra neural structures, including hippocampus and prefrontal cortex. A metabolic marker of energy use, radioactively labeled glucose analog, was used to compare differences in glucose analog uptake between delay, trace, and unpaired experimental groups (rats, Long-Evans), to identify possible new areas of involvement within forebrain and midbrain. Here, we identify increased 2-DG uptake for the delay group compared to the unpaired group in various areas including: the medial geniculate nuclei (MGN), the amygdala, cingulate cortex, auditory cortex, medial dorsal thalamus, and frontal cortices. For the trace group, compared to the unpaired group, there was an increase in 2-DG uptake for the medial orbital frontal cortex and the medial MGN. The trace group also exhibited more increases lateralized to the right hemisphere, opposite to the side of US delivery, in various areas including: CA1, subiculum, presubiculum, perirhinal cortex, ventral and dorsal MGN, and the basolateral and central amygdala. While some of these areas have been identified as important for delay or trace conditioning, some new structures have been identified such as the orbital frontal cortex for both delay and trace groups.

Avoidance learning facilitates temporal processing in the primary auditory cortex

The primary auditory cortex is now known to be involved in learning and memory, as well as auditory perception. For example, spectral tuning often shifts toward or to the frequency of the conditioned stimulus during associative learning. As previous research has focused on tonal frequency, less is known about how learning might alter temporal parameters of response in the auditory cortex. This study addressed the effects of learning on the fidelity of temporal processing. Adult male rats were trained to avoid shock that was signaled by an 8.0 kHz tone. A novel control group received non-contingent tone and shock with shock probability decreasing over days to match the reduced number of shocks received by the avoidance group as they mastered the task. An untrained (nai ve) group served as a baseline. Following training, neuronal responses to white noise and a broad spectrum of tones were determined across the primary auditory cortex in a terminal experiment with subjects under general anesthesia. Avoidance conditioning significantly improved the precision of spike-timing: the coefficient of variation of 1st spike latency was significantly reduced in avoidance animals compared to controls and nai ves, both for tones and for noise. Additionally, avoidance learning was accompanied by a reduction of the latency of peak response, by 2.0-2.5 ms relative to nai ves and approximately 1.0 ms relative to controls. The shock-matched controls also exhibited significantly shorter peak latency of response than nai ves, demonstrating the importance of this non-avoidance control. Plasticity of temporal processing showed no evidence of frequency specificity and developed independently of the non-temporal parameters magnitude of response, frequency tuning and neural threshold, none of which were facilitated. The facilitation of temporal processing suggests that avoidance learning may increase synaptic strength either within the auditory cortex, in the subcortical auditory system, or both.

Learning strategy determines auditory cortical plasticity

Learning modifies the primary auditory cortex (A1) to emphasize the processing and representation of behaviorally relevant sounds. However, the factors that determine cortical plasticity are poorly understood. While the type and amount of learning are assumed to be important, the actual strategies used to solve learning problems might be critical. To investigate this possibility, we trained two groups of adult male Sprague-Dawley rats to bar-press (BP) for water contingent on the presence of a 5.0 kHz tone using two different strategies: BP during tone presence or BP from tone-onset until receiving an error signal after tone cessation. Both groups achieved the same high levels of correct performance and both groups revealed equivalent learning of absolute frequency during training. Post-training terminal "mapping" of A1 showed no change in representational area of the tone signal frequency but revealed other substantial cue-specific plasticity that developed only in the tone-onset-to-error strategy group. Threshold was decreased approximately 10 dB and tuning bandwidth was narrowed by approximately 0.7 octaves. As sound onsets have greater perceptual weighting and cortical discharge efficacy than continual sound presence, the induction of specific learning-induced cortical plasticity may depend on the use of learning strategies that best exploit cortical proclivities. The present results also suggest a general principle for the induction and storage of plasticity in learning, viz., that the representation of specific acquired information may be selected by neurons according to a match between behaviorally selected stimulus features and circuit/network response properties.

There’s more to auditory cortex than meets the ear

The auditory cortex is typically defined as that portion of the cortex containing neurons that respond to sound. This definition is adequate in a narrow context, but does not take into account sufficiently the subtleties associated with more complex behaviors and cognitive processes. Thus, it is easy to demonstrate that cortical regions essentially unrelated to sound processing may nevertheless be activated by an auditory stimulus; conversely, it is possible to demonstrate responses within classical auditory cortical regions in the complete absence of sound. We give several examples that indicate that responses in auditory cortex cannot be predicted based solely on knowledge of stimulus features. Rather, factors such as memory, attention, and mental imagery can be shown to play a major role in modulating or producing neural responses within auditory cortex. We argue that the interactions between classically defined auditory regions and other sensory, motor, and cognitive systems underlie many behaviors of interest; and that a more complete understanding of these processes will emerge from a consideration of the distributed nature of auditory cortical function.

Does attention play a role in dynamic receptive field adaptation to changing acoustic salience in A1?

Acoustic filter properties of A1 neurons can dynamically adapt to stimulus statistics, classical conditioning, instrumental learning and the changing auditory attentional focus. We have recently developed an experimental paradigm that allows us to view cortical receptive field plasticity on-line as the animal meets different behavioral challenges by attending to salient acoustic cues and changing its cortical filters to enhance performance. We propose that attention is the key trigger that initiates a cascade of events leading to the dynamic receptive field changes that we observe. In our paradigm, ferrets were initially trained, using conditioned avoidance training techniques, to discriminate between background noise stimuli (temporally orthogonal ripple combinations) and foreground tonal target stimuli. They learned to generalize the task for a wide variety of distinct background and foreground target stimuli. We recorded cortical activity in the awake behaving animal and computed on-line spectrotemporal receptive fields (STRFs) of single neurons in A1. We observed clear, predictable task-related changes in STRF shape while the animal performed spectral tasks (including single tone and multi-tone detection, and two-tone discrimination) with different tonal targets. A different set of task-related changes occurred when the animal performed temporal tasks (including gap detection and click-rate discrimination). Distinctive cortical STRF changes may constitute a "task-specific signature". These spectral and temporal changes in cortical filters occur quite rapidly, within 2min of task onset, and fade just as quickly after task completion, or in some cases, persisted for hours. The same cell could multiplex by differentially changing its receptive field in different task conditions. On-line dynamic task-related changes, as well as persistent plastic changes, were observed at a single-unit, multi-unit and population level. Auditory attention is likely to be pivotal in mediating these task-related changes since the magnitude of STRF changes correlated with behavioral performance on tasks with novel targets. Overall, these results suggest the presence of an attention-triggered plasticity algorithm in A1 that can swiftly change STRF shape by transforming receptive fields to enhance figure/ground separation, by using a contrast matched filter to filter out the background, while simultaneously enhancing the salient acoustic target in the foreground. These results favor the view of a nimble, dynamic, attentive and adaptive brain that can quickly reshape its sensory filter properties and sensori-motor links on a moment-to-moment basis, depending upon the current challenges the animal faces. In this review, we summarize our results in the context of a broader survey of the field of auditory attention, and then consider neuronal networks that could give rise to this phenomenon of attention-driven receptive field plasticity in A1.

Brain activity associated with fear renewal

This is the first mapping study of the brain activity associated with the renewal of an extinguished conditioned response. Rats were given radiolabeled fluorodeoxyglucose, a glucose analog, to map brain effects of an extinguished tone during context-dependent renewal of conditioned fear. A tone conditioned stimulus was paired with a footshock unconditioned stimulus in a first context, followed by conditioned response extinction in a second context and conditioned response renewal in a third context. Control rats were treated identically, except that tone and shock were presented pseudorandomly. Compared with control subjects, rats with conditioned response renewal had increased tone-evoked fluorodeoxyglucose uptake in the auditory system (auditory cortex, medial geniculate, inferior colliculus and lateral lemniscal nuclei), as well as somatic and visceral sensory nuclei (external cuneate, spinal trigeminal, solitary tract and vestibular nuclei). In addition, perirhinal cortex, anterior lateral hypothalamus and ventrolateral periaqueductal gray showed conditioned response renewal effects. Brain-behavior correlations indicated that the activity of the external cuneate nucleus strongly predicted the conditioned response in the renewal group. It is suggested that context-dependent fear renewal is associated with (1) tone-evoked activation of the excitatory conditioned stimulus representation in the auditory system, (2) associative activation of the unconditioned stimulus representation in somatic and visceral sensory nuclei in the absence of the unconditioned stimulus, and (3) neural activation of the perirhinal cortex, hypothalamus and periaqueductal gray. These findings support Pavlov's stimulus-substitution theory as a neural mechanism contributing to the renewal effect.

Conditioning-specific reflex modification of rabbit (Oryctolagus cuniculus) heart rate

Conditioning-specific reflex modification (CRM) describes changes in rabbit (Oryctolagus cuniculus) nictitating membrane responses (NMR) to an unconditioned stimulus (US) when the US is tested by itself after pairings of tone and electrodermal stimulation. Although CRM has been replicated, it is unclear whether it occurs in response systems other than that of the NMR. The authors report that CRM of rabbit heart rate (HR) can occur following HR conditioning. A US that elicits HR acceleration before conditioning can elicit HR deceleration after conditioning. The rabbits' electrocardiograms showed both HR conditioning and HR CRM were correlated with an increased PQ interval--an index of parasympathetic function mediated by the vagus. The data suggest conditioned HR deceleration can generalize from conditioned stimulus to US as a function of conditioning.

Active listening: task-dependent plasticity of spectrotemporal receptive fields in primary auditory cortex

Listening is an active process in which attentive focus on salient acoustic features in auditory tasks can influence receptive field properties of cortical neurons. Recent studies showing rapid task-related changes in neuronal spectrotemporal receptive fields (STRFs) in primary auditory cortex of the behaving ferret are reviewed in the context of current research on cortical plasticity. Ferrets were trained on spectral tasks, including tone detection and two-tone discrimination, and on temporal tasks, including gap detection and click-rate discrimination. STRF changes could be measured on-line during task performance and occurred within minutes of task onset. During spectral tasks, there were specific spectral changes (enhanced response to tonal target frequency in tone detection and discrimination, suppressed response to tonal reference frequency in tone discrimination). However, only in the temporal tasks, the STRF was changed along the temporal dimension by sharpening temporal dynamics. In ferrets trained on multiple tasks, distinctive and task-specific STRF changes could be observed in the same cortical neurons in successive behavioral sessions. These results suggest that rapid task-related plasticity is an ongoing process that occurs at a network and single unit level as the animal switches between different tasks and dynamically adapts cortical STRFs in response to changing acoustic demands.

Experience-dependent changes in cortical whisker representation in the adult mouse: a 2-deoxyglucose study

Sensory experience and learning can modify cortical body maps. We have previously reported that 3 days of classical conditioning, in which stimulation of a row of whiskers was paired with tail shock, produced an expansion of the cortical representation of the "trained row" labeled with 2-deoxyglucose (2DG), in layer IIIb and IV of the barrel cortex. The present study examined plastic remodelling of the vibrissal cortical representation after pairing whisker stimulation with a drop of sweet water. Cortical representations of rows of whiskers were mapped by 2DG autoradiography after 3 days and 2 months of training. The training resulted in enlargement of the cortical representation of vibrissae involved in the stimulus pairing compared with the contralateral representation of a row of whiskers, that were not touched during the training. This modification of whisker representation was different after short-term and long-term appetitive training. After three pairing sessions, changes in the width of cortical representation were visible in layers II/IIIa (29%) and layers V/VI (28%). After 2 months of training, significant changes in the width of cortical representation row B were found only in layer IV (41%). The changes were not observed in animals, that received whisker stimulation alone or in those who were subjected to training with unpaired stimuli. The results demonstrate that stimulus-pairing-induced changes in cortical whisker representation appeared with different time courses at different levels of cortical columnar information processing.

Cholinergic modulation of experience-dependent plasticity in human auditory cortex

The factors that influence experience-dependent plasticity in the human brain are unknown. We used event-related functional magnetic resonance imaging (fMRI) and a pharmacological manipulation to measure cholinergic modulation of experience-dependent plasticity in human auditory cortex. In a differential aversive conditioning paradigm, subjects were presented with high (1600 Hz) and low tones (400 Hz), one of which was conditioned by pairing with an electrical shock. Prior to presentation, subjects were given either a placebo or an anticholinergic drug (0.4 mg iv scopolamine). Experience-dependent plasticity, expressed as a conditioning-specific enhanced BOLD response, was evident in auditory cortex in the placebo group, but not with scopolamine. This study provides in vivo evidence that experience-dependent plasticity, evident in hemodynamic changes in human auditory cortex, is modulated by acetylcholine.

Plastic changes in the central auditory system after hearing loss, restoration of function, and during learning

Traditionally the auditory system was considered a hard-wired sensory system; this view has been challenged in recent years in light of the plasticity of other sensory systems, particularly the visual and somatosensory systems. Practical experience in clinical audiology together with the use of prosthetic devices, such as cochlear implants, contributed significantly to the present view on the plasticity of the central auditory system, which was originally based on data obtained in animal experiments. The loss of auditory receptors, the hair cells, results in profound changes in the structure and function of the central auditory system, typically demonstrated by a reorganization of the projection maps in the auditory cortex. These plastic changes occur not only as a consequence of mechanical lesions of the cochlea or biochemical lesions of the hair cells by ototoxic drugs, but also as a consequence of the loss of hair cells in connection with aging or noise exposure. In light of the aging world population and the increasing amount of noise in the modern world, understanding the plasticity of the central auditory system has its practical consequences and urgency. In most of these situations, a common denominator of central plastic changes is a deterioration of inhibition in the subcortical auditory nuclei and the auditory cortex. In addition to the processes that are elicited by decreased or lost receptor function, the function of nerve cells in the adult central auditory system may dynamically change in the process of learning. A better understanding of the plastic changes in the central auditory system after sensory deafferentation, sensory stimulation, and learning may contribute significantly to improvement in the rehabilitation of damaged or lost auditory function and consequently to improved speech processing and production.

Imaging learning and memory: classical conditioning

The search for the biological basis of learning and memory has, until recently, been constrained by the limits of technology to classic anatomic and electrophysiologic studies. With the advent of functional imaging, we have begun to delve into what, for many, was a "black box." We review several different types of imaging experiments, including steady state animal experiments that image the functional labeling of fixed tissues, and dynamic human studies based on functional imaging of the intact brain during learning. The data suggest that learning and memory involve a surprising conservation of mechanisms and the integrated networking of a number of structures and processes.

Daily variation and appetitive conditioning-induced plasticity of auditory cortex receptive fields

Long-term modification of cortical receptive field maps follows learning of sensory discriminations and conditioned associations. In the process of determining whether appetitive - as opposed to aversive - conditioning is effective in causing such plastic changes, it was discovered that multineuron receptive fields, when measured in rats under ketamine-sedation, vary substantially over the course of a week, even in the absence of classical conditioning and electrode movement. Specifically, a simple correlation analysis showed that iso-intensity frequency response curves of multiunit clusters and local field potentials recorded from auditory cortex are nonstationary over 7 days. Nevertheless, significant plastic changes in receptive fields, due to conditioned pairing of a pure tone and electrical stimulation of brain reward centres, are detectable above and beyond these spontaneous daily variations. This finding is based on a novel statistical plasticity criterion which compares receptive fields recorded for three days before and three days after conditioning. Based on a more traditional criterion (i.e. one day before and after conditioning), the prevalence of learning-induced changes caused by appetitive conditioning appears to be comparable to that described in previous studies involving aversive conditioning.

A computational model of mechanisms controlling experience-dependent reorganization of representational maps in auditory cortex

Cortical representations of sound can be modified by repeatedly pairing presentation of a pure tone with electrical stimulation of neuromodulatory neurons located in the basal forebrain (Bakin & Weinberger, 1996; Kilgard & Merzenich, 1998a). We developed a computational model to investigate the possible effects of basal forebrain modulation on map reorganization in the auditory cortex. The model is a self-organizing map with acoustic response characteristics mimicking those observed in the mammalian auditory cortex. We simulated the effects of basal forebrain modulation, using parameters intrinsic to the self-organizing map, such as the learning rate (controlling the adaptability of map nodes) and the neighborhood function (controlling the excitability of map nodes). Previous research has suggested that both parameters can be useful for characterizing the effects of neuromodulation on plasticity (Kohonen, 1993; Myers et al., 1996; Myers, Ermita, Hasselmo, & Gluck, 1998). The model successfully accounts for experimentally observed effects of pairing basal forebrain stimulation with the presentation of a single tone, but not of two tones, suggesting that auditory cortical plasticity is constrained in ways not accounted for by current theories. Despite this limitation, the model provides a useful framework for describing experience-induced changes in auditory representations and for relating such changes to variations in the excitability and adaptability of cortical neurons produced by neuromodulation.

Bilateral Ablation of Auditory Cortex in Mongolian Gerbil Affects Discrimination of Frequency Modulated Tones but not of Pure Tones

This study examines the role of auditory cortex in the Mongolian gerbil in differential conditioning to pure tones and to linearly frequency-modulated (FM) tones by analyzing the effects of bilateral auditory cortex ablation. Learning behavior and performance were studied in a GO/NO-GO task aiming at avoidance of a mild foot shock by crossing a hurdle in a two-way shuttle box. Hurdle crossing as the conditioned response to the reinforced stimulus (CR+), as false alarm in response to the unreinforced stimulus (CR−), intertrial activity, and reaction times were monitored. The analysis revealed no effects of lesion on pure tone discrimination but impairment of FM tone discrimination. In the latter case lesion effects were dependent on timing of lesion relative to FM tone discrimination training. Lesions before training in naive animals led to a reduced CR+ rate and had no effect on CR− rate. Lesions in pretrained animals led to an increased CR− rate without effects on the CR+ rate. The results suggest that auditory cortex plays a more critical role in discrimination of FM tones than in discrimination of pure tones. The different lesion effects on FM tone discrimination before and after training are compatible with both the hypothesis of a purely sensory deficit in FM tone processing and the hypothesis of a differential involvement of auditory cortex in acquisition and retention, respectively.

The effects of scopolamine on changes in regional cerebral blood flow during classical conditioning of the human eyeblink response

We examined the effects of scopolamine on the functional anatomy of classical conditioning of the human eyeblink response. Ten healthy young normal female volunteers (mean age +/- SEM: 26.7 +/- 0.9 years) were administered 0.4 mg scopolamine intravenously 1 h before regional cerebral blood flow (rCBF) was measured with positron emission tomography (PET) and H215O. Scans occurred during three sequential phases: (1) explicitly unpaired presentations of the unconditioned stimulus (airpuff to the right eye) and conditioned stimulus (binaural tone), (2) paired presentations of the two stimuli (associative learning) and (3) explicitly unpaired presentation of the stimuli (extinction phase). Scopolamine impaired acquisition of the conditioned eyeblink response (54.7 +/- 4.9%) relative to 18 untreated subjects from two previous PET studies. Regions that showed significant relative increases in rCBF during conditioning included the right lateral occipital cortex, the right inferior occipital cortex, the right lateral temporo-occipital cortex, the left medial temporo-occipital cortex, the posterior cingulate, the right cerebellum/brain stem area and the medial cerebellum. Significant relative decreases in rCBF were measured in the thalamus, the left putamen/insula area, the right putamen and the left and middle cerebellar cortex. The data partially replicate previous findings in unmedicated young volunteers of conditioning-specific rCBF changes in the cingulate cortex, the cerebellar cortex, the insula and the lateral temporo-occipital cortex. Our finding of decreased rCBF in the thalamus and increased rCBF in the occipital cortex may be attributable to effects of scopolamine per se rather than conditioning. Our data lend further support to the notion that classical conditioning involves distributed changes in multiple systems within the central nervous system.

Learning-induced physiological plasticity in the thalamo-cortical sensory systems: a critical evaluation of receptive field plasticity, map changes and their potential mechanisms

The goal of this review is to give a detailed description of the main results obtained in the field of learning-induced plasticity. The review is focused on receptive field and map changes observed in the auditory, somatosensory and visual thalamo-cortical system as a result of an associative training performed in waking animals. Receptive field (RF) plasticity, 2DG and map changes obtained in the auditory and somatosensory system are reviewed. In the visual system, as there is no RF and map analysis during learning per se, the evidence presented are from increased neuronal responsiveness, and from the effects of perceptual learning in human and non human primates. Across sensory modalities, the re-tuning of neurons to a significant stimulus or map reorganizations in favour of the significant stimuli were observed at the thalamic and/or cortical level. The analysis of the literature in each sensory modality indicates that relationships between learning-induced sensory plasticity and behavioural performance can, or cannot, be found depending on the tasks that were used. The involvement (i) of Hebbian synaptic plasticity in the described neuronal changes and (ii) of neuromodulators as "gating" factors of the neuronal changes, is evaluated. The weakness of the Hebbian schema to explain learning-induced changes and the need to better define what the word "learning" means are stressed. It is suggested that future research should focus on the dynamic of information processing in sensory systems, and the concept of "effective connectivity" should be useful in that matter.

Large-scale functional connectivity in associative learning: interrelations of the rat auditory, visual, and limbic systems

Large-scale functional connectivity in associative learning: interrelations of the rat auditory, visual, and limbic systems. J. Neurophysiol. 80: 3148-3162, 1998. Functional relations between specialized parts of the brain may be important determinants of learned behaviors. To study this, we examined the interrelations of the auditory system with several extraauditory structures in two groups of rats having different behavioral histories. Both groups were trained to associate a tone conditional stimulus (CS) with an aversive unconditional stimulus (US). For one group, a light presented with the tone predicted the absence of the US (group TL-). In the other group, the light was a neutral stimulus (group TL0). Fluorodeoxyglucose (FDG) incorporation was measured in the presence of the tone-light compound. Because the tone-light compound was physically identical for both groups, neural differences between groups reflected differences in the learned associative properties of the stimuli. Covariances of FDG uptake in the auditory system and extraauditory structures were examined using partial least squares. Three strong covariance or functional connectivity patterns were identified. The first pattern mainly reflected similarities between groups, with strong interrelations between the subcortical auditory system and the thalamocortical visual system, cerebellum, deep cerebellar nuclei, and midline thalamus. This pattern of interactions may represent part of a common circuit for relaying the associative value of the tone CS to the cerebellum and the midline thalamus. The external nucleus of the inferior colliculus and medial division of the medial geniculate nucleus were associated more strongly with this pattern for group TL-, which was interpreted as representing the change of the associative value of the tone by the light, mediated through extraauditory influences on these two regions. A second pattern involved midbrain auditory regions, superior colliculus, zona incerta, and subiculum and was stronger for group TL0. The relations between midbrain structures may represent the excitatory conditioned response (CR) evoked by the tone in this group. The final pattern was strongest in group TL- and involved interrelations of the thalamocortical auditory system with hippocampus, basolateral amygdala, and hypothalamus. This pattern may represent the learned inhibition of the CR to the tone in the presence of the light. These findings are consistent with behavioral studies suggesting that at least two types of associations are formed during associative learning. One is the sensory relation of the stimuli and another is the relation between the CS and the affective components of the US. These behavioral associations are mapped to the patterns of functional connectivity between auditory and extraauditory regions.

Classical conditioning modifies cytochrome oxidase activity in the auditory system

The effects of excitatory classical conditioning on cytochrome oxidase activity in the central auditory system were investigated using quantitative histochemistry. Rats in the conditioned group were trained with consistent pairings of a compound conditional stimulus (a tone and a light) with a mild footshock, to elicit conditioned suppression of drinking. Rats in the pseudorandom group were exposed to pseudorandom presentations of the same tone, light and shock stimuli without consistent pairings. Untrained rats in a naive group did not receive presentations of the experimental stimuli. The findings demonstrated that auditory fear conditioning modifies the metabolic neuronal responses of the auditory system, supporting the hypothesis that sensory neurons are responsive to behavioural stimulus properties acquired by learning. There was a clear distinction between thalamocortical and lower divisions of the auditory system based on the differences in metabolic activity evoked by classical conditioning, which lead to an overt learned behavioural response versus pseudorandom stimulus presentations, which lead to behavioural habituation. Increases in cytochrome oxidase activity indicated that tone processing is enhanced during associative conditioning at upper auditory structures (medial geniculate nucleus and secondary auditory cortices). In contrast, metabolic activation of lower auditory structures (cochlear nuclei and inferior colliculus) in response to the pseudorandom presentation of the experimental stimuli suggest that these areas may be activated during habituation to tone stimuli. Together these findings show that mapping the metabolic activity of cytochrome oxidase with quantitative histochemistry can be successfully used to map regional long-lasting effects of learning on brain systems.

Acoustic frequency tuning of neurons in the basal forebrain of the waking guinea pig

The acoustic responses of cells in the basal forebrain were studied in the adult waking guinea pig. Frequency receptive fields were obtained across wide frequency (0.094-45.0 kHz) and intensity (0-90 dB) ranges. A total of 326 recordings were obtained in 26 electrode penetrations from five subjects; 205 from the globus pallidus (GP), 98 from the caudate-putamen (CPu) and 23 from the central nucleus of the amygdala (ACE). Twenty-nine recordings exhibited acoustic responses (GP=20 (9.8%); CPu=9 (9.2%); ACE=0). Cells in the regions of the GP that project to the primary auditory cortex (ACx) exhibited frequency tuning that was dominantly suppressive. Responses in the CPu were excitatory, but poorly tuned. The spontaneous rate of discharge of GP cells that yielded complete tuning data was positively correlated with power in the beta bands (12-25 and 25-50 Hz) and negatively correlated with power in the delta band (1-4 Hz) of the EEG of the ACx. These findings suggest that acoustically tuned neurons in the GP that are inhibited by tones are involved in the regulation of auditory cortical state, possibly promoting deactivation to unimportant sounds, and may be cholinergic in nature.

Categorical discrimination of direction in frequency-modulated tones by Mongolian gerbils

Discrimination of the direction of linearly frequency-modulated tones (FMs) was investigated in adult Mongolian gerbils (Meriones unguiculatus) using a footshock motivated shuttle box avoidance go/no go procedure. Symmetric pairs of FMs with frequency linearly increasing with time (ascending FMs) and with frequency linearly decreasing with time (descending FMs) were used as conditioned stimuli, CS+ and CS-, respectively. Stimuli were presented in randomized order in daily sessions over a period of several months. After a number of sessions, the set of conditioned stimuli was changed with respect to frequency range, steepness of modulation and duration. In experiment 1, we observed that gerbils could discriminate between the ascending 2-4 kHz CS+ and the descending 4-2 kHz CS- after a training period of 10-15 days. In experiment 2, we used FM pairs of six other frequency ranges in successive sessions (6-13; 1-2; 13-25; 0.5-1; 3 6; 0.25 0.5 kHz). We found that in the final session the last FM pair (0.25-0.5 kHz) was discriminated already after 3-4 days. Experiment 3 showed that the animals were able to discriminate five of the FM pairs learned in the separate sessions of experiment 2 (i.e. 10 different stimuli) when they were given in randomized order during one training session. In experiment 4, novel FM pairs (not heard before) and familiar FM pairs (trained in experiments 1-3) were presented within one session. It was found that, except for FMs of very short duration and small frequency range, novel FMs were discriminated according to their modulation direction. These results show that Mongolian gerbils are able to discriminate FM tones by modulation direction and, after familiarization with a number of different FM pairs, transfer the ascending-descending concept to stimuli not heard before.

Fear conditioning enhances different temporal components of tone-evoked spike trains in auditory cortex and lateral amygdala

Single neurons were recorded in freely behaving rats during fear conditioning from areas of auditory cortex that project to the lateral nucleus of the amygdala (LA). The latency and rate of conditioning and extinction were analyzed, and the results were compared to previous recordings from LA itself. Auditory cortex neurons took more trials to learn, and they responded more slowly than LA neurons within trials. Short-latency plasticity in LA, therefore, reflects inputs from the auditory thalamus rather than the auditory cortex. Unlike LA cells, some auditory cortex cells showed late conditioned responses that seemed to anticipate the unconditioned stimulus, while others showed extinction-resistant memory storage. Thus, rapid conditioning of fear responses to potentially dangerous stimuli depends on plasticity in the amygdala, while cortical areas may be particularly involved in higher cognitive (mnemonic and attentional) processing of fear experiences.

Metabolic effects of blocking tone conditioning on the rat auditory system

The Kamin blocking phenomenon occurs when behavioral expression of conditioning to a novel stimulus fails in the presence of a previously conditioned stimulus (CS). Neural metabolic effects of a tone conditioned as an excitor were compared to the effects of the same physical tone when excitatory conditioning was blocked by previous conditioning with a light. We examined the metabolic activity of the auditory system to test the hypothesis that auditory processing of a tone CS changes during blocking. Quantitative histochemistry of cytochrome oxidase (C.O.), the final mitochondrial enzyme for oxidative metabolism, was used to evaluate cumulative changes in the metabolic capacity of the auditory system resulting from blocking. Rats (Long-Evans) in the Blocking group received pairings of a light CS with a mild footshock unconditioned stimulus (US) during Phase 1 training. Rats in the Control group received random presentations of the same stimuli during Phase 1. Both groups then received the same Phase 2 training consisting of simultaneous tone and light presentations paired with footshock. The Control group exhibited significant suppression of drinking to tone alone presentations after training, whereas the Blocking group did not. Metabolic mapping results demonstrated that blocking effects were localized to auditory regions receiving direct US somatosensory projections. Significantly greater C.O. activity in the inferior colliculus and the dorsal cochlear nucleus was found for the Blocking group relative to the Control group. Input cell layers of secondary auditory cortex also demonstrated a group difference, in that layers II/III and IV had lower levels of C.O. activity in the Blocking group. These specific changes in C.O. activity linked to behavioral training demonstrated that the blocking phenomenon produced distinct neural metabolic changes in CS processing in the auditory system localized to regions with CS-US interactions.

Lateralization and behavioral correlation of changes in regional cerebral blood flow with classical conditioning of the human eyeblink response

Laterality of changes in regional cerebral blood flow (rCBF) during classical conditioning of the human eyeblink response was studied and changes in rCBF were correlated with conditioned responses. In 10 normal volunteers, rCBF was mapped with positron emission tomography and H2(15)O during pairings of a binaural tone conditioned stimulus and an air puff unconditioned stimulus to the left eye. Control conditions consisted of explicitly unpaired presentations of the tone and air puff before (control) and after (extinction) pairings. During pairings, rCBF increased significantly in right primary auditory cortex (contralateral to air puff) and decreased significantly in left and right cerebellar cortex. There were also increases in rCBF in right auditory association cortex and left temporoccipital cortex. Decreases in rCBF were noted bilaterally in the temporal poles and in the left prefrontal cortex. Positive correlations between changes in rCBF and percent conditioned responses were located in middle cerebellum, right superior temporal cortex, left dorsal premotor cortex, right middle cingulate, and right superior temporal cortex. There were negative correlations in left inferior prefrontal cortex, left middle prefrontal cortex, and right inferior parietal cortex. The data replicate our previous findings of lateralized changes in rCBF following presentations of a binaural tone and air puff to the right eye and indicate that there are pairing-specific changes in primary auditory cortex and cerebellum that are not unique to the left or right hemisphere but are a function of the side of training. The commonalities as well as differences in regional involvement in our present and previous experiment as well as in other eyeblink studies illustrate the advantage of functional neuroimaging to quantify different strategies used by the brain to perform seemingly similar functions. Indeed, the data support the notion that learning-related changes can be detected in a number of specific, but not necessarily invariant, brain regions, and that the involvement of any one region is dependent on the characteristics of the particular learning situation.

Differential frequency conditioning enhances spectral contrast sensitivity of units in auditory cortex (field Al) of the alert Mongolian gerbil

Differential aversive auditory conditioning in the awake Mongolian gerbil was performed during single- and multi-unit recording in field Al of the primary auditory cortex. Presentations of pure tone stimuli of a given frequency (reinforced conditioned stimulus; CS+) paired with electrocutaneous stimulation (unconditioned stimulus) were combined with several other non-reinforced tone stimuli (non-reinforced conditioned stimulus; CS-). Stimulus presentation during training and testing was optimized for constancy of the probability of occurrence of both the CS+ and the CS- stimulus. The paradigm led to a reorganization of both the spectral and temporal response characteristics of auditory cortical neurons with the following basic results. First, tone-evoked responses of Al neurons recorded after multiple acoustic stimulation under these conditions varied statistically around a mean value (stationarity). Conditioning produced a shift in mean values of evoked responses. The altered tone responses were also stationary (stability of the plastic effects). Second, the frequency-receptive fields (FRFs) of neurons were reorganized in a frequency-specific way such that the CS+ frequency became located in a local minimum of the FRF after training. This resulted from a training-induced increase in the responses to frequencies adjacent to the CS+ frequency in the FRF relative to the CS+ response. The effect can be interpreted as an enhancement of the 'spectral contrast' sensitivity of the unit in the CS+ neighbourhood. Third, apart from this frequency-specific plastic effect, responses to other frequencies also underwent changes during training. The non-frequency-specific changes were not generally predictable but the post-trial responses were stationary. Fourth, the analysis of the long-term behaviour of FRF reorganization revealed the stability of plastic effects under retention training and the gradual re-establishment of the pretrial FRF during extinction training. Fifth, not only the spectral characteristics but also the temporal structure of the tone-evoked responses could be affected by the training. In most cases the training-induced changes measured within the first tens of milliseconds of the response corresponded to the response changes obtained by integration over the total response period. There were some cases, however, in which the direction of the response change varied with time, indicating that excitatory and inhibitory influences on the temporal response pattern were differently affected by training.

A comparison of tone-evoked response properties of 'cluster' recordings and their constituent single cells in the auditory cortex

This study examined relationships between some acoustic response properties of 'cluster' recordings (CL) and their constituent single cells (SU) in the auditory cortex obtained from 22 clusters comprised of 63 responsive single units mainly in the anterior tonotopic field of the waking guinea pig. Response parameters included characteristic frequency (CF), threshold (Th) at CF, bandwidth 10 (BW10) and 30 (BW30) dB above Th. Clusters and single units were classified by their pattern of discharges as either 'onset' or 'sustained' response types. Comparison of CL and their constituent SU revealed differences in one or more response parameters in all CL. The CFs of onset CL were generally the same as the CFs of their constituent onset SU in contrast to sustained CL for which greater differences were observed in CF. The Th of all CL differed from that of some of their cells. The BW of approximately 50% of CL differed from their SU. The findings indicate that cluster recordings are often not good predictors of the response parameters of all of their constituent neurons.

Comparison of frequency-specific c-Fos expression and fluoro-2-deoxyglucose uptake in auditory cortex of gerbils (Meriones unguiculatus)

Induction of c-Fos in the auditory cortex of gerbils was investigated immunocytochemically 1 h after single, triple or 1 h continuous stimulation with a series of narrow band frequency-modulated tone bursts. With single stimulation c-Fos immunoreactive neurons were chiefly found in the primary auditory field (AI), where they formed a narrow frequency-specific column across layers II-VI. Side-band-like patterns adjacent to this column appeared characteristically with triple stimulation. Immunoreactive cell density in the anterior auditory field and the caudal fields was sparse and location not frequency specific with single or triple stimulation. Spatial comparisons of c-Fos immunoreactive neuron density with 2-deoxy-2-fluoro-D-glucose (FDG) autoradiography in the same animals after 1 h of stimulation revealed spreading of c-Fos expression in neurons across the tonotopic maps of the AI and in the rostral and caudal fields of the auditory cortex. The pattern of the highest density of c-Fos labelled cells in the AI still matched the peak labelling of FDG autoradiographs. The results show that the postsynaptic marker c-Fos reflects the frequency representation in the AI with single or triple stimulation yet with a higher spatial resolution than the deoxyglucose technique. Longer stimulation causes nontonotopic intracortical spreading of the c-Fos-inducing message, a phenomenon potentially reflecting the effects of cooperativity in the maps.

Functional network interactions between parallel auditory pathways during Pavlovian conditioned inhibition

Using covariance structural equation modeling and fluorodeoxyglucose (FDG) autoradiography this study examined auditory system interactions when the learned associative effects of a tone were inhibited by a light. Two groups of rats received pairings of a tone (conditioned excitor: T+) with a mild footshock. Group TL- was trained in a Pavlovian conditioned inhibition paradigm (T+/TL-) where the tone-light compound signaled the absence of footshock, making the light the inhibitor (L-). Group TL degree was trained with the tone as the excitor and the light as a 'neutral' stimulus. After FDG injection, all rats were presented with the tone-light compound. Group differences in auditory system FDG uptake were observed only in the ventral division medial geniculate nucleus (MGV), where group TL- had relatively lower incorporation. Structural equation modeling was used with the covariances of FDG activity to determine the functional influences through the auditory system anatomic connections. Differences were noted mainly at the level of the inferior colliculus (IC) and medial geniculate, possibly reflecting the unique anatomic relation of these regions with extraauditory areas. Ascending and descending influences from the IC differed with stronger influences for group TL-. Intracollicular and the ascending influence influences of MGV and medial division of the medial geniculate nucleus (MGM) on the auditory cortex also differed mainly in the sign of the functional interaction. These results demonstrate how interactions among parallel auditory pathways can code the behavioral significance of auditory stimuli and emphasize that a full appreciation of neural operations underlying learning can only be gained through examination of both regional activity and interregional interactions.

Discrimination training in a GO/NOGO-procedure alters the 2-deoxyglucose pattern in the Starling's forebrain

European starling's (Sturnus vulgaris L.) were used to measure differences in the glucose metabolism in the auditory forebrain between birds performing an auditory discrimination task and birds habituated to the same acoustic stimuli. One group (n = 5) of individuals was trained in an operant GO/NOGO-procedure to report 1-kHz tone signals in a background of 4-kHz stimuli. The other group (n = 5) was habituated to the experimental set-up and to the same sequence of tones presented to the trained birds. [14C]2-deoxyglucose (2DG) uptake was determined in the caudal auditory telencephalon and the nucleus ovoidalis of well trained and habituated birds by autoradiography of brain sections. The tissue areas having grey values above predefined threshold values of labelling were determined in every brain section of each bird and then combined to volumes of labelled tissue. No significant differences of the 2DG uptake in the nucleus ovoidalis were found between the two experimental groups. In the caudal auditory telencephalon, however, significant differences in 2DG-labelling were found. In the trained birds, the labelling in the caudal auditory telencephalon was confined to smaller brain regions than in the habituated birds. These results suggest a differential processing of sounds in the trained and habituated birds which is discussed in the context of sharpening of the frequency representation by GABAergic inhibition and processes of attention.

Mapping of stimulus features and meaning in gerbil auditory cortex with 2-deoxyglucose and c-Fos antibodies

The basic functional organization of gerbil auditory cortex was previously mapped with unit recording of best frequency and with the fluoro-2-deoxyglucose mapping (FDG) technique. Among at least seven subfields in this cortex the primary auditory cortex (AI) and the anterior auditory field (AAF) showed prominent tonotopic organization with parallel dorsoventral iso-frequency contours (electrophysiology) in correspondence to FDG labelling of frequency band laminae. In an approach to mechanisms of learning aversive tone conditioning paradigms were found to reshape frequency receptive fields of single units in AI and also produced spatial shifts of tone representation in the tonotopic maps of AI and AAF. Both results suggest that spectral features as well as aspects of behavioural meaning of sounds may be represented even in primary auditory cortex. General meaningfulness in terms of occurrence of novel and salient stimuli may be reflected by expression of immediate early genes. Mapping with an antibody against the immediate early gene product c-Fos was performed in order to identify the spatial distribution of neurons in auditory cortex which change metabolism as a result of stimulation with auditory signals in a new environment. Very short e.g. less than 3 min repetitive stimulation with a tone led to frequency-specific columnar expression of c-Fos in AI and to spare non-tonotopic expression in other fields. Longer stimulation or longer aversive conditioning with the same tone led to spreading of expression, i.e. to accessory non-tonotopic labelling in AI and other fields, particularly pronounced in the output layers V and VI. It is assumed that this spreading relates to the formation of output schemes from auditory cortex in terms of implicit behavioural meaning of stimuli.