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

Organization of orbitofrontal-auditory pathways in the Mongolian gerbil

Sound perception is highly malleable, rapidly adjusting to the acoustic environment and behavioral demands. This flexibility is the result of ongoing changes in auditory cortical activity driven by fluctuations in attention, arousal, or prior expectations. Recent work suggests that the orbitofrontal cortex (OFC) may mediate some of these rapid changes, but the anatomical connections between the OFC and the auditory system are not well characterized. Here, we used virally mediated fluorescent tracers to map the projection from OFC to the auditory midbrain, thalamus, and cortex in a classic animal model for auditory research, the Mongolian gerbil (Meriones unguiculatus). We observed no connectivity between the OFC and the auditory midbrain, and an extremely sparse connection between the dorsolateral OFC and higher order auditory thalamic regions. In contrast, we observed a robust connection between the ventral and medial subdivisions of the OFC and the auditory cortex, with a clear bias for secondary auditory cortical regions. OFC axon terminals were found in all auditory cortical lamina but were significantly more concentrated in the infragranular layers. Tissue-clearing and lightsheet microscopy further revealed that auditory cortical-projecting OFC neurons send extensive axon collaterals throughout the brain, targeting both sensory and non-sensory regions involved in learning, decision-making, and memory. These findings provide a more detailed map of orbitofrontal-auditory connections and shed light on the possible role of the OFC in supporting auditory cognition.

The perceptual categorization of multidimensional stimuli is hierarchically organized

As we interact with our surroundings, we encounter the same or similar objects from different perspectives and are compelled to generalize. For example, despite their variety we recognize dog barks as a distinct sound class. While we have some understanding of generalization along a single stimulus dimension (frequency, color), natural stimuli are identifiable by a combination of dimensions. Measuring their interaction is essential to understand perception. Using a 2-dimension discrimination task for mice and frequency or amplitude modulated sounds, we tested untrained generalization across pairs of auditory dimensions in an automatized behavioral paradigm. We uncovered a perceptual hierarchy over the tested dimensions that was dominated by the sound's spectral composition. Stimuli are thus not perceived as a whole, but as a combination of their features, each of which weights differently on the identification of the stimulus according to an established hierarchy, possibly paralleling their differential shaping of neuronal tuning.

A Narrative Review of Auditory Categorisation and Its Potential Role in Tinnitus Perception

Auditory categorisation is a phenomenon reflecting the non-linear nature of human perceptual spaces which govern sound perception. Categorisation training paradigms may reduce sensitivity toward training stimuli, decreasing the representation of these stimuli in auditory perceptual maps. Reduced cortical representation may have clinical implications for conditions that arise from disturbances in cortical activation, such as tinnitus. This review explores the categorisation of sound, with a particular focus on tinnitus. The potential of categorisation training as a sound-based tinnitus therapy is discussed. A narrative review methodological framework was followed. Four databases (PubMed, Google Scholar, Scopus, and ScienceDirect) were extensively searched for the following key words: categorisation, categorical perception, perceptual magnet effect, generalisation, and categorisation OR categorical perception OR perceptual magnet effect OR generalisation AND sound. Given the exploratory nature of the review and the fact that early works on categorisation are crucial to the understanding and development of auditory categorisation, all study types were selected for the period 1950–2022. Reference lists of articles were reviewed to identify any further relevant studies. The results of the review were catalogued and organised into themes. In total, 112 articles were reviewed in full, from which 59 were found to contain relevant information and were included in the review. Key themes identified included categorical perception of speech stimuli, warping of the auditory perceptual space, categorisation versus discrimination, the presence of categorisation across several modalities, and categorisation as an innate versus learned feature. Although a substantial amount of work focused on evaluating the effects of categorisation training on sound perception, only two studies investigated the effects of categorisation training on tinnitus. Implementation of a categorisation-based perceptual training paradigm could serve as a promising means of tinnitus management by reversing the changes in cortical plasticity that are seen in tinnitus, in turn altering the representation of sound within the auditory cortex itself. In the instance that the categorisation training is successful, this would likely mean a decrease in the level of activity within the auditory cortex (and other associated cortical areas found to be hyperactive in tinnitus) as well as a reduction in tinnitus salience.

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.

Categorical Auditory Working Memory in Crows

The ability to group sensory data into behaviorally meaningful classes and to maintain these perceptual categories active in working memory is key to intelligent behavior. Here, we show that carrion crows, highly vocal and cognitively advanced corvid songbirds, possess categorical auditory working memory. The crows were trained in a delayed match-to-category task that required them to flexibly match remembered sounds based on the upward or downward shift of the sounds' frequency modulation. After training, the crows instantaneously classified novel sounds into the correct auditory categories. The crows showed sharp category boundaries as a function of the relative frequency interval of the modulation. In addition, the crows generalized frequency-modulated sounds within a category and correctly classified novel sounds kept in working memory irrespective of other acoustic features of the sound. This suggests that crows can form and actively memorize auditory perceptual categories in the service of cognitive control of their goal-directed behaviors.

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Crows performed a delayed match-to-category task with frequency modulated sounds

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Crows classified novel sounds into upward or downward modulated sound categories

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Crows showed sharp category boundaries and within-category generalization

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Crows can actively memorize auditory perceptual categories for cognitive control

Learning Induces Transient Upregulation of Brevican in the Auditory Cortex during Consolidation of Long-Term Memories

It is a daily challenge for our brains to establish new memories via learning while providing stable storage of remote memories. In the adult vertebrate brain, bimodal regulation of the extracellular matrix (ECM) may regulate the delicate balance of learning-dependent plasticity and stable memory formation. Here, we trained adult male mice in a cortex-dependent auditory discrimination task and measured the abundance of ECM proteins brevican (BCN) and tenascin-R over the course of acquisition learning, consolidation, and long-term recall in two learning-relevant brain regions; the auditory cortex and hippocampus. Although early training led to a general downregulation of total ECM proteins, successful retrieval correlated with a region-specific and transient upregulation of BCN levels in the auditory cortex. No other parameter such as arousal or stress could account for the transient and region-specific BCN upregulation. This performance-dependent biphasic regulation of the ECM may assist transient plasticity to facilitate initial learning and subsequently promote the long-term consolidation of memory.SIGNIFICANCE STATEMENT The capacity to learn throughout life and at the same time guarantee lifelong storage and remote recall of established memories is a daily challenge. Emerging evidence suggests an important function of the extracellular matrix (ECM), a conglomerate of secreted proteins and polysaccharides in the adult vertebrate brain. We trained mice in an auditory long-term memory task and measured learning-related dynamic changes of the ECM protein brevican. Specifically, in the auditory cortex brevican is downregulated during initial learning and subsequently upregulated in exclusively those animals that have learned the task, suggesting a performance-dependent regulation in the service of memory consolidation and storage. Our data may provide novel therapeutic implications for several neuropsychiatric diseases involving dysregulation of the ECM.

Discrimination of frequency modulated sweeps by mice

Mice often produce ultrasonic vocalizations (USVs) that sweep upwards in frequency from around 60 to around 80 kHz and downwards in frequency from 80 to 60 kHz. Whether or not these USVs are used for communication purposes is still unknown. Here, mice were trained and tested using operant conditioning procedures and positive reinforcement to discriminate between synthetic upsweeps and downsweeps. The stimuli varied in bandwidth, duration, and direction of sweep. The mice performed significantly worse when discriminating between background and test stimuli when the stimuli all occupied the same bandwidths. Further, the mice's discrimination performance became much worse for stimuli that had durations similar to those natural vocalizations of the mice. Sweeps composed of different frequency ranges and longer durations had improved discrimination. These results collected using artificial stimuli created to mimic natural USVs indicate that the bandwidth of the vocalizations may be much more important for communication than the frequency contours of the vocalizations.

The Effect of Music on Cognitive Performance: Insight From Neurobiological and Animal Studies

The past 50 years have seen numerous claims that music exposure enhances human cognitive performance. Critical evaluation of studies across a variety of contexts, however, reveals important methodological weaknesses. The current article argues that an interdisciplinary approach is required to advance this research. A case is made for the use of appropriate animal models to avoid many confounds associated with human music research. Although such research has validity limitations for humans, reductionist methodology enables a more controlled exploration of music's elementary effects. This article also explores candidate mechanisms for this putative effect. A review of neurobiological evidence from human and comparative animal studies confirms that musical stimuli modify autonomic and neurochemical arousal indices, and may also modify synaptic plasticity. It is proposed that understanding how music affects animals provides a valuable conjunct to human research and may be vital in uncovering how music might be used to enhance cognitive performance.

Dopaminergic impact on local and global cortical circuit processing during learning

We have learned to detect, predict and behaviorally respond to important changes in our environment on short and longer time scales. Therefore, brains of humans and higher animals build upon a perceptual and semantic salience stored in their memories mainly generated by associative reinforcement learning. Functionally, the brain needs to extract and amplify a small number of features of sensory input with behavioral relevance to a particular situation in order to guide behavior. In this review, I argue that dopamine action, particularly in sensory cortex, orchestrates layer-dependent local and long-range cortical circuits integrating sensory associated bottom-up and semantically relevant top-down information, respectively. Available evidence reveals that dopamine thereby controls both the selection of perceptually or semantically salient signals as well as feedback processing from higher-order areas in the brain. Sensory cortical dopamine thereby governs the integration of selected sensory information within a behavioral context. This review proposes that dopamine enfolds this function by temporally distinct actions on particular layer-dependent local and global cortical circuits underlying the integration of sensory, and non-sensory cognitive and behavioral variables.

Combined Shuttle-Box Training with Electrophysiological Cortex Recording and Stimulation as a Tool to Study Perception and Learning

Shuttle-box avoidance learning is a well-established method in behavioral neuroscience and experimental setups were traditionally custom-made; the necessary equipment is now available by several commercial companies. This protocol provides a detailed description of a two-way shuttle-box avoidance learning paradigm in rodents (here Mongolian gerbils; Meriones unguiculatus) in combination with site-specific electrical intracortical microstimulation (ICMS) and simultaneous chronical electrophysiological in vivo recordings. The detailed protocol is applicable to study multiple aspects of learning behavior and perception in different rodent species. Site-specific ICMS of auditory cortical circuits as conditioned stimuli here is used as a tool to test the perceptual relevance of specific afferent, efferent and intracortical connections. Distinct activation patterns can be evoked by using different stimulation electrode arrays for local, layer-dependent ICMS or distant ICMS sites. Utilizing behavioral signal detection analysis it can be determined which stimulation strategy is most effective for eliciting a behaviorally detectable and salient signal. Further, parallel multichannel-recordings using different electrode designs (surface electrodes, depth electrodes, etc.) allow for investigating neuronal observables over the time course of such learning processes. It will be discussed how changes of the behavioral design can increase the cognitive complexity (e.g. detection, discrimination, reversal learning).

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.

Role of cortical neurodynamics for understanding the neural basis of motivated behavior - lessons from auditory category learning

Rhythmic activity appears in the auditory cortex in both microscopic and macroscopic observables and is modulated by both bottom-up and top-down processes. How this activity serves both types of processes is largely unknown. Here we review studies that have recently improved our understanding of potential functional roles of large-scale global dynamic activity patterns in auditory cortex. The experimental paradigm of auditory category learning allowed critical testing of the hypothesis that global auditory cortical activity states are associated with endogenous cognitive states mediating the meaning associated with an acoustic stimulus rather than with activity states that merely represent the stimulus for further processing.

Altered cortical spectrotemporal processing with age-related hearing loss

Presbycusis (age-related hearing loss) is a prevalent disability associated with aging that impairs spectrotemporal processing, but the mechanisms of such changes remain unclear. The goal of this study was to quantify cortical responses to frequency-modulated (FM) sweeps in a mouse model of presbycusis. Previous studies showed that cortical neurons in young mice are selective for the rate of frequency change in FM sweeps. Here single-unit data on cortical selectivity and response variability to FM sweeps of either direction and different rates (0.08–20 kHz/ms) were compared across young (1–3 mo), middle-aged (6–8 mo), and old (14–20 mo) groups. Three main findings are reported. First, there is a reduction in FM rate selectivity in the old group. Second, there is a slowing of the sweep rates at which neurons likely provide best detection and discrimination of sweep rates. Third, there is an increase in trial-to-trial variability in the magnitude and timing of spikes in response to sweeps. These changes were only observed in neurons that were selective for the fast or intermediate range of sweep rates and not in neurons that preferred slow sweeps or were nonselective. Increased variability of response magnitude, but not changes in temporal fidelity or selectivity, was seen even in the middle-aged group. The results show that spectrotemporal processing becomes slow and noisy with presbycusis in specific types of neurons, suggesting receptive field mechanisms that are altered. These data suggest neural correlates of presbycusis-related reduction in the ability of humans to process rapid spectrotemporal changes.

Representation of frequency-modulated sounds in the human brain

Frequency-modulation is a ubiquitous sound feature present in communicative sounds of various animal species and humans. Functional imaging of the human auditory system has seen remarkable advances in the last two decades and studies pertaining to frequency-modulation have centered around two major questions: a) are there dedicated feature-detectors encoding frequency-modulation in the brain and b) is there concurrent representation with amplitude-modulation, another temporal sound feature? In this review, we first describe how these two questions are motivated by psychophysical studies and neurophysiology in animal models. We then review how human non-invasive neuroimaging studies have furthered our understanding of the representation of frequency-modulated sounds in the brain. Finally, we conclude with some suggestions on how human neuroimaging could be used in future studies to address currently still open questions on this fundamental sound feature. This article is part of a Special Issue entitled Human Auditory Neuroimaging.

Left auditory cortex is involved in pairwise comparisons of the direction of frequency modulated tones

Evaluating series of complex sounds like those in speech and music requires sequential comparisons to extract task-relevant relations between subsequent sounds. With the present functional magnetic resonance imaging (fMRI) study, we investigated whether sequential comparison of a specific acoustic feature within pairs of tones leads to a change in lateralized processing in the auditory cortex (AC) of humans. For this we used the active categorization of the direction (up vs. down) of slow frequency modulated (FM) tones. Several studies suggest that this task is mainly processed in the right AC. These studies, however, tested only the categorization of the FM direction of each individual tone. In the present study we ask the question whether the right lateralized processing changes when, in addition, the FM direction is compared within pairs of successive tones. For this we use an experimental approach involving contralateral noise presentation in order to explore the contributions made by the left and right AC in the completion of the auditory task. This method has already been applied to confirm the right-lateralized processing of the FM direction of individual tones. In the present study, the subjects were required to perform, in addition, a sequential comparison of the FM direction in pairs of tones. The results suggest a division of labor between the two hemispheres such that the FM direction of each individual tone is mainly processed in the right AC whereas the sequential comparison of this feature between tones in a pair is probably performed in the left AC.

Electrical stimulation of lateral habenula during learning: frequency-dependent effects on acquisition but not retrieval of a two-way active avoidance response

The lateral habenula (LHb) is an epithalamic structure involved in signaling reward omission and aversive stimuli, and it inhibits dopaminergic neurons during motivated behavior. Less is known about LHb involvement in the acquisition and retrieval of avoidance learning. Our previous studies indicated that brief electrical stimulation of the LHb, time-locked to the avoidance of aversive footshock (presumably during the positive affective “relief” state that occurs when an aversive outcome is averted), inhibited the acquisition of avoidance learning. In the present study, we used the same paradigm to investigate different frequencies of LHb stimulation. The effect of 20 Hz vs. 50 Hz vs. 100 Hz stimulation was investigated during two phases, either during acquisition or retrieval in Mongolian gerbils. The results indicated that 50 Hz, but not 20 Hz, was sufficient to produce a long-term impairment in avoidance learning, and was somewhat more effective than 100 Hz in this regard. None of the stimulation parameters led to any effects on retrieval of avoidance learning, nor did they affect general motor activity. This suggests that, at frequencies in excess of the observed tonic firing rates of LHb neurons (>1–20 Hz), LHb stimulation may serve to interrupt the consolidation of new avoidance memories. However, these stimulation parameters are not capable of modifying avoidance memories that have already undergone extensive consolidation.

Response properties underlying selectivity for the rate of frequency modulated sweeps in the auditory cortex of the mouse

This study focused on the response properties underlying selectivity for the rate of frequency modulated (FM) sweeps in the auditory cortex of anesthetized C57bl/6 (C57) mice. Linear downward FM sweeps with rates between 0.08 and 20 kHz/ms were tested. We show that at least two different response properties predict FM rate selectivity: sideband inhibition and duration tuning. Sideband inhibition was determined using the two-tone inhibition paradigm in which excitatory and inhibitory tones were presented with different delays. Sideband inhibition was present in the majority (88%, n = 53) of neurons. The spectrotemporal properties of sideband inhibition predicted rate selectivity and exclusion of the sideband from the sweep reduced/eliminated rate tuning. The second property predictive of sweep rate selectivity was duration tuning for tones. Theoretically, if a neuron is selective for the duration that a sweep spends in the excitatory frequency tuning curve, then rate selectivity will ensue. Duration tuning for excitatory tones was present and predicted rate selectivity in ∼34% of neurons (n = 97). Both sideband inhibition and duration tuning predicted rate selectivity equally well, but sideband inhibition was present in a larger percentage of neurons suggesting that it is the dominant mechanism in the C57 mouse auditory cortex. Similar mechanisms shape sweep rate selectivity in the auditory system of bats and mice and movement-velocity selectivity in the visual system, suggesting similar solutions to analogous problems across sensory systems. This study provides baseline data on basic spectrotemporal processing in the C57 strain for elucidation of changes that occur in presbycusis.

Automated classification of mouse pup isolation syllables: from cluster analysis to an Excel-based "mouse pup syllable classification calculator"

Mouse pups vocalize at high rates when they are cold or isolated from the nest. The proportions of each syllable type produced carry information about disease state and are being used as behavioral markers for the internal state of animals. Manual classifications of these vocalizations identified 10 syllable types based on their spectro-temporal features. However, manual classification of mouse syllables is time consuming and vulnerable to experimenter bias. This study uses an automated cluster analysis to identify acoustically distinct syllable types produced by CBA/CaJ mouse pups, and then compares the results to prior manual classification methods. The cluster analysis identified two syllable types, based on their frequency bands, that have continuous frequency-time structure, and two syllable types featuring abrupt frequency transitions. Although cluster analysis computed fewer syllable types than manual classification, the clusters represented well the probability distributions of the acoustic features within syllables. These probability distributions indicate that some of the manually classified syllable types are not statistically distinct. The characteristics of the four classified clusters were used to generate a Microsoft Excel-based mouse syllable classifier that rapidly categorizes syllables, with over a 90% match, into the syllable types determined by cluster analysis.

Development of response selectivity in the mouse auditory cortex

The mouse auditory system contains neurons selective for tone duration and for a narrow range of frequency modulated (FM) sweep rates. Whether such selectivity is developmentally regulated is not known. The main goal of this study was to follow the development of neuronal responses to tones (frequency and duration tuning) and FM sweeps (direction and rate selectivity) in the core auditory cortex (A1 and AAF) of ketamine/xylazine anesthetized C57bl/6 mice. Three groups were compared: postnatal day (P) 15-20, P21-30 and P31-90. Frequency tuning bandwidth decreased during the first month indicating refinement of the excitatory receptive field. Duration tuning for tones did not change during development in terms of categories of tuning types as well as measures of selectivity such as best duration and half-maximal duration. FM rate and direction selectivity were developmentally regulated. Selectivity for linear up and down FM sweeps (0.06-22 kHz/ms) was tested. The best rate and half-maximal rate of neurons categorized as fast- or band-pass selective shifted toward faster rates during development. The percentage of fast-pass selective neurons also increased during development. These data suggest that cortical neurons' discrimination and detection abilities for relatively faster sweep rates improve during development. Although on average, direction selectivity was weak across development, there was a significant shift toward upward sweep selectivity at slow rates. Thus, the C57bl/6 mouse auditory cortex is not adult-like until at least P30. The changes in response selectivity can be explained based on known developmental changes in intrinsic and synaptic properties of mouse auditory cortical neurons.

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.

NEURODYNAMICS OF CATEGORY LEARNING: TOWARDS UNDERSTANDING THE CREATION OF MEANING IN THE BRAIN

Category learning, the formation and use of categories (equivalence classes of meaning), is an elemental function of cognition. We report our approach to study the physiological mechanisms underlying category learning using high-density multi-channel recordings of electrocorticograms in rodents. These data suggest the coexistence of separate coding principles for representing physical stimulus attributes ("stimulus representation") and subjectively relevant information (meaning) about stimuli, respectively. The implications of these findings for the construction of interactive cortical sensory neuroprostheses are discussed.

Effects of ventral tegmental area stimulation on the acquisition and long-term retention of active avoidance learning

The development of avoidance learning depends on dopamine release in forebrain regions. Previous studies indicated that rewarding brain stimulation facilitated two-way active avoidance learning. However, it is not clear whether the temporal relationship of brain stimulation to the training session (before, during or after) is important. To investigate the role of stimulation condition (no stimulation, self-stimulation only, or self-stimulation plus avoidance stimulation) and sequence of self-stimulation training (before or after avoidance training), we used a 3×2 factorial design, in which every level of stimulation was paired with every level of sequence for a total of 6 different groups. The results suggest that self-stimulation either before or after avoidance learning improved acquisition performance, but acquisition was maximal when stimulation was also given during acquisition trials. Importantly, the sequence of self-stimulation (before or after each acquisition session) was irrelevant to this beneficial effect. However, stimulation had no apparent effect on long-term retention when tested 10 days later under conditions of no stimulation, except that the performance of the group that had previously received avoidance-contingent stimulation deteriorated over the course of 60 trials. This may reflect frustration from the omission of expected reward. These results are relevant for optimizing brain stimulation to improve learning.

Fear conditioned discrimination of frequency modulated sweeps within species-specific calls of mustached bats

Social and echolocation vocalizations of bats contain different patterns of frequency modulations. An adult bat's ability to discriminate between various FM parameters, however, is not well established. Using changes in heart rate (HR) as a quantitative measure of associative learning, we demonstrate that mustached bats (Pteronotus parnellii) can be fear conditioned to linear frequency modulated (FM) sweeps typically centered at their acoustic fovea (approximately 60 kHz). We also show that HR is sensitive to a change in the direction of the conditional frequency modulation keeping all other parameters constant. In addition, a change in either depth or duration co-varied with FM rate is reflected in the change in HR. Finally, HR increases linearly with FM rate incremented by 0.1 kHz/ms from a pure tone to a target rate of 1.0 kHz/ms of the conditional stimulus. Learning is relatively rapid, occurring after a single training session. We also observed that fear conditioning enhances local field potential activity within the basolateral amygdala. Neural response enhancement coinciding with rapid learning and a fine scale cortical representation of FM sweeps shown earlier make FMs prime candidates for discriminating between different call types and possibly communicating socially relevant information within species-specific sounds.

Cortical encoding of pitch: recent results and open questions

It is widely appreciated that the key predictor of the pitch of a sound is its periodicity. Neural structures which support pitch perception must therefore be able to reflect the repetition rate of a sound, but this alone is not sufficient. Since pitch is a psychoacoustic property, a putative cortical code for pitch must also be able to account for the relationship between the amount to which a sound is periodic (i.e. its temporal regularity) and the perceived pitch salience, as well as limits in our ability to detect pitch changes or to discriminate rising from falling pitch. Pitch codes must also be robust in the presence of nuisance variables such as loudness or timbre. Here, we review a large body of work on the cortical basis of pitch perception, which illustrates that the distribution of cortical processes that give rise to pitch perception is likely to depend on both the acoustical features and functional relevance of a sound. While previous studies have greatly advanced our understanding, we highlight several open questions regarding the neural basis of pitch perception. These questions can begin to be addressed through a cooperation of investigative efforts across species and experimental techniques, and, critically, by examining the responses of single neurons in behaving animals.

Auditory discrimination learning and knowledge transfer in mice depends on task difficulty

Mice reproduce interesting effects in auditory discrimination learning and knowledge transfer discussed in human studies: (i) the advantage in the transfer from a hard to an easy task by benefits from transfer of procedural knowledge and information-integration learning, and (ii) the disadvantage in the transfer from easy to hard tasks by inability to generalize across perceptually different classes of stimuli together with initially unsuccessful attempts to transfer cognitive skills from one task to the other. House mice (NMRI strain) were trained in a shuttle-box stimulus discrimination task. They had to discriminate either between two pure tones of different frequencies (PT) or between two different modulation frequencies of an amplitude-modulated tone (AM). Then transfer of knowledge between these two tasks was tested. Mice rapidly learned PT discrimination within two to three training sessions (easy task). AM discrimination learning took longer and did not reach the high performance level of PT discrimination (hard task). No knowledge transfer was detected in animals first trained with the easy (PT) followed by the hard (AM) discrimination task. Mice benefited, however, from knowledge transfer when the AM discrimination was followed by the PT discrimination. When the task changed, confusion of conditioned stimuli occurred if the carrier frequency of the AM was the same as one of the frequencies in the PT task. These results show a hard-to-easy effect when possible knowledge transfer is tested between qualitatively different stimulus classes. The data establish mice as promising animal model for research on genetics of auditory perception and learning.

The combination of appetitive and aversive reinforcers and the nature of their interaction during auditory learning

Learned changes in behavior can be elicited by either appetitive or aversive reinforcers. It is, however, not clear whether the two types of motivation, (approaching appetitive stimuli and avoiding aversive stimuli) drive learning in the same or different ways, nor is their interaction understood in situations where the two types are combined in a single experiment. To investigate this question we have developed a novel learning paradigm for Mongolian gerbils, which not only allows rewards and punishments to be presented in isolation or in combination with each other, but also can use these opposite reinforcers to drive the same learned behavior. Specifically, we studied learning of tone-conditioned hurdle crossing in a shuttle box driven by either an appetitive reinforcer (brain stimulation reward) or an aversive reinforcer (electrical footshock), or by a combination of both. Combination of the two reinforcers potentiated speed of acquisition, led to maximum possible performance, and delayed extinction as compared to either reinforcer alone. Additional experiments, using partial reinforcement protocols and experiments in which one of the reinforcers was omitted after the animals had been previously trained with the combination of both reinforcers, indicated that appetitive and aversive reinforcers operated together but acted in different ways: in this particular experimental context, punishment appeared to be more effective for initial acquisition and reward more effective to maintain a high level of conditioned responses (CRs). The results imply that learning mechanisms in problem solving were maximally effective when the initial punishment of mistakes was combined with the subsequent rewarding of correct performance.

The neural bases underlying pitch processing difficulties

Normal listeners are often surprisingly poor at processing pitch changes. The neural bases of this difficulty were explored using magnetoencephalography (MEG) by comparing participants who obtained poor thresholds on a pitch-direction task with those who obtained good thresholds. Source-space projected data revealed that during an active listening task, the poor threshold group displayed greater activity in the left auditory cortical region when determining the direction of small pitch glides, whereas there was no difference in the good threshold group. In a passive listening task, a mismatch response (MMNm) was identified for pitch-glide direction deviants, with a tendency to be smaller in the poor listeners. The results imply that the difficulties in pitch processing are already apparent during automatic sound processing, and furthermore suggest that left hemisphere auditory regions are used by these listeners to consciously determine the direction of a pitch change. This is in line with evidence that the left hemisphere has a poor frequency resolution, and implies that normal listeners may use the sub-optimal hemisphere to process pitch changes.

Comparison of estimates for volumes of brain ablations derived from structural MRI and classical histology

Estimates of auditory cortex ablation sizes in a rodent model as derived from classical histology (volume reconstructions from Nissl-stained brain sections) and structural magnetic resonance imaging (MRI) (T1-weighted whole-brain scans from a 4.7 T animal scanner) were compared in the same specimens (Mongolian gerbils). Estimates of lesion volumes obtained with the two methods were very similar, robust, highly correlated and not significantly different from each other. Hence, the general usefulness of structural MRI for the determination of brain lesion size in small animal models is demonstrated. MRI therefore seems to be well suited to determine proper size and location of an experimental brain ablation prior to (potentially extensive) behavioral testing.

Discrimination of direction in fast frequency-modulated tones by rats

Fast frequency modulations (FM) are an essential part of species-specific auditory signals in animals as well as in human speech. Major parameters characterizing non-periodic frequency modulations are the direction of frequency change in the FM sweep (upward/downward) and the sweep speed, i.e., the speed of frequency change. While it is well established that both parameters are represented in the mammalian central auditory pathway, their importance at the perceptual level in animals is unclear. We determined the ability of rats to discriminate between upward and downward modulated FM-tones as a function of sweep speed in a two-alternative-forced-choice-paradigm. Directional discrimination in logarithmic FM-sweeps was reduced with increasing sweep speed between 20 and 1,000 octaves/s following a psychometric function. Average threshold sweep speed for FM directional discrimination was 96 octaves/s. This upper limit of perceptual FM discrimination fits well the upper limit of preferred sweep speeds in auditory neurons and the upper limit of neuronal direction selectivity in the rat auditory cortex and midbrain, as it is found in the literature. Influences of additional stimulus parameters on FM discrimination were determined using an adaptive testing-procedure for efficient threshold estimation based on a maximum likelihood approach. Directional discrimination improved with extended FM sweep range between two and five octaves. Discrimination performance declined with increasing lower frequency boundary of FM sweeps, showing an especially strong deterioration when the boundary was raised from 2 to 4 kHz. This deterioration corresponds to a frequency-dependent decline in direction selectivity of FM-encoding neurons in the rat auditory cortex, as described in the literature. Taken together, by investigating directional discrimination of FM sweeps in the rat we found characteristics at the perceptual level that can be related to several aspects of FM encoding in the central auditory pathway.

Auditory categorization of complex sounds by rats (Rattus norvegicus)

Little research has explored the auditory categorization abilities of mammals. To better understand these processes, the authors tested the abilities of rats (Rattus norvegicus) to categorize multidimensional acoustic stimuli by using a classic category-learning task developed by R. N. Shepard, C. I. Hovland, and H. M. Jenkins (1961). Rats proved to be able to categorize 8 complex sounds on the basis of either the direction or rate of frequency modulation but not on the basis of the range of frequency modulation. Rats' categorization abilities were limited but improved slowly and incrementally, suggesting that learning was not facilitated by selective attention to acoustic dimensions.

Effect of noise exposure on gap detection in rats

The effects of intense (110-120 dB) noise exposure (broadband noise for one hour) on temporal resolution was estimated in rats by measuring the behavioural gap detection threshold (GDT). Changes in GDT after 120 dB noise exposure were compared with changes in the threshold and amplitude of middle latency responses (MLR) recorded in response to tone stimuli. GDT values increased from 1.6 to 4.3 or 7.8 ms after exposure to 110 or 115 dB SPL, respectively; GDT recovered to pre-exposure values in 3-7 days. Three main types of noise-induced changes were observed after 120 dB SPL exposure: (I) GDT changes similar to those following noise exposure to 115 dB SPL and maximal hearing threshold shifts (TSs) at high frequencies of about 45 dB; (II) more pronounced changes in GDT (up to 60 ms) with maximal hearing threshold shifts of about 65 dB and (III) a lack of reliable responses to gap during the first weeks post-exposure with maximal hearing threshold shifts of about 80 dB. An increased GDT was present two months after noise exposure in animals with types II and III post-exposure changes; enhanced MLR amplitudes were also found in most of these in the first post-exposure week. The pronounced deficit in gap detection in some rats after 120 dB SPL noise exposure may signal the presence of a noise-induced tinnitus.

Macaque monkeys discriminate pitch relationships

This study demonstrates that non-human primates can categorize the direction of the pitch change of tones in a sequence. Two Macaca fascicularis were trained in a positive-reinforcement behavioral paradigm in which they listened to sequences of a variable number of different acoustic items. The training of discriminating pitch direction was divided into three phases with increasing task complexity. In the first two phases, subjects learned to employ a same/different rule. In phase 1, they discriminated acoustic items of different sound quality. Subjects had to respond when there was a change from repeating noise bursts to repeating click trains or vice versa. In phase II, acoustic items differed along one physical dimension only. Subjects had to respond to a change of the frequency of a repeating series of pure tones. In phase III, sequences consisted of three series of repeating tones of different frequency. Subjects were required to respond when the frequency of the tones changed in a downward direction and to refrain from responding when the frequency remained constant or increased. After several ten thousand trials, subjects categorized pitch direction well above chance level. The discrimination was performed over a 4.5-octave range of frequencies and was largely independent of the temporal and ordinal position of the downward pitch direction within the sequence. These results demonstrate that monkeys can recognize pitch relationships and thus that monkeys have the concept of ordinal relations between acoustic items.

Learning a new behavioral strategy in the shuttle-box increases prefrontal dopamine

Using microdialysis from medial prefrontal cortex of gerbils during aversive auditory conditioning in the shuttle-box we have previously shown a transient increase of dopamine efflux correlated with the establishment of avoidance behavior. We hypothesized that the acquisition of a new behavioral strategy is generally accompanied by this extra prefrontal dopamine release. The present experiment aimed at further testing this hypothesis. In a pre-training period in the shuttle-box the gerbils acquired an active avoidance response by generalizing two different tone signals to a GO-meaning (change of shuttle-box compartment). Thereafter, they were subjected in relearning sessions to differentially associate the known tone stimuli with GO- and NOGO- (no change of shuttle-box compartment) conditions, respectively. The following formation of discrimination behavior led to a similar extra dopamine increase as found during establishment of the avoidance strategy. This significant enhancement was limited to rapidly relearning individuals. Furthermore, the dopamine increase attenuated in these animals with increasing performance during the course of the discrimination training, similar to the retrieval stage of the avoidance strategy. Therefore, the dopamine system seems to be critically involved in the initial formation of associations for new behavioral strategies, i.e. learning. We assume that the prefrontal dopamine increase during initial learning of the complex discrimination behavior indicates an involvement of working memory principles and a goal-directed formation of a behavioral strategy.

Cats can detect repeated noise stimuli

We assessed the ability of cats to detect repeated noise (RN), a stimulus generated by seamlessly presenting short segments of white noise in a continuous loop, in a modified go-nogo task. A recent study of the gerbil suggested that animals might have an extremely limited ability to detect RN compared to human subjects. We find that cats can discriminate RN from continuous noise with reasonable accuracy until the period length of the RN sequence reaches 450-500 ms. This is slightly longer than the maximum detectable RN period length found in gerbils, but falls far short of human performance.

Auditory sensory memory for random waveforms in the Mongolian gerbil

Mongolian gerbils (N = 21) were trained to discriminate between continuous and repeated auditory white noise. While for periods up to 40 ms of the repeated noise spectral effects make this a perceptual task, longer periods require auditory sensory memory to solve the task. Short periods (20 ms) could easily be discriminated by naive gerbils (discrimination performance, i.e. hit rate minus false alarm rate >80% after 8 days of training). Discrimination was more difficult for longer periods (100 ms: discrimination performance approximately 50% after 18 days of training). By long-term training (156 days) using an optimized training paradigm two further gerbils learned to discriminate up to a period length of 360 ms but could not proceed at 400 ms. While this falls short of human performance, it demonstrates for the first time sensory memory for random waveforms in animals.

An estimate of the auditory-filter bandwidth in the Mongolian gerbil

Critical ratios (CRs) and the critical band (CB) were determined in six Mongolian gerbils (Meriones unguiculatus) using a GO/NOGO procedure and the method of constant stimuli. The test-tone frequencies were 0.5, 1, 2, 4 and 8 kHz in the CR measurements and 2 kHz in the CB measurement. Critical ratios were independent of the level of the white-noise masker. The lowest CR with a mean of 24.8 dB was found at 1 kHz for maskers with spectrum levels of 20 and 40 dB SPL. The CR increased on average by 2.1 dB per octave between the frequencies 1 and 4 kHz. The CR bandwidth at 2 kHz and at a masker spectrum level of 40 dB SPL was 417 Hz. Using the band-narrowing procedure, we determined a mean CB of 216 Hz at a test-tone frequency of 2 kHz. The results are discussed in comparison to psychophysical data from other rodent species and humans. Finally, the relation of CR and CB bandwidths to the bandwidth of tuning curves of gerbil auditory-nerve fibers and the gerbil's cochlear frequency map are explored.

Change in pattern of ongoing cortical activity with auditory category learning

Humans are able to classify novel items correctly by category; some other animals have also been shown to do this. During category learning, humans group perceptual stimuli by abstracting qualities from similarity relationships of their physical properties. Forming categories is fundamental to cognition and can be independent of a 'memory store' of information about the items or a prototype. The neurophysiological mechanisms underlying the formation of categories are unknown. Using an animal model of category learning, in which frequency-modulated tones are distinguished into the categories of 'rising' and 'falling' modulation, we demonstrate here that the sorting of stimuli into these categories emerges as a sudden change in an animal's learning strategy. Electro-corticographical recording from the auditory cortex shows that the transition is accompanied by a change in the dynamics of cortical stimulus representation. We suggest that this dynamic change represents a mechanism underlying the recognition of the abstract quality (or qualities) that defines the categories.

Spatial representation of frequency-modulated tones in gerbil auditory cortex revealed by epidural electrocorticography

The present study investigated the topography of epidurally recorded middle latency components P1 and N1 evoked by spectrally dynamic stimuli (linearly frequency-modulated (FM) tones) with respect to the tonotopic structure of the right primary auditory cortex, field AI. Whereas the gross topography corresponded to the spectral content of the FM tones, specific tonotopic offsets were found between the potential distributions evoked by FM tones of different modulation direction (i.e. 'rising' vs. 'falling'). Potentials evoked by rising FM tones were located at tonotopic positions corresponding to higher frequencies compared with potentials evoked by falling FM tones. Data indicated that the magnitude of these offsets can be attributed to the local tonotopic resolution in AI and are not dependent on the modulation rate.

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.

Right auditory cortex lesion in Mongolian gerbils impairs discrimination of rising and falling frequency-modulated tones

Mongolian gerbils (Meriones unguiculatus) were trained in a shuttle box to discriminate the direction in frequency-modulated tones (FM). Whereas control animals easily acquired FM discrimination, animals with auditory cortex lesion on the right side showed considerable difficulties in learning this task. The discrimination performance of gerbils with left auditory cortex lesion, however, was not different from controls. This study, suggesting that the right auditory cortex plays a dominant role in FM discrimination learning in gerbils, describes a useful animal model for investigation of the basic mechanisms underlying hemispheric asymmetries in auditory perception.