Role of mammalian auditory cortex in the perception of elementary sound properties

Contributions of cortical neuron firing patterns, synaptic connectivity, and plasticity to task performance

Neuronal responses during behavior are diverse, ranging from highly reliable 'classical' responses to irregular 'non-classically responsive' firing. While a continuum of response properties is observed across neural systems, little is known about the synaptic origins and contributions of diverse responses to network function, perception, and behavior. To capture the heterogeneous responses measured from auditory cortex of rodents performing a frequency recognition task, we use a novel task-performing spiking recurrent neural network incorporating spike-timing-dependent plasticity. Reliable and irregular units contribute differentially to task performance via output and recurrent connections, respectively. Excitatory plasticity shifts the response distribution while inhibition constrains its diversity. Together both improve task performance with full network engagement. The same local patterns of synaptic inputs predict spiking response properties of network units and auditory cortical neurons from in vivo whole-cell recordings during behavior. Thus, diverse neural responses contribute to network function and emerge from synaptic plasticity rules.

Differential representation of sensory information and behavioral choice across layers of the mouse auditory cortex

The activity of neurons in sensory areas sometimes covaries with upcoming choices in decision-making tasks. However, the prevalence, causal origin, and functional role of choice-related activity remain controversial. Understanding the circuit-logic of decision signals in sensory areas will require understanding their laminar specificity, but simultaneous recordings of neural activity across the cortical layers in forced-choice discrimination tasks have not yet been performed. Here, we describe neural activity from such recordings in the auditory cortex of mice during a frequency discrimination task with delayed report, which, as we show, requires the auditory cortex. Stimulus-related information was widely distributed across layers but disappeared very quickly after stimulus offset. Choice selectivity emerged toward the end of the delay period-suggesting a top-down origin-but only in the deep layers. Early stimulus-selective and late choice-selective deep neural ensembles were correlated, suggesting that the choice-selective signal fed back to the auditory cortex is not just action specific but develops as a consequence of the sensory-motor contingency imposed by the task.

Dynamics of cortical contrast adaptation predict perception of signals in noise

Neurons throughout the sensory pathway adapt their responses depending on the statistical structure of the sensory environment. Contrast gain control is a form of adaptation in the auditory cortex, but it is unclear whether the dynamics of gain control reflect efficient adaptation, and whether they shape behavioral perception. Here, we trained mice to detect a target presented in background noise shortly after a change in the contrast of the background. The observed changes in cortical gain and behavioral detection followed the dynamics of a normative model of efficient contrast gain control; specifically, target detection and sensitivity improved slowly in low contrast, but degraded rapidly in high contrast. Auditory cortex was required for this task, and cortical responses were not only similarly affected by contrast but predicted variability in behavioral performance. Combined, our results demonstrate that dynamic gain adaptation supports efficient coding in auditory cortex and predicts the perception of sounds in noise.

Altered population activity and local tuning heterogeneity in auditory cortex of Cacna2d3-deficient mice

The α2δ3 auxiliary subunit of voltage-activated calcium channels is required for normal synaptic transmission and precise temporal processing of sounds in the auditory brainstem. In mice its loss additionally leads to an inability to distinguish amplitude-modulated tones. Furthermore, loss of function of α2δ3 has been associated with autism spectrum disorder in humans. To investigate possible alterations of network activity in the higher-order auditory system in α2δ3 knockout mice, we analyzed neuronal activity patterns and topography of frequency tuning within networks of the auditory cortex (AC) using two-photon Ca2+ imaging. Compared to wild-type mice we found distinct subfield-specific alterations in the primary auditory cortex, expressed in overall lower correlations between the network activity patterns in response to different sounds as well as lower reliability of these patterns upon repetitions of the same sound. Higher AC subfields did not display these alterations but showed a higher amount of well-tuned neurons along with lower local heterogeneity of the neurons' frequency tuning. Our results provide new insight into AC network activity alterations in an autism spectrum disorder-associated mouse model.

Augmentation of learning in schizophrenia by D-serine is related to auditory and frontally-generated biomarkers: A randomized, double-blind, placebo-controlled study

Auditory cognition is impaired in schizophrenia, and typically engages a complex, distributed, hierarchical network, including both auditory and frontal input. We recently demonstrated proof of principle for the target engagement of an N-methyl-D-aspartate-type glutamate receptor (NMDAR) agonist + auditory targeted remediation (d-serine+AudRem) combination, showing significant improvement in auditory-learning induced plasticity and mismatch negativity. In this secondary analysis, we report on frontal EEG outcomes, assessing for both generalized effects and the mechanism of auditory plasticity. 21 schizophrenia or schizoaffective disorder participants were randomized to three 1x weekly AudRem + double-blind d-serine (100 mg/kg) visits. In AudRem, participants indicated which paired tone was higher in pitch. The focus of this secondary analysis was a frontally (premotor) mediated EEG outcome- event-related desynchronization in the b band (b-ERD), which was shown to be sensitive to AudRem in previous studies. d-Serine+AudRem led to significant improvement in b-ERD power across the retention and motor preparation intervals (F 1,18 =6.0, p=0.025) vs. AudRem alone. b-ERD was significantly related to baseline cognition, but not auditory-learning induced plasticity. The principal finding of this prespecified secondary analysis are that in addition to improving auditory based biomarkers, the d-serine+AudRem combination led to significant improvement in biomarkers thought to represent frontally mediated dysfunction, suggesting potential generalization of effects. Changes in auditory-learning induced plasticity were independent of these frontally mediated biomarkers. Ongoing work will assess whether d-serine+AudRem is sufficient to remediate cognition or whether targeting frontal NMDAR deficits with higher-level remediation may also be required. Trial Registration: NCT03711500.

Neuronal circuitry for recognition memory of object and place in rodent models

Rats and mice are used for studying neuronal circuits underlying recognition memory due to their ability to spontaneously remember the occurrence of an object, its place and an association of the object and place in a particular environment. A joint employment of lesions, pharmacological interventions, optogenetics and chemogenetics is constantly expanding our knowledge of the neural basis for recognition memory of object, place, and their association. In this review, we summarize current studies on recognition memory in rodents with a focus on the novel object preference, novel location preference and object-in-place paradigms. The evidence suggests that the medial prefrontal cortex- and hippocampus-connected circuits contribute to recognition memory for object and place. Under certain conditions, the striatum, medial septum, amygdala, locus coeruleus and cerebellum are also involved. We propose that the neuronal circuitry for recognition memory of object and place is hierarchically connected and constructed by different cortical (perirhinal, entorhinal and retrosplenial cortices), thalamic (nucleus reuniens, mediodorsal and anterior thalamic nuclei) and primeval (hypothalamus and interpeduncular nucleus) modules interacting with the medial prefrontal cortex and hippocampus.

Emerging experience-dependent dynamics in primary somatosensory cortex reflect behavioral adaptation

Behavioral experience and flexibility are crucial for survival in a constantly changing environment. Despite evolutionary pressures to develop adaptive behavioral strategies in a dynamically changing sensory landscape, the underlying neural correlates have not been well explored. Here, we use genetically encoded voltage imaging to measure signals in primary somatosensory cortex (S1) during sensory learning and behavioral adaptation in the mouse. In response to changing stimulus statistics, mice adopt a strategy that modifies their detection behavior in a context dependent manner as to maintain reward expectation. Surprisingly, neuronal activity in S1 shifts from simply representing stimulus properties to transducing signals necessary for adaptive behavior in an experience dependent manner. Our results suggest that neuronal signals in S1 are part of an adaptive framework that facilitates flexible behavior as individuals gain experience, which could be part of a general scheme that dynamically distributes the neural correlates of behavior during learning.

Waiblinger et al. investigate the role of primary sensory cortex in flexible behaviors. They show that neuronal signals in S1 are part of an adaptive and dynamic framework that facilitates flexible behavior as an individual gains experience, indicating a role for S1 in long-term adaptive strategies.

What can we learn from inactivation studies? Lessons from auditory cortex

Inactivation experiments in auditory cortex (AC) produce widely varying results that complicate interpretations regarding the precise role of AC in auditory perception and ensuing behaviour. The advent of optogenetic methods in neuroscience offers previously unachievable insight into the mechanisms transforming brain activity into behaviour. With a view to aiding the design and interpretation of future studies in and outside AC, here we discuss the methodological challenges faced in manipulating neural activity. While considering AC’s role in auditory behaviour through the prism of inactivation experiments, we consider the factors that confound the interpretation of the effects of inactivation on behaviour, including the species, the type of inactivation, the behavioural task employed, and the exact location of the inactivation.

Wide variation in the outcome of auditory cortex inactivation has been an impediment to clear conclusions regarding the roles of the auditory cortex in behaviour.

Inactivation methods differ in their efficacy and specificity. The likelihood of observing a behavioural deficit is additionally influenced by factors such as the species being used, task design and reward.

A synthesis of previous results suggests that auditory cortex involvement is critical for tasks that require integrating across multiple stimulus features, and less likely to be critical for simple feature discriminations.

New methods of neural silencing provide opportunities for spatially and temporally precise manipulation of activity, allowing perturbation of individual subfields and specific circuits.

Bone conducted responses in the neonatal rat auditory cortex

Rats are born deaf and start hearing at the end of the second postnatal week, when the ear canals open and low-intensity sounds start to evoke responses in the auditory cortex. Here, using μECoG electrode arrays and intracortical silicon probe recordings, we found that bone-conducted (BC) sounds evoked biphasic responses in the auditory cortex starting from postnatal day (P) 8. The initial phase of these responses, generated by thalamocortical input, was followed by intracortical propagation within supragranular layers. BC-evoked responses co-localized with the responses evoked by electrical stimulation of the cochlea and the deepest layers of the inferior colliculus prior to onset of low-threshold hearing (P13), as well as with the responses evoked by high-frequency (30 kHz) low-intensity (70 dB) air-conducted sounds after that. Thus, BC signals reach high-frequency processing regions of the auditory cortex well before the onset of low-threshold hearing, reflecting early integrity of the auditory system.

Learning speed and detection sensitivity controlled by distinct cortico-fugal neurons in visual cortex

Vertebrates can change their behavior upon detection of visual stimuli according to the outcome their actions produce. Such goal-directed behavior involves evolutionary conserved brain structures like the striatum and optic tectum, which receive ascending visual input from the periphery. In mammals, however, these structures also receive descending visual input from visual cortex (VC), via neurons that give rise to cortico-fugal projections. The function of cortico-fugal neurons in visually guided, goal-directed behavior remains unclear. Here, we address the impact of two populations of cortico-fugal neurons in mouse VC in the learning and performance of a visual detection task. We show that the ablation of striatal projecting neurons reduces learning speed, whereas the ablation of superior colliculus projecting neurons does not impact learning but reduces detection sensitivity. This functional dissociation between distinct cortico-fugal neurons in controlling learning speed and detection sensitivity suggests an adaptive contribution of cortico-fugal pathways even in simple goal-directed behavior.

Sensory and Behavioral Components of Neocortical Signal Flow in Discrimination Tasks with Short-Term Memory

In the neocortex, each sensory modality engages distinct sensory areas that route information to association areas. Where signal flow converges for maintaining information in short-term memory and how behavior may influence signal routing remain open questions. Using wide-field calcium imaging, we compared cortex-wide neuronal activity in layer 2/3 for mice trained in auditory and tactile tasks with delayed response. In both tasks, mice were either active or passive during stimulus presentation, moving their body or sitting quietly. Irrespective of behavioral strategy, auditory and tactile stimulation activated distinct subdivisions of the posterior parietal cortex, anterior area A and rostrolateral area RL, which held stimulus-related information necessary for the respective tasks. In the delay period, in contrast, behavioral strategy rather than sensory modality determined short-term memory location, with activity converging frontomedially in active trials and posterolaterally in passive trials. Our results suggest behavior-dependent routing of sensory-driven cortical signals flow from modality-specific posterior parietal cortex (PPC) subdivisions to higher association areas.

Auditory cortex reflects goal-directed movement but is not necessary for behavioral adaptation in sound-cued reward tracking

Mounting evidence suggests that the role of sensory cortices in perceptual decision making goes beyond the mere representation of the discriminative stimuli and additionally involves the representation of nonsensory variables such as reward expectation. However, the relevance of these representations for behavior is not clear. To address this issue, we trained rats to discriminate sounds in a single-interval forced-choice task and then confronted the animals with unsignaled blockwise changes of reward probabilities. We found that unequal reward probabilities for the two choice options led to substantial shifts in response bias without concomitant reduction in stimulus discrimination. Although decisional biases were on average less extreme than required to maximize overall reinforcement, a model-based analysis revealed that rats managed to harvest >97% of rewards. Neurons in auditory cortex recorded during task performance weakly differentiated the discriminative stimuli but more strongly the subsequent goal-directed movement. Although 10-20% of units exhibited significantly different firing rates between task epochs with different response biases, control experiments showed this to result from inflated false positive rates due to unspecific temporal correlations of spiking activity rather than changing reinforcement contingencies. Transient pharmacological inactivation of auditory cortex reduced sound discriminability without affecting other measures of performance, whereas inactivation of medial prefrontal cortex affected both discriminability and bias. Together, these results suggest that auditory cortex activity only weakly reflects decisional variables during flexible updating of stimulus-response-outcome contingencies and does not play a crucial role in sound-cued adaptive behavior, beyond the representation of the discriminative stimuli.NEW & NOTEWORTHY Recent evidence suggests that sensory cortex represents nonsensory variables such as reward expectation, but the relevance of these representations for behavior is not well understood. We show that rat auditory cortex (AC) is modulated during movement and reward anticipation in a sound-cued reward tracking task, whereas AC inactivation only impaired discrimination without affecting reward tracking, consistent with a predominantly sensory role of AC.

A Critical Role for Neocortical Processing of Threat Memory

Memory of cues associated with threat is critical for survival and a leading model for elucidating how sensory information is linked to adaptive behavior by learning. Although the brain-wide circuits mediating auditory threat memory have been intensely investigated, it remains unclear whether the auditory cortex is critically involved. Here we use optogenetic activity manipulations in defined cortical areas and output pathways, viral tracing, pathway-specific in vivo 2-photon calcium imaging, and computational analyses of population plasticity to reveal that the auditory cortex is selectively required for conditioning to complex stimuli, whereas the adjacent temporal association cortex controls all forms of auditory threat memory. More temporal areas have a stronger effect on memory and more neurons projecting to the lateral amygdala, which control memory to complex stimuli through a balanced form of population plasticity that selectively supports discrimination of significant sensory stimuli. Thus, neocortical processing plays a critical role in cued threat memory.

Targeted Cortical Manipulation of Auditory Perception

Driving perception by direct activation of neural ensembles in cortex is a necessary step for achieving a causal understanding of the neural code for auditory perception and developing central sensory rehabilitation methods. Here, using optogenetic manipulations during an auditory discrimination task in mice, we show that auditory cortex can be short-circuited by coarser pathways for simple sound identification. Yet when the sensory decision becomes more complex, involving temporal integration of information, auditory cortex activity is required for sound discrimination and targeted activation of specific cortical ensembles changes perceptual decisions, as predicted by our readout of the cortical code. Hence, auditory cortex representations contribute to sound discriminations by refining decisions from parallel routes.

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Auditory cortex is dispensable for discrimination of dissimilar pure tones in mice

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Auditory cortex is involved in a sound discrimination requiring temporal integration

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Focal cortical activations bias choices in cortex-dependent discriminations

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Discrimination is faster for pure tones than for optogenetic cortical activations

Auditory Cortex Contributes to Discrimination of Pure Tones

The auditory cortex is topographically organized for sound frequency and contains highly selective frequency-tuned neurons, yet the role of auditory cortex in the perception of sound frequency remains unclear. Lesion studies have shown that auditory cortex is not essential for frequency discrimination of pure tones. However, transient pharmacological inactivation has been reported to impair frequency discrimination. This suggests the possibility that successful tone discrimination after recovery from lesion surgery could arise from long-term reorganization or plasticity of compensatory pathways. Here, we compared the effects of lesions and optogenetic suppression of auditory cortex on frequency discrimination in mice. We found that transient bilateral optogenetic suppression partially but significantly impaired discrimination performance. In contrast, bilateral electrolytic lesions of auditory cortex had no effect on performance of the identical task, even when tested only 4 h after lesion. This suggests that when auditory cortex is destroyed, an alternative pathway is almost immediately adequate for mediating frequency discrimination. Yet this alternative pathway is insufficient for task performance when auditory cortex is intact but has its activity suppressed. These results indicate a fundamental difference between the effects of brain lesions and optogenetic suppression, and suggest the existence of a rapid compensatory process possibly induced by injury.

Sensory-to-Category Transformation via Dynamic Reorganization of Ensemble Structures in Mouse Auditory Cortex

The ability to group physical stimuli into behaviorally relevant categories is fundamental to perception and cognition. Despite a large body of work on stimulus categorization at the behavioral and cognitive levels, little is known about the underlying mechanisms at the neuronal level. Here, combining mouse auditory psychophysical behavior and in vivo two-photon imaging from the auditory cortex, we investigate how sensory-to-category transformation is implemented by cortical neurons during a stimulus categorization task. Distinct from responses during passive listening, many neurons exhibited emergent selectivity to stimuli near the category boundary during task performance, reshaping local tuning maps; other neurons became more selective to category membership of stimuli. At the population level, local cortical ensembles robustly encode category information and predict trial-by-trial decisions during task performance. Our data uncover a task-dependent dynamic reorganization of cortical response patterns serving as a neural mechanism for sensory-to-category transformation during perceptual decision-making.

Corticocortical Systems Underlying High-Order Motor Control

Cortical networks are characterized by the origin, destination, and reciprocity of their connections, as well as by the diameter, conduction velocity, and synaptic efficacy of their axons. The network formed by parietal and frontal areas lies at the core of cognitive-motor control because the outflow of parietofrontal signaling is conveyed to the subcortical centers and spinal cord through different parallel pathways, whose orchestration determines, not only when and how movements will be generated, but also the nature of forthcoming actions. Despite intensive studies over the last 50 years, the role of corticocortical connections in motor control and the principles whereby selected cortical networks are recruited by different task demands remain elusive. Furthermore, the synaptic integration of different cortical signals, their modulation by transthalamic loops, and the effects of conduction delays remain challenging questions that must be tackled to understand the dynamical aspects of parietofrontal operations. In this article, we evaluate results from nonhuman primate and selected rodent experiments to offer a viewpoint on how corticocortical systems contribute to learning and producing skilled actions. Addressing this subject is not only of scientific interest but also essential for interpreting the devastating consequences for motor control of lesions at different nodes of this integrated circuit. In humans, the study of corticocortical motor networks is currently based on MRI-related methods, such as resting-state connectivity and diffusion tract-tracing, which both need to be contrasted with histological studies in nonhuman primates.

Larger Auditory Cortical Area and Broader Frequency Tuning Underlie Absolute Pitch

Absolute pitch (AP), the ability of some musicians to precisely identify and name musical tones in isolation, is associated with a number of gross morphological changes in the brain, but the fundamental neural mechanisms underlying this ability have not been clear. We presented a series of logarithmic frequency sweeps to age- and sex-matched groups of musicians with or without AP and controls without musical training. We used fMRI and population receptive field (pRF) modeling to measure the responses in the auditory cortex in 61 human subjects. The tuning response of each fMRI voxel was characterized as Gaussian, with independent center frequency and bandwidth parameters. We identified three distinct tonotopic maps, corresponding to primary (A1), rostral (R), and rostral-temporal (RT) regions of auditory cortex. We initially hypothesized that AP abilities might manifest in sharper tuning in the auditory cortex. However, we observed that AP subjects had larger cortical area, with the increased area primarily devoted to broader frequency tuning. We observed anatomically that A1, R and RT were significantly larger in AP musicians than in non-AP musicians or control subjects, which did not differ significantly from each other. The increased cortical area in AP in areas A1 and R were primarily low frequency and broadly tuned, whereas the distribution of responses in area RT did not differ significantly. We conclude that AP abilities are associated with increased early auditory cortical area devoted to broad-frequency tuning and likely exploit increased ensemble encoding.SIGNIFICANCE STATEMENT Absolute pitch (AP), the ability of some musicians to precisely identify and name musical tones in isolation, is associated with a number of gross morphological changes in the brain, but the fundamental neural mechanisms have not been clear. Our study shows that AP musicians have significantly larger volume in early auditory cortex than non-AP musicians and non-musician controls and that this increased volume is primarily devoted to broad-frequency tuning. We conclude that AP musicians are likely able to exploit increased ensemble representations to encode and identify frequency.

Sensation, movement and learning in the absence of barrel cortex

For many of our senses, the role of the cerebral cortex in detecting stimuli is controversial^1–17. Here, we examine the effects of both acute and chronic inactivation of primary somatosensory cortex (S1) in mice trained to move their large facial whiskers to detect an object via touch and respond with a lever to obtain a water reward. Using transgenic animals, we expressed inhibitory opsins in excitatory cortical neurons. Transient optogenetic inactivation of S1, as well as permanent lesions, initially produced both movement and sensory deficits that impaired detection behavior, demonstrating the inextricable link between sensory and motor systems during active sensing. Surprisingly, lesioned mice rapidly recovered full behavioral capabilities by the subsequent session. Recovery was experience-dependent, and early re-exposure to the task after lesion facilitated recovery. Furthermore, primary sensory cortex ablation prior to learning did not affect task acquisition. This combined optogenetic and lesion approach suggests that manipulations of sensory cortex may be only temporarily disruptive to other brain structures, which are themselves capable of coordinating multiple, arbitrary movements with sensation. Thus, the somatosensory cortex may be dispensable for active detection of objects in the environment.

Stereotactically-guided Ablation of the Rat Auditory Cortex, and Localization of the Lesion in the Brain

The rat auditory cortex (AC) is becoming popular among auditory neuroscience investigators who are interested in experience-dependence plasticity, auditory perceptual processes, and cortical control of sound processing in the subcortical auditory nuclei. To address new challenges, a procedure to accurately locate and surgically expose the auditory cortex would expedite this research effort. Stereotactic neurosurgery is routinely used in pre-clinical research in animal models to engraft a needle or electrode at a pre-defined location within the auditory cortex. In the following protocol, we use stereotactic methods in a novel way. We identify four coordinate points over the surface of the temporal bone of the rat to define a window that, once opened, accurately exposes both the primary (A1) and secondary (Dorsal and Ventral) cortices of the AC. Using this method, we then perform a surgical ablation of the AC. After such a manipulation is performed, it is necessary to assess the localization, size, and extension of the lesions made in the cortex. Thus, we also describe a method to easily locate the AC ablation postmortem using a coordinate map constructed by transferring the cytoarchitectural limits of the AC to the surface of the brain.The combination of the stereotactically-guided location and ablation of the AC with the localization of the injured area in a coordinate map postmortem facilitates the validation of information obtained from the animal, and leads to a better analysis and comprehension of the data.

A Role for Auditory Corticothalamic Feedback in the Perception of Complex Sounds

Feedback signals from the primary auditory cortex (A1) can shape the receptive field properties of neurons in the ventral division of the medial geniculate body (MGBv). However, the behavioral significance of corticothalamic modulation is unknown. The aim of this study was to elucidate the role of this descending pathway in the perception of complex sounds. We tested the ability of adult female ferrets to detect the presence of a mistuned harmonic in a complex tone using a positive conditioned go/no-go behavioral paradigm before and after the input from layer VI in A1 to MGBv was bilaterally and selectively eliminated using chromophore-targeted laser photolysis. MGBv neurons were identified by their short latencies and sharp tuning curves. They responded robustly to harmonic complex tones and exhibited an increase in firing rate and temporal pattern changes when one frequency component in the complex tone was mistuned. Injections of fluorescent microbeads conjugated with a light-sensitive chromophore were made in MGBv, and, following retrograde transport to the cortical cell bodies, apoptosis was induced by infrared laser illumination of A1. This resulted in a selective loss of ∼60% of layer VI A1-MGBv neurons. After the lesion, mistuning detection was impaired, as indicated by decreased d' values, a shift of the psychometric curves toward higher mistuning values, and increased thresholds, whereas discrimination performance was unaffected when level cues were also available. Our results suggest that A1-MGBv corticothalamic feedback contributes to the detection of harmonicity, one of the most important grouping cues in the perception of complex sounds.SIGNIFICANCE STATEMENT Perception of a complex auditory scene is based on the ability of the brain to group those sound components that belong to the same source and to segregate them from those belonging to different sources. Because two people talking simultaneously may differ in their voice pitch, perceiving the harmonic structure of sounds is very important for auditory scene analysis. Here we demonstrate mistuning sensitivity in the thalamus and that feedback from the primary auditory cortex is required for the normal ability of ferrets to detect a mistuned harmonic within a complex sound. These results provide novel insight into the function of descending sensory pathways in the brain and suggest that this corticothalamic circuit plays an important role in scene analysis.

Effect of chronic restraint stress on inhibitory gating in the auditory cortex of rats

A fundamental adaptive mechanism of auditory function is inhibitory gating (IG), which refers to the attenuation of neural responses to repeated sound stimuli. IG is drastically impaired in individuals with emotional and cognitive impairments (i.e. posttraumatic stress disorder). The objective of this study was to test whether chronic stress impairs the IG of the auditory cortex (AC). We used the standard two-tone stimulus paradigm and examined the parametric qualities of IG in the AC of rats by recording the electrophysiological signals of a single-unit and local field potential (LFP) simultaneously. The main results of this study were that most of the AC neurons showed a weaker response to the second tone than to the first tone, reflecting an IG of the repeated input. A fast negative wave of LFP showed consistent IG across the sampled AC sites, whereas a slow positive wave of LFP had less IG effect. IG was diminished following chronic restraint stress at both, the single-unit and LFP level, due to the increase in response to the second tone. This study provided new evidence that chronic stress disrupts the physiological function of the AC. Lay Summary The effects of chronic stress on IG were investigated by recording both, single-unit spike and LFP activities, in the AC of rats. In normal rats, most of the single-unit and N25 LFP activities in the AC showed an IG effect. IG was diminished following chronic restraint stress at both, the single-unit and LFP level.

Spectral breadth and laminar distribution of thalamocortical inputs to A1

The GABAergic agonist muscimol is used to inactivate brain regions in order to reveal afferent inputs in isolation. However, muscimol's use in primary auditory cortex (A1) has been questioned on the grounds that it may unintentionally suppress thalamocortical inputs. We tested whether muscimol can preferentially suppress cortical, but not thalamocortical, circuits in urethane-anesthetized mice. We recorded tone-evoked current source density profiles to determine frequency receptive fields (RFs) for three current sinks: the "layer 4" sink (fastest onset, middle-layer sink) and current sinks 100 μm above ("layer 2/3") and 300 μm below ("layer 5/6") the main input. We first determined effects of muscimol dose (0.01-1 mM) on the characteristic frequency (CF) tone-evoked layer 4 sink. An "ideal" dose (100 μM) had no effect on CF-evoked sink onset latency or initial response but reduced peak amplitude by >80%, implying inhibition of intracortical, but not thalamocortical, activity. We extended the analysis to current sinks in layers 2/3 and 5/6 and for all three sinks determined RF breadth (quarter-octave steps, 20 dB above CF threshold). Muscimol reduced RF breadth 42% in layer 2/3 (from 2.4 ± 0.14 to 1.4 ± 0.11 octaves), 14% in layer 4 (2.2 ± 0.12 to 1.9 ± 0.10 octaves), and not at all in layer 5/6 (1.8 ± 0.10 to 1.7 ± 0.12 octaves). The results provide an estimate of the laminar and spectral extent of thalamocortical projections and support the hypothesis that intracortical pathways contribute to spectral integration in A1.

Acute off-target effects of neural circuit manipulations

Rapid and reversible manipulations of neural activity in behaving animals are transforming our understanding of brain function. An important assumption underlying much of this work is that evoked behavioural changes reflect the function of the manipulated circuits. We show that this assumption is problematic because it disregards indirect effects on the independent functions of downstream circuits. Transient inactivations of motor cortex in rats and nucleus interface (Nif) in songbirds severely degraded task-specific movement patterns and courtship songs, respectively, which are learned skills that recover spontaneously after permanent lesions of the same areas. We resolve this discrepancy in songbirds, showing that Nif silencing acutely affects the function of HVC, a downstream song control nucleus. Paralleling song recovery, the off-target effects resolved within days of Nif lesions, a recovery consistent with homeostatic regulation of neural activity in HVC. These results have implications for interpreting transient circuit manipulations and for understanding recovery after brain lesions.

Bidirectional Regulation of Innate and Learned Behaviors That Rely on Frequency Discrimination by Cortical Inhibitory Neurons

The ability to discriminate tones of different frequencies is fundamentally important for everyday hearing. While neurons in the primary auditory cortex (AC) respond differentially to tones of different frequencies, whether and how AC regulates auditory behaviors that rely on frequency discrimination remains poorly understood. Here, we find that the level of activity of inhibitory neurons in AC controls frequency specificity in innate and learned auditory behaviors that rely on frequency discrimination. Photoactivation of parvalbumin-positive interneurons (PVs) improved the ability of the mouse to detect a shift in tone frequency, whereas photosuppression of PVs impaired the performance. Furthermore, photosuppression of PVs during discriminative auditory fear conditioning increased generalization of conditioned response across tone frequencies, whereas PV photoactivation preserved normal specificity of learning. The observed changes in behavioral performance were correlated with bidirectional changes in the magnitude of tone-evoked responses, consistent with predictions of a model of a coupled excitatory-inhibitory cortical network. Direct photoactivation of excitatory neurons, which did not change tone-evoked response magnitude, did not affect behavioral performance in either task. Our results identify a new function for inhibition in the auditory cortex, demonstrating that it can improve or impair acuity of innate and learned auditory behaviors that rely on frequency discrimination.

Hearing perception relies on our ability to tell apart the spectral content of different sounds, and to learn to use this difference to distinguish behaviorally relevant (such as dangerous and safe) sounds. Recently, we demonstrated that the auditory cortex regulates frequency discrimination acuity following associative learning. However, the neuronal circuits that underlie this modulation remain unknown. In the auditory cortex, excitatory neurons serve the dominant function in transmitting information about the sensory world within and across brain areas, whereas inhibitory interneurons carry a range of modulatory functions, shaping the way information is represented and processed. Our study elucidates the function of a specific inhibitory neuronal population in sound encoding and perception. We find that interneurons in the auditory cortex, belonging to a specific class (parvalbumin-positive), modulate frequency selectivity of excitatory neurons, and regulate frequency discrimination acuity and specificity of discriminative auditory associative learning. These results expand our understanding of how specific cortical circuits contribute to innate and learned auditory behavior.

Modulating the activity of a specific type of cortical neuron can either improve or impair the ability to discriminate between tones of different frequencies and to associate danger with specific sounds.

Flexible Sensory Representations in Auditory Cortex Driven by Behavioral Relevance

Animals require the ability to ignore sensory stimuli that have no consequence yet respond to the same stimuli when they become useful. However, the brain circuits that govern this flexibility in sensory processing are not well understood. Here we show in mouse primary auditory cortex (A1) that daily passive sound exposure causes a long-lasting reduction in representations of the experienced sound by layer 2/3 pyramidal cells. This habituation arises locally in A1 and involves an enhancement in inhibition and selective upregulation in the activity of somatostatin-expressing inhibitory neurons (SOM cells). Furthermore, when mice engage in sound-guided behavior, pyramidal cell excitatory responses to habituated sounds are enhanced, whereas SOM cell responses are diminished. Together, our results demonstrate the bidirectional modulation of A1 sensory representations and suggest that SOM cells gate cortical information flow based on the behavioral relevance of the stimulus.

Auditory dysfunction in schizophrenia: integrating clinical and basic features

Schizophrenia is a complex neuropsychiatric disorder that is associated with persistent psychosocial disability in affected individuals. Although studies of schizophrenia have traditionally focused on deficits in higher-order processes such as working memory and executive function, there is an increasing realization that, in this disorder, deficits can be found throughout the cortex and are manifest even at the level of early sensory processing. These deficits are highly amenable to translational investigation and represent potential novel targets for clinical intervention. Deficits, moreover, have been linked to specific structural abnormalities in post-mortem auditory cortex tissue from individuals with schizophrenia, providing unique insights into underlying pathophysiological mechanisms.

Adaptive categorization of sound frequency does not require the auditory cortex in rats

A defining feature of adaptive behavior is our ability to change the way we interpret sensory stimuli depending on context. Rapid adaptation in behavior has been attributed to frontal cortical circuits, but it is not clear if sensory cortexes also play an essential role in such tasks. In this study we tested whether the auditory cortex was necessary for rapid adaptation in the interpretation of sounds. We used a two-alternative choice sound-categorization task for rats in which the boundary that separated two acoustic categories changed several times within a behavioral session. These shifts in the boundary resulted in changes in the rewarded action for a subset of stimuli. We found that extensive lesions of the auditory cortex did not impair the ability of rats to switch between categorization contingencies and sound discrimination performance was minimally impaired. Similar results were obtained after reversible inactivation of the auditory cortex with muscimol. In contrast, lesions of the auditory thalamus largely impaired discrimination performance and, as a result, the ability to modify behavior across contingencies. Thalamic lesions did not impair performance of a visual discrimination task, indicating that the effects were specific to audition and not to motor preparation or execution. These results suggest that subcortical outputs of the auditory thalamus can mediate rapid adaptation in the interpretation of sounds.

Plasticity of cortical excitatory-inhibitory balance

Synapses are highly plastic and are modified by changes in patterns of neural activity or sensory experience. Plasticity of cortical excitatory synapses is thought to be important for learning and memory, leading to alterations in sensory representations and cognitive maps. However, these changes must be coordinated across other synapses within local circuits to preserve neural coding schemes and the organization of excitatory and inhibitory inputs, i.e., excitatory-inhibitory balance. Recent studies indicate that inhibitory synapses are also plastic and are controlled directly by a large number of neuromodulators, particularly during episodes of learning. Many modulators transiently alter excitatory-inhibitory balance by decreasing inhibition, and thus disinhibition has emerged as a major mechanism by which neuromodulation might enable long-term synaptic modifications naturally. This review examines the relationships between neuromodulation and synaptic plasticity, focusing on the induction of long-term changes that collectively enhance cortical excitatory-inhibitory balance for improving perception and behavior.

Auditory cortex directs the input-specific remodeling of thalamus

Input-specific remodeling is observed both in the primary auditory cortex (AI) and the ventral division of the medial geniculate body of the thalamus (MGBv) through motivation such as learning. Here, we show the role of AI in the MGBv remodeling induced by the electrical stimulation (ES) of the central division of the inferior colliculus (ICc). For the MGBv neurons with frequency tunings different from those of electrically stimulated ICc neurons, their frequency tunings shifted towards the tunings of the ICc neurons. AI neurons also showed this input-specific remodeling after ES of the ICc (ESICc). Interestingly, the input-specific remodeling of MGBv was eliminated when the AI was inactivated using cortical application of muscimol. For the MGBv neurons tuned to the same frequency as the stimulated ICc neurons, their tunings were kept but their responses were facilitated after the ESICc. In contrast to the input-specific tuning shifts, this facilitation was rarely impacted by the AI inactivation. Thus, we conclude that AI directs the input-specific remodeling of MGBv induced by ESICc. It is suggested that the tuning shift in the MGBv primarily takes place in the AI and is relayed to the MGBv through the corticofugal system while the MGBv mainly highlights the frequency information emphasized in ICc.

Understanding the neurophysiological basis of auditory abilities for social communication: a perspective on the value of ethological paradigms

Acoustic communication between animals requires them to detect, discriminate, and categorize conspecific or heterospecific vocalizations in their natural environment. Laboratory studies of the auditory-processing abilities that facilitate these tasks have typically employed a broad range of acoustic stimuli, ranging from natural sounds like vocalizations to "artificial" sounds like pure tones and noise bursts. However, even when using vocalizations, laboratory studies often test abilities like categorization in relatively artificial contexts. Consequently, it is not clear whether neural and behavioral correlates of these tasks (1) reflect extensive operant training, which drives plastic changes in auditory pathways, or (2) the innate capacity of the animal and its auditory system. Here, we review a number of recent studies, which suggest that adopting more ethological paradigms utilizing natural communication contexts are scientifically important for elucidating how the auditory system normally processes and learns communication sounds. Additionally, since learning the meaning of communication sounds generally involves social interactions that engage neuromodulatory systems differently than laboratory-based conditioning paradigms, we argue that scientists need to pursue more ethological approaches to more fully inform our understanding of how the auditory system is engaged during acoustic communication. This article is part of a Special Issue entitled "Communication Sounds and the Brain: New Directions and Perspectives".

Bidirectional effects of aversive learning on perceptual acuity are mediated by the sensory cortex

Although emotional learning affects sensory acuity, little is known about how these changes are facilitated in the brain. We found that auditory fear conditioning in mice elicited either an increase or a decrease in frequency discrimination acuity depending on how specific the learned response was to the conditioned tone. Using reversible pharmacological inactivation, we found that the auditory cortex mediated learning-evoked changes in acuity in both directions.

Tone-detection training enhances spectral integration mediated by intracortical pathways in primary auditory cortex

Auditory-cued behavioral training can alter neural circuits in primary auditory cortex (A1), but the mechanisms and consequences of experience-dependent cortical plasticity are not fully understood. To address this issue, we trained adult rats to detect a 5 kHz target in order to receive a food reward. After 14 days training we identified three locations within A1: (i) the region representing the characteristic frequency (CF) 5 kHz, (ii) a nearby region with CF ∼10 kHz, and (iii) a more distant region with CF ∼20 kHz. In order to compare functional connectivity in A1 near to, vs. far from, the representation of the target frequency, we placed a 16-channel multiprobe in middle- (∼10 kHz) and high- (∼20 kHz) CF regions and obtained current-source density (CSD) profiles evoked by a range of tone stimuli (CF±1-3 octaves in quarter-octave steps). Our aim was to construct "CSD receptive fields" (CSD RFs) in order to determine the laminar and spectral profile of tone-evoked current sinks, and infer changes to thalamocortical and intracortical inputs. Behavioral training altered CSD RFs at the 10 kHz, but not 20 kHz, site relative to CSD RFs in untrained control animals. At the 10 kHz site, current sinks evoked by the target frequency were enhanced in layer 2/3, but the initial current sink in layer 4 was not altered. The results imply training-induced plasticity along intracortical pathways connecting the target representation with nearby cortical regions. Finally, we related behavioral performance (sensitivity index, d') to CSD responses in individual animals, and found a significant correlation between the development of d' over training and the amplitude of the target-evoked current sink in layer 2/3. The results suggest that plasticity along intracortical pathways is important for auditory learning.

Correlation between neural discharges in cat primary auditory cortex and tone-detection behaviors

Understanding the physiological role of the auditory cortex (AC) in acoustic perception is an essential issue in auditory neuroscience. By comparing sound discrimination behaviors in animals before and after AC lesion, many studies have demonstrated that AC is necessary for the perceptual process of human vowels and animal vocalizations, but is not necessary to discriminate simple acoustic parameters such as sound onset, intensity and duration. Because a lesion study cannot fully reveal the function of AC under normal conditions, in this study, we combined electrophysiological recording and psychophysical experiments on the same animal to investigate whether AC is involved in a simple auditory task. We recorded the neural activities of the primary auditory cortex (A1) using implanted electrodes, while freely-moving cats performed a tone-detection task in which they were required to lick a metal tube to obtain a food reward after hearing a tone pip. The performance of the cats' behavioral response increased with the increase of tone intensity, and the neural activities of A1 covaried with the behavioral performance. Also, whether the tone-detection behavior was interfered by a wideband noise was dependent on whether the tone-evoked neural response was masked by the noise-evoked response. Our results did not support that A1 neurons directly associate with the cat's behavioral decision; instead, they may mainly generate a neural representation of stimulus amplitude for further processing to determine whether a tone occurred or not.

Dissociation of detection and discrimination of pure tones following bilateral lesions of auditory cortex

It is well known that damage to the peripheral auditory system causes deficits in tone detection as well as pitch and loudness perception across a wide range of frequencies. However, the extent to which to which the auditory cortex plays a critical role in these basic aspects of spectral processing, especially with regard to speech, music, and environmental sound perception, remains unclear. Recent experiments indicate that primary auditory cortex is necessary for the normally-high perceptual acuity exhibited by humans in pure-tone frequency discrimination. The present study assessed whether the auditory cortex plays a similar role in the intensity domain and contrasted its contribution to sensory versus discriminative aspects of intensity processing. We measured intensity thresholds for pure-tone detection and pure-tone loudness discrimination in a population of healthy adults and a middle-aged man with complete or near-complete lesions of the auditory cortex bilaterally. Detection thresholds in his left and right ears were 16 and 7 dB HL, respectively, within clinically-defined normal limits. In contrast, the intensity threshold for monaural loudness discrimination at 1 kHz was 6.5±2.1 dB in the left ear and 6.5±1.9 dB in the right ear at 40 dB sensation level, well above the means of the control population (left ear: 1.6±0.22 dB; right ear: 1.7±0.19 dB). The results indicate that auditory cortex lowers just-noticeable differences for loudness discrimination by approximately 5 dB but is not necessary for tone detection in quiet. Previous human and Old-world monkey experiments employing lesion-effect, neurophysiology, and neuroimaging methods to investigate the role of auditory cortex in intensity processing are reviewed.

Minimal impairment in a rat model of duration discrimination following excitotoxic lesions of primary auditory and prefrontal cortices

We present a behavioral paradigm for the study of duration perception in the rat, and report the result of neurotoxic lesions that have the goal of identifying sites that mediate duration perception. Using a two-alternative forced-choice paradigm, rats were either trained to discriminate durations of pure tones (range = [200,500] ms; boundary = 316 ms; Weber fraction after training = 0.24 ± 0.04), or were trained to discriminate frequencies of pure tones (range = [8,16] kHz; boundary = 11.3 kHz; Weber = 0.16 ± 0.11); the latter task is a control for non-timing-specific aspects of the former. Both groups discriminate the same class of sensory stimuli, use the same motions to indicate decisions, have identical trial structures, and are trained to psychophysical threshold; the tasks are thus matched in a number of sensorimotor and cognitive demands. We made neurotoxic lesions of candidate timing-perception areas in the cerebral cortex of both groups. Following extensive bilateral lesions of the auditory cortex, the performance of the frequency discrimination group was significantly more impaired than that of the duration discrimination group. We also found that extensive bilateral lesions of the medial prefrontal cortex resulted in little to no impairment of both groups. The behavioral framework presented here provides an audition-based approach to study the neural mechanisms of time estimation and memory for durations.

Stimulus-specific adaptation in the gerbil primary auditory thalamus is the result of a fast frequency-specific habituation and is regulated by the corticofugal system

The detection of novel and therefore potentially behavioral relevant stimuli is of fundamental importance for animals. In the auditory system, stimulus-specific adaptation (SSA) resulting in stronger responses to rare compared with frequent stimuli was proposed as such a novelty detection mechanism. SSA is a now well established phenomenon found at different levels along the mammalian auditory pathway. It depends on various stimulus features, such as deviant probability, and may be an essential mechanism underlying perception of changes in sound statistics. We recorded neuronal responses from the ventral part of the medial geniculate body (vMGB) in Mongolian gerbils to determine details of the adaptation process that might indicate underlying neuronal mechanisms. Neurons in the vMGB exhibited a median spike rate change of 15.4% attributable to a fast habituation to the frequently presented standard stimulus. Accordingly, the main habituation effect could also be induced by the repetition of a few uniform tonal stimuli. The degree of habituation was frequency-specific, and comparison across simultaneously recorded units indicated that adaptation effects were apparently topographically organized. At the population level, stronger habituation effects were on average associated with the border regions of the frequency response areas. Finally, the pharmacological inactivation of the auditory cortex demonstrated that SSA in the vMGB is mainly regulated by the corticofugal system. Hence, these results indicate a more general function of SSA in the processing and analysis of auditory information than the term novelty detection suggests.

Long-term regulation in calretinin staining in the rat inferior colliculus after unilateral auditory cortical ablation

In this study we analyzed the effects in the inferior colliculus of a unilateral ablation of the auditory cortex in rats. Variations in both calretinin immunoreactivity and protein levels determined by Western blot suggest that such lesions induce changes in the regulation of this calcium-binding protein. Stereological counts of calretinin-immunoreactive neurons in the inferior colliculus 15, 90, and 180 days after the lesion showed a progressive increase in the number of immunoreactive neurons, with a parallel increase in the intensity of staining. Two hundred forty days after the cortical lesion, both the number of immunoreactive neurons and the staining intensity had returned to control values. The effects of the cortical lesion on calretinin regulation are more intense in those inferior colliculus subdivisions more densely innervated by the corticocollicular projection. This finding, along with the time course of calretinin regulation suggests that degeneration of the descending projection is linked to calretinin regulation in the inferior colliculus. We hypothesize, based on the role of calretinin, that the observed increase in immunoreactivity levels seen in the inferior colliculus after lesioning of the auditory cortex may be related to altered excitability in deafferented neurons. Our finding, may reflect adaptive mechanisms to changes in calcium influx and excitability in inferior colliculus neurons induced by lesions of the descending projection from the cortex to the inferior colliculus.

The auditory cortex mediates the perceptual effects of acoustic temporal expectation

When events occur at predictable instants, anticipation improves performance. Knowledge of event timing modulates motor circuits, improving response speed. By contrast, the neuronal mechanisms underlying changes in sensory perception due to expectation are not well understood. We have developed a novel behavioral paradigm for rats in which we manipulated expectations about sound timing. Valid expectations improved both the speed and the accuracy of subjects’ performance, indicating not only improved motor preparedness but also enhanced perception. Single neuron recordings in primary auditory cortex revealed enhanced representation of sounds during periods of heightened expectation. Furthermore, we found that activity in auditory cortex was causally linked to the performance of the task, and that changes in the neuronal representation of sounds predicted performance on a trial-by-trial basis. Our results indicate that changes in neuronal representation as early as primary sensory cortex mediate the perceptual advantage conferred by temporal expectation.

Gamma oscillations in the auditory cortex of awake rats

Numerous reports of human electrophysiology have demonstrated gamma (30-150 Hz) frequency oscillations in the auditory cortex during listening. However, only a small number of studies in non-human animals have provided evidence for gamma oscillations during listening. In this report, multi-site recordings from primary auditory cortex (A1) were carried out using a 16-channel microelectrode array in awake rats as they passively listened to tones. We addressed two fundamental questions: (i) Is passive listening associated with an increase in gamma oscillation in A1? And, if so: (ii) Are A1 gamma oscillations during passive listening coherent within local networks and/or over long distances? All sites within A1 showed a short-latency burst of activity in the low-gamma (30-70 Hz) and high-gamma (90-150 Hz) bands in the local field potential (LFP). Additionally, 53% of sites within A1 also showed longer-latency bursts of gamma oscillation that occurred episodically for up to 350 ms after tone onset, but these varied both in latency and in occurrence across trials. There was significant coherence in the low-gamma band between spike activity and the LFP recorded with the same electrode. However, neither LFPs nor the spike activity between sites spaced at least 300 μm apart showed coherent activity in the gamma band. The experiments demonstrated that gamma oscillations are present, but not uniformly expressed, throughout A1 during passive listening and that there is strong local coherence in the spatiotemporal organization of gamma activity.

Spectral integration in primary auditory cortex attributable to temporally precise convergence of thalamocortical and intracortical input

Primary sensory cortex integrates sensory information from afferent feedforward thalamocortical projection systems and convergent intracortical microcircuits. Both input systems have been demonstrated to provide different aspects of sensory information. Here we have used high-density recordings of laminar current source density (CSD) distributions in primary auditory cortex of Mongolian gerbils in combination with pharmacological silencing of cortical activity and analysis of the residual CSD, to dissociate the feedforward thalamocortical contribution and the intracortical contribution to spectral integration. We found a temporally highly precise integration of both types of inputs when the stimulation frequency was in close spectral neighborhood of the best frequency of the measurement site, in which the overlap between both inputs is maximal. Local intracortical connections provide both directly feedforward excitatory and modulatory input from adjacent cortical sites, which determine how concurrent afferent inputs are integrated. Through separate excitatory horizontal projections, terminating in cortical layers II/III, information about stimulus energy in greater spectral distance is provided even over long cortical distances. These projections effectively broaden spectral tuning width. Based on these data, we suggest a mechanism of spectral integration in primary auditory cortex that is based on temporally precise interactions of afferent thalamocortical inputs and different short- and long-range intracortical networks. The proposed conceptual framework allows integration of different and partly controversial anatomical and physiological models of spectral integration in the literature.

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.

High-resolution mapping of neuronal activity using the lipophilic thallium chelate complex TlDDC: protocol and validation of the method

In neurons the rate of K(+)-uptake increases with increasing activity. K(+)-analogues like the heavy metal ion thallium (Tl(+)) can be used, therefore, as tracers for imaging neuronal activity. However, when water-soluble Tl(+)-salts are injected systemically only minute amounts of the tracer enter the brain and the Tl(+)-uptake patterns are influenced by regional differences in blood-brain barrier (BBB) K(+)-permeability. We here show that the BBB-related limitations in using Tl(+) for imaging neuronal activity are no longer present when the lipophilic Tl(+) chelate complex thallium diethyldithiocarbamate (TlDDC) is applied. We systemically injected rodents with TlDDC and mapped the Tl(+)-distribution in the brain using an autometallographic (AMG) technique, a histochemical method for detecting heavy metals. We find that Tl(+)-doses for optimum AMG staining could be substantially reduced, and regional differences attributable to differences in BBB K(+)-permeability were no longer detectable, indicating that TlDDC crosses the BBB. At the cellular level, however, the Tl(+)-distribution was essentially the same as after injection of water-soluble Tl(+)-salts, indicating Tl(+)-release from TlDDC prior to neuronal or glial uptake. Upon sensory stimulation or intracortical microstimulation neuronal Tl(+)-uptake increased after TlDDC injection, upon muscimol treatment neuronal Tl(+)-uptake decreased. We present a protocol for mapping neuronal activity with cellular resolution, which is based on intravenous TlDDC injections during ongoing activity in unrestrained behaving animals and short stimulation times of 5 min.

Temporally dynamic frequency tuning of population responses in monkey primary auditory cortex

Frequency tuning of auditory cortical neurons is typically determined by integrating spikes over the entire duration of a tone stimulus. However, this approach may mask functionally significant variations in tuning over the time course of the response. To explore this possibility, frequency response functions (FRFs) based on population multiunit activity evoked by pure tones of 175 or 200 ms duration were examined within four time windows relative to stimulus onset corresponding to "on" (10-30 ms), "early sustained" (30-100 ms), "late sustained" (100-175 ms), and "off" (185-235 or 210-260 ms) portions of responses in primary auditory cortex (A1) of 5 awake macaques. FRFs of "on" and "early sustained" responses displayed a good concordance, with best frequencies (BFs) differing, on average, by less than 0.25 octaves. In contrast, FRFs of "on" and "late sustained" responses differed considerably, with a mean difference in BF of 0.68 octaves. At many sites, tuning of "off" responses was inversely related to that of "on" responses, with "off" FRFs displaying a trough at the BF of "on" responses. Inversely correlated "on" and "off" FRFs were more common at sites with a higher "on" BF, thus suggesting functional differences between sites with low and high "on" BF. These results indicate that frequency tuning of population responses in A1 may vary considerably over the course of the response to a tone, thus revealing a temporal dimension to the representation of sound spectrum in A1.