Central auditory onset responses, and temporal asymmetries in auditory perception

Cortical temporal integration can account for limits of temporal perception: investigations in the binaural system

The auditory system has exquisite temporal coding in the periphery which is transformed into a rate-based code in central auditory structures, like auditory cortex. However, the cortex is still able to synchronize, albeit at lower modulation rates, to acoustic fluctuations. The perceptual significance of this cortical synchronization is unknown. We estimated physiological synchronization limits of cortex (in humans with electroencephalography) and brainstem neurons (in chinchillas) to dynamic binaural cues using a novel system-identification technique, along with parallel perceptual measurements. We find that cortex can synchronize to dynamic binaural cues up to approximately 10 Hz, which aligns well with our measured limits of perceiving dynamic spatial information and utilizing dynamic binaural cues for spatial unmasking, i.e. measures of binaural sluggishness. We also find that the tracking limit for frequency modulation (FM) is similar to the limit for spatial tracking, demonstrating that this sluggish tracking is a more general perceptual limit that can be accounted for by cortical temporal integration limits.

Onset-offset cortical auditory evoked potential amplitude differences indicate auditory cortical hyperactivity and reduced inhibition in people with tinnitus

OBJECTIVE

The current study investigates evidence of hypothesized reduced central inhibition and/or increased excitation in individuals with tinnitus by evaluating cortical auditory onset versus offset responses.

METHODS

Cortical auditory evoked potentials (CAEPs) were recorded to the onset and offset of 3-second white noise stimuli in tinnitus and control groups matched in pairs by age, hearing, and sex (n = 26 total). Independent t-tests and 2-way mixed model ANOVA were used to evaluate onset-offset differences in amplitude, area, and latency of CAEP components by group. The predictive influence of tinnitus presence and associated participant characteristics on CAEP outcomes was assessed by multiple regression proportional reduction in error.

RESULTS

The tinnitus group had significantly larger onset minus offset P2 amplitudes (ΔP2 amplitudes) than control group participants. No other component variables differed significantly. ΔP2 amplitude was best predicted by tinnitus status and not significantly influenced by other variables such as hearing loss or age.

CONCLUSIONS

Hypothesized reduced central inhibition and/or increased excitation in tinnitus participants was partially supported by a group difference in ΔP2 amplitude.

SIGNIFICANCE

This was the first study to evaluate CAEP onset minus offset differences to investigate changes in central excitation/inhibition in individuals with tinnitus versus controls in matched groups.

Auditory representations for long lasting sounds: Insights from event-related brain potentials and neural oscillations

The basic features of short sounds, such as frequency and intensity including their temporal dynamics, are integrated in a unitary representation. Knowledge on how our brain processes long lasting sounds is scarce. We review research utilizing the Mismatch Negativity event-related potential and neural oscillatory activity for studying representations for long lasting simple versus complex sounds such as sinusoidal tones versus speech. There is evidence for a temporal constraint in the formation of auditory representations: Auditory edges like sound onsets within long lasting sounds open a temporal window of about 350 ms in which the sounds' dynamics are integrated into a representation, while information beyond that window contributes less to that representation. This integration window segments the auditory input into short chunks. We argue that the representations established in adjacent integration windows can be concatenated into an auditory representation of a long sound, thus, overcoming the temporal constraint.

Systematic errors in the perception of rhythm

One hypothesis for why humans enjoy musical rhythms relates to their prediction of when each beat should occur. The ability to predict the timing of an event is important from an evolutionary perspective. Therefore, our brains have evolved internal mechanisms for processing the progression of time. However, due to inherent noise in neural signals, this prediction is not always accurate. Theoretical considerations of optimal estimates suggest the occurrence of certain systematic errors made by the brain when estimating the timing of beats in rhythms. Here, we tested psychophysically whether these systematic errors exist and if so, how they depend on stimulus parameters. Our experimental data revealed two main types of systematic errors. First, observers perceived the time of the last beat of a rhythmic pattern as happening earlier than actual when the inter-beat interval was short. Second, the perceived time of the last beat was later than the actual when the inter-beat interval was long. The magnitude of these systematic errors fell as the number of beats increased. However, with many beats, the errors due to long inter-beat intervals became more apparent. We propose a Bayesian model for these systematic errors. The model fits these data well, allowing us to offer possible explanations for how these errors occurred. For instance, neural processes possibly contributing to the errors include noisy and temporally asymmetric impulse responses, priors preferring certain time intervals, and better-early-than-late loss functions. We finish this article with brief discussions of both the implications of systematic errors for the appreciation of rhythm and the possible compensation by the brain’s motor system during a musical performance.

More than the end: OFF response plasticity as a mnemonic signature of a sound’s behavioral salience

In studying how neural populations in sensory cortex code dynamically varying stimuli to guide behavior, the role of spiking after stimuli have ended has been underappreciated. This is despite growing evidence that such activity can be tuned, experience-and context-dependent and necessary for sensory decisions that play out on a slower timescale. Here we review recent studies, focusing on the auditory modality, demonstrating that this so-called OFF activity can have a more complex temporal structure than the purely phasic firing that has often been interpreted as just marking the end of stimuli. While diverse and still incompletely understood mechanisms are likely involved in generating phasic and tonic OFF firing, more studies point to the continuing post-stimulus activity serving a short-term, stimulus-specific mnemonic function that is enhanced when the stimuli are particularly salient. We summarize these results with a conceptual model highlighting how more neurons within the auditory cortical population fire for longer duration after a sound’s termination during an active behavior and can continue to do so even while passively listening to behaviorally salient stimuli. Overall, these studies increasingly suggest that tonic auditory cortical OFF activity holds an echoic memory of specific, salient sounds to guide behavioral decisions.

How to discern external acoustic waves in a piezoelectric neuron under noise?

Biological neurons keep sensitive to external stimuli and appropriate firing modes can be triggered to give effective response to external chemical and physical signals. A piezoelectric neural circuit can perceive external voice and nonlinear vibration by generating equivalent piezoelectric voltage, which can generate an equivalent trans-membrane current for inducing a variety of firing modes in the neural activities. Biological neurons can receive external stimuli from more ion channels and synapse synchronously, but the further encoding and priority in mode selection are competitive. In particular, noisy disturbance and electromagnetic radiation make it more difficult in signals identification and mode selection in the firing patterns of neurons driven by multi-channel signals. In this paper, two different periodic signals accompanied by noise are used to excite the piezoelectric neural circuit, and the signal processing in the piezoelectric neuron driven by acoustic waves under noise is reproduced and explained. The physical energy of the piezoelectric neural circuit and Hamilton energy in the neuron driven by mixed signals are calculated to explain the biophysical mechanism of auditory neuron when external stimuli are applied. It is found that the neuron prefers to respond to the external stimulus with higher physical energy and the signal which can increase the Hamilton energy of the neuron. For example, stronger inputs used to inject higher energy and it is detected and responded more sensitively. The involvement of noise is helpful to detect the external signal under stochastic resonance, and the additive noise changes the excitability of neuron as the external stimulus. The results indicate that energy controls the firing patterns and mode selection in neurons, and it provides clues to control the neural activities by injecting appropriate energy into the neurons and network.

Texture is encoded in precise temporal spiking patterns in primate somatosensory cortex

Humans are exquisitely sensitive to the microstructure and material properties of surfaces. In the peripheral nerves, texture information is conveyed via two mechanisms: coarse textural features are encoded in spatial patterns of activation that reflect their spatial layout, and fine features are encoded in highly repeatable, texture-specific temporal spiking patterns evoked as the skin moves across the surface. Here, we examined whether this temporal code is preserved in the responses of neurons in somatosensory cortex. We scanned a diverse set of everyday textures across the fingertip of awake macaques while recording the responses evoked in individual cortical neurons. We found that temporal spiking patterns are highly repeatable across multiple presentations of the same texture, with millisecond precision. As a result, texture identity can be reliably decoded from the temporal patterns themselves, even after information carried in the spike rates is eliminated. However, the combination of rate and timing is more informative than either code in isolation. The temporal precision of the texture response is heterogenous across cortical neurons and depends on the submodality composition of their input and on their location along the somatosensory neuraxis. Furthermore, temporal spiking patterns in cortex dilate and contract with decreases and increases in scanning speed, respectively, and this systematic relationship between speed and patterning may contribute to the observed perceptual invariance to speed. Finally, we find that the quality of a texture percept can be better predicted when these temporal patterns are taken into consideration. We conclude that high-precision spike timing complements rate-based signals to encode texture in somatosensory cortex.

Neuroscientists seek to understand how neuronal signals carry information and drive perception. Here, the authors show that millisecond-level spike timing in somatosensory cortex is informative about texture and shapes the evoked sensory experience.

Emergence and function of cortical offset responses in sound termination detection

Offset responses in auditory processing appear after a sound terminates. They arise in neuronal circuits within the peripheral auditory system, but their role in the central auditory system remains unknown. Here, we ask what the behavioral relevance of cortical offset responses is and what circuit mechanisms drive them. At the perceptual level, our results reveal that experimentally minimizing auditory cortical offset responses decreases the mouse performance to detect sound termination, assigning a behavioral role to offset responses. By combining in vivo electrophysiology in the auditory cortex and thalamus of awake mice, we also demonstrate that cortical offset responses are not only inherited from the periphery but also amplified and generated de novo. Finally, we show that offset responses code more than silence, including relevant changes in sound trajectories. Together, our results reveal the importance of cortical offset responses in encoding sound termination and detecting changes within temporally discontinuous sounds crucial for speech and vocalization.

Encoding of acquired sound-sequence salience by auditory cortical offset responses

Behaviorally relevant sounds are often composed of distinct acoustic units organized into specific temporal sequences. The meaning of such sound sequences can therefore be fully recognized only when they have terminated. However, the neural mechanisms underlying the perception of sound sequences remain unclear. Here, we use two-photon calcium imaging in the auditory cortex of behaving mice to test the hypothesis that neural responses to termination of sound sequences ("Off-responses") encode their acoustic history and behavioral salience. We find that auditory cortical Off-responses encode preceding sound sequences and that learning to associate a sound sequence with a reward induces enhancement of Off-responses relative to responses during the sound sequence ("On-responses"). Furthermore, learning enhances network-level discriminability of sound sequences by Off-responses. Last, learning-induced plasticity of Off-responses but not On-responses lasts to the next day. These findings identify auditory cortical Off-responses as a key neural signature of acquired sound-sequence salience.

Contralateral proximal interference

A contralateral "cue" tone presented in continuous broadband noise both lowers the threshold of a signal tone by guiding attention to it and raises its threshold by interference. Here, signal tones were fixed in duration (40 ms, 52 ms with ramps), frequency (1500 Hz), timing, and level, so attention did not need guidance. Interference by contralateral cues was studied in relation to cue-signal proximity, cue-signal temporal overlap, and cue-signal order (cue after: backward interference, BI; or cue first: forward interference, FI). Cues, also ramped, were 12 dB above the signal level. Long cues (300 or 600 ms) raised thresholds by 5.3 dB when the signal and cue overlapped and by 5.1 dB in FI and 3.2 dB in BI when cues and signals were separated by 40 ms. Short cues (40 ms) raised thresholds by 4.5 dB in FI and 4.0 dB in BI for separations of 7 to 40 ms, but by ∼13 dB when simultaneous and in phase. FI and BI are comparable in magnitude and hardly increase when the signal is close in time to abrupt cue transients. These results do not support the notion that masking of the signal is due to the contralateral cue onset/offset transient response. Instead, sluggish attention or temporal integration may explain contralateral proximal interference.

Phasic Off responses of auditory cortex underlie perception of sound duration

It has been proposed that sound information is separately streamed into onset and offset pathways for parallel processing. However, how offset responses contribute to auditory perception remains unclear. Here, loose-patch and whole-cell recordings in awake mouse primary auditory cortex (A1) reveal that a subset of pyramidal neurons exhibit a transient "Off" response, with its onset tightly time-locked to the sound termination and its frequency tuning similar to that of the transient "On" response. Both responses are characterized by excitation briefly followed by inhibition, with the latter mediated by parvalbumin (PV) inhibitory neurons. Optogenetically manipulating sound-evoked A1 responses at different temporal phases or artificially creating phantom sounds in A1 further reveals that the A1 phasic On and Off responses are critical for perceptual discrimination of sound duration. Our results suggest that perception of sound duration is dependent on precisely encoding its onset and offset timings by phasic On and Off responses.

Network dynamics underlying OFF responses in the auditory cortex

Across sensory systems, complex spatio-temporal patterns of neural activity arise following the onset (ON) and offset (OFF) of stimuli. While ON responses have been widely studied, the mechanisms generating OFF responses in cortical areas have so far not been fully elucidated. We examine here the hypothesis that OFF responses are single-cell signatures of recurrent interactions at the network level. To test this hypothesis, we performed population analyses of two-photon calcium recordings in the auditory cortex of awake mice listening to auditory stimuli, and compared them to linear single-cell and network models. While the single-cell model explained some prominent features of the data, it could not capture the structure across stimuli and trials. In contrast, the network model accounted for the low-dimensional organization of population responses and their global structure across stimuli, where distinct stimuli activated mostly orthogonal dimensions in the neural state-space.

On the generalization of tones: A detailed exploration of non-speech auditory perception stimuli

The dynamic changes in natural sounds’ temporal structures convey important event-relevant information. However, prominent researchers have previously expressed concern that non-speech auditory perception research disproportionately uses simplistic stimuli lacking the temporal variation found in natural sounds. A growing body of work now demonstrates that some conclusions and models derived from experiments using simplistic tones fail to generalize, raising important questions about the types of stimuli used to assess the auditory system. To explore the issue empirically, we conducted a novel, large-scale survey of non-speech auditory perception research from four prominent journals. A detailed analysis of 1017 experiments from 443 articles reveals that 89% of stimuli employ amplitude envelopes lacking the dynamic variations characteristic of non-speech sounds heard outside the laboratory. Given differences in task outcomes and even the underlying perceptual strategies evoked by dynamic vs. invariant amplitude envelopes, this raises important questions of broad relevance to psychologists and neuroscientists alike. This lack of exploration of a property increasingly recognized as playing a crucial role in perception suggests future research using stimuli with time-varying amplitude envelopes holds significant potential for furthering our understanding of the auditory system’s basic processing capabilities.

Why are damped sounds perceived as shorter than ramped sounds?

Hearing is the most accurate sense for perceiving duration. However, rarely it produces inaccurate estimates of duration, for example when it compares the subjective duration of tones that are increasing in intensity over time (i.e., ramped) with that of tones that are decreasing in intensity over time (i.e., damped). The literature reports that the damped tones are perceived as much being shorter than the ramped tones of the same length. The short subjective duration of damped tones may originate from a decay suppression mechanism that parses the source-informative part of many natural sounds (i.e., the beginning) from the less informative part of them (the decay): listeners may interpret the tail of damped tones like an echo or like the decay portion of an impact sound and exclude it from the account of the duration of the tone. In the natural soundscape, the tail of sounds produced in reverberant environments and the tail of impact sounds have a frequency content that is constant throughout the sound's duration. Here, the carriers used for ramped and damped sounds were a tone constant in frequency and a tone modulated in frequency. The frequency modulation was introduced to prevent the listener from interpreting the tail of these tones as the result of reverberation or the decay portion of an impact sound. Frequency constant damped tones were largely underestimated in duration whereas frequency modulated ones were not (or were only slightly), demonstrating that the decay suppression mechanism is a worthy explanation for the short subjective duration of damped tones.

A Cortico-Collicular Amplification Mechanism for Gap Detection

Auditory cortex (AC) is necessary for the detection of brief gaps in ongoing sounds, but not for the detection of longer gaps or other stimuli such as tones or noise. It remains unclear why this is so, and what is special about brief gaps in particular. Here, we used both optogenetic suppression and conventional lesions to show that the cortical dependence of brief gap detection hinges specifically on gap termination. We then identified a cortico-collicular gap detection circuit that amplifies cortical gap termination responses before projecting to inferior colliculus (IC) to impact behavior. We found that gaps evoked off-responses and on-responses in cortical neurons, which temporally overlapped for brief gaps, but not long gaps. This overlap specifically enhanced cortical responses to brief gaps, whereas IC neurons preferred longer gaps. Optogenetic suppression of AC reduced collicular responses specifically to brief gaps, indicating that under normal conditions, the enhanced cortical representation of brief gaps amplifies collicular gap responses. Together these mechanisms explain how and why AC contributes to the behavioral detection of brief gaps, which are critical cues for speech perception, perceptual grouping, and auditory scene analysis.

Age-related changes in sound onset and offset intensity coding in auditory cortical fields A1 and CL of rhesus macaques

Inhibition plays a key role in shaping sensory processing in the central auditory system and has been implicated in sculpting receptive field properties such as sound intensity coding and also in shaping temporal patterns of neuronal firing such as onset- or offset-evoked responses. There is substantial evidence supporting a decrease in inhibition throughout the ascending auditory pathway in geriatric animals. We therefore examined intensity coding of onset (ON) and offset (OFF) responses in auditory cortex of aged and young monkeys. A large proportion of cells in the primary auditory cortex (A1) and the caudolateral field (CL) displayed nonmonotonic rate-level functions for OFF responses in addition to nonmonotonic coding of ON responses. Aging differentially affected ON and OFF responses; the magnitude of effects was generally greater for ON responses. In addition to higher firing rates, neurons in old monkeys exhibited a significant increase in the proportion of monotonic rate-level functions and had higher best intensities than those in young monkeys. OFF responses in young monkeys displayed a range of intensity coding relationships with ON responses of the same cells, ranging from highly similar to highly dissimilar. Dissimilarity in ON/OFF coding was greater in CL and was reduced with aging, which was largely explained by a preferential decrease in the percentage of cells with nonmonotonic coding of ON and OFF responses. The changes we observed are consistent with previously demonstrated alterations in inhibition in the ascending auditory pathway of primates and could be involved in age-related deficits in the temporal processing of sounds.NEW & NOTEWORTHY Aging has a major impact on intensity coding of neurons in auditory cortex of rhesus macaques. Neural responses to sound onset and offset were affected to different extents, and their rate-level functions became more mutually similar, which could be accounted for by the loss of nonmonotonic intensity coding in geriatric monkeys. These findings were consistent with weakened inhibition in the central auditory system and could contribute to auditory processing deficits in elderly subjects.

A generic deviance detection principle for cortical On/Off responses, omission response, and mismatch negativity

Neural responses to sudden changes can be observed in many parts of the sensory pathways at different organizational levels. For example, deviants that violate regularity at various levels of abstraction can be observed as simple On/Off responses of individual neurons or as cumulative responses of neural populations. The cortical deviance-related responses supporting different functionalities (e.g., gap detection, chunking, etc.) seem unlikely to arise from different function-specific neural circuits, given the relatively uniform and self-similar wiring patterns across cortical areas and spatial scales. Additionally, reciprocal wiring patterns (with heterogeneous combinations of excitatory and inhibitory connections) in the cortex naturally speak in favor of a generic deviance detection principle. Based on this concept, we propose a network model consisting of reciprocally coupled neural masses as a blueprint of a universal change detector. Simulation examples reproduce properties of cortical deviance-related responses including the On/Off responses, the omitted-stimulus response (OSR), and the mismatch negativity (MMN). We propose that the emergence of change detectors relies on the involvement of disinhibition. An analysis of network connection settings further suggests a supportive effect of synaptic adaptation and a destructive effect of N-methyl-D-aspartate receptor (NMDA-r) antagonists on change detection. We conclude that the nature of cortical reciprocal wiring gives rise to a whole range of local change detectors supporting the notion of a generic deviance detection principle. Several testable predictions are provided based on the network model. Notably, we predict that the NMDA-r antagonists would generally dampen the cortical Off response, the cortical OSR, and the MMN.

Online Detection of Multiple Stimulus Changes Based on Single Neuron Interspike Intervals

Nervous systems need to detect stimulus changes based on their neuronal responses without using any additional information on the number, times, and types of stimulus changes. Here, two relatively simple, biologically realistic change point detection methods are compared with two common analysis methods. The four methods are applied to intra- and extracellularly recorded responses of a single cricket interneuron (AN2) to acoustic simulation. Solely based on these recorded responses, the methods should detect an unknown number of different types of sound intensity in- and decreases shortly after their occurrences. For this task, the methods rely on calculating an adjusting interspike interval (ISI). Both simple methods try to separate responses to intensity in- or decreases from activity during constant stimulation. The Pure-ISI method performs this task based on the distribution of the ISI, while the ISI-Ratio method uses the ratio of actual and previous ISI. These methods are compared to the frequently used Moving-Average method, which calculates mean and standard deviation of the instantaneous spike rate in a moving interval. Additionally, a classification method provides the upper limit of the change point detection performance that can be expected for the cricket interneuron responses. The classification learns the statistical properties of the actual and previous ISI during stimulus changes and constant stimulation from a training data set. The main results are: (1) The Moving-Average method requires a stable activity in a long interval to estimate the previous activity, which was not always given in our data set. (2) The Pure-ISI method can reliably detect stimulus intensity increases when the neuron bursts, but it fails to identify intensity decreases. (3) The ISI-Ratio method detects stimulus in- and decreases well, if the spike train is not too noisy. (4) The classification method shows good performance for the detection of stimulus in- and decreases. But due to the statistical learning, this method tends to confuse responses to constant stimulation with responses triggered by a stimulus change. Our results suggest that stimulus change detection does not require computationally costly mechanisms. Simple nervous systems like the cricket's could effectively apply ISI-Ratios to solve this fundamental task.

Distinct processing of tone offset in two primary auditory cortices

In the rodent auditory system, the primary cortex is subdivided into two regions, both receiving direct inputs from the auditory thalamus: the primary auditory cortex (A1) and the anterior auditory field (AAF). Although neurons in the two regions display different response properties, like response latency, firing threshold or tuning bandwidth, it is still not clear whether they process sound in a distinct way. Using in vivo electrophysiological recordings in the mouse auditory cortex, we found that AAF neurons have significantly stronger responses to tone offset than A1 neurons. AAF neurons also display faster and more transient responses than A1 neurons. Additionally, offset responses in AAF – unlike in A1, increase with sound duration. Local field potential (LFP) and laminar analyses suggest that the differences in sound responses between these two primary cortices are both of subcortical and intracortical origin. These results emphasize the potentially critical role of AAF for temporal processing. Our study reveals a distinct role of two primary auditory cortices in tone processing and highlights the complexity of sound encoding at the cortical level.

Slow NMDA-Mediated Excitation Accelerates Offset-Response Latencies Generated via a Post-Inhibitory Rebound Mechanism

In neural circuits, action potentials (spikes) are conventionally caused by excitatory inputs whereas inhibitory inputs reduce or modulate neuronal excitability. We previously showed that neurons in the superior paraolivary nucleus (SPN) require solely synaptic inhibition to generate their hallmark offset response, a burst of spikes at the end of a sound stimulus, via a post-inhibitory rebound mechanism. In addition SPN neurons receive excitatory inputs, but their functional significance is not yet known. Here we used mice of both sexes to demonstrate that in SPN neurons, the classical roles for excitation and inhibition are switched, with inhibitory inputs driving spike firing and excitatory inputs modulating this response. Hodgkin–Huxley modeling suggests that a slow, NMDA receptor (NMDAR)-mediated excitation would accelerate the offset response. We find corroborating evidence from in vitro and in vivo recordings that lack of excitation prolonged offset-response latencies and rendered them more variable to changing sound intensity levels. Our results reveal an unsuspected function for slow excitation in improving the timing of post-inhibitory rebound firing even when the firing itself does not depend on excitation. This shows the auditory system employs highly specialized mechanisms to encode timing-sensitive features of sound offsets which are crucial for sound-duration encoding and have profound biological importance for encoding the temporal structure of speech.

Changes in Neuronal Representations of Consonants in the Ascending Auditory System and Their Role in Speech Recognition

A fundamental task of the ascending auditory system is to produce representations that facilitate the recognition of complex sounds. This is particularly challenging in the context of acoustic variability, such as that between different talkers producing the same phoneme. These representations are transformed as information is propagated throughout the ascending auditory system from the inner ear to the auditory cortex (AI). Investigating these transformations and their role in speech recognition is key to understanding hearing impairment and the development of future clinical interventions. Here, we obtained neural responses to an extensive set of natural vowel-consonant-vowel phoneme sequences, each produced by multiple talkers, in three stages of the auditory processing pathway. Auditory nerve (AN) representations were simulated using a model of the peripheral auditory system and extracellular neuronal activity was recorded in the inferior colliculus (IC) and primary auditory cortex (AI) of anaesthetized guinea pigs. A classifier was developed to examine the efficacy of these representations for recognizing the speech sounds. Individual neurons convey progressively less information from AN to AI. Nonetheless, at the population level, representations are sufficiently rich to facilitate recognition of consonants with a high degree of accuracy at all stages indicating a progression from a dense, redundant representation to a sparse, distributed one. We examined the timescale of the neural code for consonant recognition and found that optimal timescales increase throughout the ascending auditory system from a few milliseconds in the periphery to several tens of milliseconds in the cortex. Despite these longer timescales, we found little evidence to suggest that representations up to the level of AI become increasingly invariant to across-talker differences. Instead, our results support the idea that the role of the subcortical auditory system is one of dimensionality expansion, which could provide a basis for flexible classification of arbitrary speech sounds.

A Hierarchy of Time Scales for Discriminating and Classifying the Temporal Shape of Sound in Three Auditory Cortical Fields

Auditory cortex is essential for mammals, including rodents, to detect temporal "shape" cues in the sound envelope but it remains unclear how different cortical fields may contribute to this ability (Lomber and Malhotra, 2008; Threlkeld et al., 2008). Previously, we found that precise spiking patterns provide a potential neural code for temporal shape cues in the sound envelope in the primary auditory (A1), and ventral auditory field (VAF) and caudal suprarhinal auditory field (cSRAF) of the rat (Lee et al., 2016). Here, we extend these findings and characterize the time course of the temporally precise output of auditory cortical neurons in male rats. A pairwise sound discrimination index and a Naive Bayesian classifier are used to determine how these spiking patterns could provide brain signals for behavioral discrimination and classification of sounds. We find response durations and optimal time constants for discriminating sound envelope shape increase in rank order with: A1 < VAF < cSRAF. Accordingly, sustained spiking is more prominent and results in more robust sound discrimination in non-primary cortex versus A1. Spike-timing patterns classify 10 different sound envelope shape sequences and there is a twofold increase in maximal performance when pooling output across the neuron population indicating a robust distributed neural code in all three cortical fields. Together, these results support the idea that temporally precise spiking patterns from primary and non-primary auditory cortical fields provide the necessary signals for animals to discriminate and classify a large range of temporal shapes in the sound envelope.SIGNIFICANCE STATEMENT Functional hierarchies in the visual cortices support the concept that classification of visual objects requires successive cortical stages of processing including a progressive increase in classical receptive field size. The present study is significant as it supports the idea that a similar progression exists in auditory cortices in the time domain. We demonstrate for the first time that three cortices provide temporal spiking patterns for robust temporal envelope shape discrimination but only the ventral non-primary cortices do so on long time scales. This study raises the possibility that primary and non-primary cortices provide unique temporal spiking patterns and time scales for perception of sound envelope shape.

Detecting temporal changes in acoustic scenes: The variable benefit of selective attention

Four experiments investigated change detection in acoustic scenes consisting of a sum of five amplitude-modulated pure tones. As the tones were about 0.7 octave apart and were amplitude-modulated with different frequencies (in the range 2-32 Hz), they were perceived as separate streams. Listeners had to detect a change in the frequency (experiments 1 and 2) or the shape (experiments 3 and 4) of the modulation of one of the five tones, in the presence of an informative cue orienting selective attention either before the scene (pre-cue) or after it (post-cue). The changes left intensity unchanged and were not detectable in the spectral (tonotopic) domain. Performance was much better with pre-cues than with post-cues. Thus, change deafness was manifest in the absence of an appropriate focusing of attention when the change occurred, even though the streams and the changes to be detected were acoustically very simple (in contrast to the conditions used in previous demonstrations of change deafness). In one case, the results were consistent with a model based on the assumption that change detection was possible if and only if attention was endogenously focused on a single tone. However, it was also found that changes resulting in a steepening of amplitude rises were to some extent able to draw attention exogenously. Change detection was not markedly facilitated when the change produced a discontinuity in the modulation domain, contrary to what could be expected from the perspective of predictive coding.

Name that Percussive Tune: Associative Memory and Amplitude Envelope

A series of experiments demonstrated novel effects of amplitude envelope on associative memory, with tones exhibiting naturally decaying amplitude envelopes (e.g., those made by two wine glasses clinking) better associated with target objects than amplitude-invariant tones. In Experiment 1 participants learned associations between household objects and 4-note tone sequences constructed of spectrally matched pure tones with either “flat” or “percussive” amplitude envelopes. Those hearing percussive tones correctly recalled significantly more sequence–object associations. Experiment 2 demonstrated that participants hearing percussive tones learned the associations more quickly. Experiment 3 used “reverse percussive” tones (percussive tones played backwards) to test whether differences in overall energy might account for this effect, finding they did not lead to the same level of performance as percussive tones. Experiment 4 varied the envelope at encoding and retrieval to determine which stage of the task was most affected by the envelope manipulation. Participants hearing percussive tones at both encoding and retrieval performed significantly better than the other three groups (i.e., flat at encoding/percussive at retrieval, etc.). We conclude that amplitude envelope plays an important role in learning and memory, a finding with relevance to psychological research on audition and associative memory, as well as practical relevance for improving human–computer interface design.

Spectral and spatial tuning of onset and offset response functions in auditory cortical fields A1 and CL of rhesus macaques

The mammalian auditory cortex is necessary for spectral and spatial processing of acoustic stimuli. Most physiological studies of single neurons in the auditory cortex have focused on the onset and sustained portions of evoked responses, but there have been far fewer studies on the relationship between onset and offset responses. In the current study, we compared spectral and spatial tuning of onset and offset responses of neurons in primary auditory cortex (A1) and the caudolateral (CL) belt area of awake macaque monkeys. Several different metrics were used to determine the relationship between onset and offset response profiles in both frequency and space domains. In the frequency domain, a substantial proportion of neurons in A1 and CL displayed highly dissimilar best stimuli for onset- and offset-evoked responses, although even for these neurons, there was usually a large overlap in the range of frequencies that elicited onset, and offset responses and distributions of tuning overlap metrics were mostly unimodal. In the spatial domain, the vast majority of neurons displayed very similar best locations for onset- and offset-evoked responses, along with unimodal distributions of all tuning overlap metrics considered. Finally, for both spectral and spatial tuning, a slightly larger fraction of neurons in A1 displayed nonoverlapping onset and offset response profiles, relative to CL, which supports hierarchical differences in the processing of sounds in the two areas. However, these differences are small compared with differences in proportions of simple cells (low overlap) and complex cells (high overlap) in primary and secondary visual areas.NEW & NOTEWORTHY In the current study, we examine the relationship between the tuning of neural responses evoked by the onset and offset of acoustic stimuli in the primary auditory cortex, as well as a higher-order auditory area-the caudolateral belt field-in awake rhesus macaques. In these areas, the relationship between onset and offset response profiles in frequency and space domains formed a continuum, ranging from highly overlapping to highly nonoverlapping.

The effect of distraction on change detection in crowded acoustic scenes

In this series of behavioural experiments we investigated the effect of distraction on the maintenance of acoustic scene information in short-term memory. Stimuli are artificial acoustic ‘scenes’ composed of several (up to twelve) concurrent tone-pip streams (‘sources’). A gap (1000 ms) is inserted partway through the ‘scene’; Changes in the form of an appearance of a new source or disappearance of an existing source, occur after the gap in 50% of the trials. Listeners were instructed to monitor the unfolding ‘soundscapes’ for these events. Distraction was measured by presenting distractor stimuli during the gap. Experiments 1 and 2 used a dual task design where listeners were required to perform a task with varying attentional demands (‘High Demand’ vs. ‘Low Demand’) on brief auditory (Experiment 1a) or visual (Experiment 1b) signals presented during the gap. Experiments 2 and 3 required participants to ignore distractor sounds and focus on the change detection task. Our results demonstrate that the maintenance of scene information in short-term memory is influenced by the availability of attentional and/or processing resources during the gap, and that this dependence appears to be modality specific. We also show that these processes are susceptible to bottom up driven distraction even in situations when the distractors are not novel, but occur on each trial. Change detection performance is systematically linked with the, independently determined, perceptual salience of the distractor sound. The findings also demonstrate that the present task may be a useful objective means for determining relative perceptual salience.

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Distraction is measured by presenting distractor stimuli during a scene gap.

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Scene maintenance in memory depends on availability of resources during the gap.

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This dependence appears to be modality specific.

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Scene maintenance also prone to bottom up distraction even when distractors not novel.

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Performance depends on the perceptual salience of the distractor sound.

Perceptual Temporal Asymmetry Associated with Distinct ON and OFF Responses to Time-Varying Sounds with Rising versus Falling Intensity: A Magnetoencephalography Study

This magnetoencephalography (MEG) study investigated evoked ON and OFF responses to ramped and damped sounds in normal-hearing human adults. Two pairs of stimuli that differed in spectral complexity were used in a passive listening task; each pair contained identical acoustical properties except for the intensity envelope. Behavioral duration judgment was conducted in separate sessions, which replicated the perceptual bias in favour of the ramped sounds and the effect of spectral complexity on perceived duration asymmetry. MEG results showed similar cortical sites for the ON and OFF responses. There was a dominant ON response with stronger phase-locking factor (PLF) in the alpha (8–14 Hz) and theta (4–8 Hz) bands for the damped sounds. In contrast, the OFF response for sounds with rising intensity was associated with stronger PLF in the gamma band (30–70 Hz). Exploratory correlation analysis showed that the OFF response in the left auditory cortex was a good predictor of the perceived temporal asymmetry for the spectrally simpler pair. The results indicate distinct asymmetry in ON and OFF responses and neural oscillation patterns associated with the dynamic intensity changes, which provides important preliminary data for future studies to examine how the auditory system develops such an asymmetry as a function of age and learning experience and whether the absence of asymmetry or abnormal ON and OFF responses can be taken as a biomarker for certain neurological conditions associated with auditory processing deficits.

Mind the Gap: Two Dissociable Mechanisms of Temporal Processing in the Auditory System

High temporal acuity of auditory processing underlies perception of speech and other rapidly varying sounds. A common measure of auditory temporal acuity in humans is the threshold for detection of brief gaps in noise. Gap-detection deficits, observed in developmental disorders, are considered evidence for "sluggish" auditory processing. Here we show, in a mouse model of gap-detection deficits, that auditory brain sensitivity to brief gaps in noise can be impaired even without a general loss of central auditory temporal acuity. Extracellular recordings in three different subdivisions of the auditory thalamus in anesthetized mice revealed a stimulus-specific, subdivision-specific deficit in thalamic sensitivity to brief gaps in noise in experimental animals relative to controls. Neural responses to brief gaps in noise were reduced, but responses to other rapidly changing stimuli unaffected, in lemniscal and nonlemniscal (but not polysensory) subdivisions of the medial geniculate body. Through experiments and modeling, we demonstrate that the observed deficits in thalamic sensitivity to brief gaps in noise arise from reduced neural population activity following noise offsets, but not onsets. These results reveal dissociable sound-onset-sensitive and sound-offset-sensitive channels underlying auditory temporal processing, and suggest that gap-detection deficits can arise from specific impairment of the sound-offset-sensitive channel.

SIGNIFICANCE STATEMENT

The experimental and modeling results reported here suggest a new hypothesis regarding the mechanisms of temporal processing in the auditory system. Using a mouse model of auditory temporal processing deficits, we demonstrate the existence of specific abnormalities in auditory thalamic activity following sound offsets, but not sound onsets. These results reveal dissociable sound-onset-sensitive and sound-offset-sensitive mechanisms underlying auditory processing of temporally varying sounds. Furthermore, the findings suggest that auditory temporal processing deficits, such as impairments in gap-in-noise detection, could arise from reduced brain sensitivity to sound offsets alone.

Attention selectively modulates cortical entrainment in different regions of the speech spectrum

Recent studies have uncovered a neural response that appears to track the envelope of speech, and have shown that this tracking process is mediated by attention. It has been argued that this tracking reflects a process of phase-locking to the fluctuations of stimulus energy, ensuring that this energy arrives during periods of high neuronal excitability. Because all acoustic stimuli are decomposed into spectral channels at the cochlea, and this spectral decomposition is maintained along the ascending auditory pathway and into auditory cortex, we hypothesized that the overall stimulus envelope is not as relevant to cortical processing as the individual frequency channels; attention may be mediating envelope tracking differentially across these spectral channels. To test this we reanalyzed data reported by Horton et al. (2013), where high-density EEG was recorded while adults attended to one of two competing naturalistic speech streams. In order to simulate cochlear filtering, the stimuli were passed through a gammatone filterbank, and temporal envelopes were extracted at each filter output. Following Horton et al. (2013), the attended and unattended envelopes were cross-correlated with the EEG, and local maxima were extracted at three different latency ranges corresponding to distinct peaks in the cross-correlation function (N1, P2, and N2). We found that the ratio between the attended and unattended cross-correlation functions varied across frequency channels in the N1 latency range, consistent with the hypothesis that attention differentially modulates envelope-tracking activity across spectral channels.

Neural dynamics of change detection in crowded acoustic scenes

Two key questions concerning change detection in crowded acoustic environments are the extent to which cortical processing is specialized for different forms of acoustic change and when in the time-course of cortical processing neural activity becomes predictive of behavioral outcomes. Here, we address these issues by using magnetoencephalography (MEG) to probe the cortical dynamics of change detection in ongoing acoustic scenes containing as many as ten concurrent sources. Each source was formed of a sequence of tone pips with a unique carrier frequency and temporal modulation pattern, designed to mimic the spectrotemporal structure of natural sounds. Our results show that listeners are more accurate and quicker to detect the appearance (than disappearance) of an auditory source in the ongoing scene. Underpinning this behavioral asymmetry are change-evoked responses differing not only in magnitude and latency, but also in their spatial patterns. We find that even the earliest (~50 ms) cortical response to change is predictive of behavioral outcomes (detection times), consistent with the hypothesized role of local neural transients in supporting change detection.

Characterization of Rebound Depolarization in Neurons of the Rat Medial Geniculate Body In Vitro

Rebound depolarization (RD) is a response to the offset from hyperpolarization of the neuronal membrane potential and is an important mechanism for the synaptic processing of inhibitory signals. In the present study, we characterized RD in neurons of the rat medial geniculate body (MGB), a nucleus of the auditory thalamus, using whole-cell patch-clamp and brain slices. RD was proportional in strength to the duration and magnitude of the hyperpolarization; was effectively blocked by Ni(2+) or Mibefradil; and was depressed when the resting membrane potential was hyperpolarized by blocking hyperpolarization-activated cyclic nucleotide-gated (HCN) channels with ZD7288 or by activating G-protein-gated inwardly-rectifying K(+) (GIRK) channels with baclofen. Our results demonstrated that RD in MGB neurons, which is carried by T-type Ca(2+) channels, is critically regulated by HCN channels and likely by GIRK channels.

Electrophysiological and psychophysical asymmetries in sensitivity to interaural correlation gaps and implications for binaural integration time

Brief deviations of interaural correlation (IAC) can provide valuable cues for detection, segregation and localization of acoustic signals. This study investigated the processing of such "binaural gaps" in continuously running noise (100-2000 Hz), in comparison to silent "monaural gaps", by measuring late auditory evoked potentials (LAEPs) and perceptual thresholds with novel, iteratively optimized stimuli. Mean perceptual binaural gap duration thresholds exhibited a major asymmetry: they were substantially shorter for uncorrelated gaps in correlated and anticorrelated reference noise (1.75 ms and 4.1 ms) than for correlated and anticorrelated gaps in uncorrelated reference noise (26.5 ms and 39.0 ms). The thresholds also showed a minor asymmetry: they were shorter in the positive than in the negative IAC range. The mean behavioral threshold for monaural gaps was 5.5 ms. For all five gap types, the amplitude of LAEP components N1 and P2 increased linearly with the logarithm of gap duration. While perceptual and electrophysiological thresholds matched for monaural gaps, LAEP thresholds were about twice as long as perceptual thresholds for uncorrelated gaps, but half as long for correlated and anticorrelated gaps. Nevertheless, LAEP thresholds showed the same asymmetries as perceptual thresholds. For gap durations below 30 ms, LAEPs were dominated by the processing of the leading edge of a gap. For longer gap durations, in contrast, both the leading and the lagging edge of a gap contributed to the evoked response. Formulae for the equivalent rectangular duration (ERD) of the binaural system's temporal window were derived for three common window shapes. The psychophysical ERD was 68 ms for diotic and about 40 ms for anti- and uncorrelated noise. After a nonlinear Z-transform of the stimulus IAC prior to temporal integration, ERDs were about 10 ms for reference correlations of ±1 and 80 ms for uncorrelated reference. Hence, a physiologically motivated peripheral nonlinearity changed the rank order of ERDs across experimental conditions in a plausible manner.

Auditory gap-in-noise detection behavior in ferrets and humans

The precise encoding of temporal features of auditory stimuli by the mammalian auditory system is critical to the perception of biologically important sounds, including vocalizations, speech, and music. In this study, auditory gap-detection behavior was evaluated in adult pigmented ferrets (Mustelid putorius furo) using bandpassed stimuli designed to widely sample the ferret's behavioral and physiological audiogram. Animals were tested under positive operant conditioning, with psychometric functions constructed in response to gap-in-noise lengths ranging from 3 to 270 ms. Using a modified version of this gap-detection task, with the same stimulus frequency parameters, we also tested a cohort of normal-hearing human subjects. Gap-detection thresholds were computed from psychometric curves transformed according to signal detection theory, revealing that for both ferrets and humans, detection sensitivity was worse for silent gaps embedded within low-frequency noise compared with high-frequency or broadband stimuli. Additional psychometric function analysis of ferret behavior indicated effects of stimulus spectral content on aspects of behavioral performance related to decision-making processes, with animals displaying improved sensitivity for broadband gap-in-noise detection. Reaction times derived from unconditioned head-orienting data and the time from stimulus onset to reward spout activation varied with the stimulus frequency content and gap length, as well as the approach-to-target choice and reward location. The present study represents a comprehensive evaluation of gap-detection behavior in ferrets, while similarities in performance with our human subjects confirm the use of the ferret as an appropriate model of temporal processing.

Speech encoding by coupled cortical theta and gamma oscillations

Many environmental stimuli present a quasi-rhythmic structure at different timescales that the brain needs to decompose and integrate. Cortical oscillations have been proposed as instruments of sensory de-multiplexing, i.e., the parallel processing of different frequency streams in sensory signals. Yet their causal role in such a process has never been demonstrated. Here, we used a neural microcircuit model to address whether coupled theta–gamma oscillations, as observed in human auditory cortex, could underpin the multiscale sensory analysis of speech. We show that, in continuous speech, theta oscillations can flexibly track the syllabic rhythm and temporally organize the phoneme-level response of gamma neurons into a code that enables syllable identification. The tracking of slow speech fluctuations by theta oscillations, and its coupling to gamma-spiking activity both appeared as critical features for accurate speech encoding. These results demonstrate that cortical oscillations can be a key instrument of speech de-multiplexing, parsing, and encoding.

DOI:http://dx.doi.org/10.7554/eLife.06213.001

Some people speak twice as fast as others, while people with different accents pronounce the same words in different ways. However, despite these differences between speakers, humans can usually follow spoken language with remarkable ease.

The different elements of speech have different frequencies: the typical frequency for syllables, for example, is about four syllables per second in speech. Phonemes, which are the smallest elements of speech, appear at a higher frequency. However, these elements are all transmitted at the same time, so the brain needs to be able to process them simultaneously.

The auditory cortex, the part of the brain that processes sound, produces various ‘waves’ of electrical activity, and these waves also have a characteristic frequency (which is the number of bursts of neural activity per second). One type of brain wave, called the theta rhythm, has a frequency of three to eight bursts per second, which is similar to the typical frequency of syllables in speech, and the frequency of another brain wave, the gamma rhythm, is similar to the frequency of phonemes. It has been suggested that these two brain waves may have a central role in our ability to follow speech, but to date there has been no direct evidence to support this theory.

Hyafil et al. have now used computer models of neural oscillations to explore this theory. Their simulations show that, as predicted, the theta rhythm tracks the syllables in spoken language, while the gamma rhythm encodes the specific features of each phoneme. Moreover, the two rhythms work together to establish the sequence of phonemes that makes up each syllable. These findings will support the development of improved speech recognition technologies.

DOI:http://dx.doi.org/10.7554/eLife.06213.002

Sensitivity to the temporal structure of rapid sound sequences - An MEG study

To probe sensitivity to the time structure of ongoing sound sequences, we measured MEG responses, in human listeners, to the offset of long tone-pip sequences containing various forms of temporal regularity. If listeners learn sequence temporal properties and form expectancies about the arrival time of an upcoming tone, sequence offset should be detectable as soon as an expected tone fails to arrive. Therefore, latencies of offset responses are indicative of the extent to which the temporal pattern has been acquired. In Exp1, sequences were isochronous with tone inter-onset-interval (IOI) set to 75, 125 or 225ms. Exp2 comprised of non-isochronous, temporally regular sequences, comprised of the IOIs above. Exp3 used the same sequences as Exp2 but listeners were required to monitor them for occasional frequency deviants. Analysis of the latency of offset responses revealed that the temporal structure of (even rather simple) regular sequences is not learnt precisely when the sequences are ignored. Pattern coding, supported by a network of temporal, parietal and frontal sources, improved considerably when the signals were made behaviourally pertinent. Thus, contrary to what might be expected in the context of an 'early warning system' framework, learning of temporal structure is not automatic, but affected by the signal's behavioural relevance.

Perceptual gap detection is mediated by gap termination responses in auditory cortex

BACKGROUND

Understanding speech in the presence of background noise often becomes increasingly difficult with age. These age-related speech processing deficits reflect impairments in temporal acuity. Gap detection is a model for temporal acuity in speech processing in which a gap inserted in white noise acts as a cue that attenuates subsequent startle responses. Lesion studies have shown that auditory cortex is necessary for the detection of brief gaps, and auditory cortical neurons respond to the end of the gap with a characteristic burst of spikes called the gap termination response (GTR). However, it remains unknown whether and how the GTR plays a causal role in gap detection. We tested this by optogenetically suppressing the activity of somatostatin- or parvalbumin-expressing inhibitory interneurons, or CaMKII-expressing excitatory neurons, in auditory cortex of behaving mice during specific epochs of a gap detection protocol.

RESULTS

Suppressing interneuron activity during the postgap interval enhanced gap detection. Suppressing excitatory cells during this interval attenuated gap detection. Suppressing activity preceding the gap had the opposite behavioral effects, whereas prolonged suppression across both intervals had no effect on gap detection.

CONCLUSIONS

In addition to confirming cortical involvement, we demonstrate here for the first time a causal relationship between postgap neural activity and perceptual gap detection. Furthermore, our results suggest that gap detection involves an ongoing comparison of pre- and postgap spiking activity. Finally, we propose a simple yet biologically plausible neural circuit that reproduces each of these neural and behavioral results.

(Dis-)Harmony in movement: effects of musical dissonance on movement timing and form

While the origins of consonance and dissonance in terms of acoustics, psychoacoustics and physiology have been debated for centuries, their plausible effects on movement synchronization have largely been ignored. The present study aimed to address this by investigating whether, and if so how, consonant/dissonant pitch intervals affect the spatiotemporal properties of regular reciprocal aiming movements. We compared movements synchronized either to consonant or to dissonant sounds and showed that they were differentially influenced by the degree of consonance of the sound presented. Interestingly, the difference was present after the sound stimulus was removed. In this case, the performance measured after consonant sound exposure was found to be more stable and accurate, with a higher percentage of information/movement coupling (tau coupling) and a higher degree of movement circularity when compared to performance measured after the exposure to dissonant sounds. We infer that the neural resonance representing consonant tones leads to finer perception/action coupling which in turn may help explain the prevailing preference for these types of tones.

Diverse cortical codes for scene segmentation in primate auditory cortex

The temporal coherence of amplitude fluctuations is a critical cue for segmentation of complex auditory scenes. The auditory system must accurately demarcate the onsets and offsets of acoustic signals. We explored how and how well the timing of onsets and offsets of gated tones are encoded by auditory cortical neurons in awake rhesus macaques. Temporal features of this representation were isolated by presenting otherwise identical pure tones of differing durations. Cortical response patterns were diverse, including selective encoding of onset and offset transients, tonic firing, and sustained suppression. Spike train classification methods revealed that many neurons robustly encoded tone duration despite substantial diversity in the encoding process. Excellent discrimination performance was achieved by neurons whose responses were primarily phasic at tone offset and by those that responded robustly while the tone persisted. Although diverse cortical response patterns converged on effective duration discrimination, this diversity significantly constrained the utility of decoding models referenced to a spiking pattern averaged across all responses or averaged within the same response category. Using maximum likelihood-based decoding models, we demonstrated that the spike train recorded in a single trial could support direct estimation of stimulus onset and offset. Comparisons between different decoding models established the substantial contribution of bursts of activity at sound onset and offset to demarcating the temporal boundaries of gated tones. Our results indicate that relatively few neurons suffice to provide temporally precise estimates of such auditory "edges," particularly for models that assume and exploit the heterogeneity of neural responses in awake cortex.

Auditory cortex is required for fear potentiation of gap detection

Auditory cortex is necessary for the perceptual detection of brief gaps in noise, but is not necessary for many other auditory tasks such as frequency discrimination, prepulse inhibition of startle responses, or fear conditioning with pure tones. It remains unclear why auditory cortex should be necessary for some auditory tasks but not others. One possibility is that auditory cortex is causally involved in gap detection and other forms of temporal processing in order to associate meaning with temporally structured sounds. This predicts that auditory cortex should be necessary for associating meaning with gaps. To test this prediction, we developed a fear conditioning paradigm for mice based on gap detection. We found that pairing a 10 or 100 ms gap with an aversive stimulus caused a robust enhancement of gap detection measured 6 h later, which we refer to as fear potentiation of gap detection. Optogenetic suppression of auditory cortex during pairing abolished this fear potentiation, indicating that auditory cortex is critically involved in associating temporally structured sounds with emotionally salient events.

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.

Timing matters: the processing of pitch relations

The human central auditory system can automatically extract abstract regularities from a variant auditory input. To this end, temporarily separated events need to be related. This study tested whether the timing between events, falling either within or outside the temporal window of integration (~350 ms), impacts the extraction of abstract feature relations. We utilized tone pairs for which tones within but not across pairs revealed a constant pitch relation (e.g., pitch of second tone of a pair higher than pitch of first tone, while absolute pitch values varied across pairs). We measured the mismatch negativity (MMN; the brain's error signal to auditory regularity violations) to second tones that rarely violated the pitch relation (e.g., pitch of second tone lower). A Short condition in which tone duration (90 ms) and stimulus onset asynchrony between the tones of a pair were short (110 ms) was compared to two conditions, where this onset asynchrony was long (510 ms). In the Long Gap condition, the tone durations were identical to Short (90 ms), but the silent interval was prolonged by 400 ms. In Long Tone, the duration of the first tone was prolonged by 400 ms, while the silent interval was comparable to Short (20 ms). Results show a frontocentral MMN of comparable amplitude in all conditions. Thus, abstract pitch relations can be extracted even when the within-pair timing exceeds the integration period. Source analyses indicate MMN generators in the supratemporal cortex. Interestingly, they were located more anterior in Long Gap than in Short and Long Tone. Moreover, frontal generator activity was found for Long Gap and Long Tone. Thus, the way in which the system automatically registers irregular abstract pitch relations depends on the timing of the events to be linked. Pending that the current MMN data mirror established abstract rule representations coding the regular pitch relation, neural processes building these templates vary with timing.

The function of offset neurons in auditory information processing

Offset neurons which respond to the termination of the sound stimulation may play important roles in auditory temporal information processing, sound signal recognition, and complex distinction. Two additional possible mechanisms were reviewed: neural inhibition and the intrinsic conductance property of offset neuron membranes. The underlying offset response was postulated to be located in the superior paraolivary nucleus of mice. The biological significance of the offset neurons was discussed as well.

Are interaural time and level differences represented by independent or integrated codes in the human auditory cortex?

Sound localization is important for orienting and focusing attention and for segregating sounds from different sources in the environment. In humans, horizontal sound localization mainly relies on interaural differences in sound arrival time and sound level. Despite their perceptual importance, the neural processing of interaural time and level differences (ITDs and ILDs) remains poorly understood. Animal studies suggest that, in the brainstem, ITDs and ILDs are processed independently by different specialized circuits. The aim of the current study was to investigate whether, at higher processing levels, they remain independent or are integrated into a common code of sound laterality. For that, we measured late auditory cortical potentials in response to changes in sound lateralization elicited by perceptually matched changes in ITD and/or ILD. The responses to the ITD and ILD changes exhibited significant morphological differences. At the same time, however, they originated from overlapping areas of the cortex and showed clear evidence for functional coupling. These results suggest that the auditory cortex contains an integrated code of sound laterality, but also retains independent information about ITD and ILD cues. This cue-related information might be used to assess how consistent the cues are, and thus, how likely they would have arisen from the same source.

No time-stretching illusion when a tone is followed by a noise

A sine tone is perceived as longer when it is preceded by a more intense noise than when presented in isolation. This is the time-stretching illusion. We conducted an experiment where the method of constant stimuli was used to examine whether a tone would also be stretched when it was followed by a noise. The duration of a tone was overestimated when it was preceded by a noise, but not when followed by a noise or when located between two consecutive noises. Moreover, the increasing of the noise intensity (from -6 to +6 dB) relative to the tone intensity resulted in larger overestimations, but only in the condition where a tone was preceded by a noise. In brief, the duration of a tone is stretched when this tone is preceded by a noise and if this tone is not followed by a noise.

Sensitivity of offset and onset cortical auditory evoked potentials to signals in noise

OBJECTIVE

The purpose of this study was to determine the effects of SNR and signal level on the offset response of the cortical auditory evoked potential (CAEP). Successful listening often depends on how well the auditory system can extract target signals from competing background noise. Both signal onsets and offsets are encoded neurally and contribute to successful listening in noise. Neural onset responses to signals in noise demonstrate a strong sensitivity to signal-to-noise ratio (SNR) rather than signal level; however, the sensitivity of neural offset responses to these cues is not known.

METHODS

We analyzed the offset response from two previously published datasets for which only the onset response was reported. For both datasets, CAEPs were recorded from young normal-hearing adults in response to a 1000-Hz tone. For the first dataset, tones were presented at seven different signal levels without background noise, while the second dataset varied both signal level and SNR.

RESULTS

Offset responses demonstrated sensitivity to absolute signal level in quiet, SNR, and to absolute signal level in noise.

CONCLUSIONS

Offset sensitivity to signal level when presented in noise contrasts with previously published onset results.

SIGNIFICANCE

This sensitivity suggests a potential clinical measure of cortical encoding of signal level in noise.

The role of sensitivity to transients in the detection of appearing and disappearing objects in complex acoustic scenes

We report a series of psychophysics experiments that investigated listeners' sensitivity to changes in complex acoustic scenes. Specifically, we sought to test the hypothesis that change detection is supported by sensitivity to change-related transients (an abrupt change in stimulus power within a certain frequency band, associated with the appearance or disappearance of a scene element). This hypothesis, in the context of natural scenes, is commonly dismissed on account that the elements of the scene may themselves be characterized by on-going energy fluctuations that would mask any genuine change-related transients. We created artificial 'scenes' populated by multiple pure-tone components. Tones were modulated (by a square wave at a distinct rate) so as to mimic the fluctuation properties of complex sounds. "Change" was defined as the appearance or disappearance of one such element. Importantly, such scenes lack semantic attributes, which may have been a limiting factor in interpreting previous auditory change-detection studies, thus allowing us to probe the low-level, pre-semantic, processes involved in auditory change perception. In Experiment 1 we measured listeners' ability to detect item appearance and disappearance in conditions where change-related transients are masked by a silent gap. In Experiment 2, we investigated the effect of an acoustic distractor - a brief signal that occurs at the time of change, but does not mask any scene components. The data show that gaps adversely affected the processing of item appearance but not disappearance. However, distractors reduced both -appearance and disappearance detection. Together our results suggest a role for sensitivity to transients in the process of auditory change detection, similar to what has been demonstrated for visual change detection.