Acoustic features and acoustic changes are represented by different central pathways

Automatic Sensory Predictions: A Review of Predictive Mechanisms in the Brain and Their Link to Conscious Processing

The human brain has the astonishing capacity of integrating streams of sensory information from the environment and forming predictions about future events in an automatic way. Despite being initially developed for visual processing, the bulk of predictive coding research has subsequently focused on auditory processing, with the famous mismatch negativity signal as possibly the most studied signature of a surprise or prediction error (PE) signal. Auditory PEs are present during various consciousness states. Intriguingly, their presence and characteristics have been linked with residual levels of consciousness and return of awareness. In this review we first give an overview of the neural substrates of predictive processes in the auditory modality and their relation to consciousness. Then, we focus on different states of consciousness - wakefulness, sleep, anesthesia, coma, meditation, and hypnosis - and on what mysteries predictive processing has been able to disclose about brain functioning in such states. We review studies investigating how the neural signatures of auditory predictions are modulated by states of reduced or lacking consciousness. As a future outlook, we propose the combination of electrophysiological and computational techniques that will allow investigation of which facets of sensory predictive processes are maintained when consciousness fades away.

Language learning as a function of infant directed speech (IDS) in Spanish: Testing neural commitment using the positive-MMR

Spanish-English bilingual families (N = 17) were recruited to assess the association between infant directed speech (IDS) in Spanish and their degree of neural commitment to the Spanish language. IDS was assessed by extracting the caregivers' Vowel Space Area (VSA) from recordings of a storybook reading task done at home. Infants' neural commitment was assessed by extracting the positive mismatch brain response (positive-MMR), an Event-Related Potential (ERP) thought to be indicative of higher attentional processes and early language commitment. A linear mixed model analysis demonstrated that caregivers' VSA predicted the amplitude of the positive-MMR in response to a native speech contrast (Spanish), but not to a non-native speech contrast (Chinese), even after holding other predictors constant (i.e., socioeconomic status, infants' age, and fundamental frequency). Our findings provide support to the view that quality of language exposure fosters language learning, and that this beneficial relationship expands to the bilingual population.

Evolving perspectives on the sources of the frequency-following response

The auditory frequency-following response (FFR) is a non-invasive index of the fidelity of sound encoding in the brain, and is used to study the integrity, plasticity, and behavioral relevance of the neural encoding of sound. In this Perspective, we review recent evidence suggesting that, in humans, the FFR arises from multiple cortical and subcortical sources, not just subcortically as previously believed, and we illustrate how the FFR to complex sounds can enhance the wider field of auditory neuroscience. Far from being of use only to study basic auditory processes, the FFR is an uncommonly multifaceted response yielding a wealth of information, with much yet to be tapped.

Topographic Distribution of Stimulus-Specific Adaptation across Auditory Cortical Fields in the Anesthetized Rat

Stimulus-specific adaptation (SSA) in single neurons of the auditory cortex was suggested to be a potential neural correlate of the mismatch negativity (MMN), a widely studied component of the auditory event-related potentials (ERP) that is elicited by changes in the auditory environment. However, several aspects on this SSA/MMN relation remain unresolved. SSA occurs in the primary auditory cortex (A1), but detailed studies on SSA beyond A1 are lacking. To study the topographic organization of SSA, we mapped the whole rat auditory cortex with multiunit activity recordings, using an oddball paradigm. We demonstrate that SSA occurs outside A1 and differs between primary and nonprimary cortical fields. In particular, SSA is much stronger and develops faster in the nonprimary than in the primary fields, paralleling the organization of subcortical SSA. Importantly, strong SSA is present in the nonprimary auditory cortex within the latency range of the MMN in the rat and correlates with an MMN-like difference wave in the simultaneously recorded local field potentials (LFP). We present new and strong evidence linking SSA at the cellular level to the MMN, a central tool in cognitive and clinical neuroscience.

This study of higher-order auditory cortex strengthens the case for long-latency stimulus-specific adaptation as a genuine neural correlate of the mismatch negativity, which flags salient stimuli.

Sensory systems automatically detect salient events in a monotonous ambient background. In humans, this change detection process is indexed by the mismatch negativity (MMN), a mid-late component of the auditory-evoked potentials that has become a central tool in cognitive and clinical neuroscience over the last 40 years. However, the neuronal correlate of MMN remains controversial. Stimulus-specific adaptation (SSA) is a special type of adaptation recorded at the neuronal level in the auditory pathway. Attenuating the response only to repetitive, background stimuli is a very efficient mechanism to enhance the saliency of any upcoming deviant or novel stimulus. Thus, SSA was originally proposed as a neural correlate of the MMN, but previous studies in the auditory cortex reported SSA only at very early latencies (circa 20–30 ms) and only within the primary auditory cortex (A1), whereas MMN analogs in the rat occur later, between 50 and 100 ms after change onset, and are generated mainly within nonprimary fields. Here, we report very strong SSA in nonprimary fields within the latency range of the MMN in the rat, providing empirical evidence of the missing link between single neuron response studies in animal models and the human MMN.

The cortical modulation of stimulus-specific adaptation in the auditory midbrain and thalamus: a potential neuronal correlate for predictive coding

To follow an ever-changing auditory scene, the auditory brain is continuously creating a representation of the past to form expectations about the future. Unexpected events will produce an error in the predictions that should “trigger” the network’s response. Indeed, neurons in the auditory midbrain, thalamus and cortex, respond to rarely occurring sounds while adapting to frequently repeated ones, i.e., they exhibit stimulus specific adaptation (SSA). SSA cannot be explained solely by intrinsic membrane properties, but likely involves the participation of the network. Thus, SSA is envisaged as a high order form of adaptation that requires the influence of cortical areas. However, present research supports the hypothesis that SSA, at least in its simplest form (i.e., to frequency deviants), can be transmitted in a bottom-up manner through the auditory pathway. Here, we briefly review the underlying neuroanatomy of the corticofugal projections before discussing state of the art studies which demonstrate that SSA present in the medial geniculate body (MGB) and inferior colliculus (IC) is not inherited from the cortex but can be modulated by the cortex via the corticofugal pathways. By modulating the gain of neurons in the thalamus and midbrain, the auditory cortex (AC) would refine SSA subcortically, preventing irrelevant information from reaching the cortex.

Enhancement of sleep slow waves: underlying mechanisms and practical consequences

Even modest sleep restriction, especially the loss of sleep slow wave activity (SWA), is invariably associated with slower electroencephalogram (EEG) activity during wake, the occurrence of local sleep in an otherwise awake brain, and impaired performance due to cognitive and memory deficits. Recent studies not only confirm the beneficial role of sleep in memory consolidation, but also point to a specific role for sleep slow waves. Thus, the implementation of methods to enhance sleep slow waves without unwanted arousals or lightening of sleep could have significant practical implications. Here we first review the evidence that it is possible to enhance sleep slow waves in humans using transcranial direct-current stimulation (tDCS) and transcranial magnetic stimulation. Since these methods are currently impractical and their safety is questionable, especially for chronic long-term exposure, we then discuss novel data suggesting that it is possible to enhance slow waves using sensory stimuli. We consider the physiology of the K-complex (KC), a peripheral evoked slow wave, and show that, among different sensory modalities, acoustic stimulation is the most effective in increasing the magnitude of slow waves, likely through the activation of non-lemniscal ascending pathways to the thalamo-cortical system. In addition, we discuss how intensity and frequency of the acoustic stimuli, as well as exact timing and pattern of stimulation, affect sleep enhancement. Finally, we discuss automated algorithms that read the EEG and, in real-time, adjust the stimulation parameters in a closed-loop manner to obtain an increase in sleep slow waves and avoid undesirable arousals. In conclusion, while discussing the mechanisms that underlie the generation of sleep slow waves, we review the converging evidence showing that acoustic stimulation is safe and represents an ideal tool for slow wave sleep (SWS) enhancement.

Adaptation in the auditory system: an overview

The early stages of the auditory system need to preserve the timing information of sounds in order to extract the basic features of acoustic stimuli. At the same time, different processes of neuronal adaptation occur at several levels to further process the auditory information. For instance, auditory nerve fiber responses already experience adaptation of their firing rates, a type of response that can be found in many other auditory nuclei and may be useful for emphasizing the onset of the stimuli. However, it is at higher levels in the auditory hierarchy where more sophisticated types of neuronal processing take place. For example, stimulus-specific adaptation, where neurons show adaptation to frequent, repetitive stimuli, but maintain their responsiveness to stimuli with different physical characteristics, thus representing a distinct kind of processing that may play a role in change and deviance detection. In the auditory cortex, adaptation takes more elaborate forms, and contributes to the processing of complex sequences, auditory scene analysis and attention. Here we review the multiple types of adaptation that occur in the auditory system, which are part of the pool of resources that the neurons employ to process the auditory scene, and are critical to a proper understanding of the neuronal mechanisms that govern auditory perception.

NR2B Antagonist CP-101,606 Abolishes Pitch-Mediated Deviance Detection in Awake Rats

Schizophrenia patients exhibit a decreased ability to detect change in their auditory environment as measured by auditory event-related potentials (ERP) such as mismatch negativity. This deficit has been linked to abnormal NMDA neurotransmission since, among other observations, non-selective channel blockers of NMDA reliably diminish automatic deviance detection in human subjects as well as in animal models. Recent molecular and functional evidence links NR2B receptor subtype to aberrant NMDA transmission in schizophrenia. However, it is unknown if NR2B receptors participate in pre-attentive deviance detection. We recorded ERP from the vertex of freely behaving rats in response to frequency mismatch protocols. We saw a robust increase in N1 response to deviants compared to standard as well as control stimuli indicating true deviance detection. Moreover, the increased negativity was highly sensitive to deviant probability. Next, we tested the effect of a non-selective NMDA channel blocker (ketamine, 30 mg/kg) and a highly selective NR2B antagonist, CP-101,606 (10 or 30 mg/kg) on deviance detection. Ketamine attenuated deviance mainly by increasing the amplitude of the standard ERP. Amplitude and/or latency of several ERP components were also markedly affected. In contrast, CP-101,606 robustly and dose-dependently inhibited the deviant's N1 amplitude, and as a consequence, completely abolished deviance detection. No other ERPs or components were affected. Thus, we report first evidence that NR2B receptors robustly participate in processes of automatic deviance detection in a rodent model. Lastly, our model demonstrates a path forward to test specific pharmacological hypotheses using translational endpoints relevant to aberrant sensory processing in schizophrenia.

Cortical mapping of mismatch negativity with deviance detection property in rat

Mismatch Negativity (MMN) is an N-methyl-d-aspartic acid (NMDA)-mediated, negative deflection in human auditory evoked potentials in response to a cognitively discriminable change. MMN-like responses have been extensively investigated in animal models, but the existence of MMN equivalent is still controversial. In this study, we aimed to investigate how closely the putative MMN (MMNp) in rats exhibited the comparable properties of human MMN. We used a surface microelectrode array with a grid of 10×7 recording sites within an area of 4.5×3.0 mm to densely map evoked potentials in the auditory cortex of anesthetized rats under the oddball paradigm. Firstly, like human MMN, deviant stimuli elicited negative deflections in auditory evoked potentials following the positive middle-latency response, termed P1. Secondly, MMNp exhibited deviance-detecting property, which could not be explained by simple stimulus specific adaptation (SSA). Thirdly, this MMNp occurred focally in the auditory cortex, including both the core and belt regions, while P1 activation focus was obtained in the core region, indicating that both P1 and MMNp are generated in the auditory cortex, yet the sources of these signals do not completely overlap. Fourthly, MMNp significantly decreased after the application of AP5 (D-(-)-2-amino-5-phosphonopentanoic acid), an antagonist at NMDA receptors. In stark contrast, AP5 affected neither P1 amplitude nor SSA of P1. These results provide compelling evidence that the MMNp we have examined in rats is functionally comparable to human MMN. The present work will stimulate translational research into MMN, which may help bridge the gap between electroencephalography (EEG)/magnetoencephalography (MEG) studies in humans and electrophysiological studies in animals.

Stimulus-specific adaptation and deviance detection in the inferior colliculus

Deviancy detection in the continuous flow of sensory information into the central nervous system is of vital importance for animals. The task requires neuronal mechanisms that allow for an efficient representation of the environment by removing statistically redundant signals. Recently, the neuronal principles of auditory deviance detection have been approached by studying the phenomenon of stimulus-specific adaptation (SSA). SSA is a reduction in the responsiveness of a neuron to a common or repetitive sound while the neuron remains highly sensitive to rare sounds (Ulanovsky et al., 2003). This phenomenon could enhance the saliency of unexpected, deviant stimuli against a background of repetitive signals. SSA shares many similarities with the evoked potential known as the “mismatch negativity,” (MMN) and it has been linked to cognitive process such as auditory memory and scene analysis (Winkler et al., 2009) as well as to behavioral habituation (Netser et al., 2011). Neurons exhibiting SSA can be found at several levels of the auditory pathway, from the inferior colliculus (IC) up to the auditory cortex (AC). In this review, we offer an account of the state-of-the art of SSA studies in the IC with the aim of contributing to the growing interest in the single-neuron electrophysiology of auditory deviance detection. The dependence of neuronal SSA on various stimulus features, e.g., probability of the deviant stimulus and repetition rate, and the roles of the AC and inhibition in shaping SSA at the level of the IC are addressed.

Mismatch brain response to speech sound changes in rats

Understanding speech is based on neural representations of individual speech sounds. In humans, such representations are capable of supporting an automatic and memory-based mechanism for auditory change detection, as reflected by the mismatch negativity (MMN) of event-related potentials. There are also findings of neural representations of speech sounds in animals, but it is not known whether these representations can support the change detection mechanism analogous to that underlying the MMN in humans. To this end, we presented synthesized spoken syllables to urethane-anesthetized rats while local field potentials were epidurally recorded above their primary auditory cortex. In an oddball condition, a deviant stimulus /ga/ or /ba/ (probability 1:12 for each) was rarely and randomly interspersed between frequently presented standard stimulus /da/ (probability 10:12). In an equiprobable condition, 12 syllables, including /da/, /ga/, and /ba/, were presented in a random order (probability 1:12 for each). We found evoked responses of higher amplitude to the deviant /ba/, albeit not to /ga/, relative to the standard /da/ in the oddball condition. Furthermore, the responses to /ba/ were higher in amplitude in the oddball condition than in the equiprobable condition. The findings suggest that anesthetized rat’s brain can form representations of human speech sounds, and that these representations can support the memory-based change detection mechanism analogous to that underlying the MMN in humans. Our findings show a striking parallel in speech processing between humans and rodents and may thus pave the way for feasible animal models of memory-based change detection.

Memory-based mismatch response to frequency changes in rats

Any occasional changes in the acoustic environment are of potential importance for survival. In humans, the preattentive detection of such changes generates the mismatch negativity (MMN) component of event-related brain potentials. MMN is elicited to rare changes ('deviants') in a series of otherwise regularly repeating stimuli ('standards'). Deviant stimuli are detected on the basis of a neural comparison process between the input from the current stimulus and the sensory memory trace of the standard stimuli. It is, however, unclear to what extent animals show a similar comparison process in response to auditory changes. To resolve this issue, epidural potentials were recorded above the primary auditory cortex of urethane-anesthetized rats. In an oddball condition, tone frequency was used to differentiate deviants interspersed randomly among a standard tone. Mismatch responses were observed at 60-100 ms after stimulus onset for frequency increases of 5% and 12.5% but not for similarly descending deviants. The response diminished when the silent inter-stimulus interval was increased from 375 ms to 600 ms for +5% deviants and from 600 ms to 1000 ms for +12.5% deviants. In comparison to the oddball condition the response also diminished in a control condition in which no repetitive standards were presented (equiprobable condition). These findings suggest that the rat mismatch response is similar to the human MMN and indicate that anesthetized rats provide a valuable model for studies of central auditory processing.

The mismatch negativity (MMN) in basic research of central auditory processing: a review

In the present article, the basic research using the mismatch negativity (MMN) and analogous results obtained by using the magnetoencephalography (MEG) and other brain-imaging technologies is reviewed. This response is elicited by any discriminable change in auditory stimulation but recent studies extended the notion of the MMN even to higher-order cognitive processes such as those involving grammar and semantic meaning. Moreover, MMN data also show the presence of automatic intelligent processes such as stimulus anticipation at the level of auditory cortex. In addition, the MMN enables one to establish the brain processes underlying the initiation of attention switch to, conscious perception of, sound change in an unattended stimulus stream.

Mismatch Negativity and Stimulus-Specific Adaptation in Animal Models

Animal models of MMN may serve both to further our understanding of neural processing beyond pure sensory coding and for unraveling the neural and pharmacological processes involved in the generation of MMN. We start this review by discussing the methodological issues that are especially important when pursuing a single-neuron correlate of MMN. Correlates of MMN have been studied in mice, rats, cats, and primates. Whereas essentially all of these studies demonstrated the presence of stimulus-specific adaptation, in the sense that responses to deviant tones are larger than the responses to standard tones, the presence of real MMN has been established only in a few. We argue for the use of more and better controls in order to clarify the situation. Finally, we discuss in detail the relationships between stimulus-specific adaptation of single-neuron responses, as established in the cat auditory cortex, and MMN. We argue that this is currently the only fully established correlate of true change detection, and hypothesize that it precedes and probably induces the neural activity that is eventually measured as MMN.

Memory-based detection of rare sound feature combinations in anesthetized rats

It is unclear whether the ability of the brain to discriminate rare from frequently repeated combinations of sound features is limited to the normal sleep/wake cycle. We recorded epidural auditory event-related potentials in urethane-anesthetized rats presented with rare tones ('deviants') interspersed with frequently repeated ones ('standards'). Deviants differed from standards either in frequency alone or in frequency combined with intensity. In both cases, deviants elicited event-related potentials exceeding in amplitude event-related potentials to standards between 76 and 108 ms from the stimulus onset, suggesting the independence of the underlying integrative and memory-based change detection mechanisms of the brain from the normal sleep/wake cycle. The relations of these event-related potentials to mismatch negativity and N1 in humans are addressed.

Novelty detector neurons in the mammalian auditory midbrain

Novel stimuli in all sensory modalities are highly effective in attracting and focusing attention. Stimulus-specific adaptation (SSA) and brain activity evoked by novel stimuli have been studied using population measures such as imaging and event-related potentials, but there have been few studies at the single-neuron level. In this study we compare SSA across different populations of neurons in the inferior colliculus (IC) of the rat and show that a subclass of neurons with rapid and pronounced SSA respond selectively to novel sounds. These neurons, located in the dorsal and external cortex of the IC, fail to respond to multiple repetitions of a sound but briefly recover their excitability when some stimulus parameter is changed. The finding of neurons that respond selectively to novel stimuli in the mammalian auditory midbrain suggests that they may contribute to a rapid subcortical pathway for directing attention and/or orienting responses to novel sounds.

Acute effect of nicotine on auditory gating in smokers and non-smokers

This paper investigates the role of cholinergic mechanisms in auditory gating by assessing the acute effects of nicotine, an acetylcholinomimetic drug, on behavioral and electrophysiological measures of consonant-vowel (CV) discrimination in quiet and in broadband noise (BBN). In a single-blind procedure, categorical boundaries and mismatch negativity (MMN) in two conditions (quiet, BBN) were obtained from 10 non-smokers and 4 smokers with normal hearing under two drug conditions (nicotine, placebo). After the nicotine sessions, plasma tests revealed a subject's nicotine concentration and subjects reported any symptoms. Larger MMN areas and steeper slopes at the boundary were interpreted as reflecting better electrophysiological and behavioral CV discrimination, respectively. Results indicate that, in non-smokers, the effects of nicotine on electrophysiological CV discrimination in quiet increase with an increase in severity of symptoms. Specifically, asymptomatic non-smokers (N = 5) demonstrate little improvement (and sometimes decrements) in performance while symptomatic non-smokers (N = 5) exhibit nicotine-enhanced discrimination, as do smokers. In noise, all subjects demonstrate nicotine-enhanced behavioral and electrophysiological discrimination. Additionally, in noise, smokers exhibit a larger number of measurable categorical boundaries as well as larger MMN areas than non-smokers in both placebo and nicotine sessions. Results are consistent with the hypothesis that nicotinic cholinergic mechanisms play a role in the gating of auditory stimuli.

Human central auditory plasticity associated with tone sequence learning

We investigated learning-related changes in amplitude, scalp topography, and source localization of the mismatch negativity (MMN), a neurophysiological response correlated with auditory discrimination ability. Participants (n = 32) underwent two EEG recordings while they watched silent films and ignored auditory stimuli. Stimuli were a standard (probability = 85%) and two deviant (probability = 7.5% each for high [HD] and low [LD]) eight-tone sequences that differed in the frequency of one tone. Between recordings, subjects practiced discriminating the HD or LD from the standard for 6 min. The amplitude of the LD MMN increased significantly across recordings in both groups, whereas the amplitude of the HD MMN did not. The LD was easier to discriminate than was the HD. Thus, practicing either discrimination increased the MMN for the easier discrimination. Learning and changes in the LD MMN amplitude were highly correlated. Source localizations of event-related potentials (ERPs) to all stimuli revealed bilateral sources in superior temporal regions. Compared with the standard ERP, the LD ERP revealed a stronger source in the left superior temporal region in both recordings, whereas the right-sided source became stronger after learning. Consistent with prior studies of auditory plasticity in animals and humans, tone sequence learning induced rapid neurophysiological plasticity in the human central auditory system. The results also suggest that there is asymmetric hemispheric involvement in tone sequence discrimination learning and that discrimination difficulty influences the time course of learning-related neurophysiological changes.

Common Neural Basis for Phoneme Processing in Infants and Adults

Investigating the degree of similarity between infants' and adults' representation of speech is critical to our understanding of infants' ability to acquire language. Phoneme perception plays a crucial role in language processing, and numerous behavioral studies have demonstrated similar capacities in infants and adults, but are these subserved by the same neural substrates or networks? In this article, we review event-related potential (ERP) results obtained in infants during phoneme discrimination tasks and compare them to results from the adult literature. The striking similarities observed both in behavior and ERPs between initial and mature stages suggest a continuity in processing and neural structure. We argue that infants have access at the beginning of life to phonemic representations, which are modified without training or implicit instruction, but by the statistical distributions of speech input in order to converge to the native phonemic categories.

Posttraining Sleep Enhances Automaticity in Perceptual Discrimination

Perceptual learning can develop over extended periods, with slow, at times sleep-dependent, improvement seen several days after training. As a result, performance can become more automatic, that is, less dependent on voluntary attention. This study investigates whether the brain correlates of this enhancement of automaticity are sleep-dependent. Event-related potentials produced in response to complex auditory stimuli were recorded while subjects' attention was focused elsewhere. We report here that following training on an auditory discrimination task, performance continued to improve, without significant further training, for 72 hr. At the same time, several event-related potential components became evident 48–72 hr after training. Posttraining sleep deprivation prevented neither the continued performance improvement nor the slow development of cortical dynamics related to an enhanced familiarity with the task. However, those brain responses associated with the automatic shift of attention to unexpected stimuli failed to develop. Thus, in this auditory learning paradigm, posttraining sleep appears to reduce the voluntary attentional effort required for successful perceptual discrimination by facilitating the intrusion of a potentially meaningful stimulus into one's focus of attention for further evaluation.

Inhibition of auditory evoked potentials and prepulse inhibition of startle in DBA/2J and DBA/2Hsd inbred mouse substrains

Previous data have shown differences among inbred mouse strains in sensory gating of auditory evoked potentials, prepulse inhibition (PPI) of startle, and startle amplitude. These measures of sensory and sensorimotor gating have both been proposed as models for genetic determinants of sensory processing abnormalities in patients with schizophrenia and their first-degree relatives. Data from our laboratory suggest that auditory evoked potentials of DBA/2J mice differ from those previously described for DBA/2Hsd. Therefore, we compared evoked potentials and PPI in these two closely related substrains based on the hypothesis that any observed endophenotypic differences are more likely to distinguish relevant from incidental genetic heterogeneity than similar approaches using inbred strains that vary across the entire genome. We found that DBA/2Hsd substrain exhibited reduced inhibition of evoked potentials and reduced startle relative to the DBA/2J substrain without alterations in auditory sensitivity, amplitude of evoked potentials or PPI of startle. These results suggest that gating of auditory evoked potentials and PPI of startle measure different aspects of neuronal function. The differences between the substrains might reflect genetic drift. Alternatively, differences could arise from different rearing environments or other non-genetic factors. Future studies will attempt to determine the cause of these differences in sensory and sensorimotor processing between these two closely related inbred mouse strains.

Spectral interactions, but no mismatch negativity, in auditory cortex of anesthetized rat

We used an 'oddball' stimulation paradigm to evoke epidural, event-related potentials over auditory cortex in urethane/xylazine-anesthetized rats, in order to study the neural mechanisms underlying the mismatch negativity (MMN). Oddball stimulus sets included deviants (10 kHz, 10% probability) and one of six standards that ranged from 1.25 to 9.75 kHz (overall stimulus rate 2/s). Controls included 'reference' responses (10 kHz, stimulus rate 1/5 s) obtained before and after each oddball set. We found that the deviant-evoked response was modified by the preceding standards, and the degree of modification varied systematically with the spectral separation between the standard and deviant. However, the modification was apparent relative to the reference, rather than relative to the standard (as with the MMN), and did not resemble the MMN. In fact, we found no evidence for the MMN in this preparation.

Processing of low-probability sounds by cortical neurons

The ability to detect rare auditory events can be critical for survival. We report here that neurons in cat primary auditory cortex (A1) responded more strongly to a rarely presented sound than to the same sound when it was common. For the rare stimuli, we used both frequency and amplitude deviants. Moreover, some A1 neurons showed hyperacuity for frequency deviants--a frequency resolution one order of magnitude better than receptive field widths in A1. In contrast, auditory thalamic neurons were insensitive to the probability of frequency deviants. These phenomena resulted from stimulus-specific adaptation in A1, which may be a single-neuron correlate of an extensively studied cortical potential--mismatch negativity--that is evoked by rare sounds. Our results thus indicate that A1 neurons, in addition to processing the acoustic features of sounds, may also be involved in sensory memory and novelty detection.

Mismatch negativity (MMN): an objective measure of sensory memory and long-lasting memories during sleep

Sleep, unlike wakefulness, facilitates the internal stimulus generation and hinders the processing of external stimulation. Nevertheless, evidence yielded by physiological studies in animals and event-related potential (ERP) studies in humans suggest that basic functions of the central auditory system are still preserved during sleep. This review is focused on the automatic change-detection function of the auditory system as revealed by a negative ERP component called mismatch negativity (MMN). MMN mainly originates in the auditory cortex, although it also receives an important contribution from subcortical areas (especially at thalamic level), as well as frontal areas. We discuss recent experiments supporting the use of MMN as an objective measure of sensory memory and long-lasting memories not only during wakefulness, but also during sleep. The outcome of the activation of MMN generating system during sleep highly differs from that in waking, especially when there is no previous information about the stimulus sequence in the neuronal network as a result of learning. We discuss these differences in MMN generation in terms of a dynamicist view of the brain that emphasizes the importance of the integration between bottom-up and top-down influences on sensory processing, independently of the processing level in the auditory hierarchy.

Speech-evoked neurophysiologic responses in children with learning problems: development and behavioral correlates of perception

OBJECTIVES

To evaluate the maturational progression of speech-evoked P1/N1/N2 cortical responses over the life span, determine whether responses are distinctive in clinical populations experiencing learning problems and elucidate the functional significance of these responses.

DESIGN

The P1/N1/N2 complex was measured in 150 normal subjects (5 to 78 yr) and 86 subjects with learning problems (LP) (8 to 15 yr) to a synthetic CV syllable. Analyses included description and comparison of the developmental time course in both groups and evaluation of the relationship between P1/N1/N2 and children's performance on speech discrimination tasks and standardized learning measures.

RESULTS

Findings revealed significant changes in waveform morphology, latency and amplitude as a function of age. Maturational patterns in the group of children with learning problems did not differ from the normal group. P1/N1/N2 parameters were significantly correlated with standardized tests of Spelling, Auditory Processing and Listening Comprehension in the LP group. Moreover, there was a predictive relationship between Auditory Processing and N2 latency.

CONCLUSIONS

The P1/N1/N2 complex changes throughout life from school-age to old age. The developmental sequence throughout the school-age years is similar in normal and LP children. Thus, differences in the rate of P1/Nl/N2 latency and amplitude development do not appear to be distinctive in these two populations. The relationship between P1/N1/N2 parameters and standardized measures of learning (particularly between Auditory Processing and N2 latency) provides new information about the role of these responses in hearing and highlights the potential value in characterizing auditory processing deficits.

Impaired auditory frequency discrimination in dyslexia detected with mismatch evoked potentials

Deficits in phonological skills appear to be at the heart of reading disability; however, the nature of this impairment is not yet known. The hypothesis that dyslexic subjects are impaired in auditory frequency discrimination was tested by using an attention-independent auditory brain potential, termed mismatch negativity (MMN) while subjects performed a visual distractor task. In separate blocks, MMN responses to graded changes in tone frequency or tone duration were recorded in 10 dyslexic and matched control subjects. MMN potentials to changes in tone frequency but not to changes in tone duration were abnormal in dyslexic subjects. This physiological deficit was corroborated by a similarly specific impairment in discriminating tone frequency, but not tone duration, which was assessed separately. Furthermore, the pitch discrimination and MMN deficit was correlated with the degree of impairment in phonological skills, as reflected in reading errors of regular words and nonwords. It is possible that in dyslexia a persistent sensory deficit in monitoring the frequency of incoming sound may impair the feedback control necessary for the normal development of phonological skills.

Speech sound perception, neurophysiology, and plasticity

An approach to understanding biological processes underlying speech perception, is to discover how speech sounds are represented in the central auditory system and to relate that representation to the perception of speech as a meaningful acoustic signal. Research from our group that pertains to the neurophysiologic representation of speech in the central pathways is reviewed here. Specifically considered is the relation between the representation of sound in the auditory pathway and the perception/misperception of speech, and neurophysiologic plasticity associated with speech-sound perceptual learning.

Is it really a mismatch negativity? An assessment of methods for determining response validity in individual subjects

Mismatch negativity (MMN) responses were collected from 86 normal school-age children in response to synthesized speech syllables, /wa/and two variants of /ba/. Waveform characteristics and statistical properties of the responses were analyzed across stimulus conditions in order to assess methods for determining response validity in individuals. Methods were compared using signal detection theory techniques. Criteria based on measurements of response area, onset latency, and duration were the best indicators of response validity. Also a promising indicator of validity was the interval of significance based on Z transformations determined by considering the variance of the underlying noise distribution. Correlations of individual responses with the grand average and integral calculations of the response negativity showed somewhat lower d' values. Statistical methods which utilized response subaverages were the poorest indicators of response validity. Likely the methods are limited primarily by the signal to noise ratio of the MMN compared to the underlying physiologic noise. Improvement of the signal to noise ratio remains a significant factor in the interpretation of MMN for individual subjects.

Auditory neurophysiologic responses and discrimination deficits in children with learning problems

Children with learning problems often cannot discriminate rapid acoustic changes that occur in speech. In this study of normal children and children with learning problems, impaired behavioral discrimination of a rapid speech change (/dalpha/versus/galpha/) was correlated with diminished magnitude of an electrophysiologic measure that is not dependent on attention or a voluntary response. The ability of children with learning problems to discriminate another rapid speech change (/balpha/versus/walpha/) also was reflected in the neurophysiology. These results indicate that some children's discrimination deficits originate in the auditory pathway before conscious perception and have implications for differential diagnosis and targeted therapeutic strategies for children with learning disabilities and attention disorders.

Interaural time and level differences: integrated or separated processing?

The processing of internal differences in time (IDT) and sound pressure level (IDL) was studied by using the mismatch negativity auditory evoked potential (MMN), which is a probe of pre-attentive auditory sensory memory. In a passive oddball experiment, subjects were reading in a book while they were presented with a standard stimulus (P = 0.88) having no IDTs or IDLs and three different deviant stimuli revealing an IDT, IDL, or both IDT and IDL. The different deviants elicited MMNs of comparable latencies indicating that memory representations of the IDTs and IDLs have been established. The MMN amplitudes to the IDT-IDL deviant were larger than those to changes in either IDT or IDL only. Moreover, the time-courses, amplitudes, and topographies of the MMNs to the IDT-IDL deviants were very similar to the sum of the MMNs elicited by the IDT and IDL deviants. These findings suggest that the representations of the binaural location cues were (at least partly) processed in parallel. It is argued that separate azimuth representations exist for IDT and IDL at a cortical level.