Pitch vs. spectral encoding of harmonic complex tones in primary auditory cortex of the awake monkey

Is there a ubiquitous spectrolaminar motif of local field potential power across primate neocortex?

Mendoza-Halliday, Major et al., 2024 ("The Paper")1 advocates a local field potential (LFP)-based approach to functional identification of cortical layers during "laminar" (simultaneous recordings from all cortical layers) multielectrode recordings in nonhuman primates (NHPs). The Paper describes a "ubiquitous spectrolaminar motif" in the primate neocortex: 1) 75-150 Hz power peaks in the supragranular layers, 2) 10-19 Hz power peaks in the infragranular layers and 3) the crossing point of their laminar power gradients identifies Layer 4 (L4). Identification of L4 is critical in general, but especially for The Paper as the "motif" discovery is couched within a framework whose central hypothesis is that gamma activity originates in the supragranular layers and reflects feedforward activity, while alpha-beta activity originates in the infragranular layers and reflects feedback activity. In an impressive scientific effort, The Paper analyzed laminar data from 14 cortical areas in 2 prior macaque studies and compared them to marmoset, mouse, and human data to further bolster the canonical nature of the motif. Identification of such canonical principles of brain operation is clearly a topic of broad scientific interest. Similarly, a reliable online method for L4 identification would be of broad scientific value for the rapidly increasing use of laminar recordings using numerous evolving technologies. Despite The Paper's strengths, and its potential for scientific impact, a series of concerns that are fundamental to the analysis and interpretation of laminar activity profile data in general, and local field potential (LFP) signals in particular, led us to question its conclusions. We thus evaluated the generality of The Paper's methods and findings using new sets of data comprised of stimulus-evoked laminar response profiles from primary and higher-order auditory cortices (A1 and belt cortex), and primary visual cortex (V1). The rationale for using these areas as a test bed for new methods is that their laminar anatomy and physiology have already been extensively characterized by prior studies, and there is general agreement across laboratories on key matters like L4 identification. Our analyses indicate that The Paper's findings do not generalize well to any of these cortical areas. In particular, we find The Paper's methods for L4 identification to be unreliable. Moreover, both methodological and statistical concerns, outlined below and in the supplement, question the stated prevalence of the motif in The Paper's published dataset. After summarizing our findings and related broader concerns, we briefly critique the evidence from biophysical modeling studies cited to support The Paper's conclusions. While our findings are at odds with the proposition of a ubiquitous spectrolaminar motif in the primate neocortex, The Paper already has, and will continue to spark debate and further experimentation. Hopefully this countervailing presentation will lead to robust collegial efforts to define optimal strategies for applying laminar recording methods in future studies.

Direct electrophysiological mapping of human pitch-related processing in auditory cortex

This work sought correlates of pitch perception, defined by neural activity above the lower limit of pitch (LLP), in auditory cortical neural ensembles, and examined their topographical distribution. Local field potentials (LFPs) were recorded in eight patients undergoing invasive recordings for pharmaco-resistant epilepsy. Stimuli consisted of bursts of broadband noise followed by regular interval noise (RIN). RIN was presented at rates below and above the LLP to distinguish responses related to the regularity of the stimulus and the presence of pitch itself. LFPs were recorded from human cortical homologues of auditory core, belt, and parabelt regions using multicontact depth electrodes implanted in Heschl's gyrus (HG) and Planum Temporale (PT), and subdural grid electrodes implanted over lateral superior temporal gyrus (STG). Evoked responses corresponding to the temporal regularity of the stimulus were assessed using autocorrelation of the evoked responses, and occurred for stimuli below and above the LLP. Induced responses throughout the high gamma range (60-200 Hz) were present for pitch values above the LLP, with onset latencies of approximately 70 ms. Mapping of the induced responses onto a common brain space demonstrated variability in the topographical distribution of high gamma responses across subjects. Induced responses were present throughout the length of HG and on PT, which is consistent with previous functional neuroimaging studies. Moreover, in each subject, a region within lateral STG showed robust induced responses at pitch-evoking stimulus rates. This work suggests a distributed representation of pitch processing in neural ensembles in human homologues of core and non-core auditory cortex.

The distribution and nature of responses to broadband sounds associated with pitch in the macaque auditory cortex

The organisation of pitch-perception mechanisms in the primate cortex is controversial, in that divergent results have been obtained, ranging from a single circumscribed 'pitch centre' to systems widely distributed across auditory cortex. Possible reasons for such discrepancies include different species, recording techniques, pitch stimuli, sampling of auditory fields, and the neural metrics recorded. In the present study, we sought to bridge some of these divisions by examining activity related to pitch in both neurons and neuronal ensembles within the auditory cortex of the rhesus macaque, a primate species with similar pitch perception and auditory cortical organisation to humans. We demonstrate similar responses, in primary and non-primary auditory cortex, to two different types of broadband pitch above the macaque lower limit in both neurons and local field potential (LFP) gamma oscillations. The majority of broadband pitch responses in neurons and LFP sites did not show equivalent tuning for sine tones.

Impairment of the Missing Fundamental Phenomenon in Individuals with Alzheimer’s Disease: A Neuropsychological and Voxel-Based Morphometric Study

Background/Aims

The missing fundamental phenomenon (MFP) is a universal pitch perception illusion that occurs in animals and humans. In this study, we aimed to determine whether the MFP is impaired in patients with Alzheimer's disease (AD) using an auditory pitch perception experiment. We further examined anatomical correlates of the MFP in patients with AD by measuring gray matter volume (GMV) on magnetic resonance images via voxel-based morphometric analysis.

Methods

We prospectively enrolled 29 patients with AD and 20 healthy older adults. Auditory stimuli included 12 melodies of Japanese nursery songs that were expected to be familiar to participants. We constructed the melodies using pure and missing fundamental tones (MFTs).

Results

Patients with AD exhibited significantly poorer performance on the MFT task than healthy controls. MFT scores were positively correlated with GMV in the bilateral insula and temporal poles, left inferior frontal gyrus, right entorhinal cortex, and right cerebellum.

Conclusions

These results suggest that impairments in the MFP represent a manifestation of the degeneration of auditory-related brain regions in AD. Further studies are required to more fully elucidate the neural mechanisms underlying auditory impairments in patients with AD and related dementia disorders.

Harmonic template neurons in primate auditory cortex underlying complex sound processing

Harmonicity is a fundamental element of music, speech, and animal vocalizations. How the auditory system extracts harmonic structures embedded in complex sounds and uses them to form a coherent unitary entity is not fully understood. Despite the prevalence of sounds rich in harmonic structures in our everyday hearing environment, it has remained largely unknown what neural mechanisms are used by the primate auditory cortex to extract these biologically important acoustic structures. In this study, we discovered a unique class of harmonic template neurons in the core region of auditory cortex of a highly vocal New World primate, the common marmoset (Callithrix jacchus), across the entire hearing frequency range. Marmosets have a rich vocal repertoire and a similar hearing range to that of humans. Responses of these neurons show nonlinear facilitation to harmonic complex sounds over inharmonic sounds, selectivity for particular harmonic structures beyond two-tone combinations, and sensitivity to harmonic number and spectral regularity. Our findings suggest that the harmonic template neurons in auditory cortex may play an important role in processing sounds with harmonic structures, such as animal vocalizations, human speech, and music.

A Neuronal Network Model for Pitch Selectivity and Representation

Pitch is a perceptual correlate of periodicity. Sounds with distinct spectra can elicit the same pitch. Despite the importance of pitch perception, understanding the cellular mechanism of pitch perception is still a major challenge and a mechanistic model of pitch is lacking. A multi-stage neuronal network model is developed for pitch frequency estimation using biophysically-based, high-resolution coincidence detector neurons. The neuronal units respond only to highly coincident input among convergent auditory nerve fibers across frequency channels. Their selectivity for only very fast rising slopes of convergent input enables these slope-detectors to distinguish the most prominent coincidences in multi-peaked input time courses. Pitch can then be estimated from the first-order interspike intervals of the slope-detectors. The regular firing pattern of the slope-detector neurons are similar for sounds sharing the same pitch despite the distinct timbres. The decoded pitch strengths also correlate well with the salience of pitch perception as reported by human listeners. Therefore, our model can serve as a neural representation for pitch. Our model performs successfully in estimating the pitch of missing fundamental complexes and reproducing the pitch variation with respect to the frequency shift of inharmonic complexes. It also accounts for the phase sensitivity of pitch perception in the cases of Schroeder phase, alternating phase and random phase relationships. Moreover, our model can also be applied to stochastic sound stimuli, iterated-ripple-noise, and account for their multiple pitch perceptions.

High-Field Functional Imaging of Pitch Processing in Auditory Cortex of the Cat

The perception of pitch is a widely studied and hotly debated topic in human hearing. Many of these studies combine functional imaging techniques with stimuli designed to disambiguate the percept of pitch from frequency information present in the stimulus. While useful in identifying potential “pitch centres” in cortex, the existence of truly pitch-responsive neurons requires single neuron-level measures that can only be undertaken in animal models. While a number of animals have been shown to be sensitive to pitch, few studies have addressed the location of cortical generators of pitch percepts in non-human models. The current study uses high-field functional magnetic resonance imaging (fMRI) of the feline brain in an attempt to identify regions of cortex that show increased activity in response to pitch-evoking stimuli. Cats were presented with iterated rippled noise (IRN) stimuli, narrowband noise stimuli with the same spectral profile but no perceivable pitch, and a processed IRN stimulus in which phase components were randomized to preserve slowly changing modulations in the absence of pitch (IRNo). Pitch-related activity was not observed to occur in either primary auditory cortex (A1) or the anterior auditory field (AAF) which comprise the core auditory cortex in cats. Rather, cortical areas surrounding the posterior ectosylvian sulcus responded preferentially to the IRN stimulus when compared to narrowband noise, with group analyses revealing bilateral activity centred in the posterior auditory field (PAF). This study demonstrates that fMRI is useful for identifying pitch-related processing in cat cortex, and identifies cortical areas that warrant further investigation. Moreover, we have taken the first steps in identifying a useful animal model for the study of pitch perception.

The harmonic organization of auditory cortex

A fundamental structure of sounds encountered in the natural environment is the harmonicity. Harmonicity is an essential component of music found in all cultures. It is also a unique feature of vocal communication sounds such as human speech and animal vocalizations. Harmonics in sounds are produced by a variety of acoustic generators and reflectors in the natural environment, including vocal apparatuses of humans and animal species as well as music instruments of many types. We live in an acoustic world full of harmonicity. Given the widespread existence of the harmonicity in many aspects of the hearing environment, it is natural to expect that it be reflected in the evolution and development of the auditory systems of both humans and animals, in particular the auditory cortex. Recent neuroimaging and neurophysiology experiments have identified regions of non-primary auditory cortex in humans and non-human primates that have selective responses to harmonic pitches. Accumulating evidence has also shown that neurons in many regions of the auditory cortex exhibit characteristic responses to harmonically related frequencies beyond the range of pitch. Together, these findings suggest that a fundamental organizational principle of auditory cortex is based on the harmonicity. Such an organization likely plays an important role in music processing by the brain. It may also form the basis of the preference for particular classes of music and voice sounds.

Evidence for pitch chroma mapping in human auditory cortex

Some areas in auditory cortex respond preferentially to sounds that elicit pitch, such as musical sounds or voiced speech. This study used human electroencephalography (EEG) with an adaptation paradigm to investigate how pitch is represented within these areas and, in particular, whether the representation reflects the physical or perceptual dimensions of pitch. Physically, pitch corresponds to a single monotonic dimension: the repetition rate of the stimulus waveform. Perceptually, however, pitch has to be described with 2 dimensions, a monotonic, "pitch height," and a cyclical, "pitch chroma," dimension, to account for the similarity of the cycle of notes (c, d, e, etc.) across different octaves. The EEG adaptation effect mirrored the cyclicality of the pitch chroma dimension, suggesting that auditory cortex contains a representation of pitch chroma. Source analysis indicated that the centroid of this pitch chroma representation lies somewhat anterior and lateral to primary auditory cortex.

Similarity of cortical activity patterns predicts generalization behavior

Humans and animals readily generalize previously learned knowledge to new situations. Determining similarity is critical for assigning category membership to a novel stimulus. We tested the hypothesis that category membership is initially encoded by the similarity of the activity pattern evoked by a novel stimulus to the patterns from known categories. We provide behavioral and neurophysiological evidence that activity patterns in primary auditory cortex contain sufficient information to explain behavioral categorization of novel speech sounds by rats. Our results suggest that category membership might be encoded by the similarity of the activity pattern evoked by a novel speech sound to the patterns evoked by known sounds. Categorization based on featureless pattern matching may represent a general neural mechanism for ensuring accurate generalization across sensory and cognitive systems.

Neural representation of harmonic complex tones in primary auditory cortex of the awake monkey

Many natural sounds are periodic and consist of frequencies (harmonics) that are integer multiples of a common fundamental frequency (F0). Such harmonic complex tones (HCTs) evoke a pitch corresponding to their F0, which plays a key role in the perception of speech and music. "Pitch-selective" neurons have been identified in non-primary auditory cortex of marmoset monkeys. Noninvasive studies point to a putative "pitch center" located in a homologous cortical region in humans. It remains unclear whether there is sufficient spectral and temporal information available at the level of primary auditory cortex (A1) to enable reliable pitch extraction in non-primary auditory cortex. Here we evaluated multiunit responses to HCTs in A1 of awake macaques using a stimulus design employed in auditory nerve studies of pitch encoding. The F0 of the HCTs was varied in small increments, such that harmonics of the HCTs fell either on the peak or on the sides of the neuronal pure tone tuning functions. Resultant response-amplitude-versus-harmonic-number functions ("rate-place profiles") displayed a periodic pattern reflecting the neuronal representation of individual HCT harmonics. Consistent with psychoacoustic findings in humans, lower harmonics were better resolved in rate-place profiles than higher harmonics. Lower F0s were also temporally represented by neuronal phase-locking to the periodic waveform of the HCTs. Findings indicate that population responses in A1 contain sufficient spectral and temporal information for extracting the pitch of HCTs by neurons in downstream cortical areas that receive their input from A1.

Dual-pitch processing mechanisms in primate auditory cortex

Pitch, our perception of how high or low a sound is on a musical scale, is a fundamental perceptual attribute of sounds and is important for both music and speech. After more than a century of research, the exact mechanisms used by the auditory system to extract pitch are still being debated. Theoretically, pitch can be computed using either spectral or temporal acoustic features of a sound. We have investigated how cues derived from the temporal envelope and spectrum of an acoustic signal are used for pitch extraction in the common marmoset (Callithrix jacchus), a vocal primate species, by measuring pitch discrimination behaviorally and examining pitch-selective neuronal responses in auditory cortex. We find that pitch is extracted by marmosets using temporal envelope cues for lower pitch sounds composed of higher-order harmonics, whereas spectral cues are used for higher pitch sounds with lower-order harmonics. Our data support dual-pitch processing mechanisms, originally proposed by psychophysicists based on human studies, whereby pitch is extracted using a combination of temporal envelope and spectral cues.

Neural mechanisms for the abstraction and use of pitch information in auditory cortex

Experiments in animals have provided an important complement to human studies of pitch perception by revealing how the activity of individual neurons represents harmonic complex and periodic sounds. Such studies have shown that the acoustical parameters associated with pitch are represented by the spiking responses of neurons in A1 (primary auditory cortex) and various higher auditory cortical fields. The responses of these neurons are also modulated by the timbre of sounds. In marmosets, a distinct region on the low-frequency border of primary and non-primary auditory cortex may provide pitch tuning that generalizes across timbre classes.

Musical experience, plasticity, and maturation: issues in measuring developmental change using EEG and MEG

The neuroscientific study of musical behavior has become a significant field of research during the last decade, and reports of this research in the popular press have caught the imagination of the public. This enterprise has also made it evident that studying the development of musical behavior can make a significant contribution to important questions in the field, such as the evolutionary origins of music, cross-cultural similarity and diversity, the effects of experience on musical processing, and relations between music and other domains. Studying musical development brings a unique set of methodological issues. We discuss a select set of these related to measurement of the electroencephalogram (EEG) and magnetoencephalogram (MEG). We use specific examples from our laboratory to illustrate the types of questions that can be answered with different data analysis techniques.

Sensitivity and selectivity of neurons in auditory cortex to the pitch, timbre, and location of sounds

We are able to rapidly recognize and localize the many sounds in our environment. We can describe any of these sounds in terms of various independent "features" such as their loudness, pitch, or position in space. However, we still know surprisingly little about how neurons in the auditory brain, specifically the auditory cortex, might form representations of these perceptual characteristics from the information that the ear provides about sound acoustics. In this article, the authors examine evidence that the auditory cortex is necessary for processing the pitch, timbre, and location of sounds, and document how neurons across multiple auditory cortical fields might represent these as trains of action potentials. They conclude by asking whether neurons in different regions of the auditory cortex might not be simply sensitive to each of these three sound features but whether they might be selective for one of them. The few studies that have examined neural sensitivity to multiple sound attributes provide only limited support for neural selectivity within auditory cortex. Providing an explanation of the neural basis of feature invariance is thus one of the major challenges to sensory neuroscience obtaining the ultimate goal of understanding how neural firing patterns in the brain give rise to perception.

Cortical encoding of pitch: recent results and open questions

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

Finding the pitch of the missing fundamental in infants

Pitch perception is critical for the perception of speech and music, for object identification, and for auditory scene analysis, whereby representations are derived for each sounding object in the environment from the complex sound wave that reaches the ears. The perceived pitch of a complex sound corresponds to its fundamental frequency. However, removal of energy at the fundamental does not alter the pitch because adults use the harmonics to derive the pitch (Bendor and Wang, 2005; Trainor, 2008). Although sound frequency is represented subcortically, the integration of harmonics into a representation of pitch does not occur until auditory cortex (Bendor and Wang, 2005). Given that auditory cortex is immature in young infants, we examined the development of cortical representations for pitch by measuring electrophysiological (EEG) responses to pitch changes that required processing the pitch of the missing fundamental. Adults and infants 4 months and older showed a mismatch negativity response to these pitch changes, but 3-month-old infants did not. Thus, cortical representations of the pitch of the missing fundamental emerge between 3 and 4 months of age, indicating that there is a profound change in auditory perception for pitch in early infancy.

Speaker normalization using cortical strip maps: a neural model for steady-state vowel categorization

Auditory signals of speech are speaker dependent, but representations of language meaning are speaker independent. The transformation from speaker-dependent to speaker-independent language representations enables speech to be learned and understood from different speakers. A neural model is presented that performs speaker normalization to generate a pitch-independent representation of speech sounds, while also preserving information about speaker identity. This speaker-invariant representation is categorized into unitized speech items, which input to sequential working memories whose distributed patterns can be categorized, or chunked, into syllable and word representations. The proposed model fits into an emerging model of auditory streaming and speech categorization. The auditory streaming and speaker normalization parts of the model both use multiple strip representations and asymmetric competitive circuits, thereby suggesting that these two circuits arose from similar neural designs. The normalized speech items are rapidly categorized and stably remembered by adaptive resonance theory circuits. Simulations use synthesized steady-state vowels from the Peterson and Barney [Peterson, G. E., and Barney, H.L., J. Acoust. Soc. Am. 24, 175-184 (1952).] vowel database and achieve accuracy rates similar to those achieved by human listeners. These results are compared to behavioral data and other speaker normalization models.

Perception and cortical neural coding of harmonic fusion in ferrets

This study examined the perception and cortical representation of harmonic complex tones, from the perspective of the spectral fusion evoked by such sounds. Experiment 1 tested whether ferrets spontaneously distinguish harmonic from inharmonic tones. In baseline sessions, ferrets detected a pure tone terminating a sequence of inharmonic tones. After they reached proficiency, a small fraction of the inharmonic tones were replaced with harmonic tones. Some of the animals confused the harmonic tones with the pure tones at twice the false-alarm rate. Experiment 2 sought correlates of harmonic fusion in single neurons of primary auditory cortex and anterior auditory field, by comparing responses to harmonic tones with those to inharmonic tones in the awake alert ferret. The effects of spectro-temporal filtering were accounted for by using the measured spectrotemporal receptive field to predict responses and by seeking correlates of fusion in the predictability of responses. Only 12% of units sampled distinguished harmonic tones from inharmonic tones, a small percentage that is consistent with the relatively weak ability of the ferrets to spontaneously discriminate harmonic tones from inharmonic tones in Experiment 1.

Cortical representations of pitch in monkeys and humans

Pitch perception is crucial for vocal communication, music perception, and auditory object processing in a complex acoustic environment. How pitch is represented in the cerebral cortex has for a long time remained an unanswered question in auditory neuroscience. Several lines of evidence now point to a distinct non-primary region of auditory cortex in primates that contains a cortical representation of pitch.

The neuronal representation of pitch in primate auditory cortex

Pitch perception is critical for identifying and segregating auditory objects, especially in the context of music and speech. The perception of pitch is not unique to humans and has been experimentally demonstrated in several animal species. Pitch is the subjective attribute of a sound's fundamental frequency (f(0)) that is determined by both the temporal regularity and average repetition rate of its acoustic waveform. Spectrally dissimilar sounds can have the same pitch if they share a common f(0). Even when the acoustic energy at f(0) is removed ('missing fundamental') the same pitch is still perceived. Despite its importance for hearing, how pitch is represented in the cerebral cortex is unknown. Here we show the existence of neurons in the auditory cortex of marmoset monkeys that respond to both pure tones and missing fundamental harmonic complex sounds with the same f(0), providing a neural correlate for pitch constancy. These pitch-selective neurons are located in a restricted low-frequency cortical region near the anterolateral border of the primary auditory cortex, and is consistent with the location of a pitch-selective area identified in recent imaging studies in humans.

Interaction of Excitatory and Inhibitory Frequency-receptive Fields in Determining Fundamental Frequency Sensitivity of Primary Auditory Cortex Neurons in Awake Cats

Harmonic complex tones produce pitch-height perception corresponding to the fundamental frequency (F0). This study investigates how the spectral cue of F0 is processed in neurons of the primary auditory cortex (A1) with sustained-response properties. We found F0-sensitive and -insensitive cells: the former discriminated between harmonics and noise, while the latter did not. F0-sensitive cells preferred F0s corresponding to the best frequency (BF) and 0.5 x BF. The F0-sensitivity to F0=0.5 x BF was preserved for missing F0, but abolished by eliminating both F0 and the second harmonic. The inhibitory subfield of the frequency-receptive field was restricted to the spectral region between the preferred harmonics in F0-sensitive cells, while it was frequency unspecific in F0-insensitive cells. We conclude that (i) A1 is well organized for discrimination between harmonics and noise; (ii) pitch-height is represented along with the tonotopic axis; (iii) all aspects of the sustained neural responses to harmonic and noise stimuli are consequences of spectral filtering; and (iv) although the observed cell behavior explains some psychophysical pitch perception behaviors, such as pitch-chroma (helical pitch perception with frequency elevation), pitch-level tolerance and adaptive behavior, F0-encoding in A1 remains at the incomplete perceptual level (dominance of the third to fifth harmonics for pitch strength is unexplainable by the cell behavior).

Evaluation of missing fundamental phenomenon in the human auditory cortex

OBJECTIVE

Harmonic complex tones consisting of four or more continuous harmonics of a certain stem frequency are perceived as the pitch of the fundamental frequency tone, it is referred to as the missing fundamental phenomenon (MFP). It is considered that the MFP is produced in the central auditory system, not in the periphery. However, it remains unclear where and how complex sounds is integrated. Using 306ch magnetoencephalography (MEG), we investigated when and where the MFP was integrated in the auditory cortex.

METHOD

We examined six subjects who were selected by MEG in 12 healthy right-handed adult volunteers with normal auditory sensation. Ears were randomly stimulated with five different complex tones consist of fundamental frequency tone and harmonic complex tones. The location and direction of equivalent current dipoles (ECD) were evaluated at P50 and N100 in the right temporal lobe by MEG. Dispersion of the source of ECD was respectively evaluated on their brain MRI.

RESULTS

Stimulation of ears with harmonic complex tones and the stem frequency tone revealed the localization of P50 and N100 ECD in the transverse temporal gyrus and their peripheral superior temporal gyrus. Although the sources of P50 ECD for harmonic complex tones and the fundamental tone were varied around the transverse temporal gyrus and superior temporal gyrus, the sources of N100 ECD were almost identical at the transverse temporal gyrus, demonstrating the MFP. This phenomenon were similarly observed, even when dichotic listening were stimulated.

CONCLUSION

These findings suggest that the MFP occurs in the transverse temporal gyrus and the superior temporal gyrus, which are the primary auditory cortex, between P50 and N100.

Interaction between the neuromagnetic responses to sound energy onset and pitch onset suggests common generators

The pitch-onset response (POR) is a negative component of the auditory evoked field which is elicited when the temporal fine structure of a continuous noise is regularized to produce a pitch perception without altering the gross spectral characteristics of the sound. Previously, we showed that the latency of the POR is inversely related to the pitch value and its amplitude is correlated with the salience of the pitch, suggesting that the underlying generators are part of a pitch-processing network [Krumbholz, K., Patterson, R.D., Seither-Preisler, A., Lammertmann, C. & Lütkenhöner, B. (2003) Cereb. Cortex,13, 765-772]. The source of the POR was located near the medial part of Heschl's gyrus. The present study was designed to determine whether the POR originates from the same generators as the energy-onset response (EOR) represented by the N100m/P200m complex. The EOR to the onset of a noise, and the POR to a subsequent transition from noise to pitch, were recorded as the time interval between the noise onset and the transition varied from 500 to 4000 ms. The mean amplitude of the POR increased by approximately 5.9 nA.m with each doubling of the time between noise onset and transition. This suggests an interaction between the POR and the EOR, which may be based on common neural generators.

Tonotopic cortical representation of periodic complex sounds

Most of the sounds that are biologically relevant are complex periodic sounds, i.e., they are made up of harmonics, whose frequencies are integer multiples of a fundamental frequency (Fo). The Fo of a complex sound can be varied by modifying its periodicity frequency; these variations are perceived as the pitch of the voice or as the note of a musical instrument. The center frequency (CF) of peaks occurring in the audio spectrum also carries information, which is essential, for instance, in vowel recognition. The aim of the present study was to establish whether the generators underlying the 100 m are tonotopically organized based on the Fo or CF of complex sounds. Auditory evoked neuromagnetic fields were recorded with a whole-head magnetoencephalography (MEG) system while 14 subjects listened to 9 different sounds (3 Fo x 3 CF) presented in random order. Equivalent current dipole (ECD) sources for the 100 m component show an orderly progression along the y-axis for both hemispheres, with higher CFs represented more medially. In the right hemisphere, sources for higher CFs were more posterior, while in the left hemisphere they were more inferior. ECD orientation also varied as a function of the sound CF. These results show that the spectral content CF of the complex sounds employed here predominates, at the latency of the 100 m component, over a concurrent mapping of their periodic frequency Fo. The effect was observed both on dipole placement and dipole orientation.

Sensitivity of human auditory evoked potentials to the harmonicity of complex tones: evidence for dissociated cortical processes of spectral and periodicity analysis

A strong subjective tendency exists for simultaneous sound frequencies forming an harmonic series (integer multiples of the fundamental) to "group" together into a unified auditory percept whose pitch is similar to that of the fundamental. The aim of the study was to determine whether cortical auditory evoked potentials (AEPs) to complex tones differ according to whether the component frequencies of the stimuli are harmonically related or not. AEPs were recorded to continuous complex tones comprising four or more sinusoids. The vertex-maximal "change-potentials" (CP1, CN1, CP2), recorded to a stimulus cycle comprising one harmonic and five inharmonic complexes changing every second, showed no sensitivity to harmonicity, although an additional mismatch negativity was possibly present to the harmonic complex. In a second study the CP2 was significantly attenuated when an harmonic complex changed to a new one in the presence of an unchanging sinusoidal background tone, harmonically related to the first complex but not to the second, and thus becoming perceptually distinct. This, however, might be caused by lateral inhibitory effects not related to harmonicity. In a third experiment, when four concurrent sinusoidal tones came to rest on steady frequencies after a 5-s period of 16/s pseudo-random frequency changes, fronto-centrally maximal "mismatch-potentials" (MN1, MP2), were recorded. Both the MN1 and the MP2 were significantly shorter in latency when the steady frequencies formed an harmonic complex. Since the harmonic complex had a short overall periodicity, equal to that of the fundamental, while that of the inharmonic complex was much longer, the effect might be explained if the latencies of the mismatch-potential are related to periodicity. The perceptual grouping of harmonically related frequencies appears not to be a function of spectral domain analysis, reflected in the change-potentials, but of periodicity analysis, reflected in the mismatch-potentials

Spectrotemporal receptive fields in the lemniscal auditory thalamus and cortex

Receptive fields have been characterized independently in the lemniscal auditory thalamus and cortex, usually with spectrotemporally simple sounds tailored to a specific task. No studies have employed naturalistic stimuli to investigate the thalamocortical transformation in temporal, spectral, and aural domains simultaneously and under identical conditions. We recorded simultaneously in the ventral division of the medial geniculate body (MGBv) and in primary auditory cortex (AI) of the ketamine-anesthetized cat. Spectrotemporal receptive fields (STRFs) of single units (n = 387) were derived by reverse-correlation with a broadband and dynamically varying stimulus, the dynamic ripple. Spectral integration, as measured by excitatory bandwidth and spectral modulation preference, was similar across both stations (mean Q(1/e) thalamus = 5.8, cortex = 5.4; upper cutoff of spectral modulation transfer function, thalamus = 1.30 cycles/octave, cortex = 1.37 cycles/octave). Temporal modulation rates slowed by a factor of two from thalamus to cortex (mean preferred rate, thalamus = 32.4 Hz, cortex = 16.6 Hz; upper cutoff of temporal modulation transfer function, thalamus = 62.9 Hz, cortex = 37.4 Hz). We found no correlation between spectral and temporal integration properties, suggesting that the excitatory-inhibitory interactions underlying preference in each domain are largely independent. A small number of neurons in each station had highly asymmetric STRFs, evidence of frequency sweep selectivity, but the population showed no directional bias. Binaural preferences differed in their relative proportions, most notably an increased prevalence of excitatory contralateral-only cells in cortex (40%) versus thalamus (23%), indicating a reorganization of this parameter. By comparing simultaneously along multiple stimulus dimensions in both stations, these observations establish the global characteristics of the thalamocortical receptive field transformation.

Functional convergence of response properties in the auditory thalamocortical system

One of the brain's fundamental tasks is to construct and transform representations of an animal's environment, yet few studies describe how individual neurons accomplish this. Our results from correlated pairs in the auditory thalamocortical system show that cortical excitatory receptive field regions can be directly inherited from thalamus, constructed from smaller inputs, and assembled by the cooperative activity of neuronal ensembles. The prevalence of functional thalamocortical connectivity is strictly governed by tonotopy, but connection strength is not. Finally, spectral and temporal modulation preferences in cortex may differ dramatically from the thalamic input. Our observations reveal a radical reconstruction of response properties from auditory thalamus to cortex, and illustrate how some properties are propagated with great fidelity while others are significantly transformed or generated intracortically.

The auditory ‘C-process’: analyzing the spectral envelope of complex sounds

OBJECTIVE

To examine the hypothesis that auditory evoked potentials (AEPs) to pitch and timbre change of complex harmonic tones reflect a process of spectral envelope analysis.

METHODS

AEPs were recorded to: (1) continuous tones of 'clarinet' timbre whose pitch abruptly rose or fell by 1 or 7 semitones every 0.5 or 1.5 s; (2) a cycle of 6 pitches changing every 0.5 s; (3) tones of constant pitch whose timbre (spectral envelope shape) changed periodically; (4) pitch change of high- and low-pass filtered 'clarinet' tones.

RESULTS

The amplitudes of the 'change-N1' (CN1) potential peaking at ca. 90 ms and the following CP2 were influenced to a far greater degree by the time interval between changes, than by the magnitude of the change or by the time interval between occurrences of the same pitch. Amplitudes were also strongly dependent on the number of partials present, irrespective of whether they were increasing or decreasing in energy. The algebraic sum of the responses to pitch change of high- and low-pass filtered tones closely approximated the response to the unfiltered tone.

CONCLUSION

The rate-sensitivity of the responses cannot be explained by the refractoriness of frequency-specific 'feature detector' neurones, but rather of a process (termed 'C-process') which analyzes amplitude modulations across the spectral envelope, the contribution of different frequency bands combining linearly in the scalp-recorded activity. On-going computation of the spectral envelope shape may be an important factor in maintaining the perceptual constancy of timbre.

Pitch perception: a dynamical-systems perspective

Two and a half millennia ago Pythagoras initiated the scientific study of the pitch of sounds; yet our understanding of the mechanisms of pitch perception remains incomplete. Physical models of pitch perception try to explain from elementary principles why certain physical characteristics of the stimulus lead to particular pitch sensations. There are two broad categories of pitch-perception models: place or spectral models consider that pitch is mainly related to the Fourier spectrum of the stimulus, whereas for periodicity or temporal models its characteristics in the time domain are more important. Current models from either class are usually computationally intensive, implementing a series of steps more or less supported by auditory physiology. However, the brain has to analyze and react in real time to an enormous amount of information from the ear and other senses. How is all this information efficiently represented and processed in the nervous system? A proposal of nonlinear and complex systems research is that dynamical attractors may form the basis of neural information processing. Because the auditory system is a complex and highly nonlinear dynamical system, it is natural to suppose that dynamical attractors may carry perceptual and functional meaning. Here we show that this idea, scarcely developed in current pitch models, can be successfully applied to pitch perception.

Brain potentials in human patients with extremely severe diffuse brain damage

To test higher cortical functions of neurological patients, oddball tasks are often used in which a frequent and a rare stimulus are randomly presented and a P3 brain wave is recorded to the rare stimulus. We examined 33 patients with extremely severe brain injury. Three oddball conditions were used: with two sine tones (ST), with two complex tones (CT) and with vowels 'o' and 'i'. Across all patients, CT elicited P3 more often than ST, and the occurrence of the P3 after vowels was intermediate. However, among patients who showed a distinct P3 wave, its amplitude in the subgroup with traumatic brain injury was larger to vowels than to CT. In patients with non-traumatic etiology, CT and vowels elicited a P3 of a nearly equal amplitude. Stimuli of sufficient complexity should be used when the P3 technique is applied for assessment of cortical functions in severely impaired patients.

Perception of complex sounds: N1 latency codes pitch and topography codes spectra

OBJECTIVES

This work aimed to find out whether the human cortical 'tonotopy' represents the true fundamental frequency (Fo) of complex sounds, or the center frequency CF at which harmonics peak in the audio spectrum. Indeed, complex periodic sounds (such as those of the human voice, musical instruments, etc.) comprise a 'fundamental component' (Fo) and its 'harmonics' (2Fo, 3Fo, ...nFo). These often peak around a certain frequency CF. As Fo and CF are confounded in pure (sinusoidal) tones, the question of whether Fo or CF is represented through tonotopy had been hitherto unresolved.

METHODS

Whole-head recordings of brain electrical activity were obtained for 16 subjects submitted to an array of 9 different series of sounds (3 Fox3 CF). Electrophysiological data were analyzed separately for each sound and each subject with brain functional imaging and dipole reconstruction.

RESULTS

Equivalent dipole sources of N1 components were, significantly for all subjects, more and more frontally oriented as CF increased, independently of Fo.

CONCLUSIONS

Sounds are mapped in both the right and the left primary auditory cortices according to the spectral profiles of their harmonics (CF), rather than their fundamental frequencies (Fo).

Congenital auditory deprivation reduces synaptic activity within the auditory cortex in a layer-specific manner

The present study investigates the functional deficits of naive auditory cortices in adult congenitally deaf cats. For this purpose, their auditory system was stimulated electrically using cochlear implants. Synaptic currents in cortical layers were revealed using current source density analyses. They were compared with synaptic currents found in electrically stimulated hearing cats. The naive auditory cortex showed significant deficits in synaptic activity in infragranular cortical layers. Furthermore, there was also a deficit of synaptic activities at longer latencies (>30 ms). The 'cortical column' was not activated in the well-defined sequence found in normal hearing cats. These results demonstrate functional deficits as a consequence of congenital auditory deprivation. Similar deficits are likely in congenitally deaf children.

Complex tone processing in primary auditory cortex of the awake monkey. II. Pitch versus critical band representation

Noninvasive neurophysiological studies in humans support the existence of an orthogonal spatial representation of pure tone frequency and complex tone pitch in auditory cortex [Langner et al., J. Comp. Physiol. A 181, 665-676 (1997)]. However, since this topographic organization is based on neuromagnetic responses evoked by wideband harmonic complexes (HCs) of variable fundamental frequency (f0), and thus interharmonic frequency separation (deltaF), critical band filtering effects due to differential resolvability of harmonics may have contributed to shaping these responses. To test this hypothesis, the present study examined responses evoked by three-component HCs of variable f0 in primary auditory cortex (A1) of the awake monkey. The center frequency of the HCs was fixed at the best frequency (BF) of the cortical site. Auditory evoked potential (AEP), multiunit activity, and current source density techniques were used to evaluate A1 responses as a function of f0 (=deltaF). Generally, amplitudes of nearly all response components increased with f0, such that maximal responses were evoked by HCs comprised of low-order resolved harmonics. Statistically significant increases in response amplitude typically occurred at deltaFs between 10% and 20% of center frequency, suggestive of critical bandlike behavior. Complex tone response amplitudes also reflected nonlinear summation in that they could not be predicted by the pure tone frequency sensitivity curves of the cortical sites. A mechanism accounting for the observed results is proposed which involves mutual lateral inhibitory interactions between responses evoked by stimulus components lying within the same critical band. As intracortical AEP components likely to be propagated to the scalp were also strongly modulated by deltaF, these findings indicate that noninvasive recordings of responses to complex sounds may require a consideration of critical band effects in their interpretation.

Complex tone processing in primary auditory cortex of the awake monkey. I. Neural ensemble correlates of roughness

Previous physiological studies [e.g., Bieser and Muller-Preuss, Exp. Brain Res. 108, 273-284 (1996); Schulze and Langner, J. Comp. Physiol. A 181, 651-663 (1997); Steinschneider et al., J. Acoust. Soc. Am. 104, 2935-2955 (1998)] have suggested that neural activity in primary auditory cortex (A1) phase-locked to the waveform envelope of complex sounds with low (<300 Hz) periodicities may represent a neural correlate of roughness perception. However, a correspondence between these temporal response patterns and human psychophysical boundaries of roughness has not yet been demonstrated. The present study examined whether the degree of synchronized phase-locked activity of neuronal ensembles in A1 of the awake monkey evoked by complex tones parallels human psychoacoustic data defining the existence region and frequency dependence of roughness. Stimuli consisted of three consecutive harmonics of fundamental frequencies (f(0)s) ranging from 25 to 4000 Hz. The center frequency of the complex tones was fixed at the best frequency (BF) of the cortical sites, which ranged from 0.3 to 10 kHz. Neural ensemble activity in the thalamorecipient zone (lower lamina III) and supragranular cortical laminae (upper lamina III and lamina II) was measured using multiunit activity and current source density techniques and the degree of phase-locking to the f0 was quantified by spectral analysis. In the thalamorecipient zone, the stimulus f0 at which phase-locking was maximal increased with BF and reached an upper limit between 75 and 150 Hz for BFs greater than about 3 kHz. Estimates of limiting phase-locking rates also increased with BF and approximated psychoacoustic values for the disappearance of roughness. These physiological relationships parallel human perceptual data and therefore support the relevance of phase-locked activity of neuronal ensembles in A1 for the physiological representation of roughness.

Current source density analysis of ON/OFF channels in the frog optic tectum

In the vertebrate retina, it is well known that an ON/OFF dichotomy is present. In other words, ON-center and OFF-center cells participate in segregated pathways morphologically and physiologically. However, there is no doubt that integration of both channels is necessary to generate the complicated response properties of visual neurons in higher optic centers. So far, functional organization of the ON and OFF channels in the optic centers has not been demonstrated at the level of neuronal populations. In this review article, we summarize our experimental approaches to demonstrate functional organization of the ON and OFF channels using current source density (CSD) analysis in the frog optic tectum. First, we show that one-dimensional CSD analysis, assuming constant conductivity, is applicable in the tectal laminated structure. The CSD depth profile of a response to electrical stimulation of the optic tract is composed of three current sinks (A, B, and D) in the retinorecipient layers and two current sinks (C and E) below those layers. This result is in agreement with previous morphological and physiological findings, and shows that CSD analysis is very useful to demonstrate the flow of visual information processing. Second, CSD analysis of tectal responses evoked by diffuse light ON and OFF stimuli reveals obviously different distributions of synaptic activity in the laminar structure. Two or three current sinks (I, II and III) are generated in response to ON stimulation only in the retinorecipient layers, while up to six current sinks (IV, V, VI, VII, VIII and IX) to OFF stimulation throughout the tectal layers. Based on well known properties of retinal ganglion cells of the frog, possible neuronal mechanisms underlying each current sinks and their functional roles in visually guided behavior are considered.

Auditory processing in primate cerebral cortex

Auditory information is relayed from the ventral nucleus of the medial geniculate complex to a core of three primary or primary-like areas of auditory cortex that are cochleotopically organized and highly responsive to pure tones. Auditory information is then distributed from the core areas to a surrounding belt of about seven areas that are less precisely cochleotopic and generally more responsive to complex stimuli than tones. Recent studies indicate that the belt areas relay to the rostral and caudal divisions of a parabelt region at a third level of processing in the cortex lateral to the belt. The parabelt and belt regions have additional inputs from dorsal and magnocellular divisions of the medial geniculate complex and other parts of the thalamus. The belt and parabelt regions appear to be concerned with integrative and associative functions involved in pattern perception and object recognition. The parabelt fields connect with regions of temporal, parietal, and frontal cortex that mediate additional auditory functions, including space perception and auditory memory.

Click train encoding in primary auditory cortex of the awake monkey: evidence for two mechanisms subserving pitch perception

Multiunit activity (MUA) and current source density (CSD) patterns evoked by click trains are examined in primary auditory cortex (A1) of three awake monkeys. Temporal and spectral features of click trains are differentially encoded in A1. Encoding of temporal features occurs at rates of 100-200 Hz through phase-locked activity in the MUA and CSD, is independent of pulse polarity pattern, and occurs in high best frequency (BF) regions of A1. The upper limit of ensemble-wide phase-locking is about 400 Hz in the input to A1, as manifested in the cortical middle laminae CSD and MUA of thalamocortical fibers. In contrast, encoding of spectral features occurs in low BF regions, and resolves both the f0 and harmonics of the stimuli through local maxima of activity determined by the tonotopic organization of the recording sites. High-pass filtered click trains decrease spectral encoding in low BF regions without modifying phase-locked responses in high BF regions. These physiological responses parallel features of human pitch perception for click trains, and support the existence of two distinct physiological mechanisms involved in pitch perception: the first using resolved harmonic components and the second utilizing unresolved harmonics that is based on encoding stimulus waveform periodicity.