Slow Temporal Modulations Research Articles

Dissipative spatiotemporal soliton in a driven waveguide laser

A distributed Kerr-lens mode locking regime can be realized in a waveguide laser by spatial profiling of the pump beam, thus creating a spatio-temporal soliton. Additional slow temporal modulation of the pump source stabilizes the spatio-temporal solution in a broad range of parameters defined by the dynamic gain saturation. We choose a Cr:ZnS waveguide laser as a practical example, but such a regime is feasible in various waveguide and fiber oscillators. A far-reaching analogy with Bose–Einstein condensates allows this approach to stabilize the weakly dissipative BECs.

Original speech and its echo are segregated and separately processed in the human brain.

Speech recognition crucially relies on slow temporal modulations (<16 Hz) in speech. Recent studies, however, have demonstrated that the long-delay echoes, which are common during online conferencing, can eliminate crucial temporal modulations in speech but do not affect speech intelligibility. Here, we investigated the underlying neural mechanisms. MEG experiments demonstrated that cortical activity can effectively track the temporal modulations eliminated by an echo, which cannot be fully explained by basic neural adaptation mechanisms. Furthermore, cortical responses to echoic speech can be better explained by a model that segregates speech from its echo than by a model that encodes echoic speech as a whole. The speech segregation effect was observed even when attention was diverted but would disappear when segregation cues, i.e., speech fine structure, were removed. These results strongly suggested that, through mechanisms such as stream segregation, the auditory system can build an echo-insensitive representation of speech envelope, which can support reliable speech recognition.

Sleep deprivation detected by voice analysis.

Sleep deprivation has an ever-increasing impact on individuals and societies. Yet, to date, there is no quick and objective test for sleep deprivation. Here, we used automated acoustic analyses of the voice to detect sleep deprivation. Building on current machine-learning approaches, we focused on interpretability by introducing two novel ideas: the use of a fully generic auditory representation as input feature space, combined with an interpretation technique based on reverse correlation. The auditory representation consisted of a spectro-temporal modulation analysis derived from neurophysiology. The interpretation method aimed to reveal the regions of the auditory representation that supported the classifiers' decisions. Results showed that generic auditory features could be used to detect sleep deprivation successfully, with an accuracy comparable to state-of-the-art speech features. Furthermore, the interpretation revealed two distinct effects of sleep deprivation on the voice: changes in slow temporal modulations related to prosody and changes in spectral features related to voice quality. Importantly, the relative balance of the two effects varied widely across individuals, even though the amount of sleep deprivation was controlled, thus confirming the need to characterize sleep deprivation at the individual level. Moreover, while the prosody factor correlated with subjective sleepiness reports, the voice quality factor did not, consistent with the presence of both explicit and implicit consequences of sleep deprivation. Overall, the findings show that individual effects of sleep deprivation may be observed in vocal biomarkers. Future investigations correlating such markers with objective physiological measures of sleep deprivation could enable "sleep stethoscopes" for the cost-effective diagnosis of the individual effects of sleep deprivation.

Dynamics of temporally modulated materials: Adiabatic transformations and frequency conversion

The recent progress in the context of elastic metamaterials and modulated waveguides with digitally controllable properties has spurred the research in the context of time-varying and space-time varying mechanical systems. In other words, the search of new functionalities, such as nonreciprocity, frequency conversion, parametric amplification, and edge-to-edge pumping, to name a few, requires advanced space-time control of the material parameters, which justifies the emergence of active times in phononics. The work presented herein discusses temporal modulations in the context of stiffness-modulated elastic structures, with emphasis on frequency conversion and wave steering. It is shown that slow temporal modulations, compliant with the adiabatic theorem, can be functionally employed to change the frequency content and the propagation direction of incident wave packets. At the same time, adiabatic variations avoid the scattering of back-propagating waves, which are instead present in case of fast modulations and, in many cases, can be undesired from a practical perspective. Both transient and steady-state behaviors of stiffness-modulated waveguides are discussed with the goal of achieving controllable transmission of elastic signals between an emitter and a receiver.

Validity of the nonlinear Schrödinger approximation for quasilinear dispersive systems with more than one derivative

For nonlinear dispersive systems, the nonlinear Schrödinger (NLS) equation can usually be derived as a formal approximation equation describing slow spatial and temporal modulations of the envelope of a spatially and temporally oscillating underlying carrier wave. Here, we justify the NLS approximation for a whole class of quasilinear dispersive systems, which also includes toy models for the waterwave problem. This is the first time that this is done for systems, where a quasilinear quadratic term is allowed to effectively lose more than one derivative. With effective loss, we here mean the loss still present after making a diagonalization of the linear part of the system such that all linear operators in this diagonalization have the same regularity properties.

Neural entrainment to speech and nonspeech in dyslexia: Conceptual replication and extension of previous investigations

Whether phonological deficits in developmental dyslexia are associated with impaired neural sampling of auditory information is still under debate. Previous findings suggested that dyslexic participants showed atypical neural entrainment to slow and/or fast temporal modulations in speech, which might affect prosodic/syllabic and phonemic processing respectively. However, the large methodological variations across these studies do not allow us to draw clear conclusions on the nature of the entrainment deficit in dyslexia. Using magnetoencephalography, we measured neural entrainment to nonspeech and speech in both groups. We first aimed to conceptually replicate previous studies on auditory entrainment in dyslexia, using the same measurement methods as in previous studies, and also using new measurement methods (cross-correlation analyses) to better characterize the synchronization between stimulus and brain response. We failed to observe any of the significant group differences that had previously been reported in delta, theta and gamma frequency bands, whether using speech or nonspeech stimuli. However, when analyzing amplitude cross-correlations between noise stimuli and brain responses, we found that control participants showed larger responses than dyslexic participants in the delta range in the right hemisphere and in the gamma range in the left hemisphere. Overall, our results are weakly consistent with the hypothesis that dyslexic individuals show an atypical entrainment to temporal modulations. Our attempt at replicating previously published results highlights the multiple weaknesses of this research area, particularly low statistical power due to small sample size, and the lack of methodological standards inducing considerable heterogeneity of measurement and analysis methods across studies.

Impaired neural response to speech edges in dyslexia

Speech comprehension has been proposed to critically rely on oscillatory cortical tracking, that is, phase alignment of neural oscillations to the slow temporal modulations (envelope) of speech. Speech-brain entrainment is readjusted over time as transient events (edges) in speech lead to speech-brain phase realignment. Auditory behavioral research suggests that phonological deficits in dyslexia are linked to difficulty in discriminating speech edges. Importantly, research to date has not specifically examined neural responses to speech edges in dyslexia. In the present study, we used MEG to record brain activity from normal and dyslexic readers while they listened to speech. We computed phase locking values (PLVs) to evaluate phase entrainment between neural oscillations and the speech envelope time-locked to edge onsets. In both groups, we observed that edge onsets induced phase resets in the auditory oscillations tracking speech, thereby enhancing their entrainment to speech. Importantly, dyslexic readers showed weaker PLVs compared to normal readers in left auditory regions from ~.15 sec to ~.65 sec after edge onset. Our results indicate that the neural mechanism that adapts cortical entrainment to the speech envelope is impaired in dyslexia. These findings here are consistent with the temporal sampling theory of developmental dyslexia.

Homology and Specificity of Natural Sound-Encoding in Human and Monkey Auditory Cortex.

Understanding homologies and differences in auditory cortical processing in human and nonhuman primates is an essential step in elucidating the neurobiology of speech and language. Using fMRI responses to natural sounds, we investigated the representation of multiple acoustic features in auditory cortex of awake macaques and humans. Comparative analyses revealed homologous large-scale topographies not only for frequency but also for temporal and spectral modulations. In both species, posterior regions preferably encoded relatively fast temporal and coarse spectral information, whereas anterior regions encoded slow temporal and fine spectral modulations. Conversely, we observed a striking interspecies difference in cortical sensitivity to temporal modulations: While decoding from macaque auditory cortex was most accurate at fast rates (> 30 Hz), humans had highest sensitivity to ~3 Hz, a relevant rate for speech analysis. These findings suggest that characteristic tuning of human auditory cortex to slow temporal modulations is unique and may have emerged as a critical step in the evolution of speech and language.

Objective and subjective assessment of envelope enhancement algorithms for assistive hearing devices

Abstract Speech perception in a noisy environment is a significant challenge for individuals with auditory processing deficits. Evidence exists that exaggerating the slow temporal modulations may enhance speech perception for these individuals in the absence or presence of background noise. Nevertheless, a comprehensive assessment of envelope enhancement algorithms is lacking. In the present research study, two different schemes of envelope enhancement (dynamic and static) were evaluated subjectively and objectively for different types and levels of background noise with and without applying a noise reduction algorithm. In the subjective assessment, the dynamic envelope enhancement algorithm was evaluated with three different subjective groups including, twelve normal adults, twelve normal children, and eleven children with suspected auditory processing disorder (APD). The subjective results revealed that the speech intelligibility scores were lower for APD subjects compared to both normal adults and normal children. The subjective results also demonstrated that enhancing the temporal envelope is much more beneficial for subjects with suspected APD when compared to normal adults and children participating in the subjective experiment. In the objective assessment, the Hearing Aid Speech Perception Index (HASPI) was employed to predict the speech intelligibility scores which correlated highly with the subjective data. Comprehensive objective experiments demonstrated that both dynamic and static envelope enhancement algorithms are only effective in improving speech perception under certain processing conditions that depended on the type, level and location of the background noise. It is also shown that the application of a noise reduction algorithm prior to the envelope enhancement algorithms will increase their range of effectiveness.

The contribution of Neal Veimeister to the modulation theory of hearing

Neal Viemeister has dedicated his scientific career to one of the most influential research programs in hearing sciences. This program aims to apply the modulation theory to auditory perception. This theory, inspired by telecommunication sciences, relies on the following assumptions: (i) communication sounds including speech and animal vocalizations convey salient and slow temporal modulation cues; (ii) the auditory system of species using communication sounds has evolved to optimize its responses to these modulation cues; (iii) the transmission of information conveyed by communication signals is constrained by the ‘modulation transfer function’ (MTF) of the transmission path (a room, a hearing aid,...). We will show how the pioneering work of Neal Viemeister on the ‘temporal MTF’ and his rigorous investigation of modulation perception by human listeners contributed to an in-depth understanding of the demodulation processes implemented in the peripheral and central auditory system. We will show how his work yielded to models of modulation processing that account for a large range of listening situations including speech intelligibility and texture perception. Finally, we will show how his work contributed to a better understanding of the perceptual consequences of ageing and cochlear damage, and a better control of information transmission via rehabilitation systems.

Corrigendum to "Slow and steady, not fast and furious: Slow temporal modulation strengthens continuous flash suppression" [Conscious Cogn. 58 (2018) 10-19

Corrigendum to "Slow and steady, not fast and furious: Slow temporal modulation strengthens continuous flash suppression" [Conscious Cogn. 58 (2018) 10-19

Slow and steady, not fast and furious: Slow temporal modulation strengthens continuous flash suppression.

Continuous flash suppression (CFS) involves the presentation of a rapidly changing Mondrian sequence to one eye and a static target in the other eye. Targets presented in this manner remain suppressed for several seconds at a time, and this has seen the prevalent use of CFS in studies of unconscious visual processes. However, the mechanisms behind CFS remain unclear, complicating its use and the comprehension of results obtained with the paradigm. For example, some studies report observations indicative of faster, visual masking processes whereas others suggest slower, rivalry processes. To reconcile this discrepancy, this study investigates the effect of temporal frequency content and Mondrian pattern structure on CFS suppression. Our results show predominant influences of spatial edges and low temporal-frequency content, which are similar to binocular rivalry, affording a parsimonious alternative in unifying the two paradigms.

Existence of long time solutions and validity of the nonlinear Schrödinger approximation for a quasilinear dispersive equation

We consider a nonlinear dispersive equation with a quasilinear quadratic term. We establish two results. First, we show that solutions to this equation with initial data of order O(ε) in Sobolev norms exist for a time span of order O(ε−2) for sufficiently small ε. Secondly, we derive the Nonlinear Schrödinger (NLS) equation as a formal approximation equation describing slow spatial and temporal modulations of the envelope of an underlying carrier wave, and justify this approximation with the help of error estimates in Sobolev norms between exact solutions of the quasilinear equation and the formal approximation obtained via the NLS equation.The proofs of both results rely on estimates of appropriate energies whose constructions are inspired by the method of normal-form transforms. To justify the NLS approximation, we have to overcome additional difficulties caused by the occurrence of resonances. We expect that the method developed in the present paper will also allow to prove the validity of the NLS approximation for a larger class of quasilinear dispersive systems with resonances.

Processing of temporally patterned sounds in the auditory cortex of Seba's short-tailed bat,Carollia perspicillata.

This article presents a characterization of cortical responses to artificial and natural temporally patterned sounds in the bat species Carollia perspicillata, a species that produces vocalizations at rates above 50Hz. Multi-unit activity was recorded in three different experiments. In the first experiment, amplitude-modulated (AM) pure tones were used as stimuli to drive auditory cortex (AC) units. AC units of both ketamine-anesthetized and awake bats could lock their spikes to every cycle of the stimulus modulation envelope, but only if the modulation frequency was below 22Hz. In the second experiment, two identical communication syllables were presented at variable intervals. Suppressed responses to the lagging syllable were observed, unless the second syllable followed the first one with a delay of at least 80ms (i.e., 12.5Hz repetition rate). In the third experiment, natural distress vocalization sequences were used as stimuli to drive AC units. Distress sequences produced by C.perspicillata contain bouts of syllables repeated at intervals of ~60ms (16Hz). Within each bout, syllables are repeated at intervals as short as 14ms (~71Hz). Cortical units could follow the slow temporal modulation flow produced by the occurrence of multisyllabic bouts, but not the fast acoustic flow created by rapid syllable repetition within the bouts. Taken together, our results indicate that even in fast vocalizing animals, such as bats, cortical neurons can only track the temporal structure of acoustic streams modulated at frequencies lower than 22Hz.

Infants' and Adults' Use of Temporal Cues in Consonant Discrimination.

Adults can use slow temporal envelope cues, or amplitude modulation (AM), to identify speech sounds in quiet. Faster AM cues and the temporal fine structure, or frequency modulation (FM), play a more important role in noise. This study assessed whether fast and slow temporal modulation cues play a similar role in infants' speech perception by comparing the ability of normal-hearing 3-month-olds and adults to use slow temporal envelope cues in discriminating consonants contrasts. English consonant-vowel syllables differing in voicing or place of articulation were processed by 2 tone-excited vocoders to replace the original FM cues with pure tones in 32 frequency bands. AM cues were extracted in each frequency band with 2 different cutoff frequencies, 256 or 8 Hz. Discrimination was assessed for infants and adults using an observer-based testing method, in quiet or in a speech-shaped noise. For infants, the effect of eliminating fast AM cues was the same in quiet and in noise: a high proportion of infants discriminated when both fast and slow AM cues were available, but less than half of the infants also discriminated when only slow AM cues were preserved. For adults, the effect of eliminating fast AM cues was greater in noise than in quiet: All adults discriminated in quiet whether or not fast AM cues were available, but in noise eliminating fast AM cues reduced the percentage of adults reaching criterion from 71 to 21%. In quiet, infants seem to depend on fast AM cues more than adults do. In noise, adults seem to depend on FM cues to a greater extent than infants do. However, infants and adults are similarly affected by a loss of fast AM cues in noise. Experience with the native language seems to change the relative importance of different acoustic cues for speech perception.

Atypical right hemisphere response to slow temporal modulations in children with developmental dyslexia

Phase entrainment of neuronal oscillations is thought to play a central role in encoding speech. Children with developmental dyslexia show impaired phonological processing of speech, proposed theoretically to be related to atypical phase entrainment to slower temporal modulations in speech (<10Hz). While studies of children with dyslexia have found atypical phase entrainment in the delta band (~2Hz), some studies of adults with developmental dyslexia have shown impaired entrainment in the low gamma band (~35–50Hz). Meanwhile, studies of neurotypical adults suggest asymmetric temporal sensitivity in auditory cortex, with preferential processing of slower modulations by right auditory cortex, and faster modulations processed bilaterally. Here we compared neural entrainment to slow (2Hz) versus faster (40Hz) amplitude-modulated noise using fNIRS to study possible hemispheric asymmetry effects in children with developmental dyslexia. We predicted atypical right hemisphere responding to 2Hz modulations for the children with dyslexia in comparison to control children, but equivalent responding to 40Hz modulations in both hemispheres. Analyses of HbO concentration revealed a right-lateralised region focused on the supra-marginal gyrus that was more active in children with dyslexia than in control children for 2Hz stimulation. We discuss possible links to linguistic prosodic processing, and interpret the data with respect to a neural ‘temporal sampling’ framework for conceptualizing the phonological deficits that characterise children with developmental dyslexia across languages.

Perception of Filtered Speech by Children with Developmental Dyslexia and Children with Specific Language Impairments.

Here we use two filtered speech tasks to investigate children’s processing of slow (<4 Hz) versus faster (∼33 Hz) temporal modulations in speech. We compare groups of children with either developmental dyslexia (Experiment 1) or speech and language impairments (SLIs, Experiment 2) to groups of typically-developing (TD) children age-matched to each disorder group. Ten nursery rhymes were filtered so that their modulation frequencies were either low-pass filtered (<4 Hz) or band-pass filtered (22 – 40 Hz). Recognition of the filtered nursery rhymes was tested in a picture recognition multiple choice paradigm. Children with dyslexia aged 10 years showed equivalent recognition overall to TD controls for both the low-pass and band-pass filtered stimuli, but showed significantly impaired acoustic learning during the experiment from low-pass filtered targets. Children with oral SLIs aged 9 years showed significantly poorer recognition of band pass filtered targets compared to their TD controls, and showed comparable acoustic learning effects to TD children during the experiment. The SLI samples were also divided into children with and without phonological difficulties. The children with both SLI and phonological difficulties were impaired in recognizing both kinds of filtered speech. These data are suggestive of impaired temporal sampling of the speech signal at different modulation rates by children with different kinds of developmental language disorder. Both SLI and dyslexic samples showed impaired discrimination of amplitude rise times. Implications of these findings for a temporal sampling framework for understanding developmental language disorders are discussed.

Dual temporal encoding mechanisms in human auditory cortex: Evidence from MEG and EEG

Current hypotheses about language processing advocate an integral relationship between encoding of temporal information and linguistic processing in the brain. All such explanations must accommodate the evident ability of the perceptual system to process both slow and fast time scales in speech. However most cortical neurons are limited in their capability to precisely synchronise to temporal modulations at rates faster than about 50Hz. Hence, a central question in auditory neurophysiology concerns how the full range of perceptually relevant modulation rates might be encoded in the cerebral cortex. Here we show with concurrent noninvasive magnetoencephalography (MEG) and electroencephalography (EEG) measurements that the human auditory cortex transitions between a phase-locked (PL) mode of responding to modulation rates below about 50Hz, and a non-phase-locked (NPL) mode at higher rates. Precisely such dual response modes are predictable from the behaviours of single neurons in auditory cortices of non-human primates. Our data point to a common mechanistic explanation for the single neuron and MEG/EEG results and support the hypothesis that two distinct types of neuronal encoding mechanisms are employed by the auditory cortex to represent a wide range of temporal modulation rates. This dual encoding model allows slow and fast modulations in speech to be processed in parallel and is therefore consistent with theoretical frameworks in which slow temporal modulations (such as rhythm or syllabic structure) are akin to the contours or edges of visual objects, whereas faster modulations (such as periodicity pitch or phonemic structure) are more like visual texture.

In young readers, the left hemisphere supports the link between temporal processing and phonological awareness

Phonological awareness, the ability to manipulate the sounds of language, is key for learning to read. The first step towards phonological competence is identification of syllables and rimes. In a continuous speech stream, syllables and rimes are marked by slow temporal rhythmic modulations, including changes in vowel intensity or amplitude rise time (ART). Prior work suggests that children's sensitivity to ART predicts reading ability and dyslexia across languages. Yet, little is known about the brain bases of this sensitivity that might be key to both language and reading acquisition. The present study explored the hypothesis that children's brain response to slow temporal modulations, tested with amplitude rise perception, relates to child reading acquisition. Fifteen young readers (ages 7–12) completed ART and a control intensity discrimination task during functional magnetic resonance imaging. Behavioral findings validated the link between ART sensitivity, language and literacy abilities in young readers. Neuroimaging findings showed that while bilateral temporal cortexes were active during the ART task, children with better phonological and ART abilities showed reduced brain activation in left temporal lobe. These findings suggest a link between amplitude rise perception and left-lateralized language abilities, and carry implications for better understanding of language, literacy, and reading disability across languages.

Justification of the 2D NLS equation for a fourth order nonlinear wave equation – quadratic resonances do not matter much in case of analytic initial conditions

The 2D Nonlinear Schrödinger (NLS) equation can be derived by multiple scaling analysis in order to describe slow temporal and spatial modulations of an oscillating wave packet in general dispersive wave systems. It is the purpose of this paper to establish estimates for the difference between the so-obtained approximation and true solutions of the original system in case that the original system possesses nontrivial quadratic resonances. We explain for a fourth order nonlinear wave equation that quadratic resonances do not matter much in case of analytic initial conditions for the NLS equation in the sense that independently of whether the resonance is stable or unstable an approximation property can be established on the natural time scale of the NLS equation if the set of resonant wave numbers is bounded away from the integer multiples of the basic wave number of the underlying wave packet. Finally, we explain that in case of an approximation by a so-called four wave interaction (FWI) system the same approach works and an even stronger result can be established.