Near-DC magnetic fields following a periodic presentation of long-duration tonebursts

Filters: When, Why, and How (Not) to Use Them

Filters are commonly used to reduce noise and improve data quality. Filter theory is part of a scientist's training, yet the impact of filters on interpreting data is not always fully appreciated. This paper reviews the issue and explains what a filter is, what problems are to be expected when using them, how to choose the right filter, and how to avoid filtering by using alternative tools. Time-frequency analysis shares some of the same problems that filters have, particularly in the case of wavelet transforms. We recommend reporting filter characteristics with sufficient details, including a plot of the impulse or step response as an inset.

Detecting changes in dynamic and complex acoustic environments

Natural sounds such as wind or rain, are characterized by the statistical occurrence of their constituents. Despite their complexity, listeners readily detect changes in these contexts. We here address the neural basis of statistical decision-making using a combination of psychophysics, EEG and modelling. In a texture-based, change-detection paradigm, human performance and reaction times improved with longer pre-change exposure, consistent with improved estimation of baseline statistics. Change-locked and decision-related EEG responses were found in a centro-parietal scalp location, whose slope depended on change size, consistent with sensory evidence accumulation. The potential's amplitude scaled with the duration of pre-change exposure, suggesting a time-dependent decision threshold. Auditory cortex-related potentials showed no response to the change. A dual timescale, statistical estimation model accounted for subjects' performance. Furthermore, a decision-augmented auditory cortex model accounted for performance and reaction times, suggesting that the primary cortical representation requires little post-processing to enable change-detection in complex acoustic environments.

DOI:http://dx.doi.org/10.7554/eLife.24910.001

Estimation of a transient response from steady-state responses by deconvolution with built-in constraints

Evidence suggests that the steady-state response (SSR) elicited by a periodic train of auditory stimuli can largely be understood as a superposition of transient responses. This study is devoted to the problem of how to estimate that transient response from measured SSRs. The proposed method differs from previous approaches in that the solution can be constrained to be consistent with physiology-based prior knowledge or educated guesses. To achieve this goal, the transient response is not represented by a time series, but by a linear combination of auxiliary functions, called components. Constraints are introduced by assigning certain properties to the components. Only few parameters are required for that purpose, because the individual components are derived from a suitably designed mother component. After adjusting the components to the problem at hand, the component amplitudes are determined by optimizing the match between predicted and measured SSRs. This requires solving a linear inverse problem. A model simulation as well as an analysis of exemplary experimental data (auditory SSRs elicited by periodically presented clicks) prove the workability of the method. Since part of the theory is quite general, it would be relatively easy to refine and extend the method. Not only could responses other than SSRs be dealt with, it could also be realized that certain key parameters of the transient response, such as amplitude and delay, depend on stimulus repetition rate.

Auditory cortex tracks the temporal regularity of sustained noisy sounds

Neuroimaging studies have revealed dramatic asymmetries between the responses to temporally regular and irregular sounds in the antero-lateral part of Heschl's gyrus. For example, the magnetoencephalography (MEG) study of Krumbholz et al. [Cereb. Cortex 13, 765-772 (2003)] showed that the transition from a noise to a similar noise with sufficient temporal regularity to provoke a pitch evoked a pronounced temporal-regularity onset response (TRon response), whereas a comparable transition in the reverse direction revealed essentially no temporal-regularity offset response (TRoff response). The current paper presents a follow-up study in which the asymmetry is examined with much greater power, and the results suggest an intriguing reinterpretation of the onset/offset asymmetry. The TR-related activity in auditory cortex appears to be composed of a transient (TRon) and a TR-related sustained response (TRsus), with a highly variable TRon/TRsus amplitude ratio. The TRoff response is generally dominated by the break-down of the TRsus activity, which occurs so rapidly as to preclude the involvement of higher-level cortical processing. The time course of the TR-related activity suggests that TR processing might be involved in monitoring the environment and alerting the brain to the onset and offset of behaviourally relevant, animate sources.

Baseline correction of overlapping event-related responses using a linear deconvolution technique

The time it takes an event-related response (ERR) to subside is often longer than the interval between successive events so that the response to a new event interferes with a baseline formed by responses to preceding events. Thus, without proper baseline correction, the interpretation of an event-triggered average (ETA) of recorded data can be problematic. As the spectral compositions of ERR and baseline typically overlap, filtering the ETA is not always an adequate solution. The approach introduced here exploits that ETA and ERR are linearly related. Unless the series of events is exactly periodic, the ERR can be derived from the ETA by linear deconvolution. The performance of the method is illustrated with simulated examples as well as data from an auditory evoked field (AEF) study. It is also outlined how to handle experiments with two or more different events. Intriguing applications beyond the scope of baseline correction arise from the fact that the ERR is invariably estimated for a time window longer than the mean interval between successive events. The method may help, for example, to better understand the relationship between transient and steady-state responses or to delineate the component structure of a specific ERR.

BOLD responses in human auditory cortex are more closely related to transient MEG responses than to sustained ones

Blood oxygen level dependent-functional magnetic resonance imaging (BOLD-fMRI) and magnetoencephalographic (MEG) signals are both coupled to postsynaptic potentials, although their relationship is incompletely understood. Here, the wide range of BOLD-fMRI and MEG responses produced by auditory cortex was exploited to better understand the BOLD-fMRI/MEG relationship. Measurements of BOLD and MEG responses were made in the same subjects using the same stimuli for both modalities. The stimuli, 24-s sequences of click trains, had duty cycles of 2.5, 25, 72, and 100%. For the 2.5% sequence, the BOLD response was elevated throughout the sequence, whereas for 100%, it peaked after sequence onset and offset and showed a diminished elevation in between. On the finer timescale of MEG, responses at 2.5% consisted of a complex of transients, including N(1)m, to each click train of the sequence, whereas for 100% the only transients occurred at sequence onset and offset between which there was a sustained elevation in the MEG signal (a sustained field). A model that separately estimated the contributions of transient and sustained MEG signals to the BOLD response best fit BOLD measurements when the transient contribution was weighted 8- to 10-fold more than the sustained one. The findings suggest that BOLD responses in the auditory cortex are tightly coupled to the neural activity underlying transient, not sustained, MEG signals.

Electrophysiological and psychophysical asymmetries in sensitivity to interaural correlation steps

The binaural auditory system's sensitivity to changes in the interaural cross correlation (IAC), as an indicator for the perceived spatial diffuseness of a sound, is of major importance for the ability to distinguish concurrent sound sources. In this article, we present electroencephalographical and corresponding psychophysical experiments with stepwise transitions of the IAC in continuously running noise. Both the transient and sustained brain response, display electrophysiological correlates of specific binaural processing in humans. The transient late auditory evoked potentials (LAEP) systematically depend on the size of the IAC transition, the reference correlation preceding the transition, the direction of the transition and on unspecific context information from the stimulus sequence. The psychophysical and electrophysiological data are characterized by two asymmetries. (1) Major asymmetry: for reference correlations of +1 and -1, psychoacoustical thresholds are comparatively lower, and the peak-to-peak-amplitudes of LAEP are larger than for a reference correlation of zero. (2) Minor asymmetry: for IAC transitions in the positive parameter range, perceptual thresholds are slightly better and peak-to-peak amplitudes are larger than in the negative range. In all experimental conditions, LAEP amplitudes are linearly related to the dB scaled power ratio of correlated (N(0)) versus anticorrelated (N(pi)) signal components. The voltage gain of LAEP per dB(N(0)/N(pi)) closely corresponds to a constant perceptual distance between two correlations. We therefore suggest that activity in the auditory cortex and perceptual IAC sensitivity are better represented by the dB-scaled N(0)/N(pi) power ratio than by the normalized IAC itself.

Neuronal current detection with low-field magnetic resonance: simulations and methods

The noninvasive detection of neuronal currents in active brain networks [or direct neuronal imaging (DNI)] by means of nuclear magnetic resonance (NMR) remains a scientific challenge. Many different attempts using NMR scanners with magnetic fields >1 T (high-field NMR) have been made in the past years to detect phase shifts or magnitude changes in the NMR signals. However, the many physiological (i.e., the contemporarily BOLD effect, the weakness of the neuronal-induced magnetic field, etc.) and technical limitations (e.g., the spatial resolution) in observing the weak signals have led to some contradicting results. In contrast, only a few attempts have been made using low-field NMR techniques. As such, this paper was aimed at reviewing two recent developments in this front. The detection schemes discussed in this manuscript, the resonant mechanism (RM) and the DC method, are specific to NMR instrumentations with main fields below the earth magnetic field (50 microT), while some even below a few microteslas (ULF-NMR). However, the experimental validation for both techniques, with differentiating sensitivity to the various neuronal activities at specific temporal and spatial resolutions, is still in progress and requires carefully designed magnetic field sensor technology. Additional care should be taken to ensure a stringent magnetic shield from the ambient magnetic field fluctuations. In this review, we discuss the characteristics and prospect of these two methods in detecting neuronal currents, along with the technical requirements on the instrumentation.

Threshold and beyond: modeling the intensity dependence of auditory responses

In many studies of auditory-evoked responses to low-intensity sounds, the response amplitude appears to increase roughly linearly with the sound level in decibels (dB), corresponding to a logarithmic intensity dependence. But the auditory system is assumed to be linear in the low-intensity limit. The goal of this study was to resolve the seeming contradiction. Based on assumptions about the rate-intensity functions of single auditory-nerve fibers and the pattern of cochlear excitation caused by a tone, a model for the gross response of the population of auditory nerve fibers was developed. In accordance with signal detection theory, the model denies the existence of a threshold. This implies that regarding the detection of a significant stimulus-related effect, a reduction in sound intensity can always be compensated for by increasing the measurement time, at least in theory. The model suggests that the gross response is proportional to intensity when the latter is low (range I), and a linear function of sound level at higher intensities (range III). For intensities in between, it is concluded that noisy experimental data may provide seemingly irrefutable evidence of a linear dependence on sound pressure (range II). In view of the small response amplitudes that are to be expected for intensity range I, direct observation of the predicted proportionality with intensity will generally be a challenging task for an experimenter. Although the model was developed for the auditory nerve, the basic conclusions are probably valid for higher levels of the auditory system, too, and might help to improve models for loudness at threshold.

DC-magnetoencephalography and time-resolved near-infrared spectroscopy combined to study neuronal and vascular brain responses

The temporal relation between vascular and neuronal responses of the brain to external stimuli is not precisely known. For a better understanding of the neuro-vascular coupling changes in cerebral blood volume and oxygenation have to be measured simultaneously with neuronal currents. With this motivation modulation dc-magnetoencephalography was combined with multi-channel time-resolved near-infrared spectroscopy to simultaneously monitor neuronal and vascular parameters on a scale of seconds. Here, the technique is described, how magnetic and optical signals can be measured simultaneously. In a simple motor activation paradigm (alternating 30 s of finger movement with 30 s of rest for 40 min) both signals were recorded non-invasively over the motor cortex of eight subjects. The off-line averaged signals from both modalities showed distinct stimulation related changes. By plotting changes in oxy- or deoxyhaemoglobin as a function of magnetic field a characteristic trajectory was created, which was similar to a hysteresis loop. A parametric analysis allowed quantitative results regarding the timing of coupling: the vascular signal increased significantly slower than the neuronal signal.

Tonic neuronal activation during simple and complex finger movements analyzed by DC-magnetoencephalography

Functional neuroimaging techniques map neuronal activation indirectly via local concomitant cortical vascular/metabolic changes. In a complementary approach, DC-magnetoencephalography measures neuronal activation dynamics directly, notably in a time range of the slow vascular/metabolic response. Here, using this technique neuronal activation dynamics and patterns for simple and complex finger movements are characterized intraindividually: in 6/6 right-handed subjects contralateral prolonged (30 s each) complex self-paced sequential finger movements revealed stronger field amplitudes over the pericentral sensorimotor cortex than simple movements. A consistent lateralization for contralateral versus ipsilateral finger movements was not found (4/6). A subsequent sensory paradigm focused on somatosensory afferences during the motor tasks and the reliability of the measuring technique. In all six subjects stable sustained neuronal activation during electrical median nerve stimulation was recorded. These neuronal quasi-tonic activation characteristics provide a new non-invasive neurophysiological measure to interpret signals mapped by functional neuroimaging techniques.

GLM-beamformer method demonstrates stationary field, alpha ERD and gamma ERS co-localisation with fMRI BOLD response in visual cortex

Recently, we introduced a new 'GLM-beamformer' technique for MEG analysis that enables accurate localisation of both phase-locked and non-phase-locked neuromagnetic effects, and their representation as statistical parametric maps (SPMs). This provides a useful framework for comparison of the full range of MEG responses with fMRI BOLD results. This paper reports a 'proof of principle' study using a simple visual paradigm (static checkerboard). The five subjects each underwent both MEG and fMRI paradigms. We demonstrate, for the first time, the presence of a sustained (DC) field in the visual cortex, and its co-localisation with the visual BOLD response. The GLM-beamformer analysis method is also used to investigate the main non-phase-locked oscillatory effects: an event-related desynchronisation (ERD) in the alpha band (8-13 Hz) and an event-related synchronisation (ERS) in the gamma band (55-70 Hz). We show, using SPMs and virtual electrode traces, the spatio-temporal covariance of these effects with the visual BOLD response. Comparisons between MEG and fMRI data sets generally focus on the relationship between the BOLD response and the transient evoked response. Here, we show that the stationary field and changes in oscillatory power are also important contributors to the BOLD response, and should be included in future studies on the relationship between neuronal activation and the haemodynamic response.

A general linear model for MEG beamformer imaging

A new general linear model (GLM) beamformer method is described for processing magnetoencephalography (MEG) data. A standard nonlinear beamformer is used to determine the time course of neuronal activation for each point in a predefined source space. A Hilbert transform gives the envelope of oscillatory activity at each location in any chosen frequency band (not necessary in the case of sustained (DC) fields), enabling the general linear model to be applied and a volumetric T statistic image to be determined. The new method is illustrated by a two-source simulation (sustained field and 20 Hz) and is shown to provide accurate localization. The method is also shown to locate accurately the increasing and decreasing gamma activities to the temporal and frontal lobes, respectively, in the case of a scintillating scotoma. The new method brings the advantages of the general linear model to the analysis of MEG data and should prove useful for the localization of changing patterns of activity across all frequency ranges including DC (sustained fields).

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.

Sound repetition rate in the human auditory pathway: representations in the waveshape and amplitude of fMRI activation

Sound repetition rate plays an important role in stream segregation, temporal pattern recognition, and the perception of successive sounds as either distinct or fused. This study was aimed at elucidating the neural coding of repetition rate and its perceptual correlates. We investigated the representations of rate in the auditory pathway of human listeners using functional magnetic resonance imaging (fMRI), an indicator of population neural activity. Stimuli were trains of noise bursts presented at rates ranging from low (1-2/s; each burst is perceptually distinct) to high (35/s; individual bursts are not distinguishable). There was a systematic change in the form of fMRI response rate-dependencies from midbrain to thalamus to cortex. In the inferior colliculus, response amplitude increased with increasing rate while response waveshape remained unchanged and sustained. In the medial geniculate body, increasing rate produced an increase in amplitude and a moderate change in waveshape at higher rates (from sustained to one showing a moderate peak just after train onset). In auditory cortex (Heschl's gyrus and the superior temporal gyrus), amplitude changed somewhat with rate, but a far more striking change occurred in response waveshape-low rates elicited a sustained response, whereas high rates elicited an unusual phasic response that included prominent peaks just after train onset and offset. The shift in cortical response waveshape from sustained to phasic with increasing rate corresponds to a perceptual shift from individually resolved bursts to fused bursts forming a continuous (but modulated) percept. Thus at high rates, a train forms a single perceptual "event," the onset and offset of which are delimited by the on and off peaks of phasic cortical responses. While auditory cortex showed a clear, qualitative correlation between perception and response waveshape, the medial geniculate body showed less correlation (since there was less change in waveshape with rate), and the inferior colliculus showed no correlation at all. Overall, our results suggest a population neural representation of the beginning and the end of distinct perceptual events that is weak or absent in the inferior colliculus, begins to emerge in the medial geniculate body, and is robust in auditory cortex.

Magnetoencephalographic correlates of audiotactile interaction

To seek for correlates of an interaction between auditory and somatosensory processing, the brain's magnetic field in response to simultaneously presented auditory (A) and tactile (T) stimuli was compared with the sum of the respective unimodal responses (A+T). The stimuli were binaural 1047-Hz tone bursts of 60 dB sensation level and tactile pressure pulses to the right thumb. The mean interval between two stimuli of the same modality was 1.95 s. The magnetic field was recorded using a 306-channel whole-scalp neuromagnetometer. A clear audiotactile interaction was revealed in the hemisphere contralateral to the side of tactile stimulation in six of eight subjects, whereas in the ipsilateral hemisphere an interaction was noticed in only three subjects. The time courses of these audiotactile interaction fields typically showed major deflections of opposite polarities around 140 and 220 ms. The first deflection appeared to arise in the region of the secondary somatosensory cortex (SII). The polarity of this interaction was consistent with the view that the auditory stimulus resulted in a partial inhibition in SII. In two subjects, strong indications of auditory contributions to the interaction were available, although in different hemispheres. The relatively high interindividual variability of the observed interaction, which represents potential neural substrates for multisensory integration, could indicate that the way subjects perceive the simultaneous presentation of auditory and tactile stimuli differs.