The enhancement of the N1 wave elicited by sensory stimuli presented at very short inter-stimulus intervals is a general feature across sensory systems

N1 facilitation at short Inter-Stimulus-Interval (ISI) occurs under 400 ms and is dependent on ISI from previous sounds: Evidence using an unpredictable auditory stimulation sequence

The N1 auditory evoked potential amplitude depends heavily on the inter-stimulus interval (ISI). Typically, shorter ISIs result in reduced N1 amplitudes, suggesting a decreased neural response with high stimulus presentation rates. However, an exception known as N1 facilitation occurs with very brief ISIs (∼150-500 ms), where the N1 amplitude increases. This study aimed to further characterize N1 facilitation using an experimental paradigm with a continuous distribution of ISIs (0.25 to 8 s) to identify the specific ISI where N1 facilitation occurs. We also examined the role of ISI history in N1 facilitation and explored correlations between N1 facilitation, overall N1 amplitude and ISI-sensitivity, and results of cognitive tasks. Twenty-nine participants passively listened to a random sequence of auditory clicks at varying intensities (65, 75, or 85 dB) and ISI ranges (0.25-0.5 s, 0.5-1 s, 1-2 s, 2-4 s, 4-8 s) while EEG was recorded. Up to 1800 sweeps were collected in the critical ISI range (0.25 to 0.5 s) where N1 facilitation is expected. Results support N1 facilitation occurring at ISIs under 400 ms (p = 0.03), where N1 amplitudes returned to values seen at longer ISIs (∼1.7 s). Notably, this effect was observed when the ISI two clicks before was shorter than 1.5 s (p = 0.001), but not otherwise (p = 0.37). These findings clarify the temporal dynamics of N1 facilitation and challenge the notion of a rigid, context-independent latent inhibition process explaining this phenomenon.

Lateralization-specific adaptation in auditory cortical evoked potentials: Comparison with frequency-specificity

'Opponent channels model' (OCM) is the widely accepted model for cortical representation of sound lateralization. Stimulus-specific 'release from adaptation' (RFA) in cortical responses has been used in previous studies to test the predictions of this model. However, these attempts were shown to be prone to confounds of spurious responses such as those to auditory motion and sound onset. The present study aims to determine whether a multiple-adaptor RFA algorithm could be employed for relatively confound-free quantification of the population response of lateralization-specific auditory cortical neurons, and provide useful data for estimation of the OCM hemifield tuning curves. Two experiments were conducted on 12 volunteers with normal hearing. In Exp.1, quadruple tone pips of either low or high frequency were presented as adaptor, followed by a single tone pip of either frequency as probe. In Exp.2, tone pips were replaced with dichotic click train pips with left-leading and right-leading interaural time difference (ITD). Frequency- and ITD-specific RFA in cortical responses N1 and P2 was quantified using global field magnitude difference between ERPs to mismatched and matched adaptor-probe pairs. RFA level measured was lower for ITD mismatch than frequency mismatch. Nonetheless, it allowed measurement of ITD-specific cortical neurons' population response, without any spurious response confound. We proposed a method for extraction of ITD-specific response magnitude from the N1 response to a lateralized sound. Using it, one can reliably measure the activity of lateralization-specific cortical neurons, i.e. elicited by moderate ITD changes. This allows estimation of hemifield tuning curves in OCM using ERP data.

Hemispheric difference of adaptation lifetime in human auditory cortex measured with MEG

Adaptation is the attenuation of a neuronal response when a stimulus is repeatedly presented. The phenomenon has been linked to sensory memory, but its exact neuronal mechanisms are under debate. One defining feature of adaptation is its lifetime, that is, the timespan over which the attenuating effect of previous stimulation persists. This can be revealed by varying the stimulus-onset interval (SOI) of the repeated stimulus. As SOI is increased, the peak amplitude of the response grows before saturating at large SOIs. The rate of this growth can be quantified and used as an estimate of adaptation lifetime. Here, we studied whether adaptation lifetime varies across the left and the right auditory cortex of the human brain. Event-related fields of whole-head magnetoencephalograms (MEG) were measured in 14 subjects during binaural presentation of pure tone stimuli. To make statistical inferences on the single-subject level, additional event-related fields were generated by resampling the original single-trial data via bootstrapping. For each hemisphere and SOI, the peak amplitude of the N1m response was then derived from both original and bootstrap-based data sets. Finally, the N1m-peak amplitudes were used for deriving subject- and hemisphere-specific estimates of adaptation lifetime. Comparing subject-specific adaptation lifetime across hemispheres, we found a significant difference, with longer adaptation lifetimes in the left than in the right auditory cortex (p = 0.004). This difference might have a functional relevance in the context of temporal binding of auditory stimuli, leading to larger integration time windows in the left than in the right hemisphere.

Time course of visual attention in rats by atomic magnetometer

Atomic magnetometer (AM) is utilized to non-invasively detect event-related magnetic fields (ERMFs) evoked by visual stimuli in rats. The aim of this study was to investigate the relationship between N2-like amplitude and visual attention. To achieve this, we combined the AM with a visual stimulation system and employed the passive single-stimulus paradigm. By measuring the ERMFs at various inter-stimulus intervals (ISIs) with a sensitivity of 20 fT/[Formula: see text], we analyzed the effects of the ISI and the 'habituation' resulting from repeated stimuli on the N2-like amplitude. Our method serves as a valuable reference for studying the passive single-stimulus paradigm and the time course of mammalian attention.

Pain-Related Vertex Evoked Potentials. Comparison of Surface Electrical to Heat Stimulation

INTRODUCTION

Demonstration of nociceptive fiber abnormality is important for diagnosing neuropathic pain and small fiber neuropathies. This is usually assessed by brief heat pulses using lasers, contact heat, or special electrodes. We hypothesized that pain-related evoked potentials to conventional surface electrical stimulation (PREPse) can index Aδ afferences despite tactile Aß fibers coactivation. PREPse may be more readily used clinically than contact heat evoked potentials (CHEPS).

METHODS

Twenty-eight healthy subjects. Vertex (Cz-A1/A2) recordings. Electrical stimulation of middle finger and second toe with conventional ring, and forearm/leg skin with cup, electrodes. Contact heat stimulation to forearm and leg. Compression ischemic nerve blockade.

RESULTS

PREPse peripheral velocities were within the midrange of Aδ fibers. N1-P1 amplitude increased with pain numerical rating scale graded (0-10) electrical stimulation (n = 25) and decreased with increasing stimulation frequency. Amplitudes were unchanged by different presentation orders of four stimulation intensities. PREPse N1 (∼130 milliseconds) and N2 (∼345 milliseconds) peaks were approximately 40 milliseconds earlier than that with CHEPS. PREPse and CHEPS N1-N2 interpeak latency (∼207 milliseconds) were similar. PREPse became unrecordable with nerve blockade of Aδ fibers.

CONCLUSIONS

PREPse earlier N1 and N2 peaks, and similar interpeak N1-N2 latencies and central conduction velocities, or synaptic delays, to CHEPS are consistent with direct stimulation of Aδ fibers. The relation of vertex PREPse amplitude and pain, or the differential effects of frequency stimulation, is similar to pain-related evoked potential to laser, special electrodes, or contact heat stimulation. The relationship to Aδ was validated by conduction velocity and nerve block. Clinical utility of PREPse compared with CHEPS needs validation in somatosensory pathways lesions.

Influence of inter-stimulus interval on 40-Hz auditory steady-state response in patients with schizophrenia

Decreased 40-Hz auditory steady-state response (ASSR) is believed to reflect abnormal gamma oscillation in patients with schizophrenia (SZ). However, previous studies have reported conflicting results due to variations in inter-stimulus interval (ISI) used. In this study, we aimed to investigate the influence of varying ISI on the 40-Hz ASSR, particularly for patients with SZ and healthy controls (HCs). Twenty-four SZ patients (aged 40.8 ± 13.9 years, male: n = 11) and 21 HCs (aged 33.3 ± 11.3 years, male: n = 8) were recruited. For every participant, 40-Hz ASSRs were acquired for three different stimulus types: 500, 2000, and 3500 ms of ISIs. Two conventional ASSR measures (total power and inter-trial coherence, ITC) were calculated. Several additional ASSR measures were also analyzed: (i) ISI-dependent power; (ii) power onset slope; (iii) power centroid latency; (iv) ISI-dependent ITC; (v) ITC onset slope (500, 2000, 3500 ms); (vi) ITC centroid latency (500, 2000, 3500 ms). As ISI increased, total power and ITC increased in patients with SZ but decreased in HCs. In addition, patients with SZ showed higher ISI-dependent ITC, which was positively correlated with the psychotic symptom severity. The abnormal ITC onset slope and centroid latency for the ISI-500 ms condition were associated with cognitive speed decline in patients with SZ. Our study confirmed that the 40-Hz ASSR could be severely influenced by ISI. Furthermore, our results showed that the additional ASSR measures (ISI-dependent ITC, ITC onset slope, ITC centroid latency) could represent psychotic symptom severity or impairment in cognitive function in patients with SZ.

Slow neural oscillations explain temporal fluctuations in distractibility

Human environments comprise various sources of distraction, which often occur unexpectedly in time. The proneness to distraction (i.e., distractibility) is posited to be independent of attentional sampling of targets, but its temporal dynamics and neurobiological basis are largely unknown. Brain oscillations in the theta band (3 - 8Hz) have been associated with fluctuating neural excitability, which is hypothesised here to explain rhythmic modulation of distractibility. In a pitch discrimination task (N = 30) with unexpected auditory distractors, we show that distractor-evoked neural responses in the electroencephalogram and perceptual susceptibility to distraction were co-modulated and cycled approximately 3 - 5 times per second. Pre-distractor neural phase in left inferior frontal and insular cortex regions explained fluctuating distractibility. Thus, human distractibility is not constant but fluctuates on a subsecond timescale. Furthermore, slow neural oscillations subserve the behavioural consequences of a hitherto largely unexplained but ever-increasing phenomenon in modern environments - distraction by unexpected sound.

Intensity and inter-stimulus-interval effects on human middle- and long-latency auditory evoked potentials in an unpredictable auditory context

It is not known how Auditory-Evoked Responses (AERs) comprising Middle Latency Responses (MLRs) and Long Latency Responses (LLRs) are modulated by stimulus intensity and inter-stimulus interval (ISI) in an unpredictable auditory context. Further, intensity and ISI effects on MLR and LLR have never been assessed simultaneously in the same humans. To address this important question, thirty participants passively listened to a random sequence of auditory clicks of three possible intensities (65, 75, and 85 dB) at five possible ISI ranges (0.25 to 0.5 s, 0.5 to 1 s, 1 to 2 s, 2 to 4 s, 4 to 8 s) over four to seven one-hour sessions while EEG was recorded. P0, Na, Pa, Nb, and Pb MLR peaks and N1 and P2 LLR peaks were measured. MLRs P0 (p = .005), Pa (p = .021), and Pb (p = <.001) were modulated by intensity, while only MLR Pb (p = <.001) was modulated by ISI. LLR N1 and P2 were modulated by both intensity and ISI (all p values < .001). Intensity and ISI interacted at Pb, N1, and P2 (all p values < .001), with greater intensity effects at longer ISIs and greater ISI effects at louder intensities. Together, these results provide a comprehensive picture of intensity and ISI effects on AER across the entire thalamocortical auditory pathway, while controlling for stimulus predictability. Moreover, they highlight P0 as the earliest MLR response sensitive to stimulus intensity and Pb (~50 ms) as the earliest cortical response coding for ISIs above 250 ms and showing an interdependence between intensity and ISI effects.

Relationship between early neural responses to syntactic and acoustic irregularities in music

Humans can detect various anomalies in a sound sequence without attending to each dimension explicitly. Event-related potentials (ERPs) have been used to examine the processes of auditory deviance detection. Previous research has shown that music-syntactic anomalies elicit early right anterior negativity (ERAN), whereas more general acoustic irregularities elicit mismatch negativity (MMN). Although these ERP components occur in a similar latency range with a similar scalp topography, the relationship between the detection processes they reflect remains unclear. This study compared these components by manipulating music-syntactic (chord progression) and acoustic (intensity) irregularities orthogonally in two experiments. Non-musicians (Experiment 1: N = 39; Experiment 2: N = 24) were asked to listen to chord sequences, each consisting of 5 four-voice chords, as they watched a silent video clip. Standard, harmonic-deviant, intensity-deviant and double-deviant chords occurred at the final position in each sequence. Deviant stimuli were presented infrequently (p = .10) in Experiment 1 and equiprobably (p = .25) in Experiment 2. Regardless of deviance probability, both harmonic and intensity deviants elicited similar negativities, which were indistinguishable in terms of latency or scalp distribution. When the two deviant types occurred simultaneously, the negativity increased in an additive manner; that is, the amplitude of the double-deviant ERP was as large as the sum of the single-deviant ERPs. These findings suggest that the detection of music-syntactic and acoustic irregularities works independently, based on different regularity representations.

Auditory cortex modelled as a dynamical network of oscillators: understanding event-related fields and their adaptation

Adaptation, the reduction of neuronal responses by repetitive stimulation, is a ubiquitous feature of auditory cortex (AC). It is not clear what causes adaptation, but short-term synaptic depression (STSD) is a potential candidate for the underlying mechanism. In such a case, adaptation can be directly linked with the way AC produces context-sensitive responses such as mismatch negativity and stimulus-specific adaptation observed on the single-unit level. We examined this hypothesis via a computational model based on AC anatomy, which includes serially connected core, belt, and parabelt areas. The model replicates the event-related field (ERF) of the magnetoencephalogram as well as ERF adaptation. The model dynamics are described by excitatory and inhibitory state variables of cell populations, with the excitatory connections modulated by STSD. We analysed the system dynamics by linearising the firing rates and solving the STSD equation using time-scale separation. This allows for characterisation of AC dynamics as a superposition of damped harmonic oscillators, so-called normal modes. We show that repetition suppression of the N1m is due to a mixture of causes, with stimulus repetition modifying both the amplitudes and the frequencies of the normal modes. In this view, adaptation results from a complete reorganisation of AC dynamics rather than a reduction of activity in discrete sources. Further, both the network structure and the balance between excitation and inhibition contribute significantly to the rate with which AC recovers from adaptation. This lifetime of adaptation is longer in the belt and parabelt than in the core area, despite the time constants of STSD being spatially homogeneous. Finally, we critically evaluate the use of a single exponential function to describe recovery from adaptation.

Different Patterns of Attention Modulation in Early N140 and Late P300 sERPs Following Ipsilateral vs. Contralateral Stimulation at the Fingers and Cheeks

Intra-hemispheric interference has been often observed when body parts with neighboring representations within the same hemisphere are stimulated. However, patterns of interference in early and late somatosensory processing stages due to the stimulation of different body parts have not been explored. Here, we explore functional similarities and differences between attention modulation of the somatosensory N140 and P300 elicited at the fingers vs. cheeks. In an active oddball paradigm, 22 participants received vibrotactile intensity deviant stimulation either ipsilateral (within-hemisphere) or contralateral (between-hemisphere) at the fingers or cheeks. The ipsilateral deviant always covered a larger area of skin than the contralateral deviant. Overall, both N140 and P300 amplitudes were higher following stimulation at the cheek and N140 topographies differed between fingers and cheek stimulation. For the N140, results showed higher deviant ERP amplitudes following contralateral than ipsilateral stimulation, regardless of the stimulated body part. N140 peak latency differed between stimulated body parts with shorter latencies for the stimulation at the fingers. Regarding P300 amplitudes, contralateral deviant stimulation at the fingers replicated the N140 pattern, showing higher responses and shorter latencies than ipsilateral stimulation at the fingers. For the stimulation at the cheeks, ipsilateral deviants elicited higher P300 amplitudes and longer latencies than contralateral ones. These findings indicate that at the fingers ipsilateral deviant stimulation leads to intra-hemispheric interference, with significantly smaller ERP amplitudes than in contralateral stimulation, both at early and late processing stages. By contrast, at the cheeks, intra-hemispheric interference is selective for early processing stages. Therefore, the mechanisms of intra-hemispheric processing differ from inter-hemispheric ones and the pattern of intra-hemispheric interference in early and late processing stages is body-part specific.

Development of a Tactile Actuator with Non-Contact and Trans-Object Characteristics Using a Time-Varying Magnetic Field

A non-contact tactile stimulation system using a time-varying magnetic field was developed. The system comprises a control unit, power unit, output unit, and actuator. The control unit adjusts stimulation parameters, particularly the signal intensity and frequency. The power unit produces high voltages for generating the magnetic field, whereas the output unit transmits the energy generated according to the signal from the control unit to the actuator. A spiral coil actuator generates the magnetic field. To validate the effectiveness of the system, preliminary experiments on 10 male adults without neurological disorders (23.2 ± 3.05 years) were conducted. Magnetic field stimuli were presented to the right palm of the subjects at three different frequencies (10, 30, and 50 Hz), and corresponding electroencephalogram (EEG) signals were measured simultaneously. Event-related potential (ERP) analysis showed that N100 and P300 components were identified in somatosensory areas. Subjective evaluations revealed that feelings such as “tingling,” “trembling,” “tapping,” and “percussing” were induced. Moreover, as the stimulus frequency changes, differences may occur in induced feeling. The system uses a time-varying magnetic field, which not only induces tactile stimulation without contact but also has trans-object characteristics that can present tactile sensations, even when there is an obstacle between an actuator and skin.

Pre-attentive auditory change detection for rapid auditory transient combinations: Insight from age-related processing changes

The N1 event-related potential (ERP) enhancement to auditory transients preceded briefly by another transient has been interpreted as a reflection of latent inhibition, or alternatively, as a superimposing mismatch negativity (MMN) to rare transient event combinations. In a previous study (Volosin, Gaál, & Horváth, 2017a), when rare glides preceded frequent gaps by 150 ms in continuous tones, gap-related N1 was enhanced in younger adults while P2 was attenuated both in younger and older adults, which could be parsimoniously explained by MMN overlap which was delayed with aging. The present study replicated and extended these results with a condition in which the roles of the two event types were reversed. Transients separated by 150 ms elicited delayed MMN in older adults, supporting the MMN interpretation over the latent inhibition account. Furthermore, the divergence of N1 and MMN elicitation patterns demonstrated the independence of N1 and MMN.

Extracting human cortical responses to sound onsets and acoustic feature changes in real music, and their relation to event rate

Evoked cortical responses (ERs) have mainly been studied in controlled experiments using simplified stimuli. Though, an outstanding question is how the human cortex responds to the complex stimuli encountered in realistic situations. Few electroencephalography (EEG) studies have used Music Information Retrieval (MIR) tools to extract cortical P1/N1/P2 to acoustical changes in real music. However, less than ten events per music piece could be detected leading to ERs due to limitations in automatic detection of sound onsets. Also, the factors influencing a successful extraction of the ERs have not been identified. Finally, previous studies did not localize the sources of the cortical generators. This study is based on an EEG/MEG dataset from 48 healthy normal hearing participants listening to three real music pieces. Acoustic features were computed from the audio signal of the music with the MIR Toolbox. To overcome limits in automatic methods, sound onsets were also manually detected. The chance of obtaining detectable ERs based on ten randomly picked onset points was less than 1:10,000. For the first time, we show that naturalistic P1/N1/P2 ERs can be reliably measured across 100 manually identified sound onsets, substantially improving the signal-to-noise level compared to <10 trials. More ERs were measurable in musical sections with slow event rates (0.2 Hz-2.5 Hz) than with fast event rates (>2.5 Hz). Furthermore, during monophonic sections of the music only P1/P2 were measurable, and during polyphonic sections only N1. Finally, MEG source analysis revealed that naturalistic P2 is located in core areas of the auditory cortex.

The reduced auditory evoked potential component N1 after repeated stimulation: Refractoriness hypothesis vs. habituation account

Similar to other event-related potential (ERP) components, the amplitude of the auditory evoked N1 depends on the interstimulus interval (ISI). At ISIs > 0.4 s, the amplitude of the N1 increases with longer ISIs, until it saturates at ISIs around 10 s. This amplitude increase with increasing ISI has been conceptualized as a function of N1 recovery or N1 refractoriness. Habituation (as a simple form of learning) represents an elaborated, opposing account for such stimulus repetition effects. For passive oddball experiments (stimulation protocols with frequent standards and rare deviants), the two accounts make different predictions. According to the habituation account, the presentation of small deviants should lead to an increased N1 for subsequent standards (= dishabituation); according to the N1 refractoriness account, there should be no or just minor effects on the N1. In the current study, we tested these predictions and compared the ERPs to standards after small deviants and to standards preceded by other standards. We observed that the ERPs to standards after small deviants were characterized by a small mismatch negativity with an onset latency > 150 ms, but the N1 to standards after deviants did not differ from the N1 to standards preceded by other standards. This negative finding is in line with other previous studies that were also not able to reveal evidence for N1 dishabituation. Aside from this repeated lack of evidence for dishabituation, the N1 habituation account is challenged by the finding that the N1 decrease is stronger for more intense stimuli. Overall, the current and previous findings are more compatible with the N1 refractoriness account, although the mechanisms underlying N1 refractoriness remain to be elucidated. Knowledge about these mechanisms would also help to understand why N1 deficits in schizophrenia are more pronounced at longer ISIs.

A STUDY ON SOMATOSENSORY EVOKED POTENTIAL PATTERNS ACCORDING TO VARIOUS VIBROTACTILE STIMULATION: FREQUENCIES AND INTENSITIES

This study investigates somatosensory evoked potential (SEP) patterns in the C3 somatosensory area with varying frequency and intensity of vibrotactile stimuli. The study subjects included 13 men ([Formula: see text] years) and seven women ([Formula: see text] years) who were right-handed and had normal cognitive function. The participants were subjected to three intensity levels (0.25, 0.38 and 1.3[Formula: see text]g) and eight frequencies (10, 50, 100, 150, 200, 225, 250 and 300[Formula: see text]Hz) of vibrotactile stimuli on the distal phalanx of their right index finger. The peak values of SEP patterns generated in response to high-frequency vibrotactile stimuli were greater than those generated because of low-frequency flutter. Moreover, the peak values increased as the stimulus intensity increased from 1[Formula: see text]g to 3[Formula: see text]g. In these results, the maximum and minimum peak, and peak to peak values of SEP pattern in the C3 somatosensory area increased with an increase in the stimulation intensity and frequency of the vibrotactile stimuli. Data on the SEP patterns generated in response to various frequencies and intensities of somatosensory stimuli and the development of relevant databases will elucidate the various clinical applications and applicable domains where SEP assessment can be beneficial.

Altered neuronal habituation to hearing others' pain in adults with autistic traits

This study tested the hypothesis that autistic traits influence the neuronal habituation that underlies the processing of others' pain. Based on their autism-spectrum quotient (AQ), two groups of participants were classified according to their autistic traits: High-AQ and Low-AQ groups. Their event-related potentials in response to trains of three identical audio recordings, exhibiting either painful or neutral feelings of others, were compared during three experimental tasks. (1) In a Pain Judgment Task, participants were instructed to focus on pain-related cues in the presented audio recordings. (2) In a Gender Judgment Task, participants were instructed to focus on non-pain-related cues in the presented audio recordings. (3) In a Passive Listening Task, participants were instructed to passively listen. In the High-AQ group, an altered empathic pattern of habituation, indexed by frontal-central P2 responses of the second repeated painful audio recordings, was found during the Passive Listening Task. Nevertheless, both High-AQ and Low-AQ groups exhibited similar patterns of habituation to hearing others' voices, both neutral and painful, in the Pain Judgment and Gender Judgment Tasks. These results suggest altered empathic neuronal habituation in the passive processing of others' vocal pain by individuals with autistic traits.

Body representation in infants: Categorical boundaries of body parts as assessed by somatosensory mismatch negativity

There is growing interest in developing and using novel measures to assess how the body is represented in human infancy. Various lines of evidence with adults and older children show that tactile perception is modulated by a high-level representation of the body. For instance, the distance between two points of tactile stimulation is perceived as being greater when these points cross a joint boundary than when they are within a body part, suggesting that the representation of the body is structured with joints acting as categorical boundaries between body parts. Investigating the developmental origins of this categorical effect has been constrained by infants’ inability to verbally report on the properties of tactile stimulation. Here we made novel use of an infant brain measure, the somatosensory mismatch negativity (sMMN), to explore categorical aspects of tactile body processing in infants aged 6–7 months. Amplitude of the sMMN elicited by tactile stimuli across the wrist boundary was significantly greater than for stimuli of equal distance that were within the boundary, suggesting a categorical effect in body processing in infants. We suggest that an early-appearing, structured representation of the body into ‘parts’ may play a role in mapping correspondences between self and other.

A review on the ongoing quest for a pain signature in the human brain

Developing an objective biomarker for pain assessment is crucial for understanding neural coding mechanisms of pain in the human brain as well as for effective treatment of pain disorders. Neuroimaging techniques have been proven to be powerful tools in the ongoing quest for a pain signature in the human brain. Although there is still a long way to go before achieving a truly successful pain signature based on neuroimaging techniques, important progresses have been made through great efforts in the last two decades by the Pain Society. Here, we focus on neural responses to transient painful stimuli in healthy people, and review the relevant studies on the identification of a neuroimaging signature for pain.

An oscillator model better predicts cortical entrainment to music

A body of research demonstrates convincingly a role for synchronization of auditory cortex to rhythmic structure in sounds including speech and music. Some studies hypothesize that an oscillator in auditory cortex could underlie important temporal processes such as segmentation and prediction. An important critique of these findings raises the plausible concern that what is measured is perhaps not an oscillator but is instead a sequence of evoked responses. The two distinct mechanisms could look very similar in the case of rhythmic input, but an oscillator might better provide the computational roles mentioned above (i.e., segmentation and prediction). We advance an approach to adjudicate between the two models: analyzing the phase lag between stimulus and neural signal across different stimulation rates. We ran numerical simulations of evoked and oscillatory computational models, showing that in the evoked case,phase lag is heavily rate-dependent, while the oscillatory model displays marked phase concentration across stimulation rates. Next, we compared these model predictions with magnetoencephalography data recorded while participants listened to music of varying note rates. Our results show that the phase concentration of the experimental data is more in line with the oscillatory model than with the evoked model. This finding supports an auditory cortical signal that (i) contains components of both bottom-up evoked responses and internal oscillatory synchronization whose strengths are weighted by their appropriateness for particular stimulus types and (ii) cannot be explained by evoked responses alone.

The somatosensory mismatch negativity as a window into body representations in infancy

How the body is represented in the developing brain is a topic of growing interest. The current study takes a novel approach to investigating neural body representations in infants by recording somatosensory mismatch negativity (sMMN) responses elicited by tactile stimulation of different body locations. Recent research in adults has suggested that sMMN amplitude may be influenced by the relative distance between representations of the stimulated body parts in somatosensory cortex. The current study uses a similar paradigm to explore whether the sMMN can be elicited in infants, and to test whether the infant sMMN response is sensitive to the somatotopic organization of somatosensory cortex. Participants were healthy infants (n = 31) aged 6 and 7 months. The protocol leveraged a discontinuity in cortical somatotopic organization, whereby the representations of the neck and the face are separated by representations of the arms, the hands and the shoulder. In a double-deviant oddball protocol, stimulation of the hand (100 trials, 10% probability) and neck (100 trials, 10% probability) was interspersed among repeated stimulation of the face (800 trials, 80% probability). Waveforms showed evidence of an infant sMMN response that was significantly larger for the face/neck contrast than for the face/hand contrast. These results suggest that, for certain combinations of body parts, early pre-attentive tactile discrimination in infants may be influenced by distance between the corresponding cortical representations. The results provide the first evidence that the sMMN can be elicited in infants, and pave the way for further applications of the sMMN in studying body representations in preverbal infants.

Neuropsychology of Human Body Parts: Exploring Categorical Boundaries of Tactile Perception Using Somatosensory Mismatch Responses

The focus of the current study is on a particular aspect of tactile perception: categorical segmentation on the body surface into discrete body parts. The MMN has been shown to be sensitive to categorical boundaries and language experience in the auditory modality. Here we recorded the somatosensory MMN (sMMN) using two tactile oddball protocols and compared sMMN amplitudes elicited by within- and across-boundary oddball pairs. Both protocols employed the identity MMN method that controls for responsivity at each body location. In the first protocol, we investigated the categorical segmentation of tactile space at the wrist by presenting pairs of tactile oddball stimuli across equal spatial distances, either across the wrist or within the forearm. Amplitude of the sMMN elicited by stimuli presented across the wrist boundary was significantly greater than for stimuli presented within the forearm, suggesting a categorical effect at an early stage of somatosensory processing. The second protocol was designed to investigate the generality of this MMN effect, and involved three digits on one hand. Amplitude of the sMMN elicited by a contrast of the third digit and the thumb was significantly larger than a contrast between the third and fifth digits, suggesting a functional boundary effect that may derive from the way that objects are typically grasped. These findings demonstrate that the sMMN is a useful index of processing of somatosensory spatial discrimination that can be used to study body part categories.

Habituation of phase-locked local field potentials and gamma-band oscillations recorded from the human insula

Salient nociceptive and non-nociceptive stimuli elicit low-frequency local field potentials (LFPs) in the human insula. Nociceptive stimuli also elicit insular gamma-band oscillations (GBOs), possibly preferential for thermonociception, which have been suggested to reflect the intensity of perceived pain. To shed light on the functional significance of these two responses, we investigated whether they would be modulated by stimulation intensity and temporal expectation - two factors contributing to stimulus saliency. Insular activity was recorded from 8 depth electrodes (41 contacts) implanted in the left insula of 6 patients investigated for epilepsy. Thermonociceptive, vibrotactile, and auditory stimuli were delivered using two intensities. To investigate the effects of temporal expectation, the stimuli were delivered in trains of three identical stimuli (S1-S2-S3) separated by a constant 1-s interval. Stimulation intensity affected intensity of perception, the magnitude of low-frequency LFPs, and the magnitude of nociceptive GBOs. Stimulus repetition did not affect perception. In contrast, both low-frequency LFPs and nociceptive GBOs showed a marked habituation of the responses to S2 and S3 as compared to S1 and, hence, a dissociation with intensity of perception. Most importantly, although insular nociceptive GBOs appear to be preferential for thermonociception, they cannot be considered as a correlate of perceived pain.

Intense pain influences the cortical processing of visual stimuli projected onto the sensitized skin

Sensitization is a form of implicit learning produced by the exposure to a harmful stimulus. In humans and other mammals, sensitization after skin injury increases the responsiveness of peripheral nociceptors and enhances the synaptic transmission of nociceptive input in the central nervous system. Here, we show that sensitization-related changes in the central nervous system are not restricted to nociceptive pathways and, instead, also affect other sensory modalities, especially if that modality conveys information relevant for the sensitized body part. Specifically, we show that after sensitizing the forearm using high-frequency electrical stimulation (HFS) of the skin, visual stimuli projected onto the sensitized forearm elicit significantly enhanced brain responses. Whereas mechanical hyperalgesia was present both 20 and 45 minutes after HFS, the enhanced responsiveness to visual stimuli was present only 20 minutes after HFS. Taken together, our results indicate that sensitization involves both nociceptive-specific and multimodal mechanisms, having distinct time courses.

Age-related processing delay reveals cause of apparent sensory excitability following auditory stimulation

When background auditory events lead to enhanced auditory event-related potentials (ERPs) for closely following sounds, this is generally interpreted as a transient increase in the responsiveness of the auditory system. We measured ERPs elicited by irrelevant probes (gaps in a continuous tone) at several time-points following rare auditory events (pitch glides) in younger and older adults, who watched movies during stimulation. Fitting previous results, in younger adults, gaps elicited increasing N1 auditory ERPs with decreasing glide-gap separation. N1 increase was paralleled by an ERP decrease in the P2 interval. In older adults, only a glide-gap separation dependent P2 decrease, but no N1-effect was observable. This ERP pattern was likely caused by a fronto-central negative waveform, which was delayed in the older adult group, thus overlapping N1 and P2 in the younger, but overlapping only P2 in the older adult group. Because the waveform exhibited a polarity reversal at the mastoids, it was identified as a mismatch negativity (MMN). This interpretation also fits previous studies showing that gap-related MMN is delayed in older adults, reflecting an age-related deterioration of fine temporal auditory resolution. These results provide a plausible alternative explanation for the ERP enhancement for sounds following background auditory events.

Cortical Sensitivity to Guitar Note Patterns: EEG Entrainment to Repetition and Key

Music is ubiquitous throughout recent human culture, and many individual's have an innate ability to appreciate and understand music. Our appreciation of music likely emerges from the brain's ability to process a series of repeated complex acoustic patterns. In order to understand these processes further, cortical responses were measured to a series of guitar notes presented with a musical pattern or without a pattern. ERP responses to individual notes were measured using a 24 electrode Bluetooth mobile EEG system (Smarting mBrainTrain) while 13 healthy non-musicians listened to structured (i.e., within musical keys and with repetition) or random sequences of guitar notes for 10 min each. We demonstrate an increased amplitude to the ERP that appears ~200 ms to notes presented within the musical sequence. This amplitude difference between random notes and patterned notes likely reflects individual's cortical sensitivity to guitar note patterns. These amplitudes were compared to ERP responses to a rare note embedded within a stream of frequent notes to determine whether the sensitivity to complex musical structure overlaps with the sensitivity to simple irregularities reflected in traditional auditory oddball experiments. Response amplitudes to the negative peak at ~175 ms are statistically correlated with the mismatch negativity (MMN) response measured to a rare note presented among a series of frequent notes (i.e., in a traditional oddball sequence), but responses to the subsequent positive peak at ~200 do not show a statistical relationship with the P300 response. Thus, the sensitivity to musical structure identified to 4 Hz note patterns appears somewhat distinct from the sensitivity to statistical regularities reflected in the traditional "auditory oddball" sequence. Overall, we suggest that this is a promising approach to examine individual's sensitivity to complex acoustic patterns, which may overlap with higher level cognitive processes, including language.

Vowels and Consonants in the Brain: Evidence from Magnetoencephalographic Studies on the N1m in Normal-Hearing Listeners

Speech sound perception is one of the most fascinating tasks performed by the human brain. It involves a mapping from continuous acoustic waveforms onto the discrete phonological units computed to store words in the mental lexicon. In this article, we review the magnetoencephalographic studies that have explored the timing and morphology of the N1m component to investigate how vowels and consonants are computed and represented within the auditory cortex. The neurons that are involved in the N1m act to construct a sensory memory of the stimulus due to spatially and temporally distributed activation patterns within the auditory cortex. Indeed, localization of auditory fields maps in animals and humans suggested two levels of sound coding, a tonotopy dimension for spectral properties and a tonochrony dimension for temporal properties of sounds. When the stimulus is a complex speech sound, tonotopy and tonochrony data may give important information to assess whether the speech sound parsing and decoding are generated by pure bottom-up reflection of acoustic differences or whether they are additionally affected by top-down processes related to phonological categories. Hints supporting pure bottom-up processing coexist with hints supporting top-down abstract phoneme representation. Actually, N1m data (amplitude, latency, source generators, and hemispheric distribution) are limited and do not help to disentangle the issue. The nature of these limitations is discussed. Moreover, neurophysiological studies on animals and neuroimaging studies on humans have been taken into consideration. We compare also the N1m findings with the investigation of the magnetic mismatch negativity (MMNm) component and with the analogous electrical components, the N1 and the MMN. We conclude that N1 seems more sensitive to capture lateralization and hierarchical processes than N1m, although the data are very preliminary. Finally, we suggest that MEG data should be integrated with EEG data in the light of the neural oscillations framework and we propose some concerns that should be addressed by future investigations if we want to closely line up language research with issues at the core of the functional brain mechanisms.

N100 as a generic cortical electrophysiological marker based on decomposition of TMS-evoked potentials across five anatomic locations

N100, the negative peak of electrical response occurring around 100 ms, is present in diverse functional paradigms including auditory, visual, somatic, behavioral and cognitive tasks. We hypothesized that the presence of the N100 across different paradigms may be indicative of a more general property of the cerebral cortex regardless of functional or anatomic specificity. To test this hypothesis, we combined transcranial magnetic stimulation (TMS) and electroencephalography (EEG) to measure cortical excitability by TMS across cortical regions without relying on specific sensory, cognitive or behavioral modalities. The five stimulated regions included left prefrontal, left motor, left primary auditory cortices, the vertex and posterior cerebellum with stimulations performed using supra- and subthreshold intensities. EEG responses produced by TMS stimulation at the five locations all generated N100s that peaked at the vertex. The amplitudes of the N100s elicited by these five diverse cortical origins were statistically not significantly different (all uncorrected p > 0.05). No other EEG response components were found to have this global property of N100. Our findings suggest that anatomy- and modality-specific interpretation of N100 should be carefully evaluated, and N100 by TMS may be used as a biomarker for evaluating local versus general cortical properties across the brain.

The neural dynamics of somatosensory processing and adaptation across childhood: a high-density electrical mapping study

Young children are often hyperreactive to somatosensory inputs hardly noticed by adults, as exemplified by irritation to seams or labels in clothing. The neurodevelopmental mechanisms underlying changes in sensory reactivity are not well understood. Based on the idea that neurodevelopmental changes in somatosensory processing and/or changes in sensory adaptation might underlie developmental differences in somatosensory reactivity, high-density electroencephalography was used to examine how the nervous system responds and adapts to repeated vibrotactile stimulation over childhood. Participants aged 6-18 yr old were presented with 50-ms vibrotactile stimuli to the right wrist over the median nerve at 5 blocked interstimulus intervals (ranging from ∼7 to ∼1 stimulus per second). Somatosensory evoked potentials (SEPs) revealed three major phases of activation within the first 200 ms, with scalp topographies suggestive of neural generators in contralateral somatosensory cortex. Although overall SEPs were highly similar for younger, middle, and older age groups (6.1-9.8, 10.0-12.9, and 13.0-17.8 yr old), there were significant age-related amplitude differences in initial and later phases of the SEP. In contrast, robust adaptation effects for fast vs. slow presentation rates were observed that did not differ as a function of age. A greater amplitude response in the later portion of the SEP was observed for the youngest group and may be related to developmental changes in responsivity to somatosensory stimuli. These data suggest the protracted development of the somatosensory system over childhood, whereas adaptation, as assayed in this study, is largely in place by ∼7 yr of age.

Towards perception awareness: Perceptual event detection for Brain computer interfaces

Brain computer interface (BCI) technology is becoming increasingly popular in many domains such as entertainment, mental state analysis, and rehabilitation. For robust performance in these domains, detecting perceptual events would be a vital ability, enabling adaptation to and act on the basis of user's perception of the environment. Here we present a framework to automatically mine spatiotemporal characteristics of a given perceptual event. As this "signature" is derived directly from subject's neural behavior, it can serve as a representation of the subject's perception of the targeted scenario, which in turn allows a BCI system to gain a new level of context awareness: perception awareness. As a proof of concept, we show the application of the proposed framework on MEG signal recordings from a face perception study, and the resulting temporal and spatial characteristics of the derived neural signature, as well as it's compatibility with the neuroscientific literature on face perception.

Assessment of nonlinear interactions in event-related potentials elicited by stimuli presented at short interstimulus intervals using single-trial data

The recording of brain event-related potentials (ERPs) is a widely used technique to investigate the neural basis of sensory perception and cognitive processing in humans. Due to the low magnitude of ERPs, averaging of several consecutive stimuli is typically employed to enhance the signal to noise ratio (SNR) before subsequent analysis. However, when the temporal interval between two consecutive stimuli is smaller than the latency of the main ERP peaks, i.e., when the stimuli are presented at a fast rate, overlaps between the corresponding ERPs may occur. These overlaps are usually dealt with by assuming that there is a simple additive superposition between the elicited ERPs and consequently performing algebraic waveform subtractions. Here, we test this assumption rigorously by providing a statistical framework that examines the presence of nonlinear additive effects between overlapping ERPs elicited by successive stimuli with short interstimulus intervals (ISIs). The results suggest that there are no nonlinear additive effects due to the time overlap per se but that, for the range of ISIs examined, the second ERP is modulated by the presence of the first stimulus irrespective of whether there is time overlap or not. In other words, two ERPs that overlap in time can still be written as an addition of two ERPs but with the second ERP being different from the first. This difference is also present in the case of nonoverlapping ERPs with short ISIs. The modulation effect elicited on the second ERP by the first stimulus is dependent on the ISI value.

Spatio-temporal dynamics of adaptation in the human visual system: a high-density electrical mapping study

When sensory inputs are presented serially, response amplitudes to stimulus repetitions generally decrease as a function of presentation rate, diminishing rapidly as inter-stimulus intervals (ISIs) fall below 1 s. This 'adaptation' is believed to represent mechanisms by which sensory systems reduce responsivity to consistent environmental inputs, freeing resources to respond to potentially more relevant inputs. While auditory adaptation functions have been relatively well characterized, considerably less is known about visual adaptation in humans. Here, high-density visual-evoked potentials (VEPs) were recorded while two paradigms were used to interrogate visual adaptation. The first presented stimulus pairs with varying ISIs, comparing VEP amplitude to the second stimulus with that of the first (paired-presentation). The second involved blocks of stimulation (N = 100) at various ISIs and comparison of VEP amplitude between blocks of differing ISIs (block-presentation). Robust VEP modulations were evident as a function of presentation rate in the block-paradigm, with strongest modulations in the 130-150 ms and 160-180 ms visual processing phases. In paired-presentations, with ISIs of just 200-300 ms, an enhancement of VEP was evident when comparing S2 with S1, with no significant effect of presentation rate. Importantly, in block-presentations, adaptation effects were statistically robust at the individual participant level. These data suggest that a more taxing block-presentation paradigm is better suited to engage visual adaptation mechanisms than a paired-presentation design. The increased sensitivity of the visual processing metric obtained in the block-paradigm has implications for the examination of visual processing deficits in clinical populations.

Auditory brainstem and middle latency responses recorded at fast rates with randomized stimulation

Randomized stimulation and averaging (RSA) allows auditory evoked potentials (AEPs) to be recorded at high stimulation rates. This method does not perform deconvolution and must therefore deal with interference derived from overlapping transient evoked responses. This paper analyzes the effects of this interference on auditory brainstem responses (ABRs) and middle latency responses (MLRs) recorded at rates of up to 300 and 125 Hz, respectively, with randomized stimulation sequences of a jitter both greater and shorter than the dominant period of the ABR/MLR components. Additionally, this paper presents an advanced approach for RSA [iterative-randomized stimulation and averaging (I-RSA)], which includes the removal of the interference associated with overlapping responses through an iterative process in the time domain. Experimental results show that (a) RSA can be efficiently used in the recording of AEPs when the jitter of the stimulation sequence is greater than the dominant period of the AEP components, and (b) I-RSA maintains all the advantages of RSA and is not constrained by the restriction of a minimum jitter. The significance of the results of this study is discussed.

Background noise can enhance cortical auditory evoked potentials under certain conditions

OBJECTIVE

To use cortical auditory evoked potentials (CAEPs) to understand neural encoding in background noise and the conditions under which noise enhances CAEP responses.

METHODS

CAEPs from 16 normal-hearing listeners were recorded using the speech syllable/ba/presented in quiet and speech-shaped noise at signal-to-noise ratios of 10 and 30dB. The syllable was presented binaurally and monaurally at two presentation rates.

RESULTS

The amplitudes of N1 and N2 peaks were often significantly enhanced in the presence of low-level background noise relative to quiet conditions, while P1 and P2 amplitudes were consistently reduced in noise. P1 and P2 amplitudes were significantly larger during binaural compared to monaural presentations, while N1 and N2 peaks were similar between binaural and monaural conditions.

CONCLUSIONS

Methodological choices impact CAEP peaks in very different ways. Negative peaks can be enhanced by background noise in certain conditions, while positive peaks are generally enhanced by binaural presentations.

SIGNIFICANCE

Methodological choices significantly impact CAEPs acquired in quiet and in noise. If CAEPs are to be used as a tool to explore signal encoding in noise, scientists must be cognizant of how differences in acquisition and processing protocols selectively shape CAEP responses.

Suppression of Subsequent N1m Amplitude When the Masker Frequency is Different from the Signal

When two tones are presented in a short interval of time, the presentation of the preceding tone (masker) suppresses the response evoked by the subsequent tone (signal). To address the processing in forward suppression, we applied 2- and 4-kHz maskers, followed by a 1-kHz signal at varying signal delays (0 to 320 ms) and measured the signal-evoked N1m. A two-way analysis of variance revealed a statistically significant effect for signal delay in both masker presentation conditions. The N1m peak amplitude at the signal delay of 320 ms was significantly larger than those of 10, 20, 40, and 80 ms (p < 0.05). No significant enhancement for the very short signal delay was observed. The results suggest that the enhancement of N1m peak amplitude for short signal delay conditions is maximized when the frequency of the masker is identical to that of the signal.

Deconvolution of magnetic acoustic change complex (mACC)

OBJECTIVE

The aim of this study was to design a novel experimental approach to investigate the morphological characteristics of auditory cortical responses elicited by rapidly changing synthesized speech sounds.

METHODS

Six sound-evoked magnetoencephalographic (MEG) responses were measured to a synthesized train of speech sounds using the vowels /e/ and /u/ in 17 normal hearing young adults. Responses were measured to: (i) the onset of the speech train, (ii) an F0 increment; (iii) an F0 decrement; (iv) an F2 decrement; (v) an F2 increment; and (vi) the offset of the speech train using short (jittered around 135ms) and long (1500ms) stimulus onset asynchronies (SOAs). The least squares (LS) deconvolution technique was used to disentangle the overlapping MEG responses in the short SOA condition only.

RESULTS

Comparison between the morphology of the recovered cortical responses in the short and long SOAs conditions showed high similarity, suggesting that the LS deconvolution technique was successful in disentangling the MEG waveforms. Waveform latencies and amplitudes were different for the two SOAs conditions and were influenced by the spectro-temporal properties of the sound sequence. The magnetic acoustic change complex (mACC) for the short SOA condition showed significantly lower amplitudes and shorter latencies compared to the long SOA condition. The F0 transition showed a larger reduction in amplitude from long to short SOA compared to the F2 transition. Lateralization of the cortical responses were observed under some stimulus conditions and appeared to be associated with the spectro-temporal properties of the acoustic stimulus.

CONCLUSIONS

The LS deconvolution technique provides a new tool to study the properties of the auditory cortical response to rapidly changing sound stimuli. The presence of the cortical auditory evoked responses for rapid transition of synthesized speech stimuli suggests that the temporal code is preserved at the level of the auditory cortex. Further, the reduced amplitudes and shorter latencies might reflect intrinsic properties of the cortical neurons to rapidly presented sounds.

SIGNIFICANCE

This is the first demonstration of the separation of overlapping cortical responses to rapidly changing speech sounds and offers a potential new biomarker of discrimination of rapid transition of sound.

The specificity of stimulus-specific adaptation in human auditory cortex increases with repeated exposure to the adapting stimulus

The neural response to a sensory stimulus tends to be more strongly reduced when the stimulus is preceded by the same, rather than a different, stimulus. This stimulus-specific adaptation (SSA) is ubiquitous across the senses. In hearing, SSA has been suggested to play a role in change detection as indexed by the mismatch negativity. This study sought to test whether SSA, measured in human auditory cortex, is caused by neural fatigue (reduction in neural responsiveness) or by sharpening of neural tuning to the adapting stimulus. For that, we measured event-related cortical potentials to pairs of pure tones with varying frequency separation and stimulus onset asynchrony (SOA). This enabled us to examine the relationship between the degree of specificity of adaptation as a function of frequency separation and the rate of decay of adaptation with increasing SOA. Using simulations of tonotopic neuron populations, we demonstrate that the fatigue model predicts independence of adaptation specificity and decay rate, whereas the sharpening model predicts interdependence. The data showed independence and thus supported the fatigue model. In a second experiment, we measured adaptation specificity after multiple presentations of the adapting stimulus. The multiple adapters produced more adaptation overall, but the effect was more specific to the adapting frequency. Within the context of the fatigue model, the observed increase in adaptation specificity could be explained by assuming a 2.5-fold increase in neural frequency selectivity. We discuss possible bottom-up and top-down mechanisms of this effect.