Stimulus Induced Desynchronization of Human Auditory 40-Hz Steady-State Responses

MK-801 disrupts and nicotine augments 40 Hz auditory steady state responses in the auditory cortex of the urethane-anesthetized rat

Patients with schizophrenia show marked deficits in processing sensory inputs including a reduction in the generation and synchronization of 40 Hz gamma oscillations in response to steady-state auditory stimulation. Such deficits are not readily demonstrable at other input frequencies. Acute administration of NMDA antagonists to healthy human subjects or laboratory animals is known to reproduce many sensory and cognitive deficits seen in schizophrenia patients. In the following study, we tested the hypothesis that the NMDA antagonist MK-801 would selectively disrupt steady-state gamma entrainment in the auditory cortex of urethane-anesthetized rat. Moreover, we further hypothesized that nicotinic receptor activation would alleviate this disruption. Auditory steady state responses were recorded in response to auditory stimuli delivered over a range of frequencies (10-80 Hz) and averaged over 50 trials. Evoked power was computed under baseline condition and after vehicle or MK-801 (0.03 mg/kg, iv). MK-801 produced a significant attenuation in response to 40 Hz auditory stimuli while entrainment to other frequencies was not affected. Time-frequency analysis revealed deficits in both power and phase-locking to 40 Hz. Nicotine (0.1 mg/kg, iv) administered after MK-801 reversed the attenuation of the 40 Hz response. Administered alone, nicotine augmented 40 Hz steady state power and phase-locking. Nicotine's effects were blocked by simultaneous administration of the α4β2 antagonist DHßE. Thus we report for the first time, a rodent model that mimics a core neurophysiological deficit seen in patients with schizophrenia and a pharmacological approach to alleviate it.

Sound envelope encoding in the auditory cortex revealed by neuromagnetic responses in the theta to gamma frequency bands

Amplitude fluctuations of natural sounds carry multiple types of information represented at different time scales, such as syllables and voice pitch in speech. However, it is not well understood how such amplitude fluctuations at different time scales are processed in the brain. In the present study we investigated the effect of the stimulus rate on the cortical evoked responses using magnetoencephalography (MEG). We used a two-tone complex sound, whose envelope fluctuated at the difference frequency and induced an acoustic beat sensation. When the beat rate was continuously swept between 3Hz and 60Hz, auditory evoked response showed distinct transient waves at slow rates, while at fast rates continuous sinusoidal oscillations similar to the auditory steady-state response (ASSR) were observed. We further derived temporal modulation transfer functions (TMTF) from amplitudes of the transient responses and from the ASSR. The results identified two critical rates of 12.5Hz and 25Hz, at which consecutive transient responses overlapped with each other. These stimulus rates roughly corresponded to the rates at which the perceptual quality of the sound envelope is known to change. Low rates (> 10Hz) are perceived as loudness fluctuation, medium rates as acoustical flutter, and rates above 25Hz as roughness. We conclude that these results reflect cortical processes that integrate successive acoustic events at different time scales for extracting complex features of natural sound.

Converging evidence for gamma synchrony deficits in schizophrenia

BACKGROUND

In electroencephalogram (EEG) studies of auditory steady-state responses (ASSRs), patients with schizophrenia show a deficit in power and/or phase-locking, particularly at the 40 Hz frequency where these responses resonate. In addition, studies of the transient gamma-band response (GBR) elicited by single tones have revealed deficits in gamma power and phase-locking in schizophrenia. We examined the degree to which the 40 Hz ASSR and the transient GBR to single tones are correlated and whether they assess overlapping or distinct gamma-band abnormalities in schizophrenia.

METHODS

EEG was recorded during 40 Hz ASSR and auditory oddball paradigms from 28 patients with schizophrenia or schizoaffective disorder (SZ) and 25 age- and gender-matched healthy controls (HC). The ASSR was elicited by 500 ms click trains, and the transient GBR was elicited by the standard tones from the oddball paradigm. Gamma phase and magnitude values, calculated using Morlet wavelet transformations, were used to derive total power and phase-locking measures.

RESULTS

Relative to HC, SZ patients had significant deficits in total gamma power and phase-locking for both ASSR- and GBR-based measures. Within both groups, the 40 Hz ASSR and GBR phase-locking measures were significantly correlated, with a similar trend evident for the total power measures. Moreover, co-varying for GBR substantially reduced 40 Hz ASSR power and phase-locking differences between the groups.

CONCLUSIONS

40 Hz ASSR and transient GBR measures provide very similar information about auditory gamma abnormalities in schizophrenia, despite the overall enhancement of 40 Hz ASSR total power and phase-locking values relative to the corresponding GBR values.

Synchronization of β and γ oscillations in the somatosensory evoked neuromagnetic steady-state response

The sensory evoked neuromagnetic response consists of superimposition of an immediately stimulus-driven component and induced changes in the autonomous brain activity, each having distinct functional relevance. Commonly, the strength of phase locking in neural activities has been used to differentiate the different responses. The steady-state response is a strong oscillatory neural activity, which is evoked with rhythmic stimulation, and provides an effective tool to investigate oscillatory brain networks. In this case, both the sensory response and intrinsic activity, representing higher order processes, are highly synchronized to the stimulus. In this study we hypothesized that temporal dynamics of oscillatory activities would characterize the differences between the two types of activities and that beta and gamma oscillations are differently involved in this distinction. We used magnetoencephalography (MEG) for studying how ongoing steady-state responses elicited by a 20-Hz vibro-tactile stimulus to the right index finger were affected by a concurrent isolated touch stimulus to the same hand ring finger. SI source activity showed oscillations at multiples of 20 Hz with characteristic differences in the beta band and the gamma band. The response amplitudes were largest at 20 Hz (beta) and significantly reduced at 40 Hz and 60 Hz (gamma), although synchronization strength, indicated by inter-trial coherence (ITC), did not substantially differ between 20 Hz and 40 Hz. Moreover, the beta oscillations showed a fast onset, whereas the amplitude of gamma oscillations increased slowly and reached the steady state 400 ms after onset of the vibration stimulus. Most importantly, the pulse stimuli interacted only with gamma oscillations in a way that gamma oscillations decreased immediately after the concurrent stimulus onset and recovered slowly, resembling the initial slope. Such time course of gamma oscillations is similar to our previous observations in the auditory system. The time constant is in line with the time required for conscious perception of the sensory stimulus. Based on the observed different spectro-temporal dynamics, we propose that while beta activities likely relate to independent representation of the sensory input, gamma oscillation likely relates to binding of sensory information for higher order processing.

β- And γ-band EEG power predicts illusory auditory continuity perception

Because acoustic landscapes are complex and rapidly changing, auditory systems have evolved mechanisms that permit rapid detection of novel sounds, sound source segregation, and perceptual restoration of sounds obscured by noise. Perceptual restoration is particularly important in noisy environments because it allows organisms to track sounds over time even when they are masked. The continuity illusion is a striking example of perceptual restoration with sounds perceived as intact even when parts of them have been replaced by gaps and rendered inaudible by being masked by an extraneous sound. The mechanisms of auditory filling-in are complex and are currently not well-understood. The present study used the high temporal resolution of EEG to examine brain activity related to continuity illusion perception. Masking noise loudness was adjusted individually for each subject so that physically identical sounds on some trials elicited a continuity illusion (failure to detect a gap in a sound) and on other trials resulted in correct gap detection. This design ensured that any measurable differences in brain activity would be due to perceptual differences rather than physical differences among stimuli. We found that baseline activity recorded immediately before presentation of the stimulus significantly predicted the occurrence of the continuity illusion in 10 out of 14 participants based on power differences in γ-band EEG (34-80 Hz). Across all participants, power in the β and γ (12- to 80-Hz range) was informative about the subsequent perceptual decision. These data suggest that a subject's baseline brain state influences the strength of continuity illusions.

Neural plasticity expressed in central auditory structures with and without tinnitus

Sensory training therapies for tinnitus are based on the assumption that, notwithstanding neural changes related to tinnitus, auditory training can alter the response properties of neurons in auditory pathways. To assess this assumption, we investigated whether brain changes induced by sensory training in tinnitus sufferers and measured by electroencephalography (EEG) are similar to those induced in age and hearing loss matched individuals without tinnitus trained on the same auditory task. Auditory training was given using a 5 kHz 40-Hz amplitude-modulated (AM) sound that was in the tinnitus frequency region of the tinnitus subjects and enabled extraction of the 40-Hz auditory steady-state response (ASSR) and P2 transient response known to localize to primary and non-primary auditory cortex, respectively. P2 amplitude increased over training sessions equally in participants with tinnitus and in control subjects, suggesting normal remodeling of non-primary auditory regions in tinnitus. However, training-induced changes in the ASSR differed between the tinnitus and control groups. In controls the phase delay between the 40-Hz response and stimulus waveforms reduced by about 10° over training, in agreement with previous results obtained in young normal hearing individuals. However, ASSR phase did not change significantly with training in the tinnitus group, although some participants showed phase shifts resembling controls. On the other hand, ASSR amplitude increased with training in the tinnitus group, whereas in controls this response (which is difficult to remodel in young normal hearing subjects) did not change with training. These results suggest that neural changes related to tinnitus altered how neural plasticity was expressed in the region of primary but not non-primary auditory cortex. Auditory training did not reduce tinnitus loudness although a small effect on the tinnitus spectrum was detected.

Desynchronizing electrical and sensory coordinated reset neuromodulation

Coordinated reset (CR) stimulation is a desynchronizing stimulation technique based on timely coordinated phase resets of sub-populations of a synchronized neuronal ensemble. It has initially been computationally developed for electrical deep brain stimulation (DBS), to enable an effective desynchronization and unlearning of pathological synchrony and connectivity (anti-kindling). Here we computationally show for ensembles of spiking and bursting model neurons interacting via excitatory and inhibitory adaptive synapses that a phase reset of neuronal populations as well as a desynchronization and an anti-kindling can robustly be achieved by direct electrical stimulation or indirect (synaptically-mediated) excitatory and inhibitory stimulation. Our findings are relevant for DBS as well as for sensory stimulation in neurological disorders characterized by pathological neuronal synchrony. Based on the obtained results, we may expect that the local effects in the vicinity of a depth electrode (realized by direct stimulation of the neurons' somata or stimulation of axon terminals) and the non-local CR effects (realized by stimulation of excitatory or inhibitory efferent fibers) of deep brain CR neuromodulation may be similar or even identical. Furthermore, our results indicate that an effective desynchronization and anti-kindling can even be achieved by non-invasive, sensory CR neuromodulation. We discuss the concept of sensory CR neuromodulation in the context of neurological disorders.

Mapping symbols to sounds: electrophysiological correlates of the impaired reading process in dyslexia

Dyslexic and control first-grade school children were compared in a Symbol-to-Sound matching test based on a non-linguistic audiovisual training which is known to have a remediating effect on dyslexia. Visual symbol patterns had to be matched with predicted sound patterns. Sounds incongruent with the corresponding visual symbol (thus not matching the prediction) elicited the N2b and P3a event-related potential (ERP) components relative to congruent sounds in control children. Their ERPs resembled the ERP effects previously reported for healthy adults with this paradigm. In dyslexic children, N2b onset latency was delayed and its amplitude significantly reduced over left hemisphere whereas P3a was absent. Moreover, N2b amplitudes significantly correlated with the reading skills. ERPs to sound changes in a control condition were unaffected. In addition, correctly predicted sounds, that is, sounds that are congruent with the visual symbol, elicited an early induced auditory gamma band response (GBR) reflecting synchronization of brain activity in normal-reading children as previously observed in healthy adults. However, dyslexic children showed no GBR. This indicates that visual symbolic and auditory sensory information are not integrated into a unitary audiovisual object representation in them. Finally, incongruent sounds were followed by a later desynchronization of brain activity in the gamma band in both groups. This desynchronization was significantly larger in dyslexic children. Although both groups accomplished the task successfully remarkable group differences in brain responses suggest that normal-reading children and dyslexic children recruit (partly) different brain mechanisms when solving the task. We propose that abnormal ERPs and GBRs in dyslexic readers indicate a deficit resulting in a widespread impairment in processing and integrating auditory and visual information and contributing to the reading impairment in dyslexia.

Interference in dichotic listening: the effect of contralateral noise on oscillatory brain networks

Coupling of thalamocortical networks through synchronous oscillations at gamma frequencies (30-80 Hz) has been suggested as a mechanism for binding of auditory sensory information into an object representation, which then becomes accessible for perception and cognition. This study investigated whether contralateral noise interferes with this step of central auditory processing. Neuromagnetic 40-Hz oscillations were examined in young healthy participants while they listened to amplitude-modulated sound in one ear and a multi-talker masking noise in the contralateral ear. Participants were engaged in a gap-detection task, for which their behavioural performance declined under masking. The amplitude modulation of the stimulus elicited steady 40-Hz oscillations with sources in bilateral auditory cortices. Analysis of the temporal dynamics of phase synchrony between source activity and the stimulus revealed two oscillatory components; the first was indicated by an instant onset in phase synchrony with the stimulus while the second showed a 200-ms time constant of gradual increase in phase synchrony after phase resetting by the gap. Masking abolished only the second component. This coincided with masking-related decrease of the P2 wave of the transient auditory-evoked responses whereas the N1 wave, reflecting early sensory processing, was unaffected. Given that the P2 response has been associated with object representation, we propose that the first 40-Hz component is related to representation of low-level sensory input whereas the second is related to internal auditory processing in thalamocortical networks. The observed modulation of oscillatory activity is discussed as reflecting a neural mechanism critical for speech understanding in noise.

Gamma oscillation in schizophrenia

Dysfunctional neural circuitry has been found to be involved in abnormalities of perception and cognition in patients with schizophrenia. Gamma oscillations are essential for integrating information within neural circuits and have therefore been associated with many perceptual and cognitive processes in healthy human subjects and animals. This review presents an overview of the neural basis of gamma oscillations and the abnormalities in the GABAergic interneuronal system thought to be responsible for gamma-range deficits in schizophrenia. We also review studies of gamma activity in sensory and cognitive processes, including auditory steady state response, attention, object representation, and working memory, in animals, healthy humans and patients with schizophrenia.

Frequency characteristics of contralateral sound suppression of 40-Hz auditory steady-state response

Sound presented to the contralateral ear suppresses the amplitude of the 40-Hz auditory steady-state response (ASSR). The frequency characteristics of this suppression of the 40-Hz ASSR for amplitude modulated (AM) tones at 1,000 Hz (79-dB SPL) were examined in 12 healthy volunteers (10 males and 2 females, mean age 32.3 years) using contralateral AM tones (500, 1,000, 2,000, and 4,000 Hz) and 1/3 octave-band noise (500, 1,000, 2,000, and 4,000 Hz). The 40-Hz ASSR at 1,000 Hz was suppressed by a relatively wide frequency range of contralateral sound than expected from the known characteristics of psychophysical central masking by contralateral sound: the greatest suppression was obtained with 500- and 1,000-Hz sounds, but considerable suppression was also obtained with 2,000- and 4,000-Hz sounds. Substantial differences in the suppression pattern were not observed between two types of contra-suppressors; i.e., AM tones and 1/3 octave-band noise. Therefore, any sound presented to the contralateral ear, regardless of the frequency, can suppress the 40-Hz ASSR. Moreover, the different frequency characteristics of the contralateral sound effects between the psychophysical central masking and the 40-Hz ASSR would support the idea that the 40-Hz ASSR has an additive role in the processing of auditory signals to simple threshold judgment. Investigation of the type of psychophysical measurement using the AM signal showing similar suppression patterns by the presentation of contralateral sound would be helpful to reveal the functional relevance of ASSRs.

Entrainment of perceptually relevant brain oscillations by non-invasive rhythmic stimulation of the human brain

The notion of driving brain oscillations by directly stimulating neuronal elements with rhythmic stimulation protocols has become increasingly popular in research on brain rhythms. Induction of brain oscillations in a controlled and functionally meaningful way would likely prove highly beneficial for the study of brain oscillations, and their therapeutic control. We here review conventional and new non-invasive brain stimulation protocols as to their suitability for controlled intervention into human brain oscillations. We focus on one such type of intervention, the direct entrainment of brain oscillations by a periodic external drive. We review highlights of the literature on entraining brain rhythms linked to perception and attention, and point out controversies. Behaviourally, such entrainment seems to alter specific aspects of perception depending on the frequency of stimulation, informing models on the functional role of oscillatory activity. This indicates that human brain oscillations and function may be promoted in a controlled way by focal entrainment, with great potential for probing into brain oscillations and their causal role.

Steady-state visual evoked potentials can be explained by temporal superposition of transient event-related responses

BACKGROUND

One common criterion for classifying electrophysiological brain responses is based on the distinction between transient (i.e. event-related potentials, ERPs) and steady-state responses (SSRs). The generation of SSRs is usually attributed to the entrainment of a neural rhythm driven by the stimulus train. However, a more parsimonious account suggests that SSRs might result from the linear addition of the transient responses elicited by each stimulus. This study aimed to investigate this possibility.

METHODOLOGY/PRINCIPAL FINDINGS

We recorded brain potentials elicited by a checkerboard stimulus reversing at different rates. We modeled SSRs by sequentially shifting and linearly adding rate-specific ERPs. Our results show a strong resemblance between recorded and synthetic SSRs, supporting the superposition hypothesis. Furthermore, we did not find evidence of entrainment of a neural oscillation at the stimulation frequency.

CONCLUSIONS/SIGNIFICANCE

This study provides evidence that visual SSRs can be explained as a superposition of transient ERPs. These findings have critical implications in our current understanding of brain oscillations. Contrary to the idea that neural networks can be tuned to a wide range of frequencies, our findings rather suggest that the oscillatory response of a given neural network is constrained within its natural frequency range.

Attention modulates beta oscillations during prolonged tactile stimulation

The inter-play between changes in beta-band (14-30-Hz) cortical rhythms and attention during somatosensation informs us about where and when relevant processes occur in the brain. As such, we investigated the effects of attention on somatosensory evoked and induced responses using vibrotactile stimulation and magnetoencephalographic recording. Subjects received trains of vibration at 23 Hz to the right index finger while watching a movie and ignoring the somatosensory stimuli or paying attention to the stimuli to detect a change in the duration of the stimulus. The amplitude of the evoked 23-Hz steady-state response in the contralateral primary somatosensory cortex (SI) was enhanced by attention and the underlying dipole source was located 2 mm more medially, indicating top-down recruitment of additional neuronal populations for the functionally relevant stimulus. Attentional modulation of the somatosensory evoked response indicates facilitation of early processing of the tactile stimulus. Beta-band activity increased after vibration offset in the contralateral primary motor cortex (MI) [event-related synchronization (ERS)] and this increase was larger for attended than ignored stimuli. Beta-band activity decreased in the ipsilateral SI prior to stimulus offset [event-related desynchronization (ERD)] for attended stimuli only. Whereas attention modulation of the evoked response was confined to the contralateral SI, event-related changes of beta-band activity involved contralateral SI-MI and inter-hemispheric SI-SI connections. Modulation of neural activity in such a large sensorimotor network indicates a role for beta activity in higher-order processing.

Evidence for modality-specific but not frequency-specific modulation of human primary auditory cortex by attention

We used the stimulus-driven 40-Hz auditory steady-state response (ASSR) that localizes tonotopically to the region of primary auditory cortex (A1) to study modulation of this region by top-down attention. Experiment 1 presented amplitude modulated (AM) auditory and visual stimuli simultaneously (AM at 40 Hz and 16 Hz, respectively) while participants responded to targets in one modality or the other. ASSR amplitude increased from an unattended passive baseline during auditory but not visual attention demonstrating modality-specific auditory attention, when attention was required for brief (1 s) but not long (2 min) time intervals. Modality-specific visual attention occurred at both time intervals. Experiment 2 asked whether attention directed to one or the other of two simultaneous auditory streams (carrier frequencies of 250 and 4100 Hz AM at 37 and 41 Hz respectively, counterbalanced) increased ASSR amplitude for the attended stream (frequency-specific auditory attention). Behaviour was strongly controlled by carrier frequency (overall target rate 1.7 Hz), and the cortical sources of the two carriers were resolved by inverse modeling. Despite these conditions favourable to frequency specificity, frequency-specific modulation of ASSR amplitude was not found at either time interval. Frequency-specific modulation of A1 may require re-entrant feedback to the auditory core from auditory percepts that possess distinct spectral attributes and are attended in higher regions of the auditory system.

Learning related activation of somatosensory cortex by an auditory stimulus recorded with magnetoencephalography

Advances in non-invasive neuroimaging technology now provide a means of directly observing learning within the brain. Classical conditioning serves as an ideal starting point for examining the dynamic expression of learning within the human brain, since this paradigm is well characterized using multiple levels of analysis in a broad range of species. We used MEG to expand the characterization of conditioned responses (CR) recorded from the human brain with a simultaneous examination of their spatial, temporal and spectral properties. We paired an auditory conditioned stimulus (CS+) with a somatosensory unconditioned stimulus (US). We found that when the US was randomly omitted, presentations of CS+ alone, elicited greater desynchronization of beta-band activity in contralateral somatosensory cortex compared to presentations of an auditory stimulus that was never paired with the US (CS-), and compared the CS+ following a non-reinforced extinction session. This differentiation was largest between 150 and 350ms following US omission. We show that cross-modal CRs in the primary sensorimotor system are predominantly characterized by modulation of ongoing cortical oscillations.

Auditory steady-state responses to 40-Hz click trains: relationship to middle latency, gamma band and beta band responses studied with deconvolution

OBJECTIVE

The nature of the auditory steady-state responses (ASSR) evoked with 40-Hz click trains and their relationship to auditory brainstem and middle latency responses (ABR/MLR), gamma band responses (GBR) and beta band responses (BBR) were investigated using superposition theory. Transient responses obtained by continuous loop averaging deconvolution (CLAD) and last click responses (LCR) were used to synthesize ASSRs and GBRs.

METHODS

ASSRs were obtained with trains of low jittered 40 Hz clicks presented monaurally and deconvolved using a modified CLAD. Resulting transient responses and modified LCRs were used to predict the ASSRs and the GBR.

RESULTS

The ABR/MLR obtained with deconvolution predicted accurately the steady portion of the ASSR but failed to predict its onset portion. The modified LCR failed to fully predict both portions. The GBRs were predicted by narrow band filtering of the ASSRs. Significant BBR activity was found both in the ASSRs and deconvolved ABR/MLRs.

CONCLUSIONS

Simulations using deconvolved ABR/MLRs obtained at 40 Hz predict fully the steady state but not the onset portion of the ASSRs, thus confirming the superposition theory.

SIGNIFICANCE

Click rate adaptation plays a significant role in ASSR generation with click trains and should be considered in evaluating convolved response generation theories.

Abnormal neural oscillations and synchrony in schizophrenia

Converging evidence from electrophysiological, physiological and anatomical studies suggests that abnormalities in the synchronized oscillatory activity of neurons may have a central role in the pathophysiology of schizophrenia. Neural oscillations are a fundamental mechanism for the establishment of precise temporal relationships between neuronal responses that are in turn relevant for memory, perception and consciousness. In patients with schizophrenia, the synchronization of beta- and gamma-band activity is abnormal, suggesting a crucial role for dysfunctional oscillations in the generation of the cognitive deficits and other symptoms of the disorder. Dysfunctional oscillations may arise owing to anomalies in the brain's rhythm-generating networks of GABA (gamma-aminobutyric acid) interneurons and in cortico-cortical connections.

Steady state responses: electrophysiological assessment of sensory function in schizophrenia

Persons with schizophrenia experience subjective sensory anomalies and objective deficits on assessment of sensory function. Such deficits could be produced by abnormal signaling in the sensory pathways and sensory cortex or later stage disturbances in cognitive processing of such inputs. Steady state responses (SSRs) provide a noninvasive method to test the integrity of sensory pathways and oscillatory responses in schizophrenia with minimal task demands. SSRs are electrophysiological responses entrained to the frequency and phase of a periodic stimulus. Patients with schizophrenia exhibit pronounced auditory SSR deficits within the gamma frequency range (35-50 Hz) in response to click trains and amplitude-modulated tones. Visual SSR deficits are also observed, most prominently in the alpha and beta frequency ranges (7-30 Hz) in response to high-contrast, high-luminance stimuli. Visual SSR studies that have used the psychophysical properties of a stimulus to target specific visual pathways predominantly report magnocellular-based deficits in those with schizophrenia. Disruption of both auditory and visual SSRs in schizophrenia are consistent with neuropathological and magnetic resonance imaging evidence of anatomic abnormalities affecting the auditory and visual cortices. Computational models suggest that auditory SSR abnormalities at gamma frequencies could be secondary to gamma-aminobutyric acid-mediated or N-methyl-D-aspartic acid dysregulation. The pathophysiological process in schizophrenia encompasses sensory processing that probably contributes to alterations in subsequent encoding and cognitive processing. The developmental evolution of these abnormalities remains to be characterized.

Dynamic crossmodal links revealed by steady-state responses in auditory-visual divided attention

Frequency tagging has been often used to study intramodal attention but not intermodal attention. We used EEG and simultaneous frequency tagging of auditory and visual sources to study intermodal focused and divided attention in detection and discrimination performance. Divided-attention costs were smaller, but still significant, in detection than in discrimination. The auditory steady-state response (SSR) showed no effects of attention at frontocentral locations, but did so at occipital locations where it was evident only when attention was divided between audition and vision. Similarly, the visual SSR at occipital locations was substantially enhanced when attention was divided across modalities. Both effects were equally present in detection and discrimination. We suggest that both effects reflect a common cause: An attention-dependent influence of auditory information processing on early cortical stages of visual information processing, mediated by enhanced effective connectivity between the two modalities under conditions of divided attention.

Source stability index: a novel beamforming based localisation metric

Many experimental studies into human brain function now use magnetoencephalography (MEG) to non-invasively investigate human neuronal activity. A number of different analysis techniques use the observed magnetic fields outside of the head to estimate the location and strength of the underlying neural generators. One such technique, a spatial filtering method known as Beamforming, produces whole-head volumetric images of activation. Typically, a differential power map throughout the head is generated between a time window containing the response to a stimulus of interest and a window containing background brain activity. A statistical test is normally performed to reveal locations which show a significantly different response in the presence of the stimulus. Despite this being a widely used measure, for both phase-locked and non-phase-locked information, it requires a number of assumptions; namely that the baseline activity defined is stable and also that a change in total power is the most effective way of revealing the neuronal sources required for the task. This paper introduces a metric which evaluates the consistency of the response at each location within a cortical volume. Such a method of localisation negates the need for a baseline period of activity to be defined and also moves away from simply considering the energy content of brain activity. The paper presents both simulated and real data. It demonstrates that this new metric of stability is able to more accurately and, crucially, more reliably draw inferences about neuronal sources of interest.

Temporal dynamics of selective attention during dichotic listening

The ability to selectively attend to one sound and ignore other competing sounds is essential for auditory communication. Subjects in our study detected occasional changes in the frequency of amplitude modulation in sounds presented to one ear while ignoring sounds in the other ear. Neuromagnetic source analysis revealed attention-related activity in a cortical network including primary auditory cortices, posterior superior temporal gyri, inferior parietal lobules (IPLs), inferior frontal gyri (IFG), and medial frontal gyri. Time courses of event-related magnetoencephalography responses were analyzed during the interval between stimulus presentation and behavioral response. Enhanced neural responses to targets and standards in the attended ear indicated early modulation of sensitivity in the attended sensory channel. A subsequent process of discriminative stimulus selection was indexed by a response increase over time for targets and decreasing activity for standards. Enhanced responses to deviants in the unattended ear indicated discriminative processing of unattended inputs as well, though to a lesser extent than for attended stimuli. Superior temporal gyrus, planum temporale, and the IPL were prominently involved in stimulus selection, whereas medial frontal regions were linked to initiation of behavioral responses and sustained activity in IFG suggested a role in attentional control.

Rate perception and the auditory 40-Hz steady-state fields evoked by two-tone sequences

The rate perception of tone sequences reflects the physical repetition rate for identical sound elements. More complex sequences are perceived at the physical rate or at lower rates, depending on perceptual organization. Here, we used magnetoencephalography and psychophysical studies to evaluate the possible relationship between rate perception of such rapid, 40-Hz tone trains and the 40-Hz steady-state response (SSR) in human primary auditory cortex. In Experiment 1, the 40-Hz SSR evoked by monotone sequences of 1000 and 600 Hz were compared to the response evoked by alternating-tone sequences of the same frequencies. The results showed that the 40-Hz SSR for the alternating-tones was attenuated compared to the monotones. In Experiment 2, frequency differences across a range of 25-300 Hz were studied. Compared to a 1000-Hz monotone sequence, the 40-Hz SSR was reduced. Amplitude reduction was most prominent for frequency differences of 200 Hz and more, which were generally perceived with half-the-physical rate. We discuss possible physiological mechanisms of this finding and its relationship to perception.

Neural representations of complex temporal modulations in the human auditory cortex

Natural sounds such as speech contain multiple levels and multiple types of temporal modulations. Because of nonlinearities of the auditory system, however, the neural response to multiple, simultaneous temporal modulations cannot be predicted from the neural responses to single modulations. Here we show the cortical neural representation of an auditory stimulus simultaneously frequency modulated (FM) at a high rate, f(FM) approximately 40 Hz, and amplitude modulation (AM) at a slow rate, f(AM) <15 Hz. Magnetoencephalography recordings show fast FM and slow AM stimulus features evoke two separate but not independent auditory steady-state responses (aSSR) at f(FM) and f(AM), respectively. The power, rather than phase locking, of the aSSR of both decreases with increasing stimulus f(AM). The aSSR at f(FM) is itself simultaneously amplitude modulated and phase modulated with fundamental frequency f(AM), showing that the slow stimulus AM is not only encoded in the neural response at f(AM) but also encoded in the instantaneous amplitude and phase of the neural response at f(FM). Both the amplitude modulation and phase modulation of the aSSR at f(FM) are most salient for low stimulus f(AM) but remain observable at the highest tested f(AM) (13.8 Hz). The instantaneous amplitude of the aSSR at f(FM) is successfully predicted by a model containing temporal integration on two time scales, approximately 25 and approximately 200 ms, followed by a static compression nonlinearity.

Steady state and induced auditory gamma deficits in schizophrenia

Steady state auditory evoked potentials (SSAEPs) in the electroencephalogram (EEG) and magnetoencephalogram (MEG) have been reported to be reduced in schizophrenia, most consistently to frequencies in the gamma range (40 Hz and greater). The current study evaluated the specificity of this deficit over a broad range of stimulus frequencies and harmonics, the relationship between phase locking and signal power, and whether induced 40 Hz activity was also affected. SSAEPs to amplitude modulated tones from 5 to 50 Hz were obtained from subjects with schizophrenia (SZ) and healthy control subjects in 5 Hz steps. Time-frequency spectral analysis was used to differentiate EEG activity synchronized in phase across trials using Phase Locking Factor (PLF) and Mean Power (MP) change from baseline activity. In the SSAEP frequency response condition, patients with SZ showed broad band reductions in both PLF and MP. In addition, the control subjects showed a more pronounced increase in PLF with increases in power compared to SZ subjects. A noise pulse embedded in 40 Hz stimuli resulted in a transient reduction of PLF and MP at 40 Hz in control subjects, while SZ showed diminished overall PLF. Finally, induced gamma (around 40 Hz) response to unmodulated tone stimuli was also reduced in SZ, indicating that disturbances in this oscillatory activity are not confined to SSAEPs. In summary, SZ subjects show impaired oscillatory responses in the gamma range across a wide variety of experimental conditions. Reduction of PLF along with reduced MP may reflect abnormalities in the auditory cortical circuits, such as a reduction in pyramidal cell volume, spine density and alterations in GABAergic neurons.

Generation of the 40-Hz auditory steady-state response (ASSR) explained using convolution

OBJECTIVE

In this study, the superposition theory of the 40-Hz auditory steady-state response (ASSR) generation is investigated using auditory brainstem response (ABR) and middle latency responses (MLRs) obtained with 40 Hz jittered sequences with the continuous loop averaging deconvolution (CLAD) algorithm.

METHODS

Click sequences at around 40 Hz with high (maximum length sequence), medium and low jitters were presented to normal hearing awake adult subjects monaurally. Overlapping MLR responses were deconvolved using the frequency domain CLAD method. In addition, conventional auditory MLRs at 4.88 Hz and ASSRs at 39.1 Hz were obtained in all subjects. Synthetic ASSRs were constructed using different rate and jitter MLRs as base recordings. Contributions of the primary components were investigated by wave elimination using phasors.

RESULTS

Findings indicate that the generation of the 40-Hz ASSRs can be explained successfully by the superposition of the ABR and MLR waves generated at that stimulation rate. N(a)-P(a) and N(b)-P(b) components of the MLR contribute about equally (45% each), while the wave V of the ABR contributes a lesser amount (10%).

CONCLUSIONS

Forty-Hertz ASSRs are composite responses generated by the superposition of the major waves of the ABR and the MLR. Dramatic amplitude increase of the ASSR at 40Hz is primarily due to the superposition of the resonating P(b) component to the P(a) wave.

SIGNIFICANCE

Several unexplained properties of the 40-Hz ASSR can be explained by the stimulus and brain state dependent characteristics of the slow ABR, the P(a) and the P(b) components of the MLR.

A Novel Type of Auditory Responses: Temporal Dynamics of 40-Hz Steady-State Responses Induced by Changes in Sound Localization

Magnetoencephalographic responses to 40-Hz amplitude-modulated tones of 4-s duration were recorded in young, middle-aged, and older healthy participants. Interaural phase difference (IPD) in the sound carrier was changed during stimulus presentation from 0 to 180°, resulting in perceptual change from focal to spacious sound. The stimulus modulation elicited synchronized gamma-band oscillations, the 40-Hz auditory steady-state response (ASSR). Equivalent current dipoles were localized in primary auditory cortices. Waveforms of cortical activity showed a decrement in ASSR amplitude 100 ms after stimulus IPD change and modification of ASSR phase, which was maximally 90°, corresponding to 6-ms delay. Time courses of ASSR phase deviation constituted a novel auditory response. The amount of ASSR phase change decreased with increasing stimulus frequency and revealed upper limits for physiological IPD detection. Thresholds for IPD detection were found close to 1,500 Hz in the young, around 1,250 Hz in the middle-aged group, and around 1,000 Hz in the older group. Whereas the ASSR change response revealed aging-related decline of binaural hearing, the amplitude of 40-Hz response and the size of the ASSR change response were not affected by aging. Additional ASSR change responses were recorded at a high rate of stimulus changes every 400 ms. ASSR response detection at this rate was superior to response detection based on the auditory-evoked P1–N1–P2 response. Responses to changes from focal to spacious sound were larger than those in the reverse direction. The ASSRs were interpreted in relation to oscillatory gamma-band activity representing auditory object representation.

Frequency-specific coupling in the cortico-cerebellar auditory system

Induced oscillatory activity in the auditory cortex peaks at around 40 Hz in humans. Using regional cerebral blood flow and positron emission tomography we previously confirmed frequency-selective cortical responses to 40-Hz tones in auditory primary cortices and concomitant bilateral activation of the cerebellar hemispheres. In this study, using functional magnetic resonance imaging (fMRI) we estimated the influence of 40-Hz auditory stimulation on the coupling between auditory cortex and superior temporal sulcus (STS) and Crus II, using a dynamic causal model of the interactions between medial geniculate nuclei, auditory superior temporal gyrus (STG)/STS, and the cerebellar Crus II auditory region. Specifically, we tested the hypothesis that 40-Hz-selective responses in the cerebellar Crus II auditory region could be explained by frequency-specific enabling of interactions in the auditory cortico–cerebellar–thalamic loop. Our model comparison results suggest that input from auditory STG/STS to cerebellum is enhanced selectively at gamma-band frequencies around 40 Hz.

Concurrent Encoding of Frequency and Amplitude Modulation in Human Auditory Cortex: Encoding Transition

Complex natural sounds (e.g., animal vocalizations or speech) can be characterized by specific spectrotemporal patterns the components of which change in both frequency (FM) and amplitude (AM). The neural coding of AM and FM has been widely studied in humans and animals but typically with either pure AM or pure FM stimuli. The neural mechanisms employed to perceptually unify AM and FM acoustic features remain unclear. Using stimuli with simultaneous sinusoidal AM (at rate fAM = 37 Hz) and FM (with varying rates ƒFM), magnetoencephalography (MEG) is used to investigate the elicited auditory steady-state response (aSSR) at relevant frequencies (ƒAM, ƒFM, ƒAM + fFM). Previous work demonstrated that for sounds with slower FM dynamics ( fFM < 5 Hz), the phase of the aSSR at ƒAM tracked the FM; in other words, AM and FM features were co-tracked and co-represented by “phase modulation” encoding. This study explores the neural coding mechanism for stimuli with faster FM dynamics (≤30 Hz), demonstrating that at faster rates ( fFM > 5 Hz), there is a transition from pure phase modulation encoding to a single-upper-sideband (SSB) response (at frequency fAM + fFM) pattern. We propose that this unexpected SSB response can be explained by the additional involvement of subsidiary AM encoding responses simultaneously to, and in quadrature with, the ongoing phase modulation. These results, using MEG to reveal a possible neural encoding of specific acoustic properties, demonstrate more generally that physiological tests of encoding hypotheses can be performed noninvasively on human subjects, complementing invasive, single-unit recordings in animals.

Cortical Steady-State Responses to Central and Peripheral Auditory Beats

Different types of generation mechanisms of 40-Hz auditory steady-state response (ASSR) were investigated using diotic and dichotic stimulation with 500- and 540-Hz pure tones of 1.0-s duration and 2.0-s stimulus onset asynchrony. When the sum of both tones was presented to both ears simultaneously, they interacted at cochlear level and resulted in perception of a 40-Hz beat termed "peripheral beat." Dichotic presentation of the 500-Hz tone to one ear and the 540-Hz tone to the other one resulted in beat perception as the effect of central interaction, most likely in the superior olivary nuclei and was termed "central beat." ASSR and transient N1m responses were found in the averaged 151-channel whole-head magnetoencephalographic recordings under both stimulus conditions and were modeled with single spatiotemporal equivalent current dipoles in both hemispheres. The ASSR sources in both conditions were more anterior, more inferior, and more medial compared with N1m sources. Right hemispheric lateralization of the magnetic field strength was found for the ASSR in both stimulus conditions. Although the central and peripheral beat interacted at different levels of the auditory system, the initial responses were projected along the afferent auditory pathway and activated common cortical sources.

Response clustering in transient stochastic synchronization and desynchronization of coupled neuronal bursters

We studied the transient dynamics of synchronized coupled neuronal bursters subjected to repeatedly applied stimuli, using a hybrid neuroelectronic system of paddlefish electroreceptors. We show experimentally that the system characteristically undergoes poststimulus transients, in which the relative phases of the oscillators may be grouped in several clusters, traversing alternate phase trajectories. These signature transient dynamics can be detected and characterized quantitatively using specific statistical measures based on a stochastic approach to transient oscillator responses.

The effect of selective attention on the gamma-band auditory steady-state response

Studies have demonstrated that selective attention can modulate the steady-state evoked potential to repetitive visual and tactile stimulation. However, examinations of the effect of attention on the auditory steady-state response (ASSR) have proven equivocal. The current experiment therefore utilized EEG to examine the effect of attention on the ASSR in healthy humans (n=15). Auditory click trains in the beta (20 Hz) and gamma (40 Hz) ranges were randomly presented binaurally in an oddball discrimination paradigm (each frequency served as the oddball (target) in each of two blocks). A Fast Fourier Transform was used to assess the effect of attention on the ASSR (signal power), and phase consistency across trials was assessed using the phase-locking factor (PLF). As expected, both 20 and 40 Hz targets elicited a robust P300 response, with maximal amplitudes over parietal regions. For the ASSR, it was found that EEG signal power was larger to 40 Hz targets compared to 40 Hz frequent stimuli across all frontocentral electrodes. No differences in signal power were observed during 20 Hz stimulation. Finally, increased PLF values were observed for 40 Hz targets compared to frequent trials. These results provide evidence that selective attention can enhance signal power and phase-locking of the ASSR, particularly to auditory stimulation in the gamma range.

Frequency organization of the 40-Hz auditory steady-state response in normal hearing and in tinnitus

We used the 40-Hz auditory steady-state response (SSR) to compare for the first time tonotopic frequency representations in the region of primary auditory cortex (PAC) between subjects with chronic tinnitus and hearing impairment and normal hearing controls. Frequency representations were measured in normal hearing (n=17) and tinnitus (n=28) subjects using eight carrier frequencies between 384 and 6561 Hz, each amplitude modulated (AM) at 40-Hz on trials of 3 min duration under passive attention. In normal hearing subjects, frequency gradients were observed in the medial-lateral, anterior-posterior, and inferior-superior axes, which were consistent with the orientation of Heschl's gyrus and with functional organization revealed by fMRI investigations. The frequency representation in the right hemisphere was approximately 5 mm anterior and approximately 7 mm lateral to that in the left hemisphere, corroborating with MEG measurements hemispheric asymmetries reported by cytoarchitectonic studies of the PAC and by MRI morphometry. In the left hemisphere, frequency gradients were inflected near 2 kHz in normal hearing subjects. These SSR frequency gradients were attenuated in both hemispheres in tinnitus subjects. Dipole power was also elevated in tinnitus, suggesting that more neurons were entrained synchronously by the AM envelope. These findings are consistent with animal experiments reporting altered tonotopy and changes in the response properties of auditory cortical neurons after hearing loss induced by noise exposure. Degraded frequency representations in tinnitus may reflect a loss of intracortical inhibition in deafferented frequency regions of the PAC after hearing injury.