Imaging the dynamics of the auditory steady-state evoked response

Transcranial Alternating Current Stimulation Alters Auditory Steady-State Oscillatory Rhythms and Their Cross-Frequency Couplings

Auditory cortical plasticity deficits in schizophrenia are evidenced with electroencephalographic (EEG)-derived biomarkers, including the 40-Hz auditory steady-state response (ASSR). Aiming to understand the underlying oscillatory mechanisms contributing to the 40-Hz ASSR, we examined its response to transcranial alternating current stimulation (tACS) applied bilaterally to the temporal lobe of 23 healthy participants. Although not responding to gamma tACS, the 40-Hz ASSR was modulated by theta tACS (vs sham tACS), with reductions in gamma power and phase locking being accompanied by increases in theta-gamma phase-amplitude cross-frequency coupling. Results reveal that oscillatory changes induced by frequency-tuned tACS may be one approach for targeting and modulating auditory plasticity in normal and diseased brains.

Cerebral Cortical Activity Following Non-invasive Cerebellar Stimulation-a Systematic Review of Combined TMS and EEG Studies

The cerebellum sends dense projections to both motor and non-motor regions of the cerebral cortex via the cerebellarthalamocortical tract. The integrity of this tract is crucial for healthy motor and cognitive function. This systematic review examines research using transcranial magnetic stimulation (TMS) and transcranial direct current stimulation (tDCS) to the cerebellum with combined cortical electroencephalography (EEG) to explore the temporal features of cerebellar-cortical connectivity. A detailed discussion of the outcomes and limitations of the studies meeting review criteria is presented. Databases were searched between 1 December 2017 and 6 December 2017, with Scopus alerts current as of 23 July 2019. Of the 407 studies initially identified, 10 met review criteria. Findings suggested that cerebellar-cortical assessment is suited to combined TMS and EEG, although work is required to ensure experimental procedures are optimal for eliciting a reliable cerebellar response from stimulation. A distinct variation in methodologies and outcome measures employed across studies, and small sample sizes limited the conclusions that could be drawn regarding the electrophysiological signatures of cerebellar-cortical communication. This review highlights the need for stringent protocols and methodologies for cerebellar-cortical assessments via combined TMS and EEG. With these in place, combined TMS and EEG will provide a valuable means for exploring cerebellar connectivity with a wide range of cortical sites. Assessments have the potential to aid in the understanding of motor and cognitive function in both healthy and clinical groups, and provide insights into long-range neural communication generally.

Computer-assisted cognitive remediation for schizophrenia: a randomized single-blind pilot study

Cognitive impairment is a core symptom in schizophrenia that has a significant impact on psychosocial function, but shows a weak response to pharmacological treatment. Consequently, a variety of cognitive remediation strategies have been evaluated to improve cognitive function in schizophrenia. The efficacy of computer-based cognitive remediation as a stand-alone intervention on general measures of neuropsychological function remains unclear. We tested the effectiveness of biweekly training using computerized cognitive remediation programs on neuropsychological and event-related potential outcome measures. Schizophrenia patients were randomly assigned to cognitive remediation training (N=17), active control (TV-watching; N=17), or treatment-as-usual (N=10) groups for ten weeks and run in parallel. Cognitive and ERP measures revealed no differential improvement over time in the cognitive remediation group. Practice effects might explain change over time on several cognitive measures for all groups, consistent with studies indicating task-specific improvement. Computer-assisted cognitive remediation alone may not be sufficient for robust or generalized effects on cognitive and electrophysiological measures in schizophrenia patients.

Stimulus variability affects the amplitude of the auditory steady-state response

In this study we investigate whether stimulus variability affects the auditory steady-state response (ASSR). We present cosinusoidal AM pulses as stimuli where we are able to manipulate waveform shape independently of the fixed repetition rate of 4 Hz. We either present sounds in which the waveform shape, the pulse-width, is fixed throughout the presentation or where it varies pseudo-randomly. Importantly, the average spectra of all the fixed-width AM stimuli are equal to the spectra of the mixed-width AM. Our null hypothesis is that the average ASSR to the fixed-width AM will not be significantly different from the ASSR to the mixed-width AM. In a region of interest beamformer analysis of MEG data, we compare the 4 Hz component of the ASSR to the mixed-width AM with the 4 Hz component of the ASSR to the pooled fixed-width AM. We find that at the group level, there is a significantly greater response to the variable mixed-width AM at the medial boundary of the Middle and Superior Temporal Gyri. Hence, we find that adding variability into AM stimuli increases the amplitude of the ASSR. This observation is important, as it provides evidence that analysis of the modulation waveform shape is an integral part of AM processing. Therefore, standard steady-state studies in audition, using sinusoidal AM, may not be sensitive to a key feature of acoustic processing.

Auditory steady state response in the schizophrenia, first-degree relatives, and schizotypal personality disorder

The power and phase synchronization of the auditory steady state response (ASSR) at 40 Hz stimulation is usually reduced in schizophrenia (SZ). The sensitivity of the 40 Hz ASSR to schizophrenia spectrum phenotypes, such as schizotypal personality disorder (SPD), or to familial risk has been less well characterized. We compared the ASSR of patients with SZ, persons with schizotypal personality disorder, first degree relatives of patients with SZ, and healthy control participants. ASSRs were obtained to 20, 30, 40 and 50 Hz click trains, and assessed using measures of power (mean trial power or MTP) and phase consistency (phase locking factor or PLF). The MTP to 40 Hz stimulation was reduced in relatives, and there was a trend for MTP reduction in SZ. The 40 Hz ASSR was not reduced in SPD participants. PLF did not differ among groups. These data suggest the 40 Hz ASSR is sensitive to familial risk factors associated with schizophrenia.

Interaction among the components of multiple auditory steady-state responses: enhancement in tinnitus patients, inhibition in controls

Amplitude and phase of steady-state signals recorded in response to amplitude-modulated (AM) sine tones vary over time, suggesting that the steady-state response (SSR) reflects not only stimulus input but also its interaction with other input streams or internally generated signals. Alterations of the interaction between simultaneous SSRs associated with tinnitus were studied by recording the magnetic field evoked by AM-tones with one of three carrier and one of three modulation frequencies. Single AM-tones were presented in single presentation mode and superpositions of three AM-tones differing in carrier and modulation frequency in multiple presentation mode. Modulation frequency-specific SSR components were recovered by bandpass filtering. Compared with single mode, in multiple mode SSR amplitude was reduced in healthy controls, but increased in tinnitus patients. Thus, while in controls multiple response components seem to reciprocally inhibit one another, in tinnitus reciprocal facilitation seems to predominate. Reciprocal inhibition was unrelated to the phase coherence among SSR components, but was correlated with the frequency of phase slips, indicating that the lateral interaction among SSR components acts in a quasi-paroxysmal manner and manifests itself in terms of a random train of phase reset events. Phase slips were more frequent in patients than controls both in single and multiple mode. Together, these findings indicate that lateral or surround inhibition of single units in auditory cortex is reduced and suggest that in-field inhibition is increased in tinnitus.

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.

A case for genetics education: collaborating with speech-language pathologists and audiologists

Because speech-language pathologists (SLPs) and audiologists (AUDs) are among the first referrals for parents of children exhibiting feeding, speech, language, hearing, and balance difficulties, it is important for SLP and AUD professionals to recognize genetic causes of and contributions to complex and Mendelian communication disorders. We review genetics in the curricula of speech-language pathology and audiology programs and obstacles to its integration throughout curricula. We present suggestions about how SLPs and AUDs can aid in diagnosis and contribute their clinical expertise in characterizing phenotypes, followed with a review of a new genetics-education website developed by the National Coalition for Health Professional Education in Genetics (NCHPEG), the University of Cincinnati, and the National Society of Genetic Counselors. The need to integrate genetics content into curricula and continuing education across disciplines is clear, as is the need for and benefit of multidisciplinary collaboration in patient care. The NCHPEG site for speech-language pathology and audiology begins to address those needs and may serve as a practical model for future multidisciplinary collaborations between genetics professionals and other health professions.

Beamformer reconstruction of correlated sources using a modified source model

This paper introduces a lead field formulation for use in beamformer analysis of MEG data. This 'dual source beamformer' is a technique to image two temporally correlated sources using beamformer methodology. We show that while the standard, single source beamformer suppresses the reconstructed power of two spatially separate but temporally correlated sources, the dual source beamformer allows for their accurate reconstruction. The technique is proven to be accurate using simulations. We also show that it can be used to image accurately the auditory steady state response, which is correlated between the left and right auditory cortices. We suggest that this technique represents a useful way of locating correlated sources, particularly if a seed location can be defined a priori for one of the two sources. Such a priori information could be based on previous studies using similar paradigms, or from other functional neuroimaging techniques.

Modulation of steady-state auditory evoked potentials by cerebellar rTMS

Steady-state auditory evoked responses (SSAER) obtained via electroencephalography (EEG) co-vary in amplitude with blood flow changes in the auditory area of the cerebellum. The aim of the present EEG study was to probe the cerebellar role in the control of such SSAER. For this purpose, we investigated changes in SSAERs due to transient disruption of the cerebellar hemisphere by repetitive transcranial magnetic stimulation (rTMS). SSAERs to click-trains of three different frequencies in the gamma-band (32, 40 and 47 Hz) were recorded from 45 scalp electrodes in six healthy volunteers immediately after 1-Hz rTMS and compared to baseline SSAERs assessed prior to magnetic stimulation. Cerebellar rTMS contralateral to the stimulated ear significantly reduced the amplitude of steady-state responses to 40-Hz click-trains and showed a tendency to reduce the amplitude to 32-Hz click-trains. No effects were observed for 47-Hz click-trains, nor for magnetic stimulation of the cerebellum ipsilateral to auditory stimulation or after sham stimulation. Our results suggest that interference with cerebellar output by rTMS modifies functional activity associated with cortical auditory processing. The finding of maximum effects on 40-Hz SSAERs provides support to the notion that the cerebellum is part of a distributed network involved in the regulation of cortical oscillatory activity and points at some frequency-specificity for the control of auditory-driven neuronal oscillations.