Isabel Lorenz

Top-down modulation of the auditory Steady State Response in a task-switch paradigm

Auditory selective attention is an important mechanism for top-down selection of the vast amount of auditory information our perceptual system is exposed to. In the present study, the impact of attention on auditory steady-state responses is investigated. This issue is still a matter of debate and recent findings point to a complex pattern of attentional effects on the auditory steady state response (aSSR). The present study aimed at shedding light on the involvement of ipsilateral and contralateral activations to the attended sound taking into account hemispheric differences and a possible dependency on modulation frequency. In aid of this, a dichotic listening experiment was designed using amplitude-modulated tones that were presented to the left and right ear simultaneously. Participants had to detect target tones in a cued ear while their brain activity was assessed using MEG. Thereby, a modulation of the aSSR by attention could be revealed, interestingly restricted to the left hemisphere and 20 Hz responses: contralateral activations were enhanced while ipsilateral activations turned out to be reduced. Thus, our findings support and extend recent findings, showing that auditory attention can influence the aSSR, but only under specific circumstances and in a complex pattern regarding the different effects for ipsilateral and contralateral activations.

Mapping cortical hubs in tinnitus

BackgroundSubjective tinnitus is the perception of a sound in the absence of any physical source. It has been shown that tinnitus is associated with hyperactivity of the auditory cortices. Accompanying this hyperactivity, changes in non-auditory brain structures have also been reported. However, there have been no studies on the long-range information flow between these regions.ResultsUsing Magnetoencephalography, we investigated the long-range cortical networks of chronic tinnitus sufferers (n = 23) and healthy controls (n = 24) in the resting state. A beamforming technique was applied to reconstruct the brain activity at source level and the directed functional coupling between all voxels was analyzed by means of Partial Directed Coherence. Within a cortical network, hubs are brain structures that either influence a great number of other brain regions or that are influenced by a great number of other brain regions. By mapping the cortical hubs in tinnitus and controls we report fundamental group differences in the global networks, mainly in the gamma frequency range. The prefrontal cortex, the orbitofrontal cortex and the parieto-occipital region were core structures in this network. The information flow from the global network to the temporal cortex correlated positively with the strength of tinnitus distress.ConclusionWith the present study we suggest that the hyperactivity of the temporal cortices in tinnitus is integrated in a global network of long-range cortical connectivity. Top-down influence from the global network on the temporal areas relates to the subjective strength of the tinnitus distress.

Alpha rhythms in audition: Cognitive and clinical perspectives

Like the visual and the sensorimotor systems, the auditory system exhibits pronounced alpha-like resting oscillatory activity. Due to the relatively small spatial extent of auditory cortical areas, this rhythmic activity is less obvious and frequently masked by non-auditory alpha-generators when recording non-invasively using magnetoencephalography (MEG) or electroencephalography (EEG). Following stimulation with sounds, marked desynchronizations can be observed between 6 and 12 Hz, which can be localized to the auditory cortex. However knowledge about the functional relevance of the auditory alpha rhythm has remained scarce so far. Results from the visual and sensorimotor system have fuelled the hypothesis of alpha activity reflecting a state of functional inhibition. The current article pursues several intentions: (1) Firstly we review and present own evidence (MEG, EEG, sEEG) for the existence of an auditory alpha-like rhythm independent of visual or motor generators, something that is occasionally met with skepticism. (2) In a second part we will discuss tinnitus and how this audiological symptom may relate to reduced background alpha. The clinical part will give an introduction into a method which aims to modulate neurophysiological activity hypothesized to underlie this distressing disorder. Using neurofeedback, one is able to directly target relevant oscillatory activity. Preliminary data point to a high potential of this approach for treating tinnitus. (3) Finally, in a cognitive neuroscientific part we will show that auditory alpha is modulated by anticipation/expectations with and without auditory stimulation. We will also introduce ideas and initial evidence that alpha oscillations are involved in the most complex capability of the auditory system, namely speech perception. The evidence presented in this article corroborates findings from other modalities, indicating that alpha-like activity functionally has an universal inhibitory role across sensory modalities.

Loss of alpha power is related to increased gamma synchronization—A marker of reduced inhibition in tinnitus?

Tinnitus is the perception of sound in the absence of any external auditory stimulus. Based on previous research we have proposed a framework which postulates that the reduction of ongoing inhibitory alpha activity in tinnitus subjects favors a synchronization of neurons in the gamma frequency range while in a resting state. In the present work we are validating the existence of an inverse relationship between auditory gamma and alpha activity in tinnitus and control subjects using Magnetoencephalography. Tinnitus subjects exhibited a significantly steeper slope of the regression line compared to controls, presumably because a greater number of subjects concurrently exhibited low alpha and high gamma power. Therefore, the role of the alpha-gamma pattern is discussed regarding its possible implication for the generation of tinnitus.

Short-term effects of single repetitive TMS sessions on auditory evoked activity in patients with chronic tinnitus.

Subjective tinnitus is the perception of a sound without any external source. Repetitive transcranial magnetic stimulation (rTMS) has been examined as a treatment tool for chronic tinnitus for several years trying to target hyperactivity/abnormal synchronization within the auditory cortex putatively underlying the auditory phantom percept. However, its exact impact on auditory cortical activity remains largely unknown. This studys objective was to systematically examine changes in auditory responses (N1, auditory steady-state response [aSSR]) measured by means of magnetoencephalography after single sessions of stimulation with different TMS paradigms. Subjects with chronic tinnitus (n = 10) underwent five sessions of rTMS in which they received one of five different stimulation protocols (1 Hz, individual alpha frequency, continuous theta burst stimulation [cTBS], intermittent theta burst stimulation [iTBS], and sham) in randomized order using a single-blind study design. Cortical steady-state responses to 40 Hz amplitude-modulated tones were measured before and after each magnetic stimulation protocol. The results demonstrate a reduction of the cortical response to the auditory steady-state stimulus after magnetic stimulation, whereas the N1 response was slightly enhanced or remained unchanged. Furthermore, reduction of the aSSR was driven by effects of iTBS, cTBS, and 1 Hz stimulation. Correspondingly, behavioral measures demonstrated the greatest reduction of tinnitus loudness after the respective rTMS protocols. The current study offers an interesting insight into the effects of rTMS on auditory cortical activity. The results of the study are discussed in the context of current limitations of TMS for the treatment of chronic tinnitus.

rTMS Induced Tinnitus Relief Is Related to an Increase in Auditory Cortical Alpha Activity

Chronic tinnitus, the continuous perception of a phantom sound, is a highly prevalent audiological symptom. A promising approach for the treatment of tinnitus is repetitive transcranial magnetic stimulation (rTMS) as this directly affects tinnitus-related brain activity. Several studies indeed show tinnitus relief after rTMS, however effects are moderate and vary strongly across patients. This may be due to a lack of knowledge regarding how rTMS affects oscillatory activity in tinnitus sufferers and which modulations are associated with tinnitus relief. In the present study we examined the effects of five different stimulation protocols (including sham) by measuring tinnitus loudness and tinnitus-related brain activity with Magnetoencephalography before and after rTMS. Changes in oscillatory activity were analysed for the stimulated auditory cortex as well as for the entire brain regarding certain frequency bands of interest (delta, theta, alpha, gamma). In line with the literature the effects of rTMS on tinnitus loudness varied strongly across patients. This variability was also reflected in the rTMS effects on oscillatory activity. Importantly, strong reductions in tinnitus loudness were associated with increases in alpha power in the stimulated auditory cortex, while an unspecific decrease in gamma and alpha power, particularly in left frontal regions, was linked to an increase in tinnitus loudness. The identification of alpha power increase as main correlate for tinnitus reduction sheds further light on the pathophysiology of tinnitus. This will hopefully stimulate the development of more effective therapy approaches.

Formerly known as inhibitory: effects of 1-Hz rTMS on auditory cortex are state-dependent.

The major repetitive transcranial magnetic stimulation (rTMS) paradigm applied to the treatment of tinnitus has been the 1‐Hz variant due to its alleged inhibitory effects. Clinical effects have, however, been hampered by great interindividual variability as well as the fact that TMS includes no explicit mechanism to modulate excitability in circumscribed regions of tonotopically organised auditory fields. Following studies showing that the effect of TMS depends on the activational state preceding the stimulation, participants were exposed to 10 min of either notch‐ or bandpass‐filtered noise prior to 1‐Hz rTMS applied to the left auditory cortex. A control group was additionally assessed using bandpass noise – albeit with subsequent sham stimulation – to assess whether effects were due to the differential sounds alone or to a genuine interaction between sound and rTMS. Electroencephalogram was recorded from 128 electrodes before and after the experimental treatment while participants performed an auditory intensity discrimination task. While state‐dependency effects from the behavioural data are not conclusive, several condition × (sound) frequency effects (some specific to the stimulated side) could be observed. Importantly, many of these could not be explained by the use of rTMS or the filtered noise alone. The resulting patterns are, however, complex and temporally variable, which currently prohibits recommendations on how to design a clinically effective approach to treat tinnitus. Nevertheless, our study gives the first evidence that state‐dependency principles can induce sound frequency‐specific effects in the auditory cortex, providing a crucial proof‐of‐principle upon which future studies can build.

A Global Brain Model of Tinnitus

1. Subjective tinnitus is characterized by the perception of a phantom sound in the absence of any physical source. 2. While transient tinnitus usually lasts only a couple of seconds to a few hours, chronic tinnitus is an ongoing conscious perception of sound for more than 6 months with low incidence of spontaneous remissions. 3. Empirical studies in animals and humans often show enhancement of cortical excitability in the auditory areas associated with the tinnitus. 4. Theoretical and experimental studies suggest an additional involvement of extra-auditory cortical regions, especially the frontal cortex, the parietal cortex, and the cingulum. 5. Using magnetoencephalograpic recordings, we found that these areas are functionally connected with each other and form a global fronto–parietal–cingulate network. 6. The top–down influence of this global network on auditory areas is associated with the distress that is perceived by many individuals with tinnitus. 7. We suggest that both entities – the enhanced excitability of the central auditory system and the integration with a global cortical network – are important to generate and maintain a conscious percept of tinnitus. 8. This chapter will concentrate on how a conscious perception of tinnitus is formed and maintained throughout a lifetime.