Neural oscillations in auditory working memory

The present thesis investigated memory load and memory decay in auditory working memory. Alpha power as a marker for memory load served as the primary indicator for load and decay fluctuations hypothetically reflecting functional inhibition of irrelevant information. Memory load was induced by presenting auditory signals (syllables and pure-tone sequences) in noise because speech-in-noise has been shown before to increase memory load. The aim of the thesis was to assess with magnetoencephalography whether a-priori temporal expectations for the onset-time of a to-be-remembered stimulus reduces memory load. It was reported previously that top-down modulations such as spatial expectations reduce memory load and improve memory performance. However, this effect has neither been investigated with temporal expectations nor in the auditory domain. The present thesis showed that temporal expectations for a syllable in noise reduced memory load. Reduced alpha power during stimulus maintenance as well as improved performance indicated the decrease in memory load. Alpha power effects emerged from the right cingulo-opercular network, presumably reflecting a reduced need for functional inhibition. Critically, symbolic cues induced temporal expectations. This effect could not be replicated for clear speech. However, more implicit temporal expectations based on the passage of time elicited a similar decrease in alpha power for clear speech reflecting reduced memory load. Memory decay was assessed with variable delay phases in an auditory sensory memory task with pure-tone sequences. Similarly to memory performance, alpha power decreased with longer delay phases. Critically, temporal expectations counteracted memory decay and led to more sustained performance as well as alpha power across different delay phases. These alpha-power effects were localized to frontal and parietal attention networks as well as primary auditory and visual sensory areas. This implies the involvement of different brain regions relevant for encoding and maintenance in auditory memory and questions a parsimonious functional inhibition explanation. A correlation of alpha power and behavioral performance underpinned the importance of alpha power for auditory working memory. Altogether, the results of the present thesis provide evidence for a beneficial effect of a-priori temporal expectations for an auditory signal on working memory. Moreover, alpha dynamics were shown to be a distinct marker for the neural efficiency of managing working memory limitations

Treatment of non-fluent aphasia through melody, rhythm and formulaic language

Left-hemisphere stroke patients often suffer a profound loss of spontaneous speech — known as non-fluent aphasia. Yet, many patients are still able to sing entire pieces of text fluently. This striking finding has inspired mainly two research questions. If the experimental design focuses on one point in time (cross section), one may ask whether or not singing facilitates speech production in aphasic patients. If the design focuses on changes over several points in time (longitudinal section), one may ask whether or not singing qualifies as a therapy to aid recovery from aphasia. The present work addresses both of these questions based on two separate experiments. A cross-sectional experiment investigated the relative effects of melody, rhythm, and lyric type on speech production in seventeen patients with non-fluent aphasia. The experiment controlled for vocal frequency variability, pitch accuracy, rhythmicity, syllable duration, phonetic complexity and other influences, such as learning effects and the acoustic setting. Contrary to earlier reports, the cross-sectional results suggest that singing may not benefit speech production in non-fluent aphasic patients over and above rhythmic speech. Previous divergent findings could very likely be due to affects from the acoustic setting, insufficient control for syllable duration, and language-specific stress patterns. However, the data reported here indicate that rhythmic pacing may be crucial, particularly for patients with lesions including the basal ganglia. Overall, basal ganglia lesions accounted for more than fifty percent of the variance related to rhythmicity. The findings suggest that benefits typically attributed to singing in the past may actually have their roots in rhythm. Moreover, the results demonstrate that lyric type may have a profound impact on speech production in non-fluent aphasic patients. Among the studied patients, lyric familiarity and formulaic language appeared to strongly mediate speech production, regardless of whether patients were singing or speaking rhythmically. Lyric familiarity and formulaic language may therefore help to explain effects that have, up until now, been presumed to result from singing. A longitudinal experiment investigated the relative long-term effects of melody and rhythm on the recovery of formulaic and non-formulaic speech. Fifteen patients with chronic non-fluent aphasia underwent either singing therapy, rhythmic therapy, or standard speech therapy. The experiment controlled for vocal frequency variability, phonatory quality, pitch accuracy, syllable duration, phonetic complexity and other influences, such as the acoustic setting and learning effects induced by the testing itself. The longitudinal results suggest that singing and rhythmic speech may be similarly effective in the treatment of non-fluent aphasia. Both singing and rhythmic therapy patients made good progress in the production of common, formulaic phrases — known to be supported by right corticostriatal brain areas. This progress occurred at an early stage of both therapies and was stable over time. Moreover, relatives of the patients reported that they were using a fixed number of formulaic phrases successfully in communicative contexts. Independent of whether patients had received singing or rhythmic therapy, they were able to easily switch between singing and rhythmic speech at any time. Conversely, patients receiving standard speech therapy made less progress in the production of formulaic phrases. They did, however, improve their production of unrehearsed, non-formulaic utterances, in contrast to singing and rhythmic therapy patients, who did not. In light of these results, it may be worth considering the combined use of standard speech therapy and the training of formulaic phrases, whether sung or rhythmically spoken. This combination may yield better results for speech recovery than either therapy alone. Overall, treatment and lyric type accounted for about ninety percent of the variance related to speech recovery in the data reported here. The present work delivers three main results. First, it may not be singing itself that aids speech production and speech recovery in non-fluent aphasic patients, but rhythm and lyric type. Second, the findings may challenge the view that singing causes a transfer of language function from the left to the right hemisphere. Moving beyond this left-right hemisphere dichotomy, the current results are consistent with the idea that rhythmic pacing may partly bypass corticostriatal damage. Third, the data support the claim that non-formulaic utterances and formulaic phrases rely on different neural mechanisms, suggesting a two-path model of speech recovery. Standard speech therapy focusing on non-formulaic, propositional utterances may engage, in particular, left perilesional brain regions, while training of formulaic phrases may open new ways of tapping into right-hemisphere language resources — even without singing

Computergestützte Befundung klinischer Elektroenzephalogramme

Das Ziel der Arbeit war es, die Befundung eines EEG aus der klinischen Praxis computergestützt durchzuführen. Die Befundung soll dabei weitgehend automatisiert ablaufen und dem Neurologen die wesentlichen Merkmale des EEG in kompakter und anschaulicher Form zur Verfügung stellen

The brain structure during language development: neural correlates of sentence comprehension in preschool children

Language skills increase as the brain matures and language specialization is linked to the left hemisphere. Among distinct language domains, sentence comprehension is particularly vital in language acquisition and, by comparison, requires a much longer time-span before full mastery in children. Although accumulating studies have revealed the neural mechanism underlying sentence comprehension acquisition, the development of the brain’s gray matter and its relation to sentence comprehension had not been fully understood. This thesis employs structural magnetic resonance imaging and diffusion-weighted imaging data to investigate the neural correlates of sentence comprehension in preschoolers both cross-sectionally and longitudinally. The first study examines how cortical thick- ness covariance is relevant for syntax in preschoolers and changes across development. Results suggest that the cortical thickness covariance of brain regions relevant for syntax increases from preschoolers to adults, whilst preschoolers with superior language abilities show a more adult-like covariance pattern. Reconstructing the white matter fiber tract connecting the left inferior frontal and superior temporal cortices using diffusion-weighted imaging data, the second study suggests that the reduced cortical thickness covariance in the left frontotemporal regions is likely due to immature white matter connectivity during preschool. The third study then investigated the cortical thickness asymmetry and its relation to sentence comprehension abilities. Results show that longitudinal cortical thick- ness asymmetry in the inferior frontal cortex was associated with improvements in sentence comprehension, further suggesting the crucial role of the inferior frontal cortex for sentence comprehension acquisition. Taken together, evidence from gray and white matter data provides new insights into the neuroscientific model of language acquisition and the emergence of syntactic processing during language development

Interactions between sentence comprehension and concurrent action: The role of movement effects and timing

Embodied approaches to language comprehension suggest that we understand sentences by using our perception and action systems for simulating their contents. In line with this assumption, the action-sentence compatibility effect (ACE) shows that sensibility judgments for sentences are faster when the direction of the described action matches the direction of the response movement. The aim of this thesis was to investigate whether this compatibility is effective between sentence direction and movement direction or between sentence direction and the direction of the movement effect. To this end, movements were dissociated from their effects in several experiments. Participants indicated whether sentences describing transfer actions toward or away from the body are sensible or not by producing a movement effect on a screen at a location near the body or far from the body. These movement effects were achieved by moving the hand from a middle button to a near or far button, i.e., toward the body or away from the body. In one condition, a movement effect resulted from pressing the button whose location corresponded with the location of the effect. Crucially for the above research question, there was another condition in which an action effect resulted from pressing the button at the opposite location. Since in the first series of experiments, the ACE turned out to be unreliable and in part seemed to be reversed, it was difficult to address the initial question. Therefore, a second series of experiments additionally investigated the role of timing between response preparation and sentence comprehension as a potential cause of the negative ACE. Results showed a positive ACE when the same directional feature was concurrently activated within the two processes, leading to priming between them. A negative ACE appeared when the directional feature was already bound into the sentence representation and thus was less accessible when needed for response preparation. In both cases, the ACE was related to the movement effect. These results suggest that the ACE occurs on the higher level of cognitive representations referring to distal information