After-effects of 10 Hz tACS over the prefrontal cortex on phonological word decisions

Introduction Previous work in the language domain has shown that 10 Hz rTMS of the left or right posterior inferior frontal gyrus (pIFG) in the prefrontal cortex impaired phonological decision-making, arguing for a causal contribution of the bilateral pIFG to phonological processing. However, the neurophysiological correlates of these effects are unclear. The present study addressed the question whether neural activity in the prefrontal cortex could be modulated by 10 Hz tACS and how this would affect phonological decisions. Methods In three sessions, 24 healthy participants received tACS at 10 Hz or 16.18 Hz (control frequency) or sham stimulation over the bilateral prefrontal cortex before task processing. Resting state EEG was recorded before and after tACS. We also recorded EEG during task processing. Results Relative to sham stimulation, 10 Hz tACS significantly facilitated phonological response speed. This effect was task-specific as tACS did not affect a simple control task. Moreover, 10 Hz tACS significantly increased theta power during phonological decisions. The individual increase in theta power was positively correlated with the behavioral facilitation after 10 Hz tACS. Conclusion Our results show a facilitation of phonological decisions after 10 Hz tACS over the bilateral prefrontal cortex. This might indicate that 10 Hz tACS increased task-related activity in the stimulated area to a level that was optimal for phonological performance. The significant correlation with the individual increase in theta power suggests that the behavioral facilitation might be related to increased theta power during language processing

Auditory cortical delta-entrainment interacts with oscillatory power in multiple fronto-parietal networks

The timing of slow auditory cortical activity aligns to the rhythmic fluctuations in speech. This entrainment is considered to be a marker of the prosodic and syllabic encoding of speech, and has been shown to correlate with intelligibility. Yet, whether and how auditory cortical entrainment is influenced by the activity in other speech–relevant areas remains unknown. Using source-localized MEG data, we quantified the dependency of auditory entrainment on the state of oscillatory activity in fronto-parietal regions. We found that delta band entrainment interacted with the oscillatory activity in three distinct networks. First, entrainment in the left anterior superior temporal gyrus (STG) was modulated by beta power in orbitofrontal areas, possibly reflecting predictive top-down modulations of auditory encoding. Second, entrainment in the left Heschl's Gyrus and anterior STG was dependent on alpha power in central areas, in line with the importance of motor structures for phonological analysis. And third, entrainment in the right posterior STG modulated theta power in parietal areas, consistent with the engagement of semantic memory. These results illustrate the topographical network interactions of auditory delta entrainment and reveal distinct cross-frequency mechanisms by which entrainment can interact with different cognitive processes underlying speech perception

Age differences in encoding-related alpha power reflect sentence comprehension difficulties

When sentence processing taxes verbal working memory, comprehension difficulties arise. This is specifically the case when processing resources decline with advancing adult age. Such decline likely affects the encoding of sentences into working memory, which constitutes the basis for successful comprehension. To assess age differences in encoding-related electrophysiological activity, we recorded the electroencephalogram from three age groups (24, 43, and 65 years). Using an auditory sentence comprehension task, age differences in encoding-related oscillatory power were examined with respect to the accuracy of the given response. That is, the difference in oscillatory power between correctly and incorrectly encoded sentences, yielding subsequent memory effects (SME), was compared across age groups. Across age groups, we observed an age-related SME inversion in the alpha band from a power decrease in younger adults to a power increase in older adults. We suggest that this SME inversion underlies age-related comprehension difficulties. With alpha being commonly linked to inhibitory processes, this shift may reflect a change in the cortical inhibition–disinhibition balance. A cortical disinhibition may imply enriched sentence encoding in younger adults. In contrast, resource limitations in older adults may necessitate an increase in cortical inhibition during sentence encoding to avoid an information overload. Overall, our findings tentatively suggest that age-related comprehension difficulties are associated with alterations to the electrophysiological dynamics subserving general higher cognitive functions

Sleep-Dependent Memory Consolidation and Incremental Sentence Comprehension : Computational Dependencies during Language Learning as Revealed by Neuronal Oscillations

We hypothesize a beneficial influence of sleep on the consolidation of the combinatorial mechanisms underlying incremental sentence comprehension. These predictions are grounded in recent work examining the effect of sleep on the consolidation of linguistic information, which demonstrate that sleep-dependent neurophysiological activity consolidates the meaning of novel words and simple grammatical rules. However, the sleep-dependent consolidation of sentence-level combinatorics has not been studied to date. Here, we propose that dissociable aspects of sleep neurophysiology consolidate two different types of combinatory mechanisms in human language: sequence-based (order-sensitive) and dependency-based (order-insensitive) combinatorics. The distinction between the two types of combinatorics is motivated both by cross-linguistic considerations and the neurobiological underpinnings of human language. Unifying this perspective with principles of sleep-dependent memory consolidation, we posit that a function of sleep is to optimize the consolidation of sequence-based knowledge (thewhen) and the establishment of semantic schemas of unordered items (thewhat) that underpin cross-linguistic variations in sentence comprehension. This hypothesis builds on the proposal that sleep is involved in the construction of predictive codes, a unified principle of brain function that supports incremental sentence comprehension. Finally, we discuss neurophysiological measures (EEG/MEG) that could be used to test these claims, such as the quantification of neuronal oscillations, which reflect basic mechanisms of information processing in the brain

Development of auditory repetition effects with age : evidence from EEG time-frequency analysis

La présentation répétée d’un son inconnu conduit à des effets de répétition comprenant la suppression (‘repetition suppression’ ou RS) ou l’augmentation (‘repetition enhancement’ ou RE) de l’activité neuronale. Ces phénomènes reflètent des mécanismes cérébraux impliquant un apprentissage perceptuel. L’objectif de ce mémoire de maitrise était d’apporter une perspective développementale de l’activité cérébrale sous-tendant l’apprentissage perceptuel auditif. L’EEG a été enregistré chez 101 participants sains âgés de 3 à 40 ans pendant un paradigme auditif passif durant lequel 30 pseudo-mots étaient répétés 6 fois chacun. Des analyses en temps- fréquence ont été calculées pour chaque répétition. La puissance spectrale enregistrées en EEG entre chaque répétition a été comparée au moyen de modèles linéaires mixtes. Les résultats montrent qu’un effet de répétition survient au cours du développement mais varie en fonction de l’âge et des bandes de fréquences. Du RS et RE ont été observés à tous les âges dans le thêta bas et le gamma respectivement. Un effet développemental a été trouvé de façon plus précoce pour le RS dans le thêta haut et de façon tardive pour le RE dans le thêta bas. Ces résultats montrent que les processus impliquant un apprentissage perceptif auditif, tel que le RS et le RE, suivent une trajectoire développementale spécifique en fonction des rythmes cérébraux. Les effets de répétition reflèteraient différents niveaux de traitement des stimuli qui se développeraient de manière indépendante. Des recherches supplémentaires seront nécessaires pour préciser le rôle fonctionnel des effets de répétitions sur le développement cognitif.The repeated presentation of unfamiliar sounds leads to repetition effects comprising repetition suppression (RS) and enhancement (RE) of neural activity. These phenomena reflect mechanisms involved in perceptual learning and are associated with a decrease or increase in EEG spectral powers. The objective of this Master’s thesis is to provide a developmental perspective of the cortical activity underlying auditory perceptual learning. EEG was recorded in 101 healthy participants ranging from 3 to 40 years during an auditory paradigm comprising 30 pseudowords repeated six times each. EEG time-frequency spectral power was calculated for each presentation and was compared to quantify repetition effects. Linear mixed model analysis revealed that some repetition effects occurred across ages and others varied with age in specific frequency bands. More precisely, RS and RE were found across ages in lower theta and gamma frequency bands respectively between the first and all subsequent pseudoword presentations. Developmental effects were seen in the RS observed in the higher theta/low alpha band and in the later occurring RE in the lower theta band. These results show that processes involved in auditory perceptual learning, such as RS and RE, are modulated by maturation. Further, repetition effects reflect different levels of stimulus processing and these levels seem to develop independently. More research is required to identify the exact functional roles of auditory repetitions effects on cognitive development

Auditory Conflict Resolution Correlates with Medial–Lateral Frontal Theta/Alpha Phase Synchrony

When multiple persons speak simultaneously, it may be difficult for the listener to direct attention to correct sound objects among conflicting ones. This could occur, for example, in an emergency situation in which one hears conflicting instructions and the loudest, instead of the wisest, voice prevails. Here, we used cortically-constrained oscillatory MEG/EEG estimates to examine how different brain regions, including caudal anterior cingulate (cACC) and dorsolateral prefrontal cortices (DLPFC), work together to resolve these kinds of auditory conflicts. During an auditory flanker interference task, subjects were presented with sound patterns consisting of three different voices, from three different directions (45° left, straight ahead, 45° right), sounding out either the letters “A” or “O”. They were asked to discriminate which sound was presented centrally and ignore the flanking distracters that were phonetically either congruent (50%) or incongruent (50%) with the target. Our cortical MEG/EEG oscillatory estimates demonstrated a direct relationship between performance and brain activity, showing that efficient conflict resolution, as measured with reduced conflict-induced RT lags, is predicted by theta/alpha phase coupling between cACC and right lateral frontal cortex regions intersecting the right frontal eye fields (FEF) and DLPFC, as well as by increased pre-stimulus gamma (60–110 Hz) power in the left inferior fontal cortex. Notably, cACC connectivity patterns that correlated with behavioral conflict-resolution measures were found during both the pre-stimulus and the pre-response periods. Our data provide evidence that, instead of being only transiently activated upon conflict detection, cACC is involved in sustained engagement of attentional resources required for effective sound object selection performance

The lexical nature of alpha-beta oscillations in context-driven word production

In context-driven word production, picture naming is faster following constrained than neutral sentential contexts (e.g., “The farmer milked the… [picture]” vs. “The child drew a… [picture]”, followed by the picture of a cow), suggesting conceptual-lexical pre-activation of the target response. Power decreases in the alpha-beta oscillatory band (8–25 Hz) are consistently found for constrained relative to neutral contexts prior to picture onset, when conceptual and lexical retrieval is ongoing. However, it remains a matter of debate whether the alpha-beta power decreases reflect (low-level) expectations of the visual input, conceptual and lexical retrieval, or motor preparation. The present study aimed at investigating the lexical-semantic nature of alpha-beta oscillations. Participants performed context-driven picture naming with constrained and neutral contexts. In addition, an auditory distractor word was presented before picture onset. Distractors were either semantically related (e.g., “goat”) or unrelated (e.g., “bean”) to the picture to be named. Picture naming was faster with constrained than neutral contexts. Distractor type did not affect naming latencies nor the behavioural context effect. In the oscillatory brain responses, the context-related alpha-beta power decreases were observed throughout the pre-picture interval when distractors were semantically unrelated to the picture, in line with previous findings. However, with semantically related distractors, the context effect was delayed until a period after distractor processing. Thus, alpha-beta power seems to be sensitive to the semantic relationship between the distractor word and the picture to be named. We interpret these results as suggesting that alpha-beta power decreases in context-driven word production reflect lexical-semantic retrieval mechanisms

Effects of acoustic periodicity and intelligibility on the neural oscillations in response to speech

Although several studies have investigated neural oscillations in response to acoustically degraded speech, it is still a matter of debate which neural frequencies reflect speech intelligibility. Part of the problem is that effects of acoustics and intelligibility have so far not been considered independently. In the current electroencephalography (EEG) study the amount of acoustic periodicity (i.e. the amount of time the stimulus sentences were voiced) was manipulated, while using the listeners’ spoken responses to control for differences in intelligibility. Firstly, the total EEG power changes in response to completely aperiodic (noise-vocoded) speech and speech with a natural mix of periodicity and aperiodicity were almost identical, while an increase in theta power (5–6.3 Hz) and a trend for less beta power (11–18 Hz) were observed in response to completely periodic speech. These two effects are taken to indicate an information processing conflict caused by the unnatural acoustic properties of the stimuli, and that the subjects may have internally rehearsed the sentences as a result of this. Secondly, we separately investigated effects of intelligibility by sorting the trials in the periodic condition according to the listeners’ spoken responses. The comparison of intelligible and largely unintelligible trials revealed that the total EEG power in the delta band (1.7–2.7 Hz) was markedly increased during the second half of the intelligible trials, which suggests that delta oscillations are an indicator of successful speech understanding

음성 의미 지각시의 고등 언어 성분 처리 디코딩

학위논문(석사) -- 서울대학교대학원 : 자연과학대학 협동과정 뇌과학전공, 2022. 8. 정천기.High-level linguistic processing in the human brain remains incompletely understood and constitutes a challenging topic in speech neuroscience. While most studies focused on decoding low-level phonetic components using intracranial recordings of the human brain during speech perception, few studies have attempted to decode high-level syntactic or semantic features. If any, most of the research targeting semantic decoding is conducted with picture naming tasks, which only deal with visual language rather than spoken language. The presenting study is focused on better characterizing the neural representations of processing spoken language perception, namely speech perception. Especially not on the lower-level language components such as phonemes or phonetics, but the higher-level components such as syntax and semantics. Since it is widely accepted that the tripartite nature of language processing consists of phonology, syntax, and semantics, a strategical method for analyzing speech perception tasks that can reject the intervention of phonetic factors was mandatory. Therefore, we conducted a question-and-answer speech task containing four questions revolving around two semantic categories (alive, body parts) with phonetically controlled words. Intracranial neural signals were recorded during the question-and-answer speech task using electrocorticography (ECoG) electrodes for 14 epilepsy patients. Post hoc brain activity analysis was conducted for three subjects who answered correctly to every trial (144 trials in total) to ensure the analyzed data contained only brain signals collected during the correct semantic processing. The decoding results suggest that absolute and relative spectral neural feature trends occur across all participants in particular time windows. Furthermore, the spatial aspect of the neural features that yield the best decoding accuracy verifies the current biophysiological brain language model explaining the circular nature of word meaning comprehension in the left hemisphere language network.인간의 고등 성분 언어 처리와 관련한 두뇌 활동을 해독하는 연구는 신경언어학 분야에서도 아직 깊이 연구되지 않은 분야 중 하나이다. 침습적 전극을 통해 얻은 뇌피질 뇌파를 이용한 대부분의 언어 디코딩 연구는 음소나 음절 수준의 하위 언어 성분에서 진행되어 왔고, 통사나 의미와 같은 고등 언어 성분에 대한 디코딩 연구는 드물다. 드물게 진행된 고등 언어 성분 디코딩 연구 또한 대다수가 시각적 언어 처리를 연구한 결과들이며, 소리 언어 디코딩 연구는 태동 단계에 머무르고 있다. 본 연구는 소리 언어 지각시의 두뇌 활성을 분석하여 그 처리 과정의 뇌파 신호 특성을 규명하고자 한다. 특히 인간 음성 언어의 하위 구성 성분보다는 통사와 의미 위주의 고등 구성 성분을 처리하는 데에 관여하는 뇌파의 시간적, 주파수적, 공간적 특성에 집중하여 분석을 진행하였다. 언어 처리의 주된 세 가지 요소는 ‘음소 (phonetics)’, ‘통사 (syntactics)’, ‘의미 (semantics)’라는 점을 고려하여, 음소 소준의 뇌파 활동을 통제할 수 있는 실험 패러다임을 구상하였으며, 구체적으로는 두개의 다른 의미 범주 (생명, 신체)에 대해서 묻는 음소적으로 동등한 단어가 포함된 질문을 들려준 후 의미를 파악해 대답하는 과정의 뇌파를 기록하는 실험을 진행하였다. 뇌파 신호는 경막하 전극 삽입술 (Electrocorticography, ECoG)을 통해 14명의 뇌전증 환자로부터 침습적 방식으로 측정되었다. 뇌파 디코딩 분석에는 피험자의 두뇌가 옳은 방식으로 처리한 고등 언어 성분이 반영된 실험만을 포함하기 위해서 모든 실험에서 옳은 대답을 한 세 명의 환자만을 대상으로 하여 분석을 진행하였다. 디코딩 분석 결과 세 명의 환자에 걸쳐 핵심 단어 (‘것은’, ‘무엇입니까’) 음성 지각 이후 특정 시간대에서 특정 주파수대의 뇌파가 양 극단의 의미를 높은 수준의 정확도(%)로 분류하는 데에 사용될 수 있다는 것을 밝혔다. 또한 이러한 높은 정확도를 기록한 뇌파의 특성에는 모든 환자에 걸쳐 절대적 혹은 상대적 트렌드가 관찰되며, 관찰되는 뇌파의 공간적 특성은 현재 통용되는 신경언어학적 언어 처리 모델이 설명하는 음성 언어 처리 방식과 일맥상통함을 밝혔다.Abstract ⅰ 1. Introduction 1 2. Materials and Methods 4 3. Results 8 4. Discussion 12 References 15 List of Figures 20 Supplementary information 28 Abstract in Korean 36석