On the effectiveness of event-related beta tACS on episodic memory formation and motor cortex excitability

Background: Transcranial alternating current stimulation (tACS) is widely used to entrain or modulate brain oscillations in order to investigate causal relationships between oscillations and cognition. Objective: In a series of experiments we here addressed the question of whether event-related, transient tACS in the beta frequency range can be used to entrain beta oscillations in two different domains: episodic memory formation and motor cortex excitability. Methods: In experiments 1 and 2, 72 healthy human participants engaged in an incidental encoding task of verbal and non-verbal material while receiving tACS to the left and right inferior frontal gyrus (IFG) at 6.8 Hz, 10.7 Hz, 18.5 Hz, 30 Hz, 48 Hz and sham stimulation for 2s during stimulus presentation. In experiment 3, tACS was administered for 10s to M1 at the individual motor beta frequency of eight subjects. We investigated the relationship between the size of TMS induced MEPs and tACS phase. Results: Beta tACS did not affect memory performance compared to sham stimulation in experiments 1 and 2. Likewise, in experiment 3, MEP size was not modulated by the tACS phase. Conclusions: Our findings suggest that event-related, transient tACS in the beta frequency range cannot be used to modulate the formation of episodic memories or motor cortex excitability. These null-results question the effectiveness of event-related tACS to entrain beta oscillations and modulate cognition

Strategic and Non-Strategic Semantic Expectations Hierarchically Modulate Neural Processing

Perception is facilitated by a hierarchy of expectations generated from context and prior knowledge. In auditory processing, violations of local (within-trial) expectations elicit a mismatch negativity (MMN), while violations of global (across-trial) expectations elicit a later positive component (P300). This result is taken as evidence of prediction errors ascending through the expectation hierarchy. However, in language comprehension, there is no evidence that violations of semantic expectations across local-global levels similarly elicit a sequence of hierarchical error signals, thus drawing into question the putative link between event-related potentials (ERPs) and prediction errors. We investigated the neural basis of such hierarchical expectations of semantics in a word-pair priming paradigm. By manipulating the overall proportion of related or unrelated word-pairs across the task, we created two global contexts that differentially encouraged strategic use of primes. Across two experiments, we replicated behavioral evidence of greater priming in the high validity context, reflecting strategic expectations of upcoming targets based on “global” context. In our preregistered EEG analyses, we observed a “local” prediction error ERP effect (i.e., semantic priming) ∼250 ms post-target, which, in exploratory analyses, was followed 100 ms later by a signal that interacted with the global context. However, the later effect behaved in an apredictive manner, i.e., was most extreme for fulfilled expectations, rather than violations. Our results are consistent with interpretations of early ERPs as reflections of prediction error and later ERPs as processes related to conscious access and in support of task demands

Event-related potentials reflect prediction errors and pop-out during comprehension of degraded speech

Comprehension of degraded speech requires higher-order expectations informed by prior knowledge. Accurate top-down expectations of incoming degraded speech cause a subjective semantic ‘pop-out’ or conscious breakthrough experience. Indeed, the same stimulus can be perceived as meaningless when no expectations are made in advance. We investigated the event-related potential (ERP) correlates of these top-down expectations, their error signals and the subjective pop-out experience in healthy participants. We manipulated expectations in a word-pair priming degraded (noise-vocoded) speech task and investigated the role of top-down expectation with a between-groups attention manipulation. Consistent with the role of expectations in comprehension, repetition priming significantly enhanced perceptual intelligibility of the noisevocoded degraded targets for attentive participants. An early ERP was larger for mismatched (i.e. unexpected) targets than matched targets, indicative of an initial error signal not reliant on top-down expectations. Subsequently, a P3a-like ERP was larger to matched targets than mismatched targets only for attending participants—i.e. a pop-out effect—while a later ERP was larger for mismatched targets and did not significantly interact with attention. Rather than relying on complex post hoc interactions between prediction error and precision to explain this apredictive pattern, we consider our data to be consistent with prediction error minimization accounts for early stages of processing followed by Global Neuronal Workspace-like breakthrough and processing in service of task goals

From alpha to perception : investigating behavior and brain activity

Il a été proposé que la perception visuelle soit organisée de façon discrète, reposant sur l'activité cérébrale oscillatoire d'environ 10Hz. Bien qu'en condition normale notre perception visuelle nous paraisse continue, nous montrons dans une première expérience que cette continuité peut être interrompue lors de la perception d'un stimulus spécifique pourtant statique, révélant les cycles d'oscillations alpha (~10Hz) à la base de la perception visuelle. La 'Flickering Wheel Illusion' ('L'illusion de la roue qui clignote') génère un clignotement illusoire régulier, réapparaissant toutes les 100ms ce qui correspond à une phase du cycle alpha. Des études précédentes ont montré que certaines phases des oscillations alpha occipitales (tout comme des oscillations theta fronto-centrales) portent des propriétés 'inhibitrices' en vue du traitement visuel tandis que leurs phases opposées tendent à augmenter la probabilité de détecter des stimuli visuels. Dans une deuxième étude, nous avons analysé comment ce rapport phase-détection temporel est représenté au niveau spatial dans le cortex visuel. Pour cela, nous avons entraîné des oscillations alpha via le clignotement d'un disque à une certaine position spatiale du champ visuel, pendant que les participants avaient pour tâche de détecter des stimuli visuels de basse luminance pouvant apparaître à de multiples positions spatiales. Les oscillations entraînées modulaient la performance de détection à toutes les positions auxquelles des cibles pourraient apparaître montrant des maxima à différentes phases de l'oscillation. De façon intéressante, nous avons pu montrer que la phase des oscillations entraînées se propageait dans l'espace comme une onde progressive, une propriété qui pourrait impliquer des rôles fonctionnels spécifiques pour le traitement sensoriel. En plus de son influence sur les processus cognitives comme la perception visuelle, la phase des oscillations lentes peut aussi moduler l'amplitude d'une oscillation rapide ('phase-amplitude cross-frequency coupling' ; 'couplage phase-amplitude inter-fréquentiel' ;'PAC'). Dans une troisième expérience, nous avons révélé l'existence d'un PAC entre des oscillations alpha et gamma pendant l'état de repos (en l'absence de tâche cognitive spécifique) ce que nous avons assigné à de 'l'inhibition pulsative', un mécanisme de contrôle du traitement sensoriel porté par le rythme alpha. Selon cette théorie, les oscillations alpha mettraient donc à disposition des moments ou bien des phases spécifiques qui mènent à une augmentation d'amplitude d'oscillations gamma entrainant une augmentation du traitement sensoriel, tandis que des phases opposées inhiberaient le traitement sensoriel. En outre, nous avons observé un PAC robuste entre des oscillations theta et beta pendant l'état de repos ainsi que pendant une tâche de détection. Ce PAC pourrait aider à maintenir un état d'activité cérébrale de base. Finalement, nous avons pu révéler un PAC entre des oscillations theta et gamma principalement dans des régions fronto-centrales, influençant la perception visuelle avec emploi d'attention visuelle. Ce résultat est en continuité et complète des résultats précédents de notre groupe qui montraient l'influence de la phase des oscillations theta sur la perception visuelle. Le travail présenté dans cette thèse contribue aux connaissances du rôle important des oscillations spontanées pour la perception visuelle en utilisant plusieurs approches expérimentales et révèlent de nouvelles questions de ce champ de recherche.It has been suggested that visual perception is organized in discrete snapshots relying on an oscillatory brain rhythm of about 10Hz. Even though, in normal conditions, our visual percept seems continuous to us, we show that a specific static stimulus pattern can lead to disruption of this continuity and uncover cycles of alpha oscillations (~10Hz) underlying visual perception. This 'Flickering Wheel Illusion' produces a regular illusory flicker recurring every ~100ms thus at one phase of the alpha cycle. According to previous work, specific phases of occipital alpha (and also fronto-central theta) oscillations are designated as 'inhibitory' for visual processing whereas opposite phases increase probability to detect visual stimuli. In a psychophysical experiment, we analyzed how this temporal phase-detection relationship is organized spatially in the visual cortex. We therefore entrained alpha oscillations with a specific spatial origin and probed visual detection of low-threshold targets at different spatial positions. Detection performance was modulated by the entrained oscillation at all target locations, showing maxima at different phases of the oscillation. We could show that the phase of the entrained alpha oscillations propagated over space like a traveling wave that could implicate important functional roles for sensory processing. Besides its influence on cognitive processes like visual perception, the phase of low frequency oscillations can also modulate the amplitude of fast oscillations (phase-amplitude cross-frequency coupling; PAC). In another experiment, we could reveal PAC between alpha and gamma oscillations during the resting state, what we referred to as pulsed inhibition, illustrating the alpha rhythm as a control mechanism of sensory processing. Alpha oscillations would thus provide specific moments, or phases that enhance gamma amplitude and thus sensory processing, whereas opposite phases would inhibit sensory processing. Moreover, we found a robust PAC between theta and beta oscillations which was also present during a visual detection task. This PAC could help maintaining a specific activity state of the brain. Finally, we could reveal PAC between theta and gamma oscillations in mainly fronto-central regions, influencing visual perception in the detection task but only within the focus of attention. This result is consistent with and complements previous findings of our group showing the influence of the phase of theta oscillations on visual perception. This work contributed to the findings of the important role of ongoing oscillations in visual perception, using multiple experimental approaches; yet our promising results uncovered new questions in this large field of research

The Flickering Wheel Illusion: When α Rhythms Make a Static Wheel Flicker.

International audienceα oscillations (8-14 Hz) greatly influence brain activity, yet we generally do not experience them consciously: the world does not appear to oscillate. Dedicated strategies must exist in the brain to prevent these oscillations from disrupting normal processing. Could suitable stimuli fool these strategies and lead to the conscious experience of our own brain oscillations? We describe and explore a novel illusion in which the center of a static wheel stimulus (with 30-40 spokes) is experienced as flickering when viewed in the visual periphery. The key feature of this illusion is that the stimulus fluctuations are experienced as a regular and consistent flicker, which our human observers estimated at ∼9 Hz during a psychophysical matching task. Correspondingly, the occipital α rhythm of the EEG was the only oscillation that showed a time course compatible with the reported illusion: when α amplitude was strong, the probability of reporting illusory flicker increased. The peak oscillatory frequency for these flicker-induced modulations was significantly correlated, on a subject-by-subject basis, with the individual α frequency measured during rest, in the absence of visual stimulation. Finally, although the effect is strongest during eye movements, we showed that stimulus motion relative to the retina is not necessary to perceive the illusion: the flicker can also be perceived on the afterimage of the wheel, yet by definition this afterimage is stationary on the retina. We conclude that this new flickering illusion is a unique way to experience the α rhythms that constantly occur in the brain but normally remain unnoticed

Global and local oscillatory entrainment of visual behavior across retinotopic space

Ongoing brain oscillations (7–10 Hz) modulate visual perception; in particular, their precise phase can predict target perception. Here, we employ this phase-dependence of perception in a psychophysical experiment to track spatial properties of entrained oscillations of visual perception across the visual field. Is this entrainment local, or a more global phenomenon? If the latter, does oscillatory phase synchronize over space, or vary with increasing distance from the oscillatory source? We presented a disc stimulus in the upper left quadrant, oscillating in luminance at different frequencies (individual alpha frequency (IAF), 5 Hz, and 15 Hz) to entrain an oscillation with specific frequency and spatial origin. Observers fixated centrally, while flash stimuli at perceptual threshold appeared at different positions and times with respect to the oscillating stimulus. IAF and 5 Hz luminance oscillations modulated detection performance at all tested positions, whereas at 15 Hz, the effect was weaker and less consistent. Furthermore, for IAF and 5 Hz entrainment, preferred phases for target detection differed significantly between spatial locations, suggesting “local” entrainment of detection performance next to the oscillatory source, whereas more distant target locations shared a “global” effect with a significantly different phase. This unexpected global component of entrainment is tentatively attributed to widespread connectivity from thalamic nuclei such as the pulvinar