Mathilde Bonnefond

Alpha Oscillations Serve to Protect Working Memory Maintenance against Anticipated Distracters

When operating in a complex world, it is essential to have mechanisms that can suppress distracting information. Such mechanisms might be related to neuronal oscillations, which are known to be involved in gating of incoming information. We here apply a working memory (WM) task to investigate how neuronal oscillations are involved in the suppression of distracting information that can be predicted in time. We used a modified Sternberg WM task in which distracters were presented in the retention interval, while we recorded the ongoing brain activity using magnetoencephalography. The data revealed a robust adjustment of the phase of alpha oscillations in anticipation of the distracter. In trials with strong phase adjustment, response times to the memory probe were reduced. Further, the power of alpha oscillations increased prior to the distracter and predicted performance. Our findings demonstrate that the doors of perception close when a distracter is expected. The phase adjustment of the alpha rhythm adds to the computational versatility of brain oscillations, because such a mechanism allows for modulating neuronal processing on a fine temporal scale.

Layer-Specific Entrainment of Gamma-Band Neural Activity by the Alpha Rhythm in Monkey Visual Cortex

Although the mammalian neocortex has a clear laminar organization, layer-specific neuronal computations remain to be uncovered. Several studies suggest that gamma band activity in primary visual cortex (V1) is produced in granular and superficial layers and is associated with the processing of visual input. Oscillatory alpha band activity in deeper layers has been proposed to modulate neuronal excitability associated with changes in arousal and cognitive factors. To investigate the layer-specific interplay between these two phenomena, we characterized the coupling between alpha and gamma band activity of the local field potential in V1 of the awake macaque. Using multicontact laminar electrodes to measure spontaneous signals simultaneously from all layers of V1, we found a robust coupling between alpha phase in the deeper layers and gamma amplitude in granular and superficial layers. Moreover, the power in the two frequency bands was anticorrelated. Taken together, these findings demonstrate robust interlaminar cross-frequency coupling in the visual cortex, supporting the view that neuronal activity in the alpha frequency range phasically modulates processing in the cortical microcircuit in a top-down manner.

Hierarchical nesting of slow oscillations, spindles and ripples in the human hippocampus during sleep

During systems-level consolidation, mnemonic representations initially reliant on the hippocampus are thought to migrate to neocortical sites for more permanent storage, with an eminent role of sleep for facilitating this information transfer. Mechanistically, consolidation processes have been hypothesized to rely on systematic interactions between the three cardinal neuronal oscillations characterizing non–rapid eye movement (NREM) sleep. Under global control of de- and hyperpolarizing slow oscillations (SOs), sleep spindles may cluster hippocampal ripples for a precisely timed transfer of local information to the neocortex. We used direct intracranial electroencephalogram recordings from human epilepsy patients during natural sleep to test the assumption that SOs, spindles and ripples are functionally coupled in the hippocampus. Employing cross-frequency phase-amplitude coupling analyses, we found that spindles were modulated by the up-state of SOs. Notably, spindles were found to in turn cluster ripples in their troughs, providing fine-tuned temporal frames for the hypothesized transfer of hippocampal memory traces.

Gamma activity coupled to alpha phase as a mechanism for top-down controlled gating

Coupling between neural oscillations in different frequency bands has been proposed to coordinate neural processing. In particular, gamma power coupled to alpha phase is proposed to reflect gating of information in the visual system but the existence of such a mechanism remains untested. Here, we recorded ongoing brain activity using magnetoencephalography in subjects who performed a modified Sternberg working memory task in which distractors were presented in the retention interval. During the anticipatory pre-distractor period, we show that the phase of alpha oscillations was coupled with the power of high (80-120Hz) gamma band activity, i.e. gamma power consistently was lower at the trough than at the peak of the alpha cycle (9-12Hz). We further show that high alpha power was associated with weaker gamma power at the trough of the alpha cycle. This result is in line with alpha activity in sensory region implementing a mechanism of pulsed inhibition silencing neuronal firing every ~100 ms.

Distinct Patterns of Brain Activity Characterise Lexical Activation and Competition in Spoken Word Production

According to a prominent theory of language production, concepts activate multiple associated words in memory, which enter into competition for selection. However, only a few electrophysiological studies have identified brain responses reflecting competition. Here, we report a magnetoencephalography study in which the activation of competing words was manipulated by presenting pictures (e.g., dog) with distractor words. The distractor and picture name were semantically related (cat), unrelated (pin), or identical (dog). Related distractors are stronger competitors to the picture name because they receive additional activation from the picture relative to other distractors. Picture naming times were longer with related than unrelated and identical distractors. Phase-locked and non-phase-locked activity were distinct but temporally related. Phase-locked activity in left temporal cortex, peaking at 400 ms, was larger on unrelated than related and identical trials, suggesting differential activation of alternative words by the picture-word stimuli. Non-phase-locked activity between roughly 350–650 ms (4–10 Hz) in left superior frontal gyrus was larger on related than unrelated and identical trials, suggesting differential resolution of the competition among the alternatives, as reflected in the naming times. These findings characterise distinct patterns of activity associated with lexical activation and competition, supporting the theory that words are selected by competition.

What's behind an Inference? An EEG Study with Conditional Arguments.

Conditional reasoning studies typically involve presenting a major conditional premise (If P then Q), a minor premise (P) and a conclusion (Q). We describe how most fMRI studies investigate reasoning and point out that these studies neglect to take into consideration the temporal sequence of cognitive steps generated by the interaction of the premises. The present study uses EEG to address this issue and compares the processing of the minor premise P when it is presented before vs. after the conditional statement (P; If P then Q vs. If P then Q; P). When the minor premise comes after the conditional statement and matches the antecedent its processing results in a P3b component, known to reflect the satisfaction of expectations, and in two later components, a PSW component and a CNV component. These two components are discussed in light of a conclusion generation phase and a maintenance phase. We also investigated the effect of violating expectations through the presentation of a minor premise that mismatches the antecedent of the conditional statement (If P then Q; R). The data indicate that the processing of such a premise yields an N2 component which is known to reflect perceptual conflict.

Oscillatory mechanisms of feedforward and feedback visual processing

Two recent monkey studies demonstrate that feedforward processing in the visual system is reflected by activity in the 40-90Hz gamma band, whereas feedback is reflected by activity in the 5-18Hz alpha and beta band. These findings can be applied to interpret human electrophysiological activity in complex visual tasks.

The role of gamma and alpha oscillations for blocking out distraction.

Although alpha activity (10 Hz) is by far the strongest signal produced by the human brain, it has for decades been considered to reflect rest or idling. However, recent studies have clearly demonstrated that alpha activity plays a pivotal role for cognitive processing. Gamma oscillations (> 30 Hz) and their role for cognition have also been the subject of intensive research. While gamma activity is thought to reflect functional processing, alpha oscillations are now thought to reflect functional inhibition in order to suppress the processing of distracting information. In our recent magnetoencephalography study we found that both power and phase of posterior alpha oscillations are top-down modulated in order to prevent the incorporation of predictable distracters in working memory. We further discuss these results here. We additionally show that the processing of the distracters is clearly distinguishable from the processing of the items to be remembered. The former induced a weaker gamma power and evoked a higher alpha activity. The higher the evoked alpha activity, the better the efficiency of distracter suppression which also depends on the pre-distracter alpha power and phase adjustment. Altogether, these results emphasize the protecting role of alpha activity and its remarkable flexibility. This ability to inhibit distracter information is crucial in our complex environment, as illustrated by the difficulties encountered by patients suffering from attentional disorders.

Communication between brain areas based on nested oscillations

Abstract Unraveling how brain regions communicate is crucial for understanding how the brain processes external and internal information. Neuronal oscillations within and across brain regions have been proposed to play a crucial role in this process. Two main hypotheses have been suggested for routing of information based on oscillations, namely communication through coherence and gating by inhibition. Here, we propose a framework unifying these two hypotheses that is based on recent empirical findings. We discuss a theory in which communication between two regions is established by phase synchronization of oscillations at lower frequencies (<25 Hz), which serve as temporal reference frame for information carried by high-frequency activity (>40 Hz). Our framework, consistent with numerous recent empirical findings, posits that cross-frequency interactions are essential for understanding how large-scale cognitive and perceptual networks operate.

Prefrontal alpha- and beta-band oscillations are involved in rule selection

A recent study in monkeys reports that oscillatory neuronal synchronization between ensembles of prefrontal neurons is involved in rule selection. The study demonstrates that beta-band synchronization (19-40 Hz) reflects the selection of a rule, whereas alpha-band synchronization (6-16 Hz) reflects the active inhibition of a not-to-be-applied rule.