Modulating brain oscillations to drive brain function

Do neuronal oscillations play a causal role in brain function? In a study in this issue of PLOS Biology, Helfrich and colleagues address this long-standing question by attempting to drive brain oscillations using transcranial electrical current stimulation. Remarkably, they were able to manipulate visual perception by forcing brain oscillations of the left and right visual hemispheres into synchrony using oscillatory currents over both hemispheres. Under this condition, human observers more often perceived an inherently ambiguous visual stimulus in one of its perceptual instantiations. These findings shed light on the mechanisms underlying neuronal computation. They show that it is the neuronal oscillations that drive the visual experience, not the experience driving the oscillations. And they indicate that synchronized oscillatory activity groups brain areas into functional networks. This points to new ways for controlled experimental and possibly also clinical interventions for the study and modulation of brain oscillations and associated functions

Alpha power increase after transcranial alternating current stimulation at alpha frequency (α-tacs) reflects plastic changes rather than entrainment

Background: Periodic stimulation of occipital areas using transcranial alternating current stimulation (tACS) at alpha (α) frequency (8–12 Hz) enhances electroencephalographic (EEG) α-oscillation long after tACS-offset. Two mechanisms have been suggested to underlie these changes in oscillatory EEG activity: tACS-induced entrainment of brain oscillations and/or tACS-induced changes in oscillatory circuits by spike-timing dependent plasticity.<p></p> Objective: We tested to what extent plasticity can account for tACS-aftereffects when controlling for entrainment “echoes.” To this end, we used a novel, intermittent tACS protocol and investigated the strength of the aftereffect as a function of phase continuity between successive tACS episodes, as well as the match between stimulation frequency and endogenous α-frequency.<p></p> Methods: 12 healthy participants were stimulated at around individual α-frequency for 15–20 min in four sessions using intermittent tACS or sham. Successive tACS events were either phase-continuous or phase-discontinuous, and either 3 or 8 s long. EEG α-phase and power changes were compared after and between episodes of α-tACS across conditions and against sham.<p></p> Results: α-aftereffects were successfully replicated after intermittent stimulation using 8-s but not 3-s trains. These aftereffects did not reveal any of the characteristics of entrainment echoes in that they were independent of tACS phase-continuity and showed neither prolonged phase alignment nor frequency synchronization to the exact stimulation frequency.<p></p> Conclusion: Our results indicate that plasticity mechanisms are sufficient to explain α-aftereffects in response to α-tACS, and inform models of tACS-induced plasticity in oscillatory circuits. Modifying brain oscillations with tACS holds promise for clinical applications in disorders involving abnormal neural synchrony

PERTURBING THE NEURONAL NETWORK DYNAMICS IN THE HIGHER-ORDER THALAMO-CORTICAL NETWORK

Understanding the brain’s dynamics and its role in cognitive functions will open up new avenues for the treatment of psychiatric illnesses. Oscillations reveal rhythmic features in the dynamics that are associated with different mental states and cognitive processes. One cognitive function of transdiagnostic importance is sustained attention. Sustained attention is the ability to focus our cognitive resources to process unpredictable information for an extended period of time. Temporary loss of sustained attention could lead to accidents while driving, and long-term sustained attention deficit is linked to many psychiatric disorders. Sustained visual attention requires an active engagement of higher-order visual circuitry to maintain a mental state for processing unpredictable visual inputs. The coordinated activity of groups of neurons in such circuitry gives rise to cortico-thalamo-cortical oscillations, which are crucial for mediating top-down control signals and bottom-up information transduction. Through its widespread interconnection with the cortex, the higher-order visual thalamus is ideally positioned to serve such a role by coordinating cortico-cortical synchrony that supports attentional engagement. However, the causal mechanisms underlying the oscillatory synchronization in the posterior visual thalamo-cortical network as well as their function during sustained attention remains unknown. Thus, basic science and translational work with causal perturbation is needed to shine a light on the role of these oscillatory dynamics in neuronal processing and behavior. The studies in this dissertation attempted to address this question by targeting the higher-order thalamo-cortical visual system and combining multiple causal perturbation tools (eg. optogenetics and transcranial brain stimulation), simultaneous multisite electrophysiological recordings, and a sustained attention task. Through a logical process of target identification, engagement, and validation, our work contributes to understanding: 1. how does the higher-order visual thalamus causally coordinate thalamo-cortical communications to enhance attentional behavior, and 2. how does transcranial alternating current stimulation affect single-neuron activity as well as long-range circuit dynamics in this network? Taken together, this work provides causal evidence underlying the mechanism of brain stimulations and its effect on behavior, which ultimately could inform the broader scientific community on how to rationally refine targeted circuit-based therapeutics for treating cognitive deficits.Doctor of Philosoph

Lasting EEG/MEG aftereffects on human brain oscillations after rhythmic transcranial brain stimulation: Level of control over oscillatory network activity

A number of rhythmic protocols have emerged for non-invasive brain stimulation (NIBS) in humans, including transcranial alternating current stimulation (tACS), oscillatory transcranial direct current stimulation (otDCS) and repetitive (also called rhythmic) transcranial magnetic stimulation (rTMS). With these techniques, it is possible to match the frequency of the externally applied electromagnetic fields to the intrinsic frequency of oscillatory neural population activity ("frequency-tuning"). Mounting evidence suggests that by this means tACS, otDCS, and rTMS can entrain brain oscillations and promote associated functions in a frequency-specific manner, in particular during (i.e. online to) stimulation. Here, we focus instead on the changes in oscillatory brain activity that persist after the end of stimulation. Understanding such aftereffects in healthy participants is an important step for developing these techniques into potentially useful clinical tools for the treatment of specific patient groups. Reviewing the electrophysiological evidence in healthy participants, we find aftereffects on brain oscillations to be a common outcome following tACS/otDCS and rTMS. However, we did not find a consistent, predictable pattern of aftereffects across studies, which is in contrast to the relative homogeneity of reported online effects. This indicates that aftereffects are partially dissociated from online, frequency-specific (entrainment) effects during tACS/otDCS and rTMS. We outline possible accounts and future directions for a better understanding of the link between online entrainment and offline aftereffects, which will be key for developing more targeted interventions into oscillatory brain activity

Manipulating neuronal communication by using low-intensity repetitive transcranial magnetic stimulation combined with electroencephalogram

Repetitive transcranial magnetic stimulation (rTMS) modulates ongoing brain rhythms by activating neuronal structures and evolving different neuronal mechanisms. In the current work, the role of stimulation strength and frequency for brain rhythms was studied. We hypothesized that a weak oscillating electric field induced by low-intensity rTMS could induce entrainment effects in the brain. To test the hypothesis, we conducted three separate experiments, in which we stimulated healthy human participants with rTMS. We individualized stimulation parameters using computational modeling of induced electric fields in the targets and individual frequency estimated by electroencephalography (EEG). We demonstrated the immediately induced entrainment of occipito-parietal and sensorimotor mu-alpha rhythm by low-intensity rTMS that resulted in phase and amplitude changes measured by EEG. Additionally, we found long-lasting corticospinal excitability changes in the motor cortex measured by motor evoked potentials from the corresponding musle.2021-11-2