Miles Wischnewski

A meta-analytic study of exogenous oscillatory electric potentials in neuroenhancement

The assumption that transcranial alternating current stimulation (tACS) enhances perceptual and cognitive ability in healthy volunteers by exposing the brain to exogenous oscillatory electric fields is increasingly finding its way into society and commercial parties. The aim of the present study is to quantify the effects of exogenous oscillatory electric field potentials on neuroenhancement in healthy volunteers. The meta-analysis included fifty-one sham controlled experiments that investigated the effects of tACS on perception and cognitive performance. Results from random effects modelling of the cumulative effect size showed small, but robust perceptual and cognitive enhancement in healthy participants to weak exogenous oscillatory electric field potentials. Analyses of tACS parameters indicate that simultaneous stimulation of the anterior and posterior locations of the scalp at >1mA intensity currently has the highest probability of increasing performance. However, technical and methodological issues currently limit the applicability of tACS in neuroenhancement. Additional research is needed to further evaluate the potential of tACS in perception and cognitive ability, and to establish the contexts and parameters under which tACS is effective.

Efficacy and time course of paired associative stimulation in cortical plasticity: Implications for neuropsychiatry

OBJECTIVE Paired associative stimulation (PAS) has been used to study normal and abnormal cortical plasticity. However, a normative review of PAS effects has not been provided so far. To this end, the magnitude and time course of PAS protocols was systematically evaluated here. METHODS A literature search in PubMed using the search term paired associative stimulation was conducted. Main inclusion criteria were that experiments were conducted in primary motor cortex of healthy volunteers without motor training before intervention and motor evoked potentials as primary outcome measure. This search yielded in total 104 experiments, which were analyzed to examine the potentiating (PASLTP) and depressing effects of PAS (PASLTD) on cortical excitability levels in healthy volunteers. RESULTS PASLTP induces reliable and stable potentiating effects (maximum ± standard error 38.5 ± 3.3%) on cortical excitability levels up to 90 min. PASLTP was most effective when applied at frequencies of 0.05 and 0.2 Hz. Analyses of the PASLTD studies demonstrated reliable and stable depression of cortical excitability levels up to 120 min (maximum ± standard error -23.0 ± 1.9%) CONCLUSIONS PAS significantly modulates cortical excitability. The potentiating effects of PASLTP are stronger than the depressing effects for PASLTD. SIGNIFICANCE Present findings offer normative insights into the magnitude and time course of PASLTP and PASLTD-induced changes in cortical excitability levels.

Effects of Theta Transcranial Alternating Current Stimulation Over the Frontal Cortex on Reversal Learning

BACKGROUND Theta oscillations in the electroencephalogram (EEG) are associated with learning and behavioral adaptation. OBJECTIVE To investigate the effects of theta transcranial alternating current stimulation (tACS) applied to the frontal cortex on reversal learning. METHODS Healthy volunteers participated in a sham-controlled between subjects design. TACS at 1 mA peak-to-peak was administered during a reward-punishment reversal learning task. Resting state EEG was measured before and after tACS and the task. RESULTS Active tACS improved learning ability, but at the same time interfered with applying the rule to optimize behavior. Furthermore, a significant decrease in frontal theta-beta EEG ratios was observed following active tACS. CONCLUSIONS Results provide behavioral and electrophysiological evidence for influencing reversal learning with exogenous oscillatory electric field potentials applied to the frontal cortex.

After-effects of transcranial alternating current stimulation on evoked delta and theta power

OBJECTIVE Phase synchronization is suggested to be among the mechanisms that can explain the effects of transcranial alternating current stimulation (tACS). However, little is known about the effects of tACS on event-related oscillatory activity. Therefore the objective was to investigate frequency-related effects of frontal tACS on event-related oscillatory power. METHODS In a double blind randomized controlled cross-over design, twenty-four participants received 12min of delta (2.5Hz), theta tACS (5Hz) and sham tACS at an intensity of 1mA peak-to-peak. Event-related delta- and theta-related oscillatory activity was recorded to reward- and punishment-related feedback signals. RESULTS Delta tACS decreased feedback-related oscillatory power in the 1.5 and 3.5Hz frequency range. This effect was driven by power changes below the tACS frequency stimulation. CONCLUSION Exogenous field potentials can attenuate event-related oscillatory activity in a rhythm slightly below the stimulation frequency. Our findings suggest an interaction between tACS and event-related rhythmic activity that extends beyond phase synchronization. SIGNIFICANCE These findings add novel insights into the mechanisms of tACS after-effects.

Stability through variability: Homeostatic plasticity and psychological resilience

According to Kalisch et al., adopting a cognitive positive appraisal style promotes internal bodily homeostasis and acts as a safeguard against the detrimental effects of stress. Here we will discuss results from recent noninvasive brain stimulation studies in humans to illustrate that homeostatic plasticity provides a neural mechanistic account for the positive appraisal style theory of resilience.

Posterior resting state EEG asymmetries are associated with hedonic valuation of food.

Research on the hedonic value of food has been important in understanding the motivational and emotional correlates of normal and abnormal eating behaviour. The aim of the present study was to explore associations between hemispheric asymmetries recorded during resting state electroencephalogram (EEG) and hedonic valuation of food. Healthy adult volunteers were recruited and four minutes of resting state EEG were recorded from the scalp. Hedonic food valuation and reward sensitivity were assessed with the hedonic attitude to food and behavioural activation scale. Results showed that parieto-occipital resting state EEG asymmetries in the alpha (8-12Hz) and beta (13-30Hz) frequency range correlate with the hedonic valuation of food. Our findings suggest that self-reported sensory-related attitude towards food is associated with interhemispheric asymmetries in resting state oscillatory activity. Our findings contribute to understanding the electrophysiological correlates of hedonic valuation, and may provide an opportunity to modulate the cortical imbalance by using non-invasive brain stimulation methods to change food consumption.

Corticospinal correlates of fast and slow adaptive processes in motor learning

Recent computational theories and behavioral observations suggest that motor learning is supported by multiple adaptation processes, operating on different timescales, but direct neural evidence is lacking. We tested this hypothesis by applying transcranial magnetic stimulation over motor cortex in 16 human subjects during a validated reach adaptation task. Motor-evoked potentials (MEPs) and cortical silent periods (CSPs) were recorded from the biceps brachii to assess modulations of corticospinal excitability as indices for corticospinal plasticity. Guided by a two-state adaptation model, we show that the MEP reflects an adaptive process that learns quickly but has poor retention, while the CSP correlates with a process that responds more slowly but retains information well. These results provide a physiological link between models of motor learning and distinct changes in corticospinal excitability. Our findings support the relationship between corticospinal gain modulations and the adaptive processes in motor learning. NEW & NOTEWORTHY Computational theories and behavioral observations suggest that motor learning is supported by multiple adaptation processes, but direct neural evidence is lacking. We tested this hypothesis by applying transcranial magnetic stimulation over human motor cortex during a reach adaptation task. Guided by a two-state adaptation model, we show that the motor-evoked potential reflects a process that adapts and decays quickly, whereas the cortical silent period reflects slow adaptation and decay.

Dissociating absolute and relative reward- and punishment-related electrocortical processing: An event-related potential study

The meaning of reward and punishment signals depends on context. Receiving a small reward where a larger reward could have been obtained can be considered a punishment, while a small loss in the context of avoiding a larger loss can be experienced as a reward. The aim of this study was to investigate the electrophysiological processes associated with absolute and relative reward and punishment signals. Twenty healthy right-handed volunteers performed a decision-making task and were instructed to judge which of two neutral objects was the most expensive. The received outcome was presented together with the non-received outcome for the alternative choice. The feedback-related potentials P200, FRN and P300 were recorded in response to absolute (i.e., received) outcome and relative (i.e., received in the context of the alternative) outcome. Absolute rewards yielded higher P200 amplitudes as compared to relative rewards, while the P200 amplitude was largest for relative as compared to absolute punishments. The P300 amplitude showed a main effect of valence with larger amplitudes for more positive relative and absolute outcomes. No effect of absolute or relative outcome was observed for the feedback-related negativity (FRN). Our findings suggest distinct processes associated with context-dependent and context-independent processing during feedback processing.

Frontal cortex electrophysiology in reward- and punishment-related feedback processing during advice-guided decision making: An interleaved EEG-DC stimulation study

During decision making, individuals are prone to rely on external cues such as expert advice when the outcome is not known. However, the electrophysiological correlates associated with outcome uncertainty and the use of expert advice are not completely understood. The feedback-related negativity (FRN), P3a, and P3b are event-related brain potentials (ERPs) linked to dissociable stages of feedback and attentional processing during decision making. Even though these ERPs are influenced by both reward- and punishment-related feedback, it remains unclear how extrinsic information during uncertainty modulates these brain potentials. In this study, the effects of advice cues on decision making were investigated in two separate experiments. In the first experiment, electroencephalography (EEG) was recorded in healthy volunteers during a decision-making task in which the participants received reward or punishment feedback preceded by novice, amateur, or expert advice. The results showed that the P3a component was significantly influenced by the subjective predictive value of an advice cue, whereas the FRN and P3b were unaffected by the advice cues. In the second, sham-controlled experiment, cathodal transcranial direct current stimulation (ctDCS) was administered in conjunction with EEG in order to explore the direct contributions of the frontal cortex to these brain potentials. Results showed no significant change in either advice-following behavior or decision times. However, ctDCS did decrease FRN amplitudes as compared to sham, with no effect on the P3a or P3b. Together, these findings suggest that advice information may act primarily on attention allocation during feedback processing, whereas the electrophysiological correlates of the detection and updating of internal prediction models are not affected.