Pre-stimulus Oscillatory Activity Research Articles

Relevance of pre-stimulus oscillatory activity for the perceived valence of emotional facial expressions

The interpretation of emotional facial expressions is crucial in everyday social interactions, and rapid processing of these expressions is necessary. Although extensive research has shed light on the mechanisms involved in facial expression processing, there is limited research on the potential role of the state of neural activity that directly precedes the occurrence of a face. Here, we investigated the potential modulatory role of pre-stimulus oscillatory activity in emotional facial expression processing. We tested emotional facial processing in two experiments, one utilizing artificial and the other natural facial expressions. The participants had to evaluate the emotional valence of the presented ambiguous facial expressions. In a univariate analysis, differences in the oscillation activity of the later rated valence of the faces were observed in both experiments, and these differences were observed even before the presentation of the facial expressions. Importantly, two different multivariate approaches directly supported the relevance of pre-stimulus oscillatory activity by exclusively using pre-stimulus oscillatory data to predict the perceived valence of the latter rated facial expression across the two experiments within as well as across subjects. The behavioral data shows the often observed negativity bias, i.e. ambiguous faces resulted in the tendency to rate them as negative. This negativity bias was related to neural activity modulations in the pre-stimulus period and also within post-stimulus processing related activity. These findings underscore the significance of pre-stimulus oscillatory activity in facial expression processing, indicating a functional role of ongoing neural states that affects the processing of facial expressions and constitute a basis for the well described negativity bias.

Transcranial magnetic stimulation effects support an oscillatory model of ERP genesis

Whether prestimulus oscillatory brain activity contributes to the generation of post-stimulus-evoked neural responses has long been debated, but findings remain inconclusive. We first investigated the hypothesized relationship via EEG recordings during a perceptual task with this correlational evidence causally probed subsequently by means of online rhythmic transcranial magnetic stimulation. Both approaches revealed a close link between prestimulus individual alpha frequency (IAF) and P1 latency, with faster IAF being related to shorter latencies, best explained via phase-reset mechanisms. Moreover, prestimulus alpha amplitude predicted P3 size, best explained via additive (correlational and causal evidence) and baseline shift mechanisms (correlational evidence), each with distinct prestimulus alpha contributors. Finally, in terms of performance, faster prestimulus IAF and shorter P1 latencies were both associated with higher task accuracy, while lower prestimulus alpha amplitudes and higher P3 amplitudes were associated with higher confidence ratings. Our results are in favor of the oscillatory model of ERP genesis and modulation, shedding new light on the mechanistic relationship between prestimulus oscillations and functionally relevant evoked components.

Prestimulus oscillatory brain activity interacts with evoked recurrent processing to facilitate conscious visual perception

We investigated whether prestimulus alpha-band oscillatory activity and stimulus-elicited recurrent processing interact to facilitate conscious visual perception. Participants tried to perceive a visual stimulus that was perceptually masked through object substitution masking (OSM). We showed that attenuated prestimulus alpha power was associated with greater negative-polarity stimulus-evoked ERP activity that resembled the visual awareness negativity (VAN), previously argued to reflect recurrent processing related to conscious perception. This effect, however, was not associated with better perception. Instead, when prestimulus alpha power was elevated, a preferred prestimulus alpha phase was associated with a greater VAN-like negativity, which was then associated with better cue perception. Cue perception was worse when prestimulus alpha power was elevated but the stimulus occurred at a nonoptimal prestimulus alpha phase and the VAN-like negativity was low. Our findings suggest that prestimulus alpha activity at a specific phase enables temporally selective recurrent processing that facilitates conscious perception in OSM.

Spatial Attention Tunes Temporal Processing in Early Visual Cortex by Speeding and Slowing Alpha Oscillations.

The perception of dynamic visual stimuli relies on two apparently conflicting perceptual mechanisms: rapid visual input must sometimes be integrated into unitary percepts but at other times must be segregated or parsed into separate objects or events. Though they have opposite effects on our perceptual experience, the deployment of spatial attention benefits both operations. Little is known about the neural mechanisms underlying this impact of spatial attention on temporal perception. Here, we record magnetoencephalography (MEG) in male and female humans to demonstrate that the deployment of spatial attention for the purpose of segregating or integrating visual stimuli impacts prestimulus oscillatory activity in retinotopic visual brain areas where the attended location is represented. Alpha band oscillations contralateral to an attended location are therefore faster than ipsilateral oscillations when stimuli appearing at this location will need to be segregated, but slower in expectation of the need for integration, consistent with the idea that α frequency is linked to perceptual sampling rate. These results demonstrate a novel interaction between temporal visual processing and the allocation of attention in space.SIGNIFICANCE STATEMENT Our environment is dynamic and visual input therefore varies over time. To make sense of continuously changing information, our visual system balances two complementary processes: temporal segregation in order to identify changes, and temporal integration to identify consistencies in time. When we know that a circumstance requires use of one or the other of these operations, we are able to prepare for this, and this preparation can be tracked in oscillatory brain activity. Here, we show how this preparation for temporal processing can be focused spatially. When we expect to integrate or segregate visual stimuli that will appear at a specific location, oscillatory brain activity changes in visual areas responsible for the representation of that location. In this way, spatial and temporal mechanisms interact to support adaptive, efficient perception.

Combined influences of strategy and selection history on attentional control.

Visual attention is guided by top-down mechanisms and pre-stimulus task preparation, but also by selection history (i.e., the bias to prioritize previously attended items). Here we examine how these influences combine. Two groups of participants completed two intermingled tasks. One task involved categorization of a unique target; one group categorized the target based on color, and the other based on shape. The other task involved searching for a target defined by unique shape while ignoring a distractor defined by unique color. Our expectation was that the search task would be difficult for the color-categorization group because their categorization task required attentional resolution of color, but the search task required that they ignore color. In some experimental blocks, trials from the two tasks appeared predictably, giving the color-categorization group an opportunity to strategically prepare by switching between color-prioritizing and shape-prioritizing attentional templates. We looked to pre-stimulus oscillatory activity as a direct index of this preparation, and to reaction times and post-stimulus ERPs for markers of resultant change in attentional deployment. Results showed that preparation in the color-categorization group optimized attentional templates, such that these participants became less sensitive to the color distractor in the search task. But preparation was not sufficient to entirely negate the influence of selection history, and participants in the color-categorization group continued to show a propensity to attend to the color distractor. These results indicate that preparatory effort can be scaled to the anticipated attentional requirements, but attention is nevertheless considerably biased by selection history.

Low pre-stimulus EEG alpha power amplifies visual awareness but not visual sensitivity.

Pre-stimulus oscillatory neural activity has been linked to the level of awareness of sensory stimuli. More specifically, the power of low-frequency oscillations (primarily in the alpha-band, i.e., 8-14Hz) prior to stimulus onset is inversely related to measures of subjective performance in visual tasks, such as confidence and visual awareness. Intriguingly, the same EEG signature does not seem to influence objective measures of task performance (i.e., accuracy). We here examined whether this dissociation holds when stringent accuracy measures are used. Previous EEG-studies have employed 2-alternative forced choice (2-AFC) discrimination tasks to link pre-stimulus oscillatory activity to correct/incorrect responses as an index of accuracy/objective performance at the single-trial level. However, 2-AFC tasks do not provide a good estimate of single-trial accuracy, as many of the responses classified as correct will be contaminated by guesses (with the chance correct response rate being 50%). Here instead, we employed a 19-AFC letter identification task to measure accuracy and the subjectively reported level of perceptual awareness on each trial. As the correct guess rate is negligible (~5%), this task provides a purer measure of accuracy. Our results replicate the inverse relationship between pre-stimulus alpha/beta-band power and perceptual awareness ratings in the absence of a link to discrimination accuracy. Pre-stimulus oscillatory phase did not predict either subjective awareness or accuracy. Our results hence confirm a dissociation of the pre-stimulus EEG power-task performance link for subjective versus objective measures of performance, and further substantiate pre-stimulus alpha power as a neural predictor of visual awareness.

Temporal Signatures of Criticality in Human Cortical Excitability as Probed by Early Somatosensory Responses.

Brain responses vary considerably from moment to moment, even to identical sensory stimuli. This has been attributed to changes in instantaneous neuronal states determining the system's excitability. Yet the spatiotemporal organization of these dynamics remains poorly understood. Here we test whether variability in stimulus-evoked activity can be interpreted within the framework of criticality, which postulates dynamics of neural systems to be tuned toward the phase transition between stability and instability as is reflected in scale-free fluctuations in spontaneous neural activity. Using a novel noninvasive approach in 33 male human participants, we tracked instantaneous cortical excitability by inferring the magnitude of excitatory postsynaptic currents from the N20 component of the somatosensory evoked potential. Fluctuations of cortical excitability demonstrated long-range temporal dependencies decaying according to a power law across trials, a hallmark of systems at critical states. As these dynamics covaried with changes in prestimulus oscillatory activity in the alpha band (8-13 Hz), we establish a mechanistic link between ongoing and evoked activity through cortical excitability and argue that the co-emergence of common temporal power laws may indeed originate from neural networks poised close to a critical state. In contrast, no signatures of criticality were found in subcortical or peripheral nerve activity. Thus, criticality may represent a parsimonious organizing principle of variability in stimulus-related brain processes on a cortical level, possibly reflecting a delicate equilibrium between robustness and flexibility of neural responses to external stimuli.SIGNIFICANCE STATEMENT Variability of neural responses in primary sensory areas is puzzling, as it is detrimental to the exact mapping between stimulus features and neural activity. However, such variability can be beneficial for information processing in neural networks if it is of a specific nature, namely, if dynamics are poised at a so-called critical state characterized by a scale-free spatiotemporal structure. Here, we demonstrate the existence of a link between signatures of criticality in ongoing and evoked activity through cortical excitability, which fills the long-standing gap between two major directions of research on neural variability: the impact of instantaneous brain states on stimulus processing on the one hand and the scale-free organization of spatiotemporal network dynamics of spontaneous activity on the other.

Detecting Pre-Stimulus Source-Level Effects on Object Perception with Magnetoencephalography.

Pre-stimulus oscillatory brain activity influences upcoming perception. The characteristics of this pre-stimulus activity can predict whether a near-threshold stimulus will be perceived or not perceived, but can they also predict which one of two competing stimuli with different perceptual contents is perceived? Ambiguous visual stimuli, which can be seen in one of two possible ways at a time, are ideally suited to investigate this question. Magnetoencephalography (MEG) is a neurophysiological measurement technique that records magnetic signals emitted as a result of brain activity. The millisecond temporal resolution of MEG allows for a characterization of oscillatory brain states from as little as 1 second of recorded data. Presenting an empty screen around 1 second prior to the ambiguous stimulus onset therefore provides a time window in which one can investigate whether pre-stimulus oscillatory activity biases the content of upcoming perception, as indicated by participants'reports. The spatial resolution of MEG is not excellent, but sufficient to localise sources of brain activity at the centimetre scale. Source reconstruction of MEG activity then allows for testing hypotheses about the oscillatory activity of specific regions of interest, as well as the time- and frequency-resolved connectivity between regions of interest. The described protocol enables a better understanding of the influence of spontaneous, ongoing brain activity on visual perception.

Transcranial alternating current stimulation at 10 Hz modulates response bias in the Somatic Signal Detection Task.

Ongoing, pre-stimulus oscillatory activity in the 8–13 Hz alpha range has been shown to correlate with both true and false reports of peri-threshold somatosensory stimuli. However, to directly test the role of such oscillatory activity in behaviour, it is necessary to manipulate it. Transcranial alternating current stimulation (tACS) offers a method of directly manipulating oscillatory brain activity using a sinusoidal current passed to the scalp. We tested whether alpha tACS would change somatosensory sensitivity or response bias in a signal detection task in order to test whether alpha oscillations have a causal role in behaviour. Active 10 Hz tACS or sham stimulation was applied using electrodes placed bilaterally at positions CP3 and CP4 of the 10–20 electrode placement system. Participants performed the Somatic Signal Detection Task (SSDT), in which they must detect brief somatosensory targets delivered at their detection threshold. These targets are sometimes accompanied by a light flash, which could also occur alone. Active tACS did not modulate sensitivity to targets but did modulate response criterion. Specifically, we found that active stimulation generally increased touch reporting rates, but particularly increased responding on light trials. Stimulation did not interact with the presence of touch, and thus increased both hits and false alarms. TACS stimulation increased reports of touch in a manner consistent with our observational reports, changing response bias, and consistent with a role for alpha activity in somatosensory detection.

Prestimulus EEG Power Predicts Conscious Awareness But Not Objective Visual Performance.

Prestimulus oscillatory neural activity has been linked to perceptual outcomes during performance of psychophysical detection and discrimination tasks. Specifically, the power and phase of low frequency oscillations have been found to predict whether an upcoming weak visual target will be detected or not. However, the mechanisms by which baseline oscillatory activity influences perception remain unclear. Recent studies suggest that the frequently reported negative relationship between α power and stimulus detection may be explained by changes in detection criterion (i.e., increased target present responses regardless of whether the target was present/absent) driven by the state of neural excitability, rather than changes in visual sensitivity (i.e., more veridical percepts). Here, we recorded EEG while human participants performed a luminance discrimination task on perithreshold stimuli in combination with single-trial ratings of perceptual awareness. Our aim was to investigate whether the power and/or phase of prestimulus oscillatory activity predict discrimination accuracy and/or perceptual awareness on a trial-by-trial basis. Prestimulus power (3–28 Hz) was inversely related to perceptual awareness ratings (i.e., higher ratings in states of low prestimulus power/high excitability) but did not predict discrimination accuracy. In contrast, prestimulus oscillatory phase did not predict awareness ratings or accuracy in any frequency band. These results provide evidence that prestimulus α power influences the level of subjective awareness of threshold visual stimuli but does not influence visual sensitivity when a decision has to be made regarding stimulus features. Hence, we find a clear dissociation between the influence of ongoing neural activity on conscious awareness and objective performance.

Neural Oscillation Reveals Deficits in Visuospatial Working Memory in Children With Developmental Coordination Disorder.

The electroencephalographic (EEG) oscillations associated with visuospatial working memory (VSWM) were examined in children with developmental coordination disorder (DCD; 10-11years; N=29) and typically developing (TD) children (10-11years; N=29). Behaviorally, DCD showed poorer VSWM than TD, which coincided with the diminished ability of DCD in modulating neural oscillations. Furthermore, prestimulus oscillatory alpha activity was correlated with VSWM performance. The results suggest that children with DCD might have a reduced ability to encode and recognize new information, and in particular have difficulty in maintaining task-relevant information, resulting in poorer VSWM. This study thus concludes that changes in oscillatory EEG activity reflect some of the problems leading to cognitive deficits in DCD.

Pre-stimulus alpha oscillations over somatosensory cortex predict tactile misperceptions

Fluctuations of pre-stimulus oscillatory activity in the somatosensory alpha band (8–14Hz) observed using human EEG and MEG have been shown to influence the detection of supra- and peri-threshold somatosensory stimuli. However, some reports of touch occur even without a stimulus. We investigated the possibility that pre-stimulus alpha oscillations might also influence these false reports of touch – known as tactile misperceptions. We recorded EEG while participants performed the Somatic Signal Detection Task (SSDT), in which participants must detect brief, peri-threshold somatosensory targets. We found that pre-stimulus oscillatory power in the somatosensory alpha range exhibited a negative linear relationship with reporting of touch at electrode clusters over both contralateral and ipsilateral somatosensory regions. As pre-stimulus alpha power increased, the probability of reporting a touch declined; as it decreased, the probability of reporting a touch increased. This relationship was stronger on trials without a somatosensory stimulus than on trials with a somatosensory stimulus, although was present for both trial types. Spatio-temporal cluster-based permutation analysis also found that pre-stimulus alpha was lower on trials when touch was reported – irrespective of whether it was present – over contralateral and ipsilateral somatosensory cortices, as well as left frontocentral areas. We argue that alpha power may reflect changes in response criterion rather than sensitivity alone. Low alpha power relates to a low barrier to reporting a touch even when one is not present, while high alpha power is linked to less frequent reporting of touch overall.

Intention to encode boosts memory-related pre-stimulus EEG beta power

Pre-stimulus oscillatory brain activity can predict the degree to which an upcoming stimulus will be remembered at a later point in time. Recently, increased pre-stimulus power in ongoing theta (5–8Hz) and low beta (13–17Hz) bands during encoding has been associated with enhanced memory performance. When a cue is presented before stimulus onset, encoding-related brain activations may be regarded as a sign of preparation for the upcoming stimulus. Here, we investigated whether the intention to encode the following stimulus into long-term memory affects these preparatory pre-stimulus activations during encoding. Two groups of 18 participants took part in a subsequent memory paradigm. Electroencephalogram (EEG) was recorded while participants were presented with a series of pictures, each one preceded by a cue, which were supposed to be classified according to animacy. One group was informed about the upcoming recognition task and therefore was enabled to develop the intention to encode the stimuli (intentional encoding), whereas the other group did not receive this information (incidental encoding). Afterwards, recognition of the pictures was tested. During intentional encoding only, power in theta and low beta bands was found to be significantly increased before the onset of pictures that were later remembered compared to later forgotten ones. Group comparisons confirmed greater memory-related power increases in the low beta band for intentional than incidental encoding. These findings indicate that oscillatory states that are associated with successful encoding can be initiated voluntarily if the intention to encode the stimuli is given. We therefore suggest low beta band activation before stimulus onset to be an indicator of memory-specific preparation for an upcoming stimulus.

Dissociation of the functional relevance of different pre-stimulus oscillatory activity for memory formation

The state of a neural assembly preceding an incoming stimulus modulates the processing of that subsequently presented stimuli. For human memory formation, the role of oscillatory brain activity within different frequency ranges has been discussed but a more functional relation could not be established. In the present Experiment I, an increase of pre-stimulus theta- (3–7Hz) and beta- (13–17Hz) band oscillations during encoding for later remembered stimuli was observed. To establish a more direct functional relation, we adopted a novel brain–computer-interface (BCI) method to selectively detect oscillatory activity in real-time combined with an adaptive stimulus presentation at different levels of activity. Therefore, in the second experiment the BCI was used to present the visual stimuli with a high temporal resolution directly within defined brain states of beta- or theta-band activity. The quality of the subsequent processing of the stimuli was assessed at the behavioral level with a surprise recognition task. Results revealed a variation of memory performance in direct relation to the amount of pre-stimulus beta- but not theta-band oscillations, suggesting a functional relevance of beta-band oscillations for memory encoding. Thus, the BCI method enabled a more functional differentiation of the effective role of ongoing oscillatory activity.

Effects of concurrent caffeine and mobile phone exposure on local target probability processing in the human brain.

Millions of people use mobile phones (MP) while drinking coffee or other caffeine containing beverages. Little is known about the potential combined effects of MP irradiation and caffeine on cognitive functions. Here we investigated whether caffeine intake and concurrent exposure to Universal Mobile Telecommunications System (UMTS) MP-like irradiation may interactively influence neuro-cognitive function in an active visual oddball paradigm. In a full factorial experimental design, 25 participants performed a simple visual target detection task while reaction time (RT) and electroencephalogram (EEG) was recorded. Target trials were divided into Low and High probability sets based on target-to-target distance. We analyzed single trial RT and alpha-band power (amplitude) in the pre-target interval. We found that RT was shorter in High vs. Low local probability trials, and caffeine further shortened RT in High probability trials relative to the baseline condition suggesting that caffeine improves the efficiency of implicit short-term memory. Caffeine also decreased pre-target alpha amplitude resulting in higher arousal level. Furthermore, pre-target gamma power positively correlated with RT, which may have facilitated target detection. However, in the present pharmacologically validated study UMTS exposure either alone or in combination with caffeine did not alter RT or pre-stimulus oscillatory brain activity.

The contribution of pre-stimulus neural oscillatory activity to spontaneous response time variability

Large variability between individual response times, even in identical conditions, is a ubiquitous property of animal behavior. However, the origins of this stochasticity and its relation to action decisions remain unclear. Here we focus on the state of the perception–action network in the pre-stimulus period and its influence on subsequent saccadic response time and choice in humans. We employ magnetoencephalography (MEG) and a correlational source reconstruction approach to identify the brain areas where pre-stimulus oscillatory activity predicted saccadic response time to visual targets. We find a relationship between future response time and pre-stimulus power, but not phase, in occipital (including V1), parietal, posterior cingulate and superior frontal cortices, consistently across alpha, beta and low gamma frequencies, each accounting for between 1 and 4% of the RT variance. Importantly, these correlations were not explained by deterministic sources of variance, such as experimental factors and trial history. Our results further suggest that occipital areas mainly reflect short-term (trial to trial) stochastic fluctuations, while the frontal contribution largely reflects longer-term effects such as fatigue or practice. Parietal areas reflect fluctuations at both time scales. We found no evidence of lateralization: these effects were indistinguishable in both hemispheres and for both saccade directions, and non-predictive of choice — a finding with fundamental consequences for models of action decision, where independent, not coupled, noise is normally assumed.

Local Entrainment of Alpha Oscillations by Visual Stimuli Causes Cyclic Modulation of Perception

Prestimulus oscillatory neural activity in the visual cortex has large consequences for perception and can be influenced by top-down control from higher-order brain regions. Making a causal claim about the mechanistic role of oscillatory activity requires that oscillations be directly manipulated independently of cognitive instructions. There are indications that a direct manipulation, or entrainment, of visual alpha activity is possible through visual stimulation. However, three important questions remain: (1) Can the entrained alpha activity be endogenously maintained in the absence of continuous stimulation?; (2) Does entrainment of alpha activity reflect a global or a local process?; and (3) Does the entrained alpha activity influence perception? To address these questions, we presented human subjects with rhythmic stimuli in one visual hemifield, and arhythmic stimuli in the other. After rhythmic entrainment, we found a periodic pattern in detection performance of near-threshold targets specific to the entrained hemifield. Using magnetoencephalograhy to measure ongoing brain activity, we observed strong alpha activity contralateral to the rhythmic stimulation outlasting the stimulation by several cycles. This entrained alpha activity was produced locally in early visual cortex, as revealed by source analysis. Importantly, stronger alpha entrainment predicted a stronger phasic modulation of detection performance in the entrained hemifield. These findings argue for a cortically focal entrainment of ongoing alpha oscillations by visual stimulation, with concomitant consequences for perception. Our results support the notion that oscillatory brain activity in the alpha band provides a causal mechanism for the temporal organization of visual perception.

P 66. TMS entrainment of pre-stimulus oscillatory activity in tactile processing

It is widely recognized that oscillatory activity plays an important functional role in neural systems. Decreases in alpha (∼10 Hz) EEG/MEG activity in the parietal cortex correlate with the deployment of spatial attention controlateral to target location in visual, auditory and tactile domains ( Bauer et al., 2012 , Foxe and Snyder, 2011 , Linkenkaer-Hansen et al., 2004 ). Recently, repetitive Transcranial Magnetic Stimulation (rTMS) has been successfully applied to entrain a specific frequency at the parietal cortex (IPS) and the visual cortex. A short burst of 10 Hz rTMS impaired contralateral visual target detection and improved it ipsilaterally, compared to other control frequencies ( Romei et al., 2010 ). This finding suggests a causal role of rhythmic activity in the alfa range in perception ( Thut et al., 2011 ). The aim of the present study is to address whether entraining alpha frequency in the IPS plays a role in tactile orienting, indicating similarities between senses (vision and touch) in the communication between top-down (parietal) and primary sensory areas (V1 or S1). We applied rhythmic TMS at 10 Hz and 20 Hz to the (right or left) IPS and S1, immediately before a masked vibrotactile target stimulus (present in 50% of the trials) to the left or right hand. Preliminary results lean towards the consequential effects of entraining alpha frequency into IPS for tactile detection such that it decreases tactile perception contralaterally and increases it ipsilaterally, compared to Sham. The results suggest the existence of a similar mechanism of orienting attention in vision and touch, therefore paving the way for further testing the role of parietal alpha band oscillation in other tactile information processing, such as in tactile remapping, a mechanism that allow our brain to localize tactile events in external space. European Research Council (StG-2010 263145).

Temporal Characteristics of the Influence of Punishment on Perceptual Decision Making in the Human Brain

Perceptual decision making is the process by which information from sensory systems is combined and used to influence our behavior. In addition to the sensory input, this process can be affected by other factors, such as reward and punishment for correct and incorrect responses. To investigate the temporal dynamics of how monetary punishment influences perceptual decision making in humans, we collected electroencephalography (EEG) data during a perceptual categorization task whereby the punishment level for incorrect responses was parametrically manipulated across blocks of trials. Behaviorally, we observed improved accuracy for high relative to low punishment levels. Using multivariate linear discriminant analysis of the EEG, we identified multiple punishment-induced discriminating components with spatially distinct scalp topographies. Compared with components related to sensory evidence, components discriminating punishment levels appeared later in the trial, suggesting that punishment affects primarily late postsensory, decision-related processing. Crucially, the amplitude of these punishment components across participants was predictive of the size of the behavioral improvements induced by punishment. Finally, trial-by-trial changes in prestimulus oscillatory activity in the alpha and gamma bands were good predictors of the amplitude of these components. We discuss these findings in the context of increased motivation/attention, resulting from increases in punishment, which in turn yields improved decision-related processing.

Prestimulus Beta Power and Phase Synchrony Influence the Sound-Induced Flash Illusion

We investigated the sound-induced flash illusion, an example for the influence of auditory information on visual perception. It consists of the perception of 2 visual stimuli upon the presentation of a single visual stimulus alongside 2 auditory stimuli. We used magnetoencephalography to assess the influence of prestimulus oscillatory activity on varying the perception of invariant stimuli. We compared cortical activity from trials in which subjects perceived an illusion with trials in which subjects did not perceive the illusion, keeping the stimulation fixed. Subjects perceived the illusion in approximately 50% of trials. Prior to the illusion, we found stronger beta-band power in left temporal sensors, localized to the left middle temporal gyrus. Illusory perceptions were preceded by increased beta-band phase synchrony between the left middle temporal gyrus and auditory areas as well as by decreased phase synchrony with visual areas. Alpha-band phase synchrony between visual and temporal, parietal, and frontal cortical areas as well as alpha-band phase synchrony between auditory and visual areas were modulated. This supports and extends reports on the influence of brain states prior to stimulation on subsequent perception. We suggest that prestimulus local and network activities form predispositions if sensory streams will be integrated.