P300 and response selection: a new look using independent-components analysis

The P300 wave is decomposed into components reflecting response selection and automatic reactivation of stimulus-response links

The parietal P300 wave of event-related potentials (ERPs) has been associated with various psychological operations in numerous laboratory tasks. This study aims to decompose the P3 wave of ERPs into subcomponents and link them with behavioral parameters, such as the strength of stimulus-response (S-R) links and GO/NOGO responses. EEGs (31 channels), referenced to linked ears, were recorded from 172 healthy adults (107 women) who participated in two cued GO/NOGO tasks, where the strength of S-R links was manipulated through instructions. P300 waves were observed in active conditions in response to cues, GO/NOGO stimuli, and in passive conditions when no manual response was required. Utilizing a combination of current source density transformation and blind source separation methods, we decomposed the P300 wave into two distinct components, purportedly originating from different parts of the parietal lobules. The amplitude of the parietal midline component (with current sources around Pz) closely mirrored the strength of the S-R link across proactive, reactive, and passive conditions. The amplitude of the lateral parietal component (with current sources around P3 and P4) resembled the push-pull activity of the output nuclei of the basal ganglia in action selection-inhibition operations. These findings provide insights into the neural mechanisms underlying action selection processes and the reactivation of S-R links.

The response relevance of visual stimuli modulates the P3 component and the underlying sensorimotor network

The functional meaning and neural basis of the P3b component of ERPs are still under debate. One of the main issues is whether P3b reflects only stimulus-related processes (stimulus evaluation hypothesis) or response-related processes as well (stimulus-response or S-R link activation hypothesis). Here, we conducted an EEG experiment examining whether P3b may indeed reflect an S-R link activation, followed by an fMRI experiment in which we explored the brain areas and functional connectivity possibly constituting the neural basis of these sensorimotor links. In both experiments, two successive visual stimuli, S1 and S2, were presented with a 1 sec interval, and responses were defined either by S1 or S2, while participants responded only after S2 onset. The obtained EEG results suggest that P3b may be interpreted in terms of the S-R link activation account, although further studies are needed to disentangle P3-related activity from overlapping anticipatory activity. The obtained fMRI results showed that processing of the relevant S1 involved activation of a distributed postero-anterior sensorimotor network, and increased strength of functional connectivity within this network. This network may underlie activation of the S-R links, thus possibly also the P3b component, forming a bridging step between sensory encoding and response execution.

ERP effects and perceived exclusion in the Cyberball paradigm: Correlates of expectancy violation?

A virtual ball-tossing game called Cyberball has allowed the identification of neural structures involved in the processing of social exclusion by using neurocognitive methods. However, there is still an ongoing debate if structures involved are either pain- or exclusion-specific or part of a broader network. In electrophysiological Cyberball studies we have shown that the P3b component is sensitive to exclusion manipulations, possibly modulated by the probability of ball possession of the participant (event "self") or the presumed co-players (event "other"). Since it is known from oddball studies that the P3b is not only modulated by the objective probability of an event, but also by subjective expectancy, we independently manipulated the probability of the events "self" and "other" and the expectancy for these events. Questionnaire data indicate that social need threat is only induced when the expectancy for involvement in the ball-tossing game is violated. Similarly, the P3b amplitude of both "self" and "other" events was a correlate of expectancy violation. We conclude that both the subjective report of exclusion and the P3b effect induced in the Cyberball paradigm are primarily based on a cognitive process sensitive to expectancy violations, and that the P3b is not related to the activation of an exclusion-specific neural alarm system.

Accelerative and decelerative effects of hedonic valence and emotional arousal during visual scene processing

Perceptual processing of natural scene pictures is enhanced when the scene conveys emotional content. Such “motivated attention” to pleasant and unpleasant pictures has been shown to improve identification accuracy in non-speeded behavioural tasks. An open question is whether emotional content also modulates the speed of visual scene processing. In the present studies we show that unpleasant content reliably slowed two-choice categorization of pictures, irrespective of physical image properties, perceptual complexity, and categorization instructions. Conversely, pleasant content did not slow or even accelerated choice reactions, relative to neutral scenes. As indicated by lateralized readiness potentials, these effects occurred at cognitive processing rather than motor preparation/execution stages. Specifically, analysis of event-related potentials showed a prolongation of early scene discrimination for stimuli perceived as emotionally arousing, regardless of valence, and reflected in delayed peaks of the N1 component. In contrast, the timing of other processing steps, reflected in the P2 and late positive potential components and presumably related to post-discriminatory processes such as stimulus–response mapping, appeared to be determined by hedonic valence, with more pleasant scenes eliciting faster processing. Consistent with this model, varying arousal (low/high) within the emotional categories mediated the effects of valence on choice reaction speed. Functionally, arousal may prolong stimulus analysis in order to prevent erroneous and potentially harmful decisions. Pleasantness may act as a safety signal allowing rapid initiation of overt responses.

An electrophysiological index of changes in risk decision-making strategies

Human decision-making is significantly modulated by previously experienced outcomes. Using event-related potentials (ERPs), we examined whether ERP components evoked by outcome feedbacks could serve as biomarkers to signal the influence of current outcome evaluation on subsequent decision-making. In this study, 18 adult volunteers participated in a simple monetary gambling task, in which they were asked to choose between two options that differed in risk. Their decisions were immediately followed by outcome presentation. Temporospatial principle component analysis (PCA) was applied to the outcome-onset locked ERPs in the 200-1000 ms time window. The PCA factors that approximated classical ERP components (P2, feedback-related negativity, P3a, and P3b) in terms of time course and scalp distribution were tested for their association with subsequent decision-making strategies. Our results revealed that a fronto-central PCA factor approximating the classical P3a was related to changes of decision-making strategies on subsequent trials. The decision to switch between high- and low-risk options resulted in a larger P3a relative to the decision to retain the same choice. According to the results, we suggest that the amplitude of the fronto-central P3a is an electrophysiological index of the influence of current outcome on subsequent risk decision-making. Furthermore, the ERP source analysis indicated that the activations of the frontopolar cortex and sensorimotor cortex were involved in subsequent changes of strategies, which enriches our understanding of the neural mechanisms of adjusting decision-making strategies based on previous experience.

Comparing the neural distance effect derived from the non-symbolic comparison and the same-different task

As a result of the representation of numerosities, more accurate and faster discrimination between two numerosities is observed when the distance between them increases. In previous studies, the comparison and same-different task were most frequently used to investigate this distance effect. Recently, it was questioned whether the non-symbolic distance effects derived from these tasks originate at the same level. In the current study, we examined the behavioral and neural distance effects of the comparison and same-different task to assess potential differences between both tasks. Participants were first year university students. Each participant completed both tasks, while their reaction time, accuracy and brain activity on predefined components was measured. The early N1-P2p transition and the P2p component on temporo-occipital (TO) and inferior parietal (IP) electrode groups were considered, as well as the late P3 component on a central (C) electrode group. The results showed that the behavioral distance effects from both tasks were comparable, although participants' performance was worse on the same-different task. The neural results revealed similar effects of distance on the mean amplitudes for the early components for both tasks (all p′s < 0.02) and an additional effect of task difficulty on the mean amplitudes of these components. Similar as in previous studies, we found a (marginally) significant increase in mean amplitude of the later P3 component with increasing distance for the comparison (p = 0.07), but not for the same-different task. Apparently, the initial stages of number processing are comparable for both tasks, but an additional later stage is only present for the comparison task. The P3 effect would be indicative of this decisional stage, which was previously proposed to underlie the comparison distance effect (CDE).

The cortical chronometry of electrogustatory event-related potentials

Electrogustometry (EGM) is the standard tool to assess gustatory functions in clinical environments. The stimulation elicits a percept often described as metallic, sour or salty, also referred to as electric taste. To date, the neuronal mechanisms that underlie electric taste perception are not yet fully understood. Electroencephalographic (EEG) approaches will certainly complement behavioral procedures and, furthermore, extend the understanding of gustatory processing in general and disturbances of gustatory functions in particular. We used anodal pulses applied to the tip of the participants' tongue while EEG was recorded. The major disadvantage of combining EEG and EGM, namely the electrical stimulation artifact, was overcome by means of Independent Component Analysis (ICA), which separated the EGM artifact from the neural portion of the EEG. After artifact correction, we found a largely uncontaminated electrogustatory event-related potential (eGERP) at both individual and group level. Furthermore, source analysis revealed an early involvement of bilateral insular cortices and the adjacent operculi, the areas comprising the primary taste cortex. The procedures, described in detail, pave the way for the eGERP to become an affordable and objective tool for the assessment of taste function, and thus to complement behavioral measures (i.e. EGM detection thresholds). Furthermore, they render the access to different levels of the electrogustatory processing pathway possible and by doing so they may aid the identification and localisation of lesions that cause taste disturbances.

Event-Related Potential Study of Executive Dysfunctions in a Speeded Reaction Task in Cocaine Addiction

This study used a flanker task with NoGo elements to investigate frontal executive function deficits in 19 cocaine abusers. The executive functions of interest in this study were cortical inhibition or ability to withhold motor response, the ability to select an appropriate response among several competing ones, the ability to inhibit inappropriate responses, and the ability to detect error and exercise corrective control. These processes were evaluated with specific frontal and parietal event-related potentials (ERP) registered during performance on this speeded reaction time task with conflicting motor response demands. Specifically we used behavioral response measures, stimulus-locked anterior (frontal N200, N450) ERP markers of conflict detection, response inhibition (NoGo-N2 and NoGo-P3), and response-locked error-related negativity (ERN) that represent different time points of signal classification, motor response conflict detection, response inhibition, and error monitoring processes. The results revealed that the higher-level executive motor control attributed to the prefrontal cortex is hypoactive in cocaine abusers, and therefore is incapable to effectively resolve response conflicts arising between the competing motor response alternatives. It was also demonstrated that the mesial frontal structures, such as the anterior cingulate cortex, implicated in motor response conflict detection and error monitoring functions were also compromised in addicts. It is reasonable to propose that a 'hypofunctional' prefrontal and midfrontal processing results in a diminished ability to effectively override strong habitual automated response tendencies controlled by the lower-level neural mechanisms triggered by the external stimuli. The results propose a neurobiological basis for the understanding why cocaine abusers are facing difficulties in controlling their drug-seeking and drug-taking behaviors, and why their drug-related habitual behavior is so vulnerable to be triggered by external (e.g., drug-related items and environment) cues.

Evaluation of PCA and ICA of simulated ERPs: Promax vs. Infomax rotations

Independent components analysis (ICA) and principal components analysis (PCA) are methods used to analyze event-related potential (ERP) and functional imaging (fMRI) data. In the present study, ICA and PCA were directly compared by applying them to simulated ERP datasets. Specifically, PCA was used to generate a subspace of the dataset followed by the application of PCA Promax or ICA Infomax rotations. The simulated datasets were composed of real background EEG activity plus two ERP simulated components. The results suggest that Promax is most effective for temporal analysis, whereas Infomax is most effective for spatial analysis. Failed analyses were examined and used to devise potential diagnostic strategies for both rotations. Finally, the results also showed that decomposition of subject averages yield better results than of grand averages across subjects.

The independent components of auditory P300 and CNV evoked potentials derived from single-trial recordings

The back-projected independent components (BICs) of single-trial, auditory P300 and contingent negative variation (CNV) evoked potentials (EPs) were derived using independent component analysis (ICA) and cluster analysis. The method was tested in simulation including a study of the electric dipole equivalents of the signal sources. P300 data were obtained from healthy and Alzheimer's disease (AD) subjects. The BICs were of approximately 100 ms duration and approximated positive- and negative-going half-sinusoids. Some positively and negatively peaking BICs constituting the P300 coincided with known peaks in the averaged P300. However, there were trial-to-trial differences in their occurrences, particularly where a positive or a negative BIC could occur with the same latency in different trials, a fact which would be obscured by averaging them. These variations resulted in marked differences in the shapes of the reconstructed, artefact-free, single-trial P300s. The latencies of the BIC associated with the P3b peak differed between healthy and AD subjects (p < 0.01). More reliable evidence than that obtainable from single-trial or averaged P300s is likely to be found by studying the properties of the BICs over a number of trials. For the CNV, BICs corresponding to both the orienting and the expectancy components were found.

Abnormal frontal and parietal activity during working memory updating in post-traumatic stress disorder

This study used event-related potentials (ERPs) to investigate the timing and scalp topography of working memory in post-traumatic stress disorder (PTSD). This study was designed to investigate ERPs associated with a specific working memory updating process. ERPs were recorded from 10 patients and 10 controls during two visual tasks where (a) targets were a specific word or (b) targets were consecutive matching words. In the first task, nontarget words are not retained in working memory. In the second task, as in delay-match-to-sample tasks, a non-target word defines a new target identity, so these words are retained in working memory. This working memory updating process was related to large positive ERPs over frontal and parietal areas at 400-800 ms, which were smaller in PTSD. Estimation of cortical source activity indicated abnormal patterns of frontal and parietal activity in PTSD, which were also observed in regional cerebral blood flow [Clark, C.R., McFarlane, A.C., Morris, P., Weber, D.L., Sonkkilla, C., Shaw, M., Marcina, J., Tochon-Danguy, H., Egan, G., 2003. Cerebral function in posttraumatic stress disorder during verbal working memory updating: a positron emission tomography study. Biological Psychiatry 53, 474-481]. Frontal and parietal cortex are known to be involved in distributed networks for working memory processes, interacting with medial temporal areas during episodic memory processes. Abnormal function in these brain networks helps to explain everyday concentration and memory difficulties in PTSD.

EEG-dependent ERP recording: using TMS to increase the incidence of a selected pre-stimulus pattern

EEG dependent event-related potential (ERP) recording (interactive ERP) is an extension to ERP paradigms whereby stimuli are initiated in response to a selected pattern of background EEG. This form of recording is critically dependent upon the incidence of the particular pattern of interest. We introduce here a process that modifies the EEG in a predictable manner so as to increase the incidence of a particular pattern. Transcranial magnetic stimulation (TMS) stimuli are applied in response to a selected pattern of pre-TMS activity, and the post-TMS response is characterized by the incidence of a defined pattern of EEG activity. Analysis of validation test results obtained with the TMS modification part of the process verifies an increased incidence of the response pattern after TMS stimuli, compared with placebo stimuli. The TMS modification procedure is then combined with interactive ERP recording in a two step process to affect the ERP response to sensory stimuli. The post-TMS pattern from the first step becomes the pre-stimulus pattern of the interactive ERP recording in the second step. The TMS modified interactive ERP (TMIERP) process is illustrated here using an auditory oddball paradigm. The amplitude of the P300 peak obtained using this process was significantly higher than that obtained using the standard auditory oddball paradigm.

Overview of artifact reduction and removal in evoked potential and event-related potential recordings

Artifact in one form or another needs to be contended with in all EEG and EP studies. Various methods have been employed to avoid, eliminate, or minimize artifact. This article has described the methods that have been available for some time and newer methods and their advantages. It is the authors' hope that through the use of these methods the accuracy of all EP and ERP measurements will be improved and promote the validity and general acceptance of EP and ERP recordings. Genuine and valuable data are contained in EPs and ERPs. The challenge is to extract relatively small voltage signals often occurring within a higher voltage background of artifact. The computing power required to perform the artifact removal/reduction procedures now is available with basic laptop and desktop computers, as are the software programs that provide the artifact removal/reduction capabilities. It may be of interest for a prudent researcher to integrate the currently available artifact rejection methods before subjecting the ERP and EP data for further analysis and subsequent publication.

Investigating the generators of the scalp recorded visuo-verbal P300 using cortically constrained source localization

Considerable ambiguity exists about the generators of the scalp recorded P300, despite a vast body of research employing a diverse range of methodologies. Previous investigations employing source localization techniques have been limited largely to equivalent current dipole models, with most studies identifying medial temporal and/or hippocampal sources, but providing little information about the contribution of other cortical regions to the generation of the scalp recorded P3. Event-related potentials (ERPs) were recorded from 5 subjects using a 124-channel sensor array during the performance of a visuo-verbal Oddball task. Cortically constrained, MRI-guided boundary element modeling was used to identify the cortical generators of this target P3 in individual subjects. Cortical generators of the P3 were localized principally to the intraparietal sulcus (IPS) and surrounding superior parietal lobes (SPL) bilaterally in all subjects, though with some variability across subjects. Two subjects also showed activity in the lingual/inferior occipital gyrus and mid-fusiform gyrus. A group cortical surface was calculated by non-linear warping of each subject's segmented cortex followed by averaging and creation of a group mesh. Source activity identified across the group reflected the individual subject activations in the IPS and SPL bilaterally and in the lingual/inferior occipital gyrus primarily on the left. Activation of IPS and SPL is interpreted to reflect the role of this region in working memory and related attention processes and visuo-motor integration. The activity in left lingual/inferior occipital gyrus is taken to reflect activation of regions associated with modality-specific analysis of visual word forms.

Smoking/Nicotine Affects the Magnitude and Onset of Lateralized Readiness Potentials

AbstractSmoking/nicotine improves cognitive performance for a variety of tasks. In most cases, reaction time (RT) is generally shorter after smoking/nicotine. While there may be some slight facilitation of stimulus-evaluation processing, most of the RT effects of nicotine appear to take place following the response-selection stage. This study investigated possible effects (in smokers) of smoking/nicotine on response preparation and execution processes using the lateralized readiness potential (LRP). On each trial, a warning stimulus preceded an imperative stimulus by 1.2s. The warning stimulus completely specified the correct response to the imperative stimulus. The study was completed in two morning sessions in which 4 cigarettes were smoked in each session. The nicotine yield of the cigarettes varied between sessions (0.05mg or 1.1mg). Maximum amplitudes of both the stimulus and response-locked LRPs were larger in the 1.1 mg session. For both stimulus- and response-locked LRPs, smoking the 1.1 mg cigarette (but not the 0.05 mg cigarette) shortened onset latency. However, the magnitude of the effect was much larger for the stimulus-locked LRPs, suggesting that response preparation is facilitated by smoking/nicotine to a greater degree than response execution.