Reference-independent ERP old/new effects of auditory and visual word recognition memory: Joint extraction of stimulus- and response-locked neuronal generator patterns

The late positive event-related potential component is time locked to the decision in recognition memory tasks

Two event-related potential (ERP) components are commonly observed in recognition memory tasks: the Frontal Negativity (FN400) and the Late Positive Component (LPC). These components are widely interpreted as neural correlates of familiarity and recollection, respectively. However, the interpretation of LPC effects is complicated by inconsistent results regarding the timing of ERP amplitude differences. There are also mixed findings regarding how LPC amplitudes covary with decision confidence. Critically, LPC effects have almost always been measured using fixed time windows relative to memory probe stimulus onset, yet it has not been determined whether LPC effects are time locked to the stimulus or the recognition memory decision. To investigate this, we analysed a large (n = 132) existing dataset recorded during recognition memory tasks with old/new decisions followed by post-decisional confidence ratings. We used ERP deconvolution to disentangle contributions to LPC effects (defined as differences between hits and correct rejections) that were time locked to either the stimulus or the vocal old/new response. We identified a left-lateralised parietal LPC effect that was time locked to the vocal response rather than probe stimulus onset. We also isolated a response-locked, midline parietal ERP correlate of confidence that influenced measures of LPC amplitudes at left parietal electrodes. Our findings demonstrate that, contrary to widespread assumptions, the LPC effect is time locked to the recognition memory decision and is best measured using response-locked ERPs. By extension, differences in response time distributions across conditions of interest may lead to substantial measurement biases when analysing stimulus-locked ERPs. Our findings highlight important confounding factors that further complicate the interpretation of existing stimulus-locked LPC effects as neural correlates of recollection. We recommend that future studies adopt our analytic approach to better isolate LPC effects and their sensitivity to manipulations in recognition memory tasks.

Actions do not clearly impact auditory memory

When memorizing a list of words, those that are read aloud are remembered better than those read silently, a phenomenon known as the production effect. There have been several attempts to understand the production effect, however, actions alone have not been examined as possible contributors. Stimuli that coincide with our own actions are processed differently compared to stimuli presented passively to us. These sensory response modulations may have an impact on how action-revolving inputs are stored in memory. In this study, we investigated whether actions could impact auditory memory. Participants listened to sounds presented either during or in between their actions. We measured electrophysiological responses to the sounds and tested participants' memory of them. Results showed attenuation of sensory responses for action-coinciding sounds. However, we did not find a significant effect on memory performance. The absence of significant behavioral findings suggests that the production effect may be not dependent on the effects of actions per se. We conclude that action alone is not sufficient to improve memory performance, and thus elicit a production effect.

Using temporo-spatial principal component analysis as tool to dissociate latent ERP components of episodic memory retrieval: Objectifying time-window selection for overlapping ERP components

This report details how principal component analysis (PCA) can be used as a valuable tool to dissociate latent ERP components, even when considerable temporal and spatial overlap makes it difficult to discern ERP effects in standard time windows. We illustrate our methodological approach in a data set from a recognition memory paradigm, in which event-related potential (ERP) correlates of familiarity, recollection and the late parietal negativity (LPN) were partially overlapping. By adapting standard time windows based on the results of a temporo-spatial PCA, small yet reliable ERP correlates reflecting familiarity and recollection for identical items and late recollection for changed items were identified, complementing the result pattern observed in behavioral performance. Due to similar temporo-spatial characteristics and opposing polarities in late parietal ERP correlates associated with memory retrieval, component overlap is often observed in this field of research. Hence, the complex interplay of several processes underlying higher cognitive functions such as memory retrieval may interfere with standard ERP assessment. In such instances, PCA can provide promising ways to objectively assess time window selection for subsequent ERP analyses.

The role of electroencephalography electrical reference in the assessment of functional brain-heart interplay: From methodology to user guidelines

BACKGROUND

The choice of EEG reference has been widely studied. However, the choice of the most appropriate re-referencing for EEG data is still debated. Moreover, the role of EEG reference in the estimation of functional Brain-Heart Interplay (BHI), together with different multivariate modelling strategies, has not been investigated yet.

METHODS

This study identifies the best methodology combining a proper EEG electrical reference and signal processing methods for an effective functional BHI assessment. The effects of the EEG reference among common average, mastoids average, Laplacian reference, Cz reference, and the reference electrode standardization technique (REST) were explored throughout different BHI methods including synthetic data generation (SDG) model, heartbeat-evoked potentials, heartbeat-evoked oscillations, and maximal information coefficient.

RESULTS

The SDG model exhibited high robustness between EEG references, whereas the maximal information coefficient method exhibited a high sensitivity. The common average and REST references for EEG showed a good consistency in the between-method comparisons. Laplacian, and Cz references significantly bias a BHI measurement.

COMPARISON WITH EXISTING METHODS

The use of EEG reference based on a common average outperforms on the use of other references for consistency in estimating directed functional BHI. We do not recommend the use of EEG references based on analytical derivations as the experimental conditions may not meet the requirements of their optimal estimation, particularly in clinical settings.

CONCLUSION

The use of a common average for EEG electrical reference is concluded to be the most appropriate choice for a quantitative, functional BHI assessment.

Family Risk for Depression and Prioritization of Religion or Spirituality: Early Neurophysiological Modulations of Motivated Attention

The personal importance of religion or spirituality (R/S) has been associated with a lower risk for major depression (MDD), suicidal behavior, reduced cortical thinning and increased posterior EEG alpha, which has also been linked to antidepressant treatment response in MDD. Building on prior event-related potential (ERP) findings using an emotional hemifield paradigm, this study examined whether abnormal early (preconscious) responsivity to negative arousing stimuli, which is indicative of right parietotemporal dysfunction in both MDD patients and individuals at clinical high risk for MDD, is likewise moderated by R/S. We reanalyzed 72-channel ERP data from 127 individuals at high or low family risk for MDD (Kayser et al., 2017, NeuroImage Clin. 14, 692-707) after R/S stratification (low R/S importance, low/high risk, n = 38/61; high R/S importance, n = 15/13). ERPs were transformed to reference-free current source density (CSD) and quantified by temporal principal components analysis (tPCA). This report focused on N2 sink (peak latency 212 ms), the earliest prominent CSD-tPCA component previously found to be sensitive to emotional content. While overall N2 sink reflected activation of occipitotemporal cortex (prestriate/cuneus), as estimated via a distributed inverse solution, affective significance was marked by a relative (i.e., superimposed) positivity. Statistical analyses employed both non-parametric permutation tests and repeated measures ANOVA for mixed factorial designs with unstructured covariance matrix, including sex, age, and clinical covariates. Participants with low R/S importance, independent of risk status, showed greater ERP responsivity to negative than neutral stimuli, particularly over the right hemisphere. In contrast, early emotional ERP responsivity and asymmetry was substantially reduced for high risk individuals with high R/S importance, however, enhanced for low risk individuals with high R/S importance. Hemifield modulations of these effects (i.e., emotional ERP enhancements with left visual field/right hemisphere stimulus presentations) further corroborated these observations. Results suggest down-regulation of a right-lateralized network for salience detection at an early processing stage in high risk and high R/S importance individuals, presumably to prevent overactivation of ventral brain regions further downstream. These findings may point to a neurophysiological mechanism underlying resilience of families at risk for depression with high R/S prioritization.

Quantifying evidential value and selective reporting in recent and 10-year past psychophysiological literature: A pre-registered P-curve analysis

Selective reporting (i.e., only reporting significant findings as opposed to all analyses or results) is a questionable research practice that undermines the integrity of published research. Psychophysiology research may be susceptible to selective reporting, given the high number of decision points and methodological complexity in analyses of psychophysiology data. We aimed to assess the presence of selective reporting and evidential value (i.e., that significant results are due to true underlying effects) in recent and past psychophysiological research by utilizing p-curve analyses. Study protocols and methods were pre-registered on the Open Science Framework (OSF). P-values and the associated test statistics were extracted from articles in the most recent issue (as of January 2018) and 10-year previous counterpart issue of three major psychophysiology journals: Psychophysiology, International Journal of Psychophysiology, and Journal of Psychophysiology. Using the p-curve application, 10 primary p-curves were conducted: all recent articles, all past articles, recent articles split by journal, past articles split by journal, recent cognitive electrophysiology articles, and past cognitive electrophysiology articles. Evidential value and generally adequate average power (≥78% average power) were present in all p-curves, except those that only included articles from the Journal of Psychophysiology because of the small number of articles published in the journal. Findings provide some positive news and indicate that, generally, results were not selectively reported, and selective reporting may not be a primary issue for this sample of psychophysiological research. Future p-curve analyses examining sub-disciplines of psychophysiology are recommended.

Electrophysiological Responses to Expectancy Violations in Semantic and Gambling Tasks: A Comparison of Different EEG Reference Approaches

Dynamically evaluating the outcomes of our actions and thoughts is a fundamental cognitive ability. Given its excellent temporal resolution, the event-related potential (ERP) technology has been used to address this issue. The feedback-related negativity (FRN) component of ERPs has been studied intensively with the averaged linked mastoid reference method (LM). However, it is unknown whether FRN can be induced by an expectancy violation in an antonym relations context and whether LM is the most suitable reference approach. To address these issues, the current research directly compared the ERP components induced by expectancy violations in antonym expectation and gambling tasks with a within-subjects design and investigated the effect of the reference approach on the experimental effects. Specifically, we systematically compared the influence of the LM, reference electrode standardization technique (REST) and average reference (AVE) approaches on the amplitude, scalp distribution and magnitude of ERP effects as a function of expectancy violation type. The expectancy deviation in the antonym expectation task elicited an N400 effect that differed from the FRN effect induced in the gambling task; this difference was confirmed by all the three reference methods. Both the amplitudes of the ERP effects (N400 and FRN) and the magnitude as the expectancy violation increased were greater under the LM approach than those under the REST approach, followed by those under the AVE approach. Based on the statistical results, the electrode sites that showed the N400 and FRN effects critically depended on the reference method, and the results of the REST analysis were consistent with previous ERP studies. Combined with evidence from simulation studies, we suggest that REST is an optional reference method to be used in future ERP data analysis.

Different Topological Properties of EEG-Derived Networks Describe Working Memory Phases as Revealed by Graph Theoretical Analysis

Several non-invasive imaging methods have contributed to shed light on the brain mechanisms underlying working memory (WM). The aim of the present study was to depict the topology of the relevant EEG-derived brain networks associated to distinct operations of WM function elicited by the Sternberg Item Recognition Task (SIRT) such as encoding, storage, and retrieval in healthy, middle age (46 ± 5 years) adults. High density EEG recordings were performed in 17 participants whilst attending a visual SIRT. Neural correlates of WM were assessed by means of a combination of EEG signal processing methods (i.e., time-varying connectivity estimation and graph theory), in order to extract synthetic descriptors of the complex networks underlying the encoding, storage, and retrieval phases of WM construct. The group analysis revealed that the encoding phase exhibited a significantly higher small-world topology of EEG networks with respect to storage and retrieval in all EEG frequency oscillations, thus indicating that during the encoding of items the global network organization could “optimally” promote the information flow between WM sub-networks. We also found that the magnitude of such configuration could predict subject behavioral performance when memory load increases as indicated by the negative correlation between Reaction Time and the local efficiency values estimated during the encoding in the alpha band in both 4 and 6 digits conditions. At the local scale, the values of the degree index which measures the degree of in- and out- information flow between scalp areas were found to specifically distinguish the hubs within the relevant sub-networks associated to each of the three different WM phases, according to the different role of the sub-network of regions in the different WM phases. Our findings indicate that the use of EEG-derived connectivity measures and their related topological indices might offer a reliable and yet affordable approach to monitor WM components and thus theoretically support the clinical assessment of cognitive functions in presence of WM decline/impairment, as it occurs after stroke.

Electrophysiological Correlates of Change Detection during Delayed Matching Task: A Comparison of Different References

Detecting the changed information between memory representation and incoming sensory inputs is a fundamental cognitive ability. By offering the promise of excellent temporal resolution, event-related potential (ERP) technique has served as a primary tool for studying this process with reference of the linked mastoid (LM). However, given that LM may distort the ERP signals, it is still undetermined whether LM is the best reference choice. The goal of the current study was to systematically compare LM, reference electrode standardization technique (REST) and average reference (AR) for assessing the ERP correlates of change detection during a delayed matching task. Colored shapes were adopted as materials while both the task-relevant shape feature and -irrelevant color feature could be changed. The results of the ERP amplitude showed that both of the task-relevant and -conjunction feature changes elicited significantly more positive posterior P2 in REST and AR, but not in LM. Besides, significantly increased N270 was observed in task-relevant and -conjunction feature changes in both the REST and LM, but in the conjunction feature change in AR. Only the REST-obtained N270 revealed a significant increment in task-irrelevant feature change, which was compatible with the delayed behavioral performance. Statistical parametric scalp mapping (SPSM) results showed a left posterior distribution for AR, an anterior distribution for LM, and both the anterior and left posterior distributions for REST. These results indicate that different types of references may provide distinct cognitive interpretations. Interestingly, only the SPSM of REST was consistent with previous fMRI findings. Combined with the evidence of simulation studies and the current observations, we take the REST-based results as the objective one, and recommend using REST technology in the future ERP data analysis.

A Comparative Study of Average, Linked Mastoid, and REST References for ERP Components Acquired during fMRI

In simultaneous electroencephalogram (EEG) and functional magnetic resonance imaging (fMRI) studies, average reference (AR), and digitally linked mastoid (LM) are popular re-referencing techniques in event-related potential (ERP) analyses. However, they may introduce their own physiological signals and alter the EEG/ERP outcome. A reference electrode standardization technique (REST) that calculated a reference point at infinity was proposed to solve this problem. To confirm the advantage of REST in ERP analyses of synchronous EEG-fMRI studies, we compared the reference effect of AR, LM, and REST on task-related ERP results of a working memory task during an fMRI scan. As we hypothesized, we found that the adopted reference did not change the topography map of ERP components (N1 and P300 in the present study), but it did alter the task-related effect on ERP components. LM decreased or eliminated the visual working memory (VWM) load effect on P300, and the AR distorted the distribution of VWM location-related effect at left posterior electrodes as shown in the statistical parametric scalp mapping (SPSM) of N1. ERP cortical source estimates, which are independent of the EEG reference choice, were used as the golden standard to infer the relative utility of different references on the ERP task-related effect. By comparison, REST reference provided a more integrated and reasonable result. These results were further confirmed by the results of fMRI activations and a corresponding EEG-only study. Thus, we recommend the REST, especially with a realistic head model, as the optimal reference method for ERP data analysis in simultaneous EEG-fMRI studies.

Motivated attention and family risk for depression: Neuronal generator patterns at scalp elicited by lateralized aversive pictures reveal blunted emotional responsivity

Behavioral and electrophysiologic evidence suggests that major depression (MDD) involves right parietotemporal dysfunction, a region activated by arousing affective stimuli. Building on prior event-related potential (ERP) findings (Kayser et al. 2016 NeuroImage 142:337–350), this study examined whether these abnormalities also characterize individuals at clinical high risk for MDD. We systematically explored the impact of family risk status and personal history of depression and anxiety on three distinct stages of emotional processing comprising the late positive potential (LPP). ERPs (72 channels) were recorded from 74 high and 53 low risk individuals (age 13–59 years, 58 male) during a visual half-field paradigm using highly-controlled pictures of cosmetic surgery patients showing disordered (negative) or healed (neutral) facial areas before or after treatment. Reference-free current source density (CSD) transformations of ERP waveforms were quantified by temporal principal components analysis (tPCA). Component scores of prominent CSD-tPCA factors sensitive to emotional content were analyzed via permutation tests and repeated measures ANOVA for mixed factorial designs with unstructured covariance matrix, including gender, age and clinical covariates. Factor-based distributed inverse solutions provided descriptive estimates of emotional brain activations at group level corresponding to hierarchical activations along ventral visual processing stream. Risk status affected emotional responsivity (increased positivity to negative-than-neutral stimuli) overlapping early N2 sink (peak latency 212 ms), P3 source (385 ms), and a late centroparietal source (630 ms). High risk individuals had reduced right-greater-than-left emotional lateralization involving occipitotemporal cortex (N2 sink) and bilaterally reduced emotional effects involving posterior cingulate (P3 source) and inferior temporal cortex (630 ms) when compared to those at low risk. While the early emotional effects were enhanced for left hemifield (right hemisphere) presentations, hemifield modulations did not differ between risk groups, suggesting top-down rather than bottom-up effects of risk. Groups did not differ in their stimulus valence or arousal ratings. Similar effects were seen for individuals with a lifetime history of depression or anxiety disorder in comparison to those without. However, there was no evidence that risk status and history of MDD or anxiety disorder interacted in their impact on emotional responsivity, suggesting largely independent attenuation of attentional resource allocation to enhance perceptual processing of motivationally salient stimuli. These findings further suggest that a deficit in motivated attention preceding conscious awareness may be a marker of risk for depression.

•

Emotional hemifield ERP task with 127 individuals at high and low family risk for MDD

•

CSD-PCA methods summarized affective modulation of late positive potential (LPP).

•

High risk and prior diagnosis of MDD or anxiety disorder independently reduced LPP.

•

Suggested hypoarousal (top-down) of right temporoparietal and other emotional regions

•

Left hemifield (bottom-up) modulations of early emotional asymmetries were preserved.

Neuronal generator patterns at scalp elicited by lateralized aversive pictures reveal consecutive stages of motivated attention

Event-related potential (ERP) studies have provided evidence for an allocation of attentional resources to enhance perceptual processing of motivationally salient stimuli. Emotional modulation affects several consecutive components associated with stages of affective-cognitive processing, beginning as early as 100-200ms after stimulus onset. In agreement with the notion that the right parietotemporal region is critically involved during the perception of arousing affective stimuli, some ERP studies have reported asymmetric emotional ERP effects. However, it is difficult to separate emotional from non-emotional effects because differences in stimulus content unrelated to affective salience or task demands may also be associated with lateralized function or promote cognitive processing. Other concerns pertain to the operational definition and statistical independence of ERP component measures, their dependence on an EEG reference, and spatial smearing due to volume conduction, all of which impede the identification of distinct scalp activation patterns associated with affective processing. Building on prior research using a visual half-field paradigm with highly controlled emotional stimuli (pictures of cosmetic surgery patients showing disordered [negative] or healed [neutral] facial areas before or after treatment), 72-channel ERPs recorded from 152 individuals (ages 13-68years; 81 female) were transformed into reference-free current source density (CSD) waveforms and submitted to temporal principal components analysis (PCA) to identify their underlying neuronal generator patterns. Using both nonparametric randomization tests and repeated measures ANOVA, robust effects of emotional content were found over parietooccipital regions for CSD factors corresponding to N2 sink (212ms peak latency), P3 source (385ms) and a late centroparietal source (630ms), all indicative of greater positivity for negative than neutral stimuli. For the N2 sink, emotional effects were right-lateralized and modulated by hemifield, with larger amplitude and asymmetry for left hemifield (right hemisphere) presentations. For all three factors, more positive amplitudes at parietooccipital sites were associated with increased ratings of negative valence and greater arousal. Distributed inverse solutions of the CSD-PCA-based emotional effects implicated a sequence of maximal activations in right occipitotemporal cortex, bilateral posterior cingulate cortex, and bilateral inferior temporal cortex. These findings are consistent with hierarchical activations of the ventral visual pathway reflecting subsequent processing stages in response to motivationally salient stimuli.

Implicit memory for words heard during sleep

When we fall asleep, our awareness of the surrounding world fades. Yet, the sleeping brain is far from being dormant and recent research unraveled the preservation of complex sensory processing during sleep. In wakefulness, such processes usually lead to the formation of long-term memory traces, being it implicit or explicit. We examined here the consequences upon awakening of the processing of sensory information at a high level of representation during sleep. Participants were instructed to classify auditory stimuli as words or pseudo-words, through left and right hand responses, while transitioning toward sleep. An analysis of the electroencephalographic (EEG) signal revealed the preservation of lateralized motor activations in response to sounds, suggesting that stimuli were correctly categorized during sleep. Upon awakening, participants did not explicitly remember words processed during sleep and failed to distinguish them from new words (old/new recognition test). However, both behavioral and EEG data indicate the presence of an implicit memory trace for words presented during sleep. In addition, the underlying neural signature of such implicit memories markedly differed from the explicit memories formed during wakefulness, in line with dual-process accounts arguing for two independent systems for explicit and implicit memory. Thus, our results reveal that implicit learning mechanisms can be triggered during sleep and provide a novel approach to explore the neural implementation of memory without awareness.

Estimating a neutral reference for electroencephalographic recordings: the importance of using a high-density montage and a realistic head model

Objective. In electroencephalography (EEG) measurements, the signal of each recording electrode is contrasted with a reference electrode or a combination of electrodes. The estimation of a neutral reference is a long-standing issue in EEG data analysis, which has motivated the proposal of different re-referencing methods, among which linked-mastoid re-referencing (LMR), average re-referencing (AR) and reference electrode standardization technique (REST). In this study we quantitatively assessed the extent to which the use of a high-density montage and a realistic head model can impact on the optimal estimation of a neutral reference for EEG recordings. Approach. Using simulated recordings generated by projecting specific source activity over the sensors, we assessed to what extent AR, REST and LMR may distort the scalp topography. We examined the impact electrode coverage has on AR and REST, and how accurate the REST reconstruction is for realistic and less realistic (three-layer and single-layer spherical) head models, and with possible uncertainty in the electrode positions. We assessed LMR, AR and REST also in the presence of typical EEG artifacts that are mixed in the recordings. Finally, we applied them to real EEG data collected in a target detection experiment to corroborate our findings on simulated data. Main results. Both AR and REST have relatively low reconstruction errors compared to LMR, and that REST is less sensitive than AR and LMR to artifacts mixed in the EEG data. For both AR and REST, high electrode density yields low re-referencing reconstruction errors. A realistic head model is critical for REST, leading to a more accurate estimate of a neutral reference compared to spherical head models. With a low-density montage, REST shows a more reliable reconstruction than AR either with a realistic or a three-layer spherical head model. Conversely, with a high-density montage AR yields better results unless precise information on electrode positions is available. Significance. Our study is the first to quantitatively assess the performance of EEG re-referencing techniques in relation to the use of a high-density montage and a realistic head model. We hope our study will help researchers in the choice of the most effective re-referencing approach for their EEG studies.

Posterior EEG alpha at rest and during task performance: Comparison of current source density and field potential measures

Resting and task-related EEG alpha are used in studies of cognition and psychopathology. Although Laplacian methods have been applied, apprehensions about loss of global activity dissuade researchers from greater use except as a supplement to reference-dependent measures. The unfortunate result has been continued reliance on reference strategies that differ across labs, and a systemic preference for a montage-dependent average reference over true reference-free measures. We addressed these concerns by comparing resting- and task-related EEG alpha using three EEG transformations: nose- (NR) and average-referenced (AR) EEG, and the corresponding CSD. Amplitude spectra of resting and prestimulus task-related EEG (novelty oddball) and event-related spectral perturbations were scaled to equate each transformation. Alpha measures quantified for 8-12 Hz bands were: 1) net amplitude (eyes-closed minus eyes-open) and 2) overall amplitude (eyes-closed plus eyes-open); 3) task amplitude (prestimulus baseline) and 4) task event-related desynchronization (ERD). Mean topographies unambiguously represented posterior alpha for overall, net and task, as well as poststimulus alpha ERD. Topographies were similar for the three transformations, but differed in dispersion, CSD being sharpest and NR most broadly distributed. Transformations also differed in scale, AR showing less attenuation or spurious secondary maxima at anterior sites, consistent with simulations of distributed posterior generators. Posterior task alpha and alpha ERD were positively correlated with overall alpha, but not with net alpha. CSD topographies consistently and appropriately represented posterior EEG alpha for all measures.

Issues and considerations for using the scalp surface Laplacian in EEG/ERP research: A tutorial review

Despite the recognition that the surface Laplacian may counteract adverse effects of volume conduction and recording reference for surface potential data, electrophysiology as a discipline has been reluctant to embrace this approach for data analysis. The reasons for such hesitation are manifold but often involve unfamiliarity with the nature of the underlying transformation, as well as intimidation by a perceived mathematical complexity, and concerns of signal loss, dense electrode array requirements, or susceptibility to noise. We revisit the pitfalls arising from volume conduction and the mandated arbitrary choice of EEG reference, describe the basic principle of the surface Laplacian transform in an intuitive fashion, and exemplify the differences between common reference schemes (nose, linked mastoids, average) and the surface Laplacian for frequently-measured EEG spectra (theta, alpha) and standard event-related potential (ERP) components, such as N1 or P3. We specifically review common reservations against the universal use of the surface Laplacian, which can be effectively addressed by employing spherical spline interpolations with an appropriate selection of the spline flexibility parameter and regularization constant. We argue from a pragmatic perspective that not only are these reservations unfounded but that the continued predominant use of surface potentials poses a considerable impediment on the progress of EEG and ERP research.

Hemifield-dependent N1 and event-related theta/delta oscillations: An unbiased comparison of surface Laplacian and common EEG reference choices

Surface Laplacian methodology has been used to reduce the impact of volume conduction and arbitrary choice of EEG recording reference for the analysis of surface potentials. However, the empirical implications of employing these different transformations to the same EEG data remain obscure. This study directly compared the statistical effects of four commonly-used (nose, linked mastoids, average) or recommended (reference electrode standardization technique [REST]) references and their spherical spline current source density (CSD) transformation for a large data set stemming from a well-understood experimental manipulation. ERPs (72 sites) recorded from 130 individuals during a visual half-field paradigm with highly-controlled emotional stimuli were characterized by mid-parietooccipital N1 (125 ms peak latency) and event-related synchronization (ERS) of theta/delta (160 ms), which were most robust over the contralateral hemisphere. All five data transformations were rescaled to the same covariance and submitted to a single temporal or time-frequency PCA (Varimax) to yield simplified estimates of N1 or theta/delta ERS. Unbiased nonparametric permutation tests revealed that these hemifield-dependent asymmetries were by far most focal and prominent for CSD data, despite all transformations showing maximum effects at mid-parietooccipital sites. Employing smaller subsamples (signal-to-noise) or window-based ERP/ERS amplitudes did not affect these comparisons. Furthermore, correlations between N1 and theta/delta ERS at these sites were strongest for CSD and weakest for nose-referenced data. Contrary to the common notion that the spatial high pass filter properties of a surface Laplacian reduce important contributions of neuronal generators to the EEG signal, the present findings demonstrate that instead volume conduction inherent in surface potentials weakens the representation of neuronal activation patterns at scalp that directly reflect regional brain activity.

Permutation Tests in the Educational and Behavioral Sciences

Over the past two decades, permutation tests (PTs) have received much attention in the educational and behavioral sciences. The aim of this article is to review the theoretical developments of PTs, the active areas in the educational and behavioral research using PTs, and the types of analysis under which PTs have been applied. We obtained 224 published articles, which included 141 theoretical articles and 83 application articles. After scrutinizing each article, we are happy to see that (1) some researchers began to advocate introducing PTs into basic statistics training; (2) computing load for PTs may be reduced dramatically by some intelligent algorithms; (3) PTs began to be applied in new areas such as studies on the relationship between brain and behavior and the relationship between gene and behavior; (4) besides simple types of analysis such as independent two-group comparison, PTs can also be carried out under more complex situations such as multivariate analysis. However, we should also notice that PTs are still mostly used for simple analyses (e.g., randomness analysis).

Auditory event-related potentials and α oscillations in the psychosis prodrome: neuronal generator patterns during a novelty oddball task

Prior research suggests that event-related potentials (ERP) obtained during active and passive auditory paradigms, which have demonstrated abnormal neurocognitive function in schizophrenia, may provide helpful tools in predicting transition to psychosis. In addition to ERP measures, reduced modulations of EEG alpha, reflecting top-down control required to inhibit irrelevant information, have revealed attentional deficits in schizophrenia and its prodromal stage. Employing a three-stimulus novelty oddball task, nose-referenced 48-channel ERPs were recorded from 22 clinical high-risk (CHR) patients and 20 healthy controls detecting target tones (12% probability, 500Hz; button press) among nontargets (76%, 350Hz) and novel sounds (12%). After current source density (CSD) transformation of EEG epochs (-200 to 1000ms), event-related spectral perturbations were obtained for each site up to 30Hz and 800ms after stimulus onset, and simplified by unrestricted time-frequency (TF) principal components analysis (PCA). Alpha event-related desynchronization (ERD) as measured by TF factor 610-9 (spectral peak latency at 610ms and 9Hz; 31.9% variance) was prominent over right posterior regions for targets, and markedly reduced in CHR patients compared to controls, particularly in three patients who later developed psychosis. In contrast, low-frequency event-related synchronization (ERS) distinctly linked to novels (260-1; 16.0%; mid-frontal) and N1 sink across conditions (130-1; 3.4%; centro-temporoparietal) did not differ between groups. Analogous time-domain CSD-ERP measures (temporal PCA), consisting of N1 sink, novelty mismatch negativity (MMN), novelty vertex source, novelty P3, P3b, and frontal response negativity, were robust and closely comparable between groups. Novelty MMN at FCz was, however, absent in the three converters. In agreement with prior findings, alpha ERD and MMN may hold particular promise for predicting transition to psychosis among CHR patients.

Olfaction in the psychosis prodrome: electrophysiological and behavioral measures of odor detection

Smell identification deficits (SIDs) are relatively specific to schizophrenia and its negative symptoms, and may predict transition to psychosis in clinical high-risk (CHR) individuals. Moreover, event-related potentials (ERPs) to odors are reduced in schizophrenia. This study examined whether CHR patients show SIDs and abnormal olfactory N1 and P2 potentials. ERPs (49 channels) were recorded from 21 CHR and 20 healthy participants (13 males/group; ages 13-27 years) during an odor detection task using three concentrations of hydrogen sulfide (H2S) or blank air presented unilaterally by a constant-flow olfactometer. Neuronal generator patterns underlying olfactory ERPs were identified and measured by principal components analysis (unrestricted Varimax) of reference-free current source densities (CSD). Replicating previous findings, CSD waveforms to H2S stimuli were characterized by an early N1 sink (345 ms, lateral-temporal) and a late P2 source (600 ms, mid-frontocentroparietal). N1 and P2 varied monotonically with odor intensity (strong > medium > weak) and did not differ across groups. Patients and controls also showed comparable odor detection and had normal odor identification and thresholds (Sniffin' Sticks). However, olfactory ERPs strongly reflected differences in odor intensity and detection in controls, but these associations were substantially weaker in patients. Moreover, severity of negative symptoms in patients was associated with reduced olfactory ERPs and poorer odor detection, identification and thresholds. Three patients who developed psychosis had poorer odor detection and thresholds, and marked reductions of N1 and P2. Thus, despite the lack of overall group differences, olfactory measures may be of utility in predicting transition to psychosis among CHR patients.

Electrophysiological Correlates of Perceptual Auditory Priming Without Explicit Recognition Memory

The aim of this study was to identify an event-related potential (ERP correlate) of perceptual auditory priming using a method that can dissociate it from explicit memory similar to Rugg et al. (1998) . EEG was recorded during performance of an auditory word recognition test, where 17 participants discriminated “old” from “new” aural words, encoded using either a “deep” or “shallow” levels-of-processing (LOP) study task. A right-lateralized P200 effect was modulated by words’ old/new status but not by accuracy of recognition or LOP manipulation. Because this effect was driven by simple repetition rather than factors known to influence episodic recognition memory, a “bottom-up” perceptual priming function was inferred which was substantiated by its early temporal appearance. A similar ERP amplitude modulation was evident across a broader topographical region during the subsequent N400 time interval. Conversely the late posterior component (LPC; 500–800 ms) for deeply-encoded, correctly-recognized words was of higher amplitude than LPCs for shallowly-encoded and new words, consistent with proposals that this ERP component indexes episodic memory. To our knowledge this is the first report of an ERP correlate of auditory perceptual priming dissociated from explicit episodic memory.

Generator localization by current source density (CSD): implications of volume conduction and field closure at intracranial and scalp resolutions

The topographic ambiguity and reference-dependency that has plagued EEG/ERP research throughout its history are largely attributable to volume conduction, which may be concisely described by a vector form of Ohm's Law. This biophysical relationship is common to popular algorithms that infer neuronal generators via inverse solutions. It may be further simplified as Poisson's source equation, which identifies underlying current generators from estimates of the second spatial derivative of the field potential (Laplacian transformation). Intracranial current source density (CSD) studies have dissected the "cortical dipole" into intracortical sources and sinks, corresponding to physiologically-meaningful patterns of neuronal activity at a sublaminar resolution, much of which is locally cancelled (i.e., closed field). By virtue of the macroscopic scale of the scalp-recorded EEG, a surface Laplacian reflects the radial projections of these underlying currents, representing a unique, unambiguous measure of neuronal activity at scalp. Although the surface Laplacian requires minimal assumptions compared to complex, model-sensitive inverses, the resulting waveform topographies faithfully summarize and simplify essential constraints that must be placed on putative generators of a scalp potential topography, even if they arise from deep or partially-closed fields. CSD methods thereby provide a global empirical and biophysical context for generator localization, spanning scales from intracortical to scalp recordings.

A neurophysiological deficit in early visual processing in schizophrenia patients with auditory hallucinations

Existing 67-channel event-related potentials, obtained during recognition and working memory paradigms with words or faces, were used to examine early visual processing in schizophrenia patients prone to auditory hallucinations (AH, n = 26) or not (NH, n = 49) and healthy controls (HC, n = 46). Current source density (CSD) transforms revealed distinct, strongly left- (words) or right-lateralized (faces; N170) inferior-temporal N1 sinks (150 ms) in each group. N1 was quantified by temporal PCA of peak-adjusted CSDs. For words and faces in both paradigms, N1 was substantially reduced in AH compared with NH and HC, who did not differ from each other. The difference in N1 between AH and NH was not due to overall symptom severity or performance accuracy, with both groups showing comparable memory deficits. Our findings extend prior reports of reduced auditory N1 in AH, suggesting a broader early perceptual integration deficit that is not limited to the auditory modality.

Effect of audiovisual training on monaural spatial hearing in horizontal plane

The article aims to test the hypothesis that audiovisual integration can improve spatial hearing in monaural conditions when interaural difference cues are not available. We trained one group of subjects with an audiovisual task, where a flash was presented in parallel with the sound and another group in an auditory task, where only sound from different spatial locations was presented. To check whether the observed audiovisual effect was similar to feedback, the third group was trained using the visual feedback paradigm. Training sessions were administered once per day, for 5 days. The performance level in each group was compared for auditory only stimulation on the first and the last day of practice. Improvement after audiovisual training was several times higher than after auditory practice. The group trained with visual feedback demonstrated a different effect of training with the improvement smaller than the group with audiovisual training. We conclude that cross-modal facilitation is highly important to improve spatial hearing in monaural conditions and may be applied to the rehabilitation of patients with unilateral deafness and after unilateral cochlear implantation.

Explicit and implicit memory in female college students with schizotypal traits: an event-related potential study

The explicit and implicit memory of nonclinical individuals with schizotypal traits was investigated using event-related potentials. Explicit and implicit memory was measured with continuous recognition and categorization tasks, respectively. On the recognition task, the control group demonstrated a greater old/new effect in response to the old than to the new words during the 250-750 ms post-stimulus period, whereas schizotypal trait group did not exhibit an old/new effect during the 550-650 ms period. The control group demonstrated faster response times to the old than to the new words, whereas the schizotypal group demonstrated longer response times to the old than to the new words. On the categorization task, both groups showed old/new effects during the 250-550 ms after stimulus onset and responded more rapidly and with fewer errors to the old than to the new words. These results suggest that individuals with schizotypal traits have impaired explicit but preserved implicit memory.

Neuronal generator patterns of olfactory event-related brain potentials in schizophrenia

To better characterize neurophysiologic processes underlying olfactory dysfunction in schizophrenia, nose-referenced 30-channel electroencephalogram was recorded from 32 patients and 35 healthy adults (18 and 18 male) during detection of hydrogen sulfide (constant-flow olfactometer, 200 ms unirhinal exposure). Event-related potentials (ERPs) were transformed to reference-free current source density (CSD) waveforms and analyzed by unrestricted Varimax-PCA. Participants indicated when they perceived a high (10 ppm) or low (50% dilution) odor concentration. Patients and controls did not differ in detection of high (23% misses) and low (43%) intensities and also had similar olfactory ERP waveforms. CSDs showed a greater bilateral frontotemporal N1 sink (305 ms) and mid-parietal P2 source (630 ms) for high than low intensities. N1 sink and P2 source were markedly reduced in patients for high intensity stimuli, providing further neurophysiological evidence of olfactory dysfunction in schizophrenia.

ERP generator patterns in schizophrenia during tonal and phonetic oddball tasks: effects of response hand and silent count

Greater left than right reductions of P3 amplitude in schizophrenia during auditory oddball tasks have been interpreted as evidence of left-lateralized dysfunction. However, the contributions of methodological factors (response mode, stimulus properties, recording reference), which affect event-related potential (ERP) topographies, remain unclear. We recorded 31-channel ERPs from 23 schizophrenic patients and 23 age- and gender-matched healthy controls (all right-handed) during tonal and phonetic oddball tasks, varying response mode (left press, right press, silent count) within subjects. Performance accuracy was high in both groups but patients were slower. ERP generator patterns were summarized by temporal Principal Components Analysis (PCA; unrestricted Varimax) from reference-free current source density (CSD; spherical spline Laplacians) waveforms, which sharpen scalp topographies. CSD represents the magnitude of the radial current flow entering (source) and leaving (sink) the scalp. Both patients and controls showed asymmetric frontolateral and parietotemporal N2 sinks peaking at 240 ms and asymmetric parietal P3 sources (355 ms) for targets (tonal R > L, phonetic L > R), but frontocentral N2 sinks and parietal P3 sources were bilaterally reduced in patients. A response-related midfrontal sink and accompanying centroparietal source (560 ms) were highly comparable across groups. However, a superimposed left temporal source was larger for silent count compared to button press, and this difference was smaller in patients. In both groups, left or right press produced opposite, region-specific asymmetries originating from central sites, modulating the N2/P3 complex. The results suggest bilaterally reduced neural generators of N2 and P3 in schizophrenia during auditory oddball tasks, but both groups showed comparable topographic effects of task and response mode. However, additional working memory demands during silent count may partially overlap in time the generation of the N2/P3 complex and differentially affect the asymmetry of P3 subcomponents, particularly when employing conventional ERP measures.

Novelty P3 reductions in depression: characterization using principal components analysis (PCA) of current source density (CSD) waveforms

We previously reported a novelty P3 reduction in depressed patients compared to healthy controls (n=20 per group) in a novelty oddball task using a 31-channel montage. In an independent replication and extension using a 67-channel montage (n=49 per group), reference-free current source density (CSD) waveforms were simplified and quantified by a temporal, covariance-based principal components analysis (PCA) (unrestricted Varimax rotation), yielding factor solutions consistent with other oddball tasks. A factor with a loadings peak at 343 ms summarized the target P3b source as well as a secondary midline frontocentral source for novels and targets. An earlier novelty vertex source (NVS) at 241 ms was present for novels, but not targets, and was reduced in patients. Compatible CSD-PCA findings were also confirmed for the original low-density sample. Results are consistent with a reduced novelty response in clinical depression, involving the early phase of the frontocentral novelty P3.

Current source density (CSD) old/new effects during recognition memory for words and faces in schizophrenia and in healthy adults

We previously reported a preserved 'old-new effect' (enhanced parietal positivity 300-800 ms following correctly-recognized repeated words) in schizophrenia over mid-parietal sites using 31-channel nose-referenced event-related potentials (ERP) and reference-free current source densities (CSD). However, patients showed poorer word recognition memory and reduced left lateral-parietal P3 sources. The present study investigated whether these abnormalities are specific to words. High-density ERPs (67 channels) were recorded from 57 schizophrenic (24 females) and 44 healthy (26 females) right-handed adults during parallel visual continuous recognition memory tasks using common words or unknown faces. To identify and measure neuronal generator patterns underlying ERPs, unrestricted Varimax-PCA was performed using CSD estimates (spherical spline surface Laplacian). Two late source factors peaking at 442 ms (lateral parietal maximum) and 723 ms (centroparietal maximum) accounted for most of the variance between 250 and 850 ms. Poorer (76.6+/-20.0% vs. 85.7+/-12.4% correct) and slower (824+/-170 vs. 755+/-147 ms) performance in patients was accompanied by reduced stimulus-locked parietal sources. However, both controls and patients showed mid-frontal (442 ms) and left parietal (723 ms) old/new effects in both tasks. Whereas mid-frontal old/new effects were comparable across groups and tasks, later left parietal old/new effects were markedly reduced in patients over lateral temporoparietal but not mid-parietal sites, particularly for words, implicating impaired phonological processing. In agreement with prior results, ERP correlates of recognition memory deficits in schizophrenia suggest functional impairments of lateral posterior cortex (stimulus representation) associated with conscious recollection. This deficit was more pronounced for common words despite a greater difficulty to recall unknown faces, indicating that it is not due to a generalized cognitive deficit in schizophrenia.

Stimulus- and response-locked neuronal generator patterns of auditory and visual word recognition memory in schizophrenia

Examining visual word recognition memory (WRM) with nose-referenced EEGs, we reported a preserved ERP 'old-new effect' (enhanced parietal positivity 300-800 ms to correctly-recognized repeated items) in schizophrenia ([Kayser, J., Bruder, G.E., Friedman, D., Tenke, C.E., Amador, X.F., Clark, S.C., Malaspina, D., Gorman, J.M., 1999. Brain event-related potentials (ERPs) in schizophrenia during a word recognition memory task. Int. J. Psychophysiol. 34(3), 249-265.]). However, patients showed reduced early negative potentials (N1, N2) and poorer WRM. Because group differences in neuronal generator patterns (i.e., sink-source orientation) may be masked by choice of EEG recording reference, the current study combined surface Laplacians and principal components analysis (PCA) to clarify ERP component topography and polarity and to disentangle stimulus- and response-related contributions. To investigate the impact of stimulus modality, 31-channel ERPs were recorded from 20 schizophrenic patients (15 male) and 20 age-, gender-, and handedness-matched healthy adults during parallel visual and auditory continuous WRM tasks. Stimulus- and response-locked reference-free current source densities (spherical splines) were submitted to unrestricted Varimax-PCA to identify and measure neuronal generator patterns underlying ERPs. Poorer (78.2+/-18.7% vs. 87.8+/-11.3% correct) and slower (958+/-226 vs. 773+/-206 ms) performance in patients was accompanied by reduced stimulus-related left-parietal P3 sources (150 ms pre-response) and vertex N2 sinks (both overall and old/new effects) but modality-specific N1 sinks were not significantly reduced. A distinct mid-frontal sink 50-ms post-response was markedly attenuated in patients. Reductions were more robust for auditory stimuli. However, patients showed increased lateral-frontotemporal sinks (T7 maximum) concurrent with auditory P3 sources. Electrophysiologic correlates of WRM deficits in schizophrenia suggest functional impairments of posterior cortex (stimulus representation) and anterior cingulate (stimulus categorization, response monitoring), primarily affecting memory for spoken words.

Hemispatial PCA dissociates temporal from parietal ERP generator patterns: CSD components in healthy adults and depressed patients during a dichotic oddball task

Event-related potentials (31-channel ERPs) were recorded from 38 depressed, unmedicated outpatients and 26 healthy adults (all right-handed) in tonal and phonetic oddball tasks developed to exploit the perceptual challenge of a dichotic stimulation. Tonal nontargets were pairs of complex tones (corresponding to musical notes G and B above middle C) presented simultaneously to each ear (L/R) in an alternating series (G/B or B/G; 2-s fixed SOA). A target tone (note A) replaced one of the pair on 20% of the trials (A/B, G/A, B/A, A/G). Phonetic nontargets were L/R pairs of syllables (/ba/, /da/) with a short voice onset time (VOT), and targets contained a syllable (/ta/) with a long VOT. Subjects responded with a left or right button press to targets (counterbalanced across blocks). Target detection was poorer in patients than controls and for tones than syllables. Reference-free current source densities (CSDs; spherical spline Laplacian) derived from ERP waveforms were simplified and measured using temporal, covariance-based PCA followed by unrestricted Varimax rotation. Target-related N2 sinks and mid-parietal P3 sources were represented by CSD factors peaking at 245 and 440 ms. The P3 source topography included a secondary, left-lateralized temporal lobe maximum for both targets and nontargets. However, a subsequent hemispheric spatiotemporal PCA disentangled temporal lobe N1 and P3 sources as distinct factors. P3 sources were reduced in patients compared with controls, even after using performance as a covariate. Results are consistent with prior reports of P3 reduction in depression and implicate distinct parietal and temporal generators of P3 when using a dichotic oddball paradigm.