Task-Related EEG Asymmetry: Effects of Age and Ability

EEG was recorded over parietal sites in 16 adult males while they were engaged in verbal and spatial tasks. A portion of the Modified Minnesota Paper Form Board Test was administered to obtain a measure of spatial ability. Computer analysis of 1 min of EEG during each task revealed relatively reduced amplitude of EEG alpha frequency (7–13 Hz) from the left hemisphere during the verbal task, while the opposite pattern held during the spatial task, with right hemisphere alpha reduced compared to the left hemisphere alpha. No task-related asymmetry was seen for the whole-band EEG. No relationship was found between spatial test scores and degree of task-related asymmetry, but degree of asymmetry increased with increasing age.     

Interhemisphere differences during tasks involving attention and selection of lateralized stimuli

The state of cortical activation in the parietal and temporal areas of the right and left hemispheres was evaluated using evoked potentials (EP) during tasks consisting of selection of visual stimuli lateralized in the right and left visual fields and needing three different types of attention: to stimulus shape, to stimulus position, and simultaneously to stimulus shape and position. EP were recorded in 15 young healthy experimental subjects using six cortical leads: P3, P4, T3, T4, T5, and T6; the following endogenous EP components (in standard terminology) were analyzed: contingent negative variation (CNV), N1, P3, and the N1-P3 complex. Asymmetry in evoked potentials was assessed in terms of differences to contra-and ipsilateral stimuli in the right and left hemispheres. EP asymmetry was detected in the right hemisphere in all types of selection of lateralized stimuli. The magnitude of asymmetry in the right hemisphere depended on the level (or intensity) of attention: the degree of asymmetry increased with increases in the need for attention to analyze the stimuli. There was a significant relationship between the magnitude of asymmetry and the latent periods of the subjects’ responses. The functional significance of these data demonstrating asymmetry may be that it provides better spatial differentiation of visual signals in the right hemisphere, along with dominance of the right hemisphere in attention tasks. __________ Translated from Rossiiskii Fiziologicheskii Zhurnal imeni I. M. Sechenova, Vol. 92, No. 6, pp. 709–722, June, 2006.     

Stability of EEG Laterality Effects

Task related interhemispheric asymmetries of ongoing EEG activity (alpha band) were tested for stability across three separate occasions. Seven cognitive tasks, presumed to differentially engage either the left or right cerebral hemisphere, were presented to 6 subjects. Involvement with the tasks was determined by evaluation of performance and with a subjective report on degree of involvement. Bilateral EEG was recorded over the parietal areas referenced to an ipsilateral ear, vertex and a central placement, on separate channels. Spectral intensity estimates of 8–13 Hz activity were used to form left/right ratios for determination of asymmetry. Intra-subject stability of alpha band activity was found to improve with the establishment of task validity and to be related to the electrode placement used.     

Increased frontal phase-locking of event-related alpha oscillations during task processing.

Recent findings substantiate the view that electroencephalographic (EEG) alpha rhythm (7-13 Hz) is functionally involved in cognitive stimulus processing. Our previous results have shown that enhanced alpha responses to auditory task stimuli can be well synchronized with stimulus until 800-1000 ms. The present study analyzed the effect of perceptual uncertainty and difficulty in decision making on event-related alpha oscillations in single auditory event-related brain potentials (ERPs). EEG was recorded from Fz, Cz and Pz electrodes in 10 subjects participating in two experimental sessions, in which auditory stimuli with equal physical parameters were presented under passive and task instructions. Separate measurements of single alpha response amplitude and phase-locking were performed and statistically analyzed for consecutive time windows in the post-stimulus epoch. Major results show that, during the cognitive task, the phase-locking of alpha oscillations at the frontal site is significantly increased in the time window of 500-1000 ms after stimulation. Thus, the involvement of enhanced and synchronized frontal alpha activity in higher brain processes is strongly emphasized.     

P300 hemispheric amplitude asymmetries from a visual oddball task

The P3(00) event-related potential (ERP) was elicited in 80 normal, right-handed male subjects using a simple visual discrimination task, with electroencephalographic (EEG) activity recorded at 19 electrodes. P3 amplitude was larger over the right than over the left hemisphere electrode sites primarily at anteromedial locations (F3/4, C3/4) for target, novel, and standard stimuli. The N1, P2, and N2 components also demonstrated hemispheric asymmetries. The strongest P3 hemispheric asymmetries for all stimuli were observed at anterior locations, suggesting a frontal right hemisphere localization for initial stimulus processing, although target stimuli produced larger P3 amplitudes at parietal locations than did novel stimuli. The relationships of hemispheric asymmetries to anatomical variables, background EEG activity, and neurocognitive factors are discussed.     

Effects of feedback control on EEG alpha asymmetry during covert mental tasks

EEG alpha (8-12 Hz) was recorded bilaterally from O1-P3 and O2-P4 during two eyes-closed sessions in 40 women. During one session subjects performed two covert right-hemisphere activation tasks and two covert left-hemisphere activation tasks (no-biofeedback condition). During the other session subjects performed the same tasks while trying to control their EEG asymmetries, being provided with EEG asymmetry auditory feedback (biofeedback condition). The ratios of integrated EEG alpha amplitudes were calculated as measure of hemispheric asymmetry, and the number of times that the right and left alpha had the same phase angle was determined as index of hemispheric synchrony (alpha phase index). In both conditions, subjects rated orally after each task their degree of subjective fullfilment of the task. There were significant differences between the EEG asymmetry during right- and left-hemisphere tasks. Significant differences of integrated alpha amplitudes and ratios between the no-biofeedback and biofeedback condition were also found. There was more hemispheric asymmetry in the biofeedback than in the no-biofeedback condition, and a significantly higher alpha phase index. There was no correlation between fullfilment ratings and alpha ratios in the no-biofeedback condition, but there was such correlation during right and left hemisphere tasks in the biofeedback condition. The study demonstrated that biofeedback regulation and covert mental activity can have effects on EEG alpha asymmetry which are cumulative.     

The processing of sound duration after left hemisphere stroke: Event-related potential and behavioral evidence

The ability of left-hemisphere stroke patients ( n = 8) and healthy control subjects ( n = 8) to process sounds preattentively and attentively was studied by recording auditory event-related potentials (ERPs) and behavioral responses. For the right-ear stimulation, the mismatch negativity (MMN) was significantly smaller in the patients than control subjects over both hemispheres. For the left-ear stimuli, the MMN was significantly smaller in the patient group than in the control group over the left hemisphere, whereas no group differences were obtained over the right hemisphere. In addition, the N1 amplitude was reduced bilaterally for the right-ear stimulation (with the reduction being larger over the left hemisphere), whereas no significant effects on the N1 amplitude were found for the left-ear stimulation. Behaviorally, the patients detected significantly fewer deviant tones than did the control subjects irrespective of the stimulated ear. The present results thus suggest that the long-latency ERPs can be used to probe such auditory processing deficits that are difficult to define with behavioral measures. Especially by recording MMN to monaural stimuli, the discrimination accuracy can be separately determined for the left and right temporal lobes.     

Hemispheric differences in auditory oddball responses during monaural versus binaural stimulation

Hemispheric lateralization of early event-related potentials (ERPs; e.g. N1) is largely based on anatomy of the afferent pathway; lateralization of later auditory ERPs (P2/N2, P250, P3b) is less clear. Using 257-channel EEG, the present study examined hemispheric laterality of auditory ERPs by comparing binaural and monaural versions of an auditory oddball task. N1 showed a contralateral bias over auditory cortex in both hemispheres as a function of ear of stimulation, although right hemisphere sources were activated regardless of which ear received input. Beginning around N1 and continuing through the time of P3b, right hemisphere temporal–parietal and frontal areas were more activated than their left hemisphere counterparts for stimulus evaluation/comparison and target detection. P250 and P3b component amplitudes, topographies, and source estimations were significantly influenced by ear of stimulation, with right hemisphere activity being stronger. This was particularly true for anterior temporal and inferior frontal sources which were more strongly associated with the later, more cognitive components (P250, P3b). Results are consistent with theories of a right hemisphere network that is prominently involved in sustained attention, stimulus evaluation, target detection, and working memory/context updating.     

Asymmetrical hemispheric EEG activation evoked by stimulus position during the Simon task

The Simon effect has been previously shown to be asymmetric at both the behavioral and electrophysiological levels. The present investigation was aimed to clarify whether, during a Simon task, hemispheric asymmetry is also observed in the early phases of stimulus processing. In a group of healthy subjects performing the Simon task, we analyzed scalp potentials evoked by the first lateralized cue (left or right), instead of the classical readiness potential preceding the motor response. ERP results showed a significant left cortical activation to stimuli presented in the right visual field at the 140–160 ms time window. Instead, left stimuli elicited a significant activation of the right versus left hemisphere starting at the next 160–180 ms time interval. We linked this asymmetry to that observed in behavioral data: the Simon effect recorded with left stimuli is smaller than the Simon effect recorded with right stimuli. Results confirm the hypothesis that in right handed subjects, left hemisphere is specialized for motor response selection and is able to process right stimuli faster than the right hemisphere does for left stimuli.     

音调与语音Oddball实验的脑电流源模型分析

采用语音和音调Oddball实验范式,研究大脑离散电流源在靶刺激响应过程中的电流时间过程。采集了健康受试者的64导联脑电数据,建立并比较音调和语音靶刺激ERPs数据的区域源电流模型。结果显示:听觉靶刺激识别的主要神经源包括双侧颞上回和双侧脑岛。音调靶刺激识别引起右侧区域源的N2/P3幅度大于左侧,呈现右偏侧化,其中脑岛的右偏侧化较明显,而语音靶刺激识别引起左侧区域源N2/P3幅度大于右侧,呈现左偏侧化,双侧脑岛和双侧颞上回都出现明显的左偏侧化。研究表明:靶刺激识别这一脑处理过程随刺激模式的不同存在差异,音调靶刺激识别以右侧脑岛和右侧颞上回的激活更强,而语音靶刺激识别以左侧颞上回的激活更强。     

EEG Asymmetry During Covert Mental Activity

Twenty subjects selected for (overt) task-induced EEG asymmetry performed 17 different covert mental tasks while EEG was recorded from right and left temporoparietal leads. After each task, subjects rated their experiences on four scales: (covert) verbalization, visual imagery, affect, and concentration. Zero-order and semipartial correlations between integrated EEG measures and subjective ratings showed that verbalization and concentration were systematically related to EEG asymmetry. The results suggest that verbal processes may be a more important influence on temporoparietal EEG asymmetry than are visual imagery processes, at least during purely covert tasks. The results also demonstrate that EEG asymmetry reflects lateralized cognitive processes occurring in the absence of external stimuli or overt responses.     

Stability of Pathway-Hemisphere Differences in the Auditory Event-Related Potential (ERP) to Monaural Stimulation

The evidence for ERP manifestations of contralateral dominance in the auditory pathways is not entirely compelling. The effect when found has involved either latency or amplitude of a variety of components obtained from averaged or subtraction waveforms. Also, the effect appears to have as much to do with hemisphere as pathway differences. The objectives of this experiment were to establish whether a specific ERP peak reflected contralateral dominance, to assess its long-term stability, and to clarify the apparent pathway-hemisphere interactions which result from monaural stimulation. Ten male subjects were tested on 6 occasions (generally weekly intervals). On each occasion, 76 monaural tones were presented while EEG was recorded at C₂, P₂, T₃, and T₄ referred to linked earlobes. P70, N120, and P200 peak amplitudes and latencies and N1–P2 amplitudes were examined. N120 amplitudes proved to be larger at the temporal site contralateral to the ear stimulated. This contralateral dominance effect was observed on all 6 occasions for right ear stimuli and on all but one occasion for left ear stimuli. N1–P2 amplitudes less reliably reflected contralateral dominance because possible hemisphere effects appeared with this measure. Methodological issues related to assessment of ERP contralateral dominance and to the variety of effects seen in the literature are discussed. The data are examined with reference to neurophysiological substrates thought to underlie auditory ERPs. Finally, the utility of a stable ERP phenomenon in assessing drug or pathology effects is evaluated.     

Consistency of Task-Related EEG Asymmetries

Temporoparietal EEG was recorded bilaterally while 11 subjects performed 4 verbal and 4 spatial tasks on two occasions. Significant differences in integrated EEG asymmetry (8-12 Hz) were found between verbal and spatial tasks in both sessions. Subjects’ verbal-minus-spatial differences in the ratio of left to right integrated amplitude were reliable both between- (r =.88) and within-(mean r =.75) subjects. A percent time measure did not show significant task effects and had poor reliability. For integrated amplitude, the means of the left/right ratios of the 8 tasks were also very consistent between sessions.     

Evaluating the feasibility of the steady‐state visual evoked potential (SSVEP) to study temporal attention

Improvements in perceptual performance can be obtained when events in the environment are temporally predictable—and temporal predictability improves attention and sensory processing. The amplitude of the steady‐state visual evoked potential (SSVEP) has been shown to correlate with attention paid to a flickering stimulus even if the flickering stimulus is irrelevant for the task. However, to our knowledge, the validity of the SSVEP to study temporal attention has not been established. Therefore, we designed an SSVEP temporal attention task to evaluate whether the SSVEP and its temporal dynamics can be used to study temporal attention. We used a forced‐choice perceptual detection task while presenting task‐irrelevant visual flicker at alpha (10 Hz) and two surrounding frequencies (6 or 15 Hz). Temporal predictability was manipulated by having the interstimulus intervals (ISI) be constant or variable. Behavioral results replicated previous studies confirming the benefits of temporal expectations on performance for trials with constant ISI. EEG analyses revealed robust SSVEP amplitudes for all flicker frequencies, although a main effect of temporal expectations on SSVEP amplitude was not significant. Additional analyses revealed temporal predictability‐related modulations of SSVEP amplitude at 10 Hz and its second harmonic (20 Hz). The effect of temporal predictability was also observed for the 6 Hz flicker, but not for 15 Hz for any ISI condition. These results provide some evidence for the feasibility of the SSVEP technique to study temporal attention for stimuli with flicker frequencies around the alpha band.     

Spectral EEG characteristics during increases in the complexity of the context of cognitive activity

The spatial and frequency characteristics of cortical electrical activity were studied in healthy human subjects in two series of experiments involving solution of sequentially presented visual tasks. The first task was to assess the relative sizes of two circles and was identical in both series. In the first series, this was supplemented by a task consisting of recognition of pseudowords/words, presumptively also requiring predominant involvement of the ventral “what?” visual system. In the second series, the additional task (spatial localization of a target stimulus in a matrix of letters) was associated with the predominant involvement of the dorsal “where?” visual system. Cortical electrical activity immediately before presentation of pairs of tasks was analyzed. Measures of EEG spectral power in the frontal, central, occipital, and temporal areas of the cortex was subjected to dispersion analysis. The power of electrical potentials in the delta and beta1 frequency ranges was greater when both tasks were associated predominantly with activation of the ventral visual system (first series of experiments). Power in the occipital alpha rhythm was lesser in the left hemisphere in both series of experiments. The interaction of the “experimental series” and “hemisphere” factors was significant in the temporal areas for EEG activity in the alpha2 range, where the predominant involvement of the ventral visual system on solution of both tasks corresponded to greater asymmetry in the electrical oscillations in the rapid alpha2 rhythm and its neighboring beta1 range with greater desynchronization (lesser power) on the left side. Thus, the nature of the ongoing activity is reflected in the spatial-frequency characteristics of the “background” electrical activity of the cortex.     

Dynamic movement of N100m current sources in auditory evoked fields: comparison of ipsilateral versus contralateral responses in human auditory cortex

We recorded auditory evoked magnetic fields (AEFs) to monaural 400 Hz tone bursts and investigated spatio-temporal features of the N100m current sources in the both hemispheres during the time before the N100m reaches at the peak strength and 5 ms after the peak. A hemispheric asymmetry was evaluated as the asymmetry index based on the ratio of N100m peak dipole strength between right and left hemispheres for either ear stimulation. The results of asymmetry indices showed right-hemispheric dominance for left ear stimulation but no hemispheric dominance for right ear stimulation. The current sources for N100m in both hemispheres in response to monaural 400 Hz stimulation moved toward anterolateral direction along the long axis of the Heschl gyri during the time before it reaches the peak strength; the ipsilateral N100m sources were located slightly posterior to the contralateral N100m ones. The onset and peak latencies of the right hemispheric N100m in response to right ear stimulation are shorter than those of the left hemispheric N100m to left ear stimulation. The traveling distance of the right hemispheric N100m sources following right ear stimulation was longer than that for the left hemispheric ones following left ear stimulation. These results suggest the right-dominant hemispheric asymmetry in pure tone processing.     

Bilateral Asymmetry of Skin Conductance Responses During Auditory and Visual Tasks

Bilateral skin conductance (SC) was recorded while dextral subjects engaged in tasks designed to differentially engage the right (RH) and left (LH) hemispheres. Subjects compared strings of speech sounds (LH) and musical chords (RH). They also decided whether written words rhymed (LH) and viewed pictures of faces in a continuous recognition paradigm (RH). SCRs in the right hand were larger during the chords task than the syllables task. The left hand did not differ for the two stimuli. In the visual experiment a comparable effect was obtained in males only. SCRs in the left hand were larger for rhymes than faces; the right hand did not differentiate between stimuli. Sex differences in laterality are considered. Subjects who were more equally balanced in awareness of the two sides of their bodies and subjects with familial sinistrals were more likely to show task appropriate SC changes. Using rote repetition and visual imagery as mnemonics did not affect SC asymmetries.     

Frequency tagging of steady‐state evoked potentials to explore the crossmodal links in spatial attention between vision and touch

The sustained periodic modulation of a stimulus induces an entrainment of cortical neurons responding to the stimulus, appearing as a steady‐state evoked potential (SS‐EP) in the EEG frequency spectrum. Here, we used frequency tagging of SS‐EPs to study the crossmodal links in spatial attention between touch and vision. We hypothesized that a visual stimulus approaching the left or right hand orients spatial attention toward the approached hand, and thereby enhances the processing of vibrotactile input originating from that hand. Twenty‐five subjects took part in the experiment: 16‐s trains of vibrotactile stimuli (4.2 and 7.2 Hz) were applied simultaneously to the left and right hand, concomitantly with a punctate visual stimulus blinking at 9.8 Hz. The visual stimulus was approached toward the left or right hand. The hands were either uncrossed (left and right hands to the left and right of the participant) or crossed (left and right hands to the right and left of the participant). The vibrotactile stimuli elicited two distinct SS‐EPs with scalp topographies compatible with activity in the contralateral primary somatosensory cortex. The visual stimulus elicited a third SS‐EP with a topography compatible with activity in visual areas. When the visual stimulus was over one of the hands, the amplitude of the vibrotactile SS‐EP elicited by stimulation of that hand was enhanced, regardless of whether the hands were uncrossed or crossed. This demonstrates a crossmodal effect of spatial attention between vision and touch, integrating proprioceptive and/or visual information to map the position of the limbs in external space.     

A right‐ear bias of auditory selective attention is evident in alpha oscillations

Auditory selective attention makes it possible to pick out one speech stream that is embedded in a multispeaker environment. We adapted a cued dichotic listening task to examine suppression of a speech stream lateralized to the nonattended ear, and to evaluate the effects of attention on the right ear's well‐known advantage in the perception of linguistic stimuli. After being cued to attend to input from either their left or right ear, participants heard two different four‐word streams presented simultaneously to the separate ears. Following each dichotic presentation, participants judged whether a spoken probe word had been in the attended ear's stream. We used EEG signals to track participants' spatial lateralization of auditory attention, which is marked by interhemispheric differences in EEG alpha (8–14 Hz) power. A right‐ear advantage (REA) was evident in faster response times and greater sensitivity in distinguishing attended from unattended words. Consistent with the REA, we found strongest parietal and right frontotemporal alpha modulation during the attend‐right condition. These findings provide evidence for a link between selective attention and the REA during directed dichotic listening.     

Left hemisphere lateralisation of auditory hallucinations in schizophrenia: A dichotic listening study

Introduction. We propose that auditory hallucinations are internally generated speech misrepresentations that are lateralised to the left temporal lobe. If hallucinations are misrepresentations involving the speech perception area of the left temporal lobe, then hallucinating patients should have problems identifying a simultaneously presented external speech sound, especially when the sound is lateralised to the left hemisphere. Lateralisation of speech perception can be experimentally studied with the dichotic listening task with consonant-vowel syllables. We predicted a negative relation between frequency of auditory hallucinations and performance on the dichotic listening task.

Method. We studied 87 right-handed patients with schizophrenia. Hallucination scores were taken from the BPRS symptom scale. Right and left ear scores in the dichotic listening task were recorded. A right ear advantage is expected in healthy individuals, indicating left temporal lobe processing superiority. The patients were compared with 36 right-handed healthy reference subjects.

Results. A gradual decrease in the ability to process and report the right ear stimulus with increasing frequency of hallucinations was seen in the schizophrenia patients. No such relationship was found for processing and reporting of the left ear stimulus. There were no significant correlations with negative symptoms. Thus, the results were not the consequence of illness severity. There was however a significant correlation with unusual thought content symptom, pointing to a relationship also between delusions and auditory hallucinations.

Conclusion. The results support that auditory hallucinations may be internally generated speech misrepresentations, originating in the left temporal lobe.