Electrical source analysis of auditory ERPs in medial temporal lobe amnestic syndrome

Impact of Reward and Loss Anticipation on Cognitive Control: An Event-Related Potential Study in Subjects With Schizophrenia and Healthy Controls

INTRODUCTION

Deficits of cognitive functions and motivation are core aspects of schizophrenia. The interaction of these deficits might contribute to impair the ability to flexibly adjust behavior in accordance with one's intentions and goals. Many studies have focused on the anterior N2 as a correlate of cognitive control based on motivational value.

AIMS

Given the key role of motivation impairment in schizophrenia as a predictor of functional outcome, we aimed to study the impact of reward- and avoidance-based motivation on cognitive control using N2.

METHOD

Event-related potentials were recorded during the execution of the "Monetary Incentive Delay (MID)" task in 34 patients with schizophrenia (SCZ) stabilized on second-generation antipsychotics and 22 healthy controls (HC). Cognitive domains were assessed using the MATRICS Consensus Cognitive Battery. Negative symptom domains (Avolition/apathy and Expressive deficit), as well as positive and disorganization dimensions were also assessed in SCZ.

RESULTS

We did not observe any group difference in N2 amplitude or latency. In HC, N2 amplitude was significantly larger for anticipation of large loss with regard to all reward conditions and for all incentive versus neutral conditions. In SCZ, N2 amplitude did not discriminate between large loss and reward or between incentive and neutral conditions. N2 amplitude was not correlated with psychopathological dimensions or MCCB-assessed cognitive deficits in SCZ.

CONCLUSION

Our data in HC are in line with the hypothesis that N2 amplitude reflects the impact of motivational salience on cognitive control. Our results in SCZ indicate a deficit in the discrimination of motivational salience to the service of cognitive control, independently of psychopathology and other cognitive deficits.

Distributed cortical network for visual attention

The contribution of prefrontal and posterior association cortex to voluntary and involuntary visual attention was as sessed using electrophysiological techniques in patients with focal lesions in prefrontal (n = 11), temporal-parietal (n = 10), or lateral parietal cortex (n = 7). Subjects participated in a task requiring detection of designated target stimuli embedded in trains of repetitive stimuli. Infrequent and irrelevant novel visual stimuli were randomly interspersed with the target and background stimuli. Controls generated attention dependent N1 (170 msec) and N2 (243 msec) potentials maximal over extrastriate cortex. Anterior and posterior association cortex lesions reduced the amplitude of both the N1 and N2 potentials recorded over extrastriate cortex of the lesioned hemisphere. The pattern of results obtained reveals that an intrahemispheric network involving prefrontal and posterior association cortex modulates early visual processing in extrastriate regions. Voluntary target detection generated a parietal maximal P300 response (P3b) and irrelevant novel stimuli generated a more frontocentrally distributed P300 (P3a). Cortical lesions had differential effects on P3a and P3b potentials. The P3b was not significantly reduced in any cortical lesioned group. Conversely, the P3a was reduced by both prefrontal and posterior lesions with decrements most severe throughout the lesioned hemisphere. These data provide evidence that an association cortex network involving prefrontal and posterior regions is activated during orientation to novel events. The lack of a significant effect on the visual target P3b in patients with novelty P3a reductions supports the notion that different neural systems are engaged during voluntary vs involuntary atten- tion to visual stimuli.

Somatosensory system deficits in schizophrenia revealed by MEG during a median-nerve oddball task

Although impairments related to somatosensory perception are common in schizophrenia, they have rarely been examined in functional imaging studies. In the present study, magnetoencephalography (MEG) was used to identify neural networks that support attention to somatosensory stimuli in healthy adults and abnormalities in these networks in patient with schizophrenia. A median-nerve oddball task was used to probe attention to somatosensory stimuli, and an advanced, high-resolution MEG source-imaging method was applied to assess activity throughout the brain. In nineteen healthy subjects, attention-related activation was seen in a sensorimotor network involving primary somatosensory (S1), secondary somatosensory (S2), primary motor (M1), pre-motor (PMA), and paracentral lobule (PCL) areas. A frontal-parietal-temporal "attention network", containing dorsal- and ventral-lateral prefrontal cortex (DLPFC and VLPFC), orbitofrontal cortex (OFC), anterior cingulate cortex (ACC), superior parietal lobule (SPL), inferior parietal lobule (IPL)/supramarginal gyrus (SMG), and temporal lobe areas, was also activated. Seventeen individuals with schizophrenia showed early attention-related hyperactivations in S1 and M1 but hypo-activation in S1, S2, M1, and PMA at later latency in the sensorimotor network. Within this attention network, hypoactivation was found in SPL, DLPFC, orbitofrontal cortex, and the dorsal aspect of ACC. Hyperactivation was seen in SMG/IPL, frontal pole, and the ventral aspect of ACC in patients. These findings link attention-related somatosensory deficits to dysfunction in both sensorimotor and frontal-parietal-temporal networks in schizophrenia.

A parietal–frontal network studied by somatosensory oddball MEG responses, and its cross-modal consistency

Previous studies using functional magnetic resonance imaging (fMRI) and event-related potentials (ERPs) of the brain have found that a distributed parietal-frontal neuronal network is activated in normals during both auditory and visual oddball tasks. The common cortical regions in this network are inferior parietal lobule (IPL)/supramarginal gyrus (SMG), anterior cingulate cortex (ACC), and dorsolateral prefrontal cortex (DLPFC). It is not clear whether the same network is activated by oddball tasks during somatosensory stimulation. The present study addressed this question by testing healthy adults as they performed a novel median-nerve oddball paradigm while undergoing magnetoencephalography (MEG). An automated multiple dipole analysis technique, the Multi-Start Spatio-Temporal (MSST) algorithm, localized multiple neuronal generators, and identified their time-courses. IPL/SMG, ACC, and DLPFC were reliably localized in the MEG median-nerve oddball responses, with IPL/SMG activation significantly preceding ACC and DLPFC activation. Thus, the same parietal-frontal neuronal network that shows activation during auditory and visual oddball tests is activated in a median-nerve oddball paradigm. Regions uniquely related to somatosensory oddball responses (e.g., primary and secondary somatosensory, dorsal premotor, primary motor, and supplementary motor areas) were also localized. Since the parietal-frontal network supports attentional allocation during performance of the task, this study may provide a novel method, as well as normative baseline data, for examining attention-related deficits in the somatosensory system of patients with neurological or psychiatric disorders.

Linking Brainwaves to the Brain: An ERP Primer

This article reviews literature on the characteristics and possible interpretations of the event-related potential (ERP) peaks commonly identified in research. The description of each peak includes typical latencies, cortical distributions, and possible brain sources of observed activity as well as the evoking paradigms and underlying psychological processes. The review is intended to serve as a tutorial for general readers interested in neuropsychological research and as a reference source for researchers using ERP techniques.

Hippocampal involvement in detection of deviant auditory and visual stimuli

Recent models of hippocampal function have emphasized its role in processing sequences of events. In this study, we used an oddball task to investigate hippocampal responses to the detection of deviant "target" stimuli that were embedded in a sequence of repetitive "standard" stimuli. Evidence from intracranial event-related potential studies has suggested a critical role for the hippocampus in oddball tasks. However, functional neuroimaging experiments have failed to detect activation in the hippocampus in response to deviant stimuli. Our study aimed to resolve this discrepancy by using a novel functional magnetic resonance imaging (fMRI) technique that drastically improves signal detection in the hippocampus. Significant hippocampal activation was observed during both auditory and visual oddball tasks. Although there was no difference in the overall level of hippocampal activation in the two modalities, significant modality differences in the profile of activation along the long axis of the hippocampus were observed. In both left and right hippocampi, an anterior-to-posterior gradient in the activation (anterior to posterior) was observed during the auditory oddball task, whereas a posterior-to-anterior gradient (posterior to anterior) was observed during the visual oddball task. These results indicate that the hippocampus is involved in the detection of deviant stimuli regardless of stimulus modality, and that there are prominent modality differences along the long axis of the hippocampus. The implications of our findings for understanding hippocampal involvement in processing sequences of events are discussed.

Impact of instructed relevance on the visual ERP

Event-related potentials have been used to study the distinction between attention to novelty and effortful orienting to task-relevant items. However, effects due to stimulus frequency, response production, and instructed task-relevance have not been fully separated. The current study used a visual oddball design in which two stimuli were presented, one frequently (P = 0.8) and one infrequently (P = 0.2). Subjects (n = 21) participated in 4 blocks, two in which the infrequent stimulus was the instructed target and two in which the frequent stimulus was the instructed target. This was crossed with two blocks in which subjects were instructed to respond to the targets with a keypress and two blocks in which subjects were instructed to press a key to the ignored non-targets and respond to the targets by withholding a keypress. The results showed N1 effects for instruction, suggesting enhancement due to an attentional set established prior to stimulus delivery, and for frequency, suggesting orienting to the infrequent stimuli. A prefrontal positivity (P2a) was enhanced to instructed targets, but instruction only entered as an interactive factor on the posterior N2 at the same latency, suggesting interaction between frontal areas of evaluation and posterior areas of perceptual representation in identification of task-relevant stimuli. The P300 was enhanced to the infrequent stimuli, but there was no main effect for target instruction, although instruction did impact a higher-order interaction.

The N200 abnormalities of auditory event-related potentials in patients with panic disorder

Cognitive impairment in patients with panic disorder (PD) has been studied using event-related potentials (ERPs). However, previous studies obtained ERP data only at a few scalp sites, and seldom investigated N200 measurements from the difference waveforms. In the present study, auditory ERPs were recorded at 16 scalp sites during an active discrimination task of oddball paradigm. Fourteen PD patients (8 with agoraphobia; 6 without agoraphobia) were compared with 14 sex- and age-matched control subjects. For the nontarget waveforms, P2 amplitude was reduced in PD patients. For the target waveforms, a topographical difference between female PD patients and female controls was found for N200 amplitude, which attenuated in female PD patients over the parietal area. Two subcomponents of N200, N2a and N2b, were measured from the difference waveforms. A significant group difference was found for N2b amplitude, which reduced in PD patients compared with unaffected control subjects. It is suggested that N2b reduction reflects an abnormally controlled processing of stimulus information in PD.

Event-related potentials and genetic risk for schizophrenia

BACKGROUND

Event-related potentials (ERPs) during an auditory oddball task were investigated in patients with schizophrenia and in their healthy siblings to explore the question of whether abnormalities of two-dimensional topographic scalp-distribution of P300 amplitude and latency relate to genetic risk for schizophrenia. We also examined the P50, N100, and P200-waves, elicited during the same task.

METHODS

We investigated 42 schizophrenic patients, 62 of their healthy siblings, and 34 unrelated normal control subjects with a standard auditory oddball paradigm and 16 electroencephalogram electrodes. Amplitudes and latencies of the ERPs P50, N100, P200, and P300 were topographically analyzed.

RESULTS

In the patients, P300 amplitude was significantly decreased in the range of 54%-58% over the left parietotemporal area. Siblings did not show decreased P300 amplitudes when compared with normal subjects. P300 latencies were unchanged in both groups. No significant group differences were observed for the other event-related potentials.

CONCLUSIONS

In line with previous studies, the P300 amplitude in schizophrenic patients was decreased over the left temporoparietal area; however, we found no evidence for a genetic trait effect in the event-related potential abnormality. Possible reasons for these largely negative findings are discussed.

Age-related changes of evoked potentials

The aim of this review is to analyse the current state of our knowledge on evoked potentials (EPs) in ageing and to report some conclusions on the relation between EPs and elder age. Evoked potentials provide a measure of the function of sensory systems that change during the different stages of life. Each sensory system has its own time of maturation. The individuation of the exact period of life when brain ageing starts is difficult to define. Normally, the amplitude of EPs decreases, and their latency increases from adult to elder life. Many authors speculate that these modifications might depend on neuronal loss, changes in cell membrane, composition or senile plaques present in older patients, but there is no evidence that these changes might modify the cerebral function in healthy aged individuals. This review emphasises some incongruities present in different studies confirmed by daily neurophysiologic practice. Different techniques as event-related desynchronization (ERD), contingent negative variation (CNV) and Bereitschaftspotential, are available to study central neuronal changes in normal and pathologic ageing.

Auditory event-related potentials in an amnesic patient with a left temporal lobe lesion

We studied auditory event-related potentials (ERPs) in correlation with Rey's auditory verbal learning test (AVLT) in an amnesic patient with a left temporal lobe lesion. On the AVLT, the number of words recalled was limited to immediate memory capacity (nine words) throughout five trials, and recall after interference was remarkably decreased. In the ERPs, the P300 component was elicited normally, but no N400 component was apparent. Present findings suggest that these two ERP components may be associated with independent processes and support the view that the N400 may index a neural process involved in transfer of information from immediate memory to long-term storage.

Differential effects of normal aging on sources of standard N1, target N1 and target P300 auditory event-related brain potentials revealed by low resolution electromagnetic tomography (LORETA)

The P300 event-related potential (ERP) is considered to be closely related to cognitive processes. In normal aging, P300 scalp latencies increase, parietal P300 scalp amplitudes decrease and the scalp potential field shifts to a relatively more frontal distribution. Based on ERPs recorded in 172 normal healthy subjects aged between 20 and 88 years in an auditory oddball paradigm, the effects of age on the electrical activity in the brain corresponding to N1 and P300 components were estimated by means of low resolution electromagnetic tomography (LORETA). This distributed approach directly computes a unique 3-dimensional electrical source distribution by assuming that neighbouring neurons are simultaneously and synchronously active. N1 LORETA generators, located predominantly in both auditory cortices and also symmetrically in prefrontal areas, increased with advancing age for standards but remained stable for targets. P300 LORETA generators, located symmetrically in the prefrontal cortex, in the parieto-occipital junction and in the inferior parietal cortex (supramarginal gyrus) and medially in the superior parietal cortex, were differentially affected by age. While age did not affect parieto-occipital sources, superior parietal and right prefrontal sources decreased pronouncedly. Thus, in normal aging, P300 current density decreased in regions were a fronto-parietal network for sustained attention was localized.

Cortical-hippocampal auditory processing identified by magnetoencephalography

We recorded magnetic and electrical responses simultaneously in an auditory detection task to elucidate the brain areas involved in auditory processing. Target stimuli evoked magnetic fields peaking at approximately the same latency of around about 400 msec (M400) over the anterior temporal, superior temporal, and parietal regions on each hemisphere. Equivalent current dipoles (ECDs) were analyzed with a time-varying multidipole model and superimposed on each subject's magnetic resonance image (MRI). Multiple independent dipoles located in the superior temporal plane, inferior parietal lobe, and mesial temporal region best accounted for the recorded M400 fields. These findings suggest that distributed activity in multiple structures including the mesial temporal, superior temporal, and inferior parietal regions on both hemispheres is engaged during auditory attention and memory updating.

Event related potentials recorded in patients with locked-in syndrome

OBJECTIVE

To determine the possibility of recording "cognitive" event related potentials (ERPs) in locked-in patients and therefore to determine whether ERPs can have a role in differential diagnosis of coma.

METHODS

ERPs to classic auditory or visual "odd ball paradigms" were recorded three to four days, seven to eight days, and 30 to 60 days after admission to the intensive care unit, in four patients affected by basilar artery thromboembolism resulting in locked-in syndrome. Two patients (one 32 year old man, one 31 year old woman) could move the eyes laterally and vertically spontaneously and on command. One patient (a 39 year old man) had a "one and half syndrome", one patient (a 40 year old woman) could only elevate the left eyelid and eye. Results were compared with data from 30 age matched controls. In the last recording session a letter recognition paradigm was applied, in which ERPs were produced by the identification of letters forming a word. Results were compared with five age matched controls. Brainstem lesions extending to the pontomesencephalic junction were found on MRI and CT.

RESULTS

ERPs to the oddball paradigms were recorded in three patients in the first recording session, in all patients in the second recording session. Latency, amplitude, and topographic distribution of ERP components were inside normal limits. With the letter recognition paradigm the patients could emit a P3 component to correspond with target letters, with the same margin of error as controls.

CONCLUSION

It is possible to record ERPs in patients with locked-in syndrome shortly after the acute ischaemic lesion, and therefore to assess objectively cognitive activities. Furthermore the letter recognition paradigm could be implemented to facilitate linguistic communication with patients with locked-in syndrome.

Generators for human P300 elicited by somatosensory stimuli using multiple dipole source analysis

Cognitive event-related potentials, such as P300, are sensitive to manipulations of psychological variables and may provide evidence to support theories of brain mechanisms involved in cognition. However, the relationship between event-related potentials and the active neural structures is not yet understood. Electrical stimulation of the index and little fingers of the left hand in the context of a somatosensory target discrimination task, performed by healthy human subjects, elicited the middle-latency component of somatosensory evoked potentials, N60, the long-latency component, N140, and the P300 component. Identification of the generators for both the earlier components and P300, using equivalent electrical dipole modeling, was performed. Individual spatiotemporal seven-dipole models were developed in order to suggest locations of the sources generating each subject's scalp-recorded wave forms. Three dipoles with fairly weak moments, located in the primary and secondary sensory areas, explained the middle- and long-latency somatosensory evoked potential components, and the remaining four dipoles (4-7), with stronger dipole moments, were active during P300. There was a clear temporal separation of dipole activity between the somatosensory evoked potential components and the P300 component. Dipoles 4 and 5 were found quite symmetrically in the parahippocampal areas of the two hemispheres, while dipoles 6 and 7 were slightly asymmetrical. Dipole 7 was found in the left hippocampal area. Dipole 6 appeared in the right insular cortex. The locations of the four dipoles implicated in the generation of the somatosensory P300 were compared with the locations of four dipoles accounting for the auditory evoked P300 described in our previous paper [Tarkka et al. (1995) Electroenceph. clin. Neurophysiol. 96, 538-545]. No substantial difference in source locations of the P300 was found between auditory and somatosensory modality other than an asymmetrical activity in the somatosensory modality contralateral to the stimulated hand.

Working memory in temporal lobe epilepsy: an event-related potential study

Event-related potentials (ERPs) were recorded to a digit-probe identification and matching task (modified 'Sternberg paradigm') in 29 patients with temporal lobe epilepsy (TLE) and 26 healthy subjects. Our main aim was to identify the neurophysiological correlates of abnormal short term memory function in patients with TLE. Neuropsychological tests allowed the definition and comparison of two patient groups according to the presence or absence of memory dysfunction. These two groups did not differ significantly in mean age, education years, IQ, seizure duration, seizure frequency, anti-epileptic drug (AED) regimes, or on findings on neuroimaging. ERPs recorded under different levels of memory load were analysed both by conventional component identification and by an objective computer method of determining mean amplitudes of multiple 50 ms epochs (MMA analysis). We found that some significant abnormalities were common to both groups of patients; these included slow reaction times, a reduced amplitude of the N170 wave (and the corresponding 157-210 ms epoch in the MMA analysis) and a broad late negative shift between 577 and 735 ms. Other findings, including a significantly reduced performance accuracy as the level of memory load increased, were restricted to patients with abnormal memory function. The ERP changes that were specific to these patients occurred within a latency band of 200-420 ms and included a relatively preserved, but delayed P250 component and a delayed and attenuated N290 wave. When compared with either healthy subjects or with patients with normal memory, the responses in patients with abnormal memory showed an abnormal 'positive shift' between 262 and 315 ms after probe presentation and a further positive shift between 315 and 420 ms as memory load increased. These abnormalities of 'memory scanning' ERPs in patients with TLE which paralleled neuropsychological and behavioural evidence of memory dysfunction, and which occurred in the section of the response that is sensitive to memory loading in healthy subjects, provide further objective evidence that abnormalities of short term memory processes contribute to the memory deficits of TLE.

Dipole-modeling of the visual evoked P300

We collected visual event-related potentials (ERPs) from 6 normal subjects using an "oddball" paradigm. Subjects were required to count the occurrences of matching shapes presented in the left and right visual field. Shapes matched on 20% of trials. ERPs were recorded from 20 or 43 electrodes distributed over the scalp. A multiple spatio-temporal equivalent dipole (ED) model was used to fit the early sensory and P300 component. A latency window to analyze the P300 was determined using the global field power statistic. The spatial topography of the P300 over this window was characterized by a midline positivity that decreased in amplitude with spatial distance from the peak. After sensory components were fit, the source of P300 could be accounted for by 1 or 2 EDs, which were usually located near medial temporal areas. This result is at odds with evidence from depth recordings during the oddball paradigm, showing that multiple regions of the brain are active during this interval.

Electric source localization of the auditory P300 agrees with magnetic source localization

The event-related cortical potential elicited in the context of auditory target detection tasks includes the N1, P2 and P3 components. The aim of the present study was to identify the sources of these scalp-recorded components using an electrical multiple dipole model. Nine healthy adults volunteered for the study. An auditory oddball paradigm was used. Stimuli (18% target and 82% non-target tones) were delivered through ear-phones and subjects were required to silently count the targets. Event-related potentials (ERPs) to these stimuli were recorded by 30 electrodes placed on the scalp. The identification of the sources of the ERP was attempted using the brain electric source analysis (BESA) program. The instantaneous source locations of N1, P2 and P3 reported in magnetoencephalographic (MEG) literature were used as initial starting locations for the spatio-temporal multiple dipole modeling of the EEG data. First the auditory long latency responses were modeled separately. Bilateral superior temporal plane sources with almost vertical orientations explained the first 250 msec window of the non-target tone recording including N1/P2 complex. This agrees with MEG source localization of N1m/P2m. Two slightly deeper dipoles in superior temporal gyri and bilateral dipoles in hippocampi or parahippocampal areas explained P3 (analysis window 250-600 msec). The final model explained the complete epoch of 600 msec with 6 dipoles and the residual variances of individual models ranged from 3.83% to 7.77%. The concordance between MEG and BESA source localization results supports the notion of generators in temporal lobes for the N1/P2 complex and generators in temporal and hippocampal areas for the P3 component.

Transient P300 abnormality of event-related potentials following unilateral temporal lobectomy

Event-related potentials (ERP) were recorded during auditory oddball tasks for a patient prior to and soon after left anterior temporal lobectomy. The N100 amplitude decreased bilaterally although the latency did not change after the lobectomy. The P300 amplitude decreased in the left hemisphere at 1 and 2 weeks after surgery, then recovered to the pre-operative level at 4 weeks. These findings suggest that the medial temporal structure participates in the generating system of P300.

P300 asymmetries in focal brain lesions are reference dependent

In 6 patients with lesions of frontal, parietal and temporal lobe, in 4 patients affected by primary progressive aphasia (PPA) and in 56 age-matched controls, event-related potentials (ERPs) to an auditory odd-ball paradigm were recorded with the linked earlobe reference (LER) and with a computer calculated average reference (AR), excluding the two linked earlobe derivations. Latencies, amplitudes and scalp distribution of the earlier ERP components (P1, N1, P2, N2) were within normal limits for both LER and AR recordings. P300 scalp distribution in patients was normal when LER was used. When P300 was recorded using AR, the scalp distribution was statistically different from normal distributions in all patients. A negativity, instead of the positive P300 observed in controls, was recorded in patients from leads corresponding to the affected hemisphere.

A test of brain electrical source analysis (BESA): a simulation study

The present report summarizes the results of a simulation study on the accuracy of Scherg's implementation of spatio-temporal analysis (BESA) for estimating the parameters (wave shape, location, orientation) of the intracranial sources of event-related brain potentials (ERPs) recorded from the scalp. In view of the subjective factors that might influence a solution, 10 subjects, ranging from those with much experience with ERPs and extensive background in the use of BESA to those with little experience with BESA and/or no knowledge of ERPs, independently analyzed a set of simulated somatosensory ERP data. The simulation contained wave forms from 32 electrode sites generated by a combination of 10 dipole sources. The primary question was how faithfully the different subjects would depict the source wave shapes, locations and orientations. Based on the 9 subjects who were familiar with ERPs, the grand-average cross-correlation coefficient between subjects' estimated and actual source wave shapes was 0.89 (standard deviation (S.D.) = 0.17). The grand-average location error, based upon a head diameter of 17 cm, was 1.4 cm (S.D. = 1.0 cm). The grand-average orientation error was 24.4 degrees (S.D. = 20 degrees).