Prefrontal Electrodes Research Articles

Oscillatory EEG correlates of an implicit activation of multiplication facts in the number bisection task

Neuroimaging evidence points towards the left inferior parietal cortex to be crucial for the representation and retrieval of multiplication facts. However, to date studies allowing a functional interpretation of neuroimaging data are still scarce. In the current study we aimed at evaluating the functional involvement of left inferior parietal cortex areas in the implicit retrieval of multiplication fact knowledge in a number bisection task by examining event-related desynchronization (ERD) in the upper alpha band. Upper alpha ERD is generally agreed to be modulated by processes of memory retrieval. It was observed that upper alpha ERD decreased for multiplicative triplets (e.g. 3_6_9) but not for non-multiplicative (e.g. 2_5_8) triplets at left parietal electrodes but increased at left prefrontal electrodes. These results are interpreted to suggest that after multiplicativity has been recognized further magnitude evaluations in the left hemisphere may be abated by prefrontal processes of executive control.

Brain Electrical Source Differences between Depressed Subjects and Healthy Controls

Many brain regions show metabolic and perfusion abnormalities in major depressive disorder (MDD), including anterior cingulate and prefrontal cortices. Some of these same areas also show abnormal function with low resolution electromagnetic tomography (LORETA). However, LORETA results are not always consistent across studies, nor with findings from other imaging modalities. These discrepancies may be due, among other factors, to the sensitivity of EEG source localization to different electrode montages. Thirty-six channel EEG was collected from healthy controls and age- and gender-matched unmedicated subjects with MDD (n = 74). EEGs were analyzed with LORETA to assess resting state current density at each of 2,394 cortical voxels. For comparison to previous studies, LORETA was performed using all electrodes or with specific prefrontal electrodes removed. Voxel-by-voxel differences between the depressed and healthy groups were calculated using non-parametric statistics. MDD subjects showed significantly elevated current density in delta, theta, alpha, beta1, and beta2 frequency bands relative to controls in anterior cingulate and prefrontal cortices. Removal of certain prefrontal electrodes from input to LORETA decreased or eliminated significant differences between groups. LORETA detects differences in brain activity between MDD subjects and healthy controls that are consistent with previous findings using other imaging modalities. Inconsistent findings among LORETA studies, and between LORETA studies and those using other functional imaging techniques, may result from differences in electrode montages.

Transmission of information during Continuous Attention Test.

The Short-Time Directed Transfer Function (SDTF) is an estimator based on a multivariate autoregressive model which has proved to be successful in ERP experiments, e.g. those connected with motor action and its imagination. The aim of this study is the evaluation of the performance of SDTF in the cognitive experiment. We have applied SDTF for the estimation of the pattern of EEG signal transmissions during a Continuous Attention Test (CAT). Time-frequency patterns of propagation were estimated for two experimental conditions. Statistical procedures based on thin-plate spline model were used for estimation of significant changes in respect to the reference epoch. The repeatability of the results for a subject and across the subjects were investigated. The effect of prolonged transmission in the gamma band from the prefrontal electrodes found in all subjects was explained by the active inhibition in the case when a subject had to sustain from performing the action.

Identification of wave‐like spatial structure in the SSVEP: Comparison of simultaneous EEG and MEG

Steady-state visual-evoked potentials/fields (SSVEPs/SSVEFs) are used in cognitive and clinical electroencephalogram (EEG) and magnetoencephalogram (MEG) studies because of their excellent signal-to-noise ratios and relative immunity to artifact. Steady-state paradigms are also used to characterize preferred frequencies of dynamic neocortical processes. In this study, SSVEPs and SSVEFs were simultaneously recorded while subjects viewed checkerboard patterns alternating (black to white, white to black) with fixed driving frequency between 2 and 20 Hz. Distinct peaks in SSVEP/SSVEF power were observed in the theta (4-8 Hz) and upper alpha (10-14 Hz) bands. A distinct peak in SSVEP power was also observed in the beta band (between 15 and 20 Hz) which had no counterpart in the MEG. One-dimensional spatial spectra indicate that distinct large-scale source distributions contribute to SSVEP power in the upper alpha band in the form of long wavelength (lambda>20 cm) traveling waves propagating from occipital to prefrontal electrodes. In the beta band, spatial spectra and SSVEF indicate that long-wavelength source distributions over posterior and anterior regions form standing wave patterns. These results suggest that simple models of SSVEP based on a single dipole source in the occipital lobe are inadequate to explain the dynamic spatial patterns of SSVEP magnitude and phase. Theoretical models of SSVEP should include multiple local and distributed sources and exhibit both traveling and standing wave dynamics.

Steady-State Visual Evoked Potentials: Distributed Local Sources and Wave-Like Dynamics Are Sensitive to Flicker Frequency

Steady-state visual evoked potentials (SSVEPs) are used in cognitive and clinical studies of brain function because of excellent signal-to-noise ratios and relative immunity to artifacts. SSVEPs also provide a means to characterize preferred frequencies of neocortical dynamic processes. In this study, SSVEPs were recorded with 110 electrodes while subjects viewed random dot patterns flickered between 3 and 30 Hz. Peaks in SSVEP power were observed at delta (3 Hz), lower alpha (7 and 8 Hz), and upper alpha band (12 and 13 Hz) frequencies; the spatial distribution of SSVEP power is also strongly dependent on the input frequency suggesting cortical resonances. We characterized the cortical sources that generate SSVEPs at different input frequencies by applying surface Laplacians and spatial spectral analysis. Laplacian SSVEPs recorded are sensitive to small changes (1-2 Hz) in the input frequency at occipital and parietal electrodes indicating distinct local sources. At 10 Hz, local source activity occurs in multiple cortical regions; Laplacian SSVEPs are also observed in lateral frontal electrodes. Laplacian SSVEPs are negligible at many frontal electrodes that elicit strong potential SSVEPs at delta, lower alpha, and upper alpha bands. One-dimensional (anterior-posterior) spatial spectra indicate that distinct large-scale source distributions contribute SSVEP power in these frequency bands. In the upper alpha band, spatial spectra indicate the presence of long-wavelength (> 15 cm) traveling waves propagating from occipital to prefrontal electrodes. In the delta and lower alpha band, spatial spectra indicate that long-wavelength source distributions over posterior and anterior regions form standing-wave patterns. These results suggest that the SSVEP is generated by both (relatively stationary) localized sources and distributed sources that exhibit characteristics of wave phenomena.

Detrimental Effects of Irrelevant Speech on Serial Recall of Visual Items are Reflected in Reduced Visual N1 and Reduced Theta Activity

The term irrelevant sound effect (ISE) describes an empirically robust finding in which serial recall performance of visual items is reduced by irrelevant speech. At present little is known about its neurophysiological basis. Although some previous neuroelectric studies have concentrated on responses elicited by irrelevant background sound, whether the processing of visually presented to-be-remembered digits itself is affected by irrelevant speech has yet to be studied. An experiment (n = 20) was conducted in which serial recall performance for visually presented digits was tested during exposure to either irrelevant speech, continuous white noise or silence while measuring EEG activity. White noise was chosen as a control condition, because it constitutes auditory stimulation while leaving serial recall performance unimpaired. In addition to replicating the detrimental behavioural effect of irrelevant speech, an analogous speech-specific early decrease ( approximately 160 ms) in the visual ERP (N1) and subsequently a reduced theta response (4-6 Hz; 400-800 ms) at right prefrontal electrodes were observed. Although irrelevant sound presentation was restricted to the visual presentation phase, power spectra reveal that the weaker theta response for speech persisted in a silent retention phase before serial recall. Based on such data we propose re-evaluating the role attention plays in explaining the ISE.

Decreased EEG coherence between prefrontal electrodes: a correlate of high language proficiency?

To investigate the influence of proficiency level on the cortical organization of foreign language processing, two groups of German speaking students, differing only in their proficiency in English as a second language, were subjected to EEG coherence analysis during foreign and native language processing (news reports, alpha1 frequency band). In the group with minor experience with English, coherence increase was observed with all electrode combinations, with left hemisphere (LH) predominance. In the high proficiency group, coherence increase was limited to temporal electrodes over LH. In the latter group only, coherence between prefrontal electrodes was significantly lower during the language tasks than during the baseline task (silence, noisy screen). Both results were obtained with foreign as well as native language processing. We suggest that reduced EEG coherence in highly proficient foreign language speakers reflects a more efficient operating strategy not only for their second, but also for their native language.

Phase-coupling of theta–gamma EEG rhythms during short-term memory processing

Because of the importance of oscillations as a general phenomenon of neuronal activity the use of EEG spectral analysis is among the most important approaches for studying human information processing. Usually, oscillations at different frequencies occur simultaneously during information processing. Thus, the question for synchronisation of different frequencies by phase coupling and its possible functional significance is of primary importance. An answer may be given by bispectral analysis. Estimation of the (cross-) bispectrum allows to identify synchronised frequencies and possibly, the existence of non-linear phase coupling of different oscillators. Previous studies have demonstrated the simultaneous occurrence of slow (4–7 Hz) and fast (20–30 Hz) oscillations at frontal and prefrontal electrode positions during memory processing. However, interrelations between these rhythms have not been investigated up to now. In order to test short-term memory, the Sternberg task with random figures and number words was carried out on 10 female subjects. During the task EEG was recorded. Power and bispectral analyses from frontal, prefrontal and frontopolar regions were performed off-line. Increased power was found in both the theta and the gamma bands. Strong phase-coupling between theta at Fz and gamma at F3 and at Fp1, respectively, was shown for memorising number words by means of cross-bicoherence. A possible reason for this is an amplitude modulation of gamma frequencies by slow oscillations. The correspondent coherence analysis between the envelope of gamma frequencies at Fp1 and the raw EEG at Fz supports this presumption. This finding is interpreted as an EEG aspect of the functional linking between the prefrontal areas and the G.cinguli (as part of the limbic system), which are both extremely important for memory functions.

Quantified EEG changes associated with a positive clinical response to clozapine in schizophrenia

1. 1- The authors conducted a retrospective exploratory computerized EEG study on the effect of clozapine in treatment-refractory schizophrenics, 10 high-responders (HRs) and 10 low-responders ( LRs ). in an attempt to correlate amplitude but especially coherence changes with a positive clinical response to clozapine. 2. 2- EEGs with eyes closed were obtained before and during a clozapine treatment. Both groups had a similar drug profile with regards to non-clozapine medication. Probability maps were computed to illustrate changes of amplitude and coherence (for all combinations of paired electrodes) for 4 frequency bands (theta, alpha, beta 1 and 2). 3. 3- The effect on AMPLITUDE was a generalized increase in lower bands and a decrease in anterior regions in higher bands of both HRs and LRs. Considerable changes of COHERENCE were observed on a wide set of paired electrodes in most frequency bands with some electrodes involved in HRs but uninvolved or differently involved in LRs suggesting differences in selected brain regions. 4. 4- Changes of coherence but not amplitude were correlated with changes on the BPRS, thus to clinical improvement, and concerned mostly the right anterior-medial temporal (T4) and central (C4) electrodes paired with prefrontal electrodes, left central (C3), temporal (T3) and parietal (P3) electrodes in the theta band. 5. 5- Clozapine has both generalized and regional effects as can be suspected considering its effect on many subtypes of brain receptors. A brain dysfunction centralized on the right anterior-medial temporal region may characterize treatment-refractory schizophrenics.

Brain activation with a maze test

The maze test is a complex cognitive task involving visuoperceptual, planning and foresight abilities. EEGs were recorded from 20 healthy subjects according to the 10/20 system. Coherence is analogous to a correlation coefficient between the signals recorded by two electrodes and is computed between each electrode paired to all other electrodes. The maze test provoked changes of coherence mainly within the prefrontal regions, between parietal and central electrodes paired with frontal electrodes, especially the right frontopolar and the left prefrontal electrodes, and in the posterior temporal, parietal and occipital regions. EEG coherence analysis detects changes in regions known to be involved in visual processing and executive functions.

Chronic morphine fails to enhance the reward value of prefrontal cortex self-stimulation

Dopamine (DA) plays an important role in the rewarding effects of drugs of abuse and intracranial self-stimulation (ICSS). We previously reported that ICSS derived from the prefrontal cortex appears insensitive to the reward-enhancing effects of amphetamine, a drug that increases DA release and reward at other ICSS sites. In the present study, rats with prefrontal electrodes were tested to see if morphine (7.5 or 10.0 mg/kg, IP) given once per day for 10 days enhanced prefrontal reward as assessed with the curve-shift method. Morphine initially produced sedation; however, after 3–4 days response rates increased sharply while frequency thresholds were unaffected. These results demonstrate that morphine does not enhance prefrontal ICSS reward and provide further evidence that prefrontal brain stimulation reward does not display the same characterics as other ICSS sites.