Localization of Brainstem Auditory Evoked Potentials in Primates: A Comparison of Localization Techniques Applied to Deep Brain Sources

Summary The objective of this study was to evaluate the performance of source localization techniques through localization of deep brain sources. To accomplish this, two replications of a brainstem auditory evoked potential (BAEP, left ear 60 dB nHL clicks) were recorded from five normal rhesus monkeys. We analyzed waves III and IV, as this portion of the BAEP corresponds to the deepest signal. Data were analyzed using five different source localization techniques: Moving Dipoles, Fixed Dipoles, MUSIC (Multiple Signal Classification) dipole scan, LORETA (Low Resolution Tomography), and LCMV (Linearly Constrained Minimum Variance) spatial filtering. The moving dipole, fixed dipole and MUSIC solutions were found to be, on average, 25.1 mm from the brainstem generators. LORETA detected sources within the brainstem 65 of the time. However, 90 of these localization results also included false detections defined as regions of the brain that were more than 2 cm away from the auditory pathway. LCMV included the brainstem in 90 of the trials and false detections in 40 of the cases. These findings indicate that evoked electrical activity from deep brain sources can be localized with cm accuracy. The dipole methods performed better than LORETA and LCMV. Given the depth and amplitude of the sources analyzed in this study, these results can be interpreted as an upper bound on the accuracy of each technique.     

Correspondence of Visual Evoked Potentials with FMRI Signals in Human Visual Cortex

Functional magnetic resonance imaging (FMRI) and event related potentials (ERPs) are tools that can be used to image brain activity with relatively good spatial and temporal resolution, respectively. Utilizing both of these methods is therefore desirable in neuroimaging studies to explore the spatio-temporal characteristics of brain function. While several studies have investigated the relationship between EEG and positive (+) BOLD (activation), little is known about the relationship between EEG signals and negative (−) BOLD (deactivation) responses. In this study, we used a visual stimuli designed to shift cortical activity from anterior to posterior regions of the visual cortex. Using EEG and FMRI, we investigated how shifts in +BOLD and −BOLD location were correlated to shifts in the N75 and P100 visual evoked potential (VEP) dipolar sources. The results show that the N75 dipole along with +BOLD, were indeed shifted from posterior to anterior regions of the visual cortex. The P100 VEP component, along with the −BOLD were not shifted to the same extent, indicating that N75 is better correlated to +BOLD than to −BOLD. These findings indicate how different components of the EEG signal are related to the positive and negative BOLD responses, which may aid in interpreting the relationship between visually evoked EEG and FMRI signals. An erratum to this article can be found at     

Discriminant Analysis and Equivalent Source Localization of the EEG Related to Cognitive Functions

Discriminant analysis and EEG source localization methods were employed to compare groups of normal subjects during different cognitive conditions using 43-channel EEG recordings in the alpha (8-13 Hz) frequency band. Recordings were obtained from 69 dextral females during 2 passive conditions, Eyes-Open and Eyes-Closed, and 2 active conditions, Word-Finding and Dot-Localization. The cross-spectral matrix between all of the electrode sites was used to characterize the EEGs obtained during each condition. The subjects were partitioned into training and test sets and quadratic discriminant functions were constructed from the training sets to classify the EEGs. The discriminant functions successfully classified both the training and test sets at rates approaching 80%. The classification was repeated using only the diagonal (power spectral) elements of the cross-spectral matrices in the discriminant functions and this approach was successful in discriminating between the EEGs from the passive cognitive conditions but failed to discriminate between the EEGs from the active conditions. Source localization using a modified MUSIC algorithm indicated that the centers of brain electrical activity that distinguished the Eyes-Closed condition from the Eyes-Open condition were located in the medial occipital and right frontal regions. Centers of electrical activity that distinguished the Word-Finding condition from the Dot-Localization condition were located in the right medial posterior and left temporal regions. Validation of the locations of the centers of activity was accomplished by repeating the classification procedures using the spatial patterns generated on the scalp by dipole current sources placed at these locations.     

颈椎病患者短潜时体感诱发电位的初步研究

为探讨短潜时体感诱发电位对颈椎病诊断的意义，检测了15例颈椎病患的短潜时体感诱发电位(SSEP)，并与对照组比较。结果表明：颈椎病患SSEP的N20、P25波潜伏期及N13-N20峰间潜伏期(即中枢神经传导时间，CCT)和N13-P25峰间潜伏期均比对照组明显延长，其SSEP异常率为73，3％。提示短潜时体感诱发电位检查能客观评价颈椎病患的神经功能，有助于评价疗效和判断预后。     

Localization of multiple dipoles: Mathematical programming approaches

Summary Although there has been progress in EEG and evoked potential analysis, the identification of underlying neural activity has eluded researchers despite its importance. This paper introduces the application of mathematical programming techniques to the "inverse" problem of three-dimensional localization of brain activity from scalp potentials. Preliminary computer experience with these optimization methods reported herein, yields encouraging results on simulated data and points new directions for research.     

视听联合刺激的跨感觉通道诱发电位源的分析

运用偶极子源分析方法分析视听单独刺激和联合刺激下脑区的区别和联系.采用64道脑电采集系统,记录了7名被试者在视觉、听觉和视听联合刺激下的脑诱发电位,并分别在初期、早期和晚期时间段对这3种任务做偶极子源分析.通过比较视听联合刺激和视听单独刺激下偶极子源的变化,可以得到视听联合刺激下的源不等于单独视听刺激下的源的简单叠加,从而提示存在视听相互作用的区域.     

Frontal theta and posterior alpha in resting EEG: A critical examination of convergent and discriminant validity

Prior research has identified two resting EEG biomarkers with potential for predicting functional outcomes in depression: theta current density in frontal brain regions (especially rostral anterior cingulate cortex) and alpha power over posterior scalp regions. As little is known about the discriminant and convergent validity of these putative biomarkers, a thorough evaluation of these psychometric properties was conducted toward the goal of improving clinical utility of these markers. Resting 71-channel EEG recorded from 35 healthy adults at two sessions (1-week retest) were used to systematically compare different quantification techniques for theta and alpha sources at scalp (surface Laplacian or current source density [CSD]) and brain (distributed inverse; exact low resolution electromagnetic tomography [eLORETA]) level. Signal quality was evaluated with signal-to-noise ratio, participant-level spectra, and frequency PCA covariance decomposition. Convergent and discriminant validity were assessed within a multitrait-multimethod framework. Posterior alpha was reliably identified as two spectral components, each with unique spatial patterns and condition effects (eyes open/closed), high signal quality, and good convergent and discriminant validity. In contrast, frontal theta was characterized by one low-variance component, low signal quality, lack of a distinct spectral peak, and mixed validity. Correlations between candidate biomarkers suggest that posterior alpha components constitute reliable, convergent, and discriminant biometrics in healthy adults. Component-based identification of spectral activity (CSD/eLORETA-fPCA) was superior to fixed, a priori frequency bands. Improved quantification and conceptualization of frontal theta is necessary to determine clinical utility.     

头皮EEG偶极子定位在癫癎外科中的应用研究

目的：研究头皮EEG偶极子定位方法在癫痫手术病人中的应用价值及其准确性。方法：在80例难治性癫痫手术病人中．术前在发作间期头皮EEG上用偶极子定位方法定出致痫源放电位置．术中以皮质电图(ECoG)及脑深部电极记录确定致痫区．对比偶极子定位的准确性．按ECoG定出的位置直接对致痫区进行手术处理．术后随访手术效果。结果：在颞叶癫痫．偶极子定位与ECoG及脑深部电极定位完全一致；在额、顶、枕叶癫痫，偶极子位置误差为10～15mm。随访6～24个月(平均13个月)，80例中71％的病人无癫痫发作，25％的病人发作减少75％以上．手术有效率96%。结论：头皮EEG的偶极子定位方法无创、准确，相当于脑磁图，可避免创伤性检查而用于癫痫病人的术前定位。     

Use-dependent cortical plasticity in thalidomide-induced upper extremity dysplasia: evidence from somaesthesia and neuroimaging

In this study cerebral reorganization was investigated in thalidomide-damaged subjects who use their feet to compensate for their malformed upper extremities. Tactile localization across toes was combined with fMRI to study use-dependent plasticity of the human somatosensory cortex. The manner of compensatory foot use was assessed by a questionnaire. In the behavioural experiment toes were stimulated with above threshold monofilaments and subjects had to report which toe was stimulated. When feet were employed for all everyday actions subjects made significantly fewer errors in the localization task. In subjects who use their feet only for specific actions such as grasping objects there were as many localization errors as in the control group of thalidomide-affected subjects with normal extremities. However, the patterns of mislocalizations were different with less errors occurring for the toe of the dominant foot involved in these actions. Functional MRI showed stronger haemodynamic responses to electrical stimulation of the toes in subjects using their feet for everyday actions as compared to controls. Our data show that long-term use of the feet for fine sensorimotor skills leads to better performance in tactile localization and changes in cerebral SI representation supporting the notion of use-dependent plasticity in the somatosensory cortex.This paper is part of a doctoral thesis at the Heinrich-Heine-University, Düsseldorf     

Reliability and validity of neuroelectric source imaging in primary somatosensory cortex of human upper limb amputees

The present study investigated the test-retest reliability of EEG source localizations of somatosensory evoked potentials (SEPs) in human upper limb amputees over a long time frame (several months) and examined the validity of source reconstruction. In two sessions spaced several months apart five unilateral upper limb amputees were stimulated at the first and fifth digit of the intact hand and at the left and right lower corner of the mouth. To examine the validity of the results of the neuroelectric source reconstruction a comparison with neuromagnetic source localization was performed for two subjects. The source localizations of the SEP components were found to be highly reproducible: the mean standard deviation of the dipole locations was 8.80 mm in the x-, 7.00 mm in the y- and 4.15 mm in the z-direction. The match of the comparison of EEG and MEG data was in the range of one centimeter. These results support the use of multi-electrode EEG recordings combined with MRI as an adequate method for the investigation of the functional organization of the somatosensory cortex in upper limb amputees and suggest high stability of cortical reorganization in these subjects.     

3-D Source Localization of Epileptic Foci Integrating EEG and MRI Data

This study evaluates the utility of 3-D localization of interictal spike activity on the electroencephalographs (EEG) superimposed on magnetic resonance imagery (MRI) in a pediatric population with extra-temporal lesional epileptic foci. 3-D software programming based on the CURRY platform (a multimodal neuro-imaging software) was adapted for analyzing scalp EEG data and reconstructing superimposed images in 10 children who underwent extensive pre-surgical evaluation for intractable partial seizures. The results of 3-D spike source localization were assessed in relationship to focal lesions evident on the patient's MRI scans. Calculated spike sources were closest to the lesions during intervals corresponding to the spike peaks. The information was useful in surgical planning in six children that underwent successful resections.     

Reproducibility and stability of neuroelectric source imaging in primary somatosensory cortex

The present study investigated the test-retest reliability of EEG source localization of somatosensory evoked potentials (SEPs) over an extended time period and tested the accuracy of source reconstruction co-registred with individual brain morphology (MRIs). Seven healthy subjects were stimulated pneumatically at the first digit and fifth digit of each hand and at the left and right lower corner of the mouth in two sessions spaced several weeks apart. At each location 1000 stimuli were presented. The overlay of the dipole localizations with the individual anatomic structure of the subjects' cortex was accomplished by the use of magnetic resonance images. A spherical 4-shell model of the head was used to localize the neuroelectric sources of the EEG data. In two cases a more realistically shaped 3 compartment model was computed using the boundary element method (BEM). The source localizations of the SEP component were found to be highly reproducible: the mean standard deviation of the dipole locations was 5.21 mm in the x-, 5.98 mm in the y- and 4.22 mm in the z-direction. BEM was not found to be superior to a 4-shell model. These data support the use of multi-electrode EEG recordings combined with MRI as an adequate method for the investigation of the functional organization of the somatosensory cortex.     

Laminar Expression of Ephrin‐A2 in Primary Somatosensory Cortex of Postnatal Rats

Several Eph receptors, prominently EphA4 and EphA7, and their corresponding ligands are known to influence neocortical development, including topographic sorting of thalamocortical axons within primary somatosensory cortex (SI). This study investigated postnatal expression of a ligand that can bind to these receptors, ephrin‐A2. Quantitative methods revealed that expression of ephrin‐A2 mRNA in SI reached maximum levels on postnatal day (P) 4 and dropped thereafter to background by P18. Ephrin‐A2 mRNA expression assessed by in situ hybridization qualitatively revealed a similar time course and localized the expression pattern primarily in two broad laminae in SI, comprising the supragranular and infragranular layers, and with additional expression in the subplate. This expression pattern was investigated in greater detail using immunohistochemistry for ephrin‐A2 protein. Immunoreactivity generally showed the same laminar distribution as seen with in situ hybridization, except that it persisted longer, lasting to approximately P14. Expression in the cortical plate was low or absent within presumptive layer IV, and it remained so as cortical lamination progressed. Double‐labeling immunohistochemistry with confocal microscopy revealed that cortical neurons were the principal elements expressing ephrin‐A2 protein. These findings are consistent with possible involvement of ephrin‐A2, in concert with one or more Eph receptors, in influencing arbor development of thalamocortical axons at cortical layer IV boundaries. Anat Rec, 2012. © 2011 Wiley Periodicals, Inc.     

Dynamic Brain Topography of Somatosensory Evoked Potentials and Equivalent Dipoles in Response to Graded Painful Skin and Muscle Stimulation

The differential effects of painful stimulation of skin vs. muscle on the cerebral electrophysiology have been poorly described. This study examined the somatosensory evoked potentials (SEPs) and the associated dipole models of non-painful and graded painful electrical stimulation applied to the skin and muscle in 20 healthy subjects. With the psychophysical stimulus-response functions determined, the skin stimulation showed a steeper slope than muscle stimulation. For both types of stimulation, the SEPs indicated a similar temporo-spatial activation sequence: F4/N90-P4/P95, Fc2/N135, Cz/P250, Cz/P300, and Cz/N460. The SEP amplitudes increased significantly with the stimulus intensities in these components. The peak SEP latencies of skin stimulation were in general shorter than that of muscle stimulation. The SEP amplitudes to skin stimulation were significantly larger than those caused by muscle stimulation at every stimulus intensity level, except the early mid-latency component. In this case, muscle stimulation caused higher amplitudes over the contralateral parietal-frontal sites. For both types of stimulation, the topographic maps were quite similar. Equivalent dipole modeling revealed identical site parameters (<1.0 cm) between skin and muscle stimulation. However, the electrical skin stimulation did not correlate with the pain intensity. Pain intensity, in contrast, was uniquely associated with the Cz/P250 amplitudes for the muscle stimulation. It is concluded that non-nociceptive and nociceptive electrical stimuli applied to skin and muscle are processed in the common cerebral areas, but exhibit differential SEP effects.     

Measurement of Electroencephalic Evoked Response: Comparison of Univariate and Multivariate

Two univariate peak-to-peak measures of electroencephalic average evoked response (AER) were compared with two multivariate measures of AER with respect to: (1) ability to discriminate between stimuli of different intensity; and 2) test-retest reliability. AER data from 47 sleeping newborns for each of 2 test sessions were utilized to calculate P₂-N₂ amplitude,t_p2-n2 (the test statistic which measures the degree to which the P₂-N₂ amplitude differed from zero), F₁ (a multivariate measure of the degree to which changes in potential differed from zero), and F_jk (a multivariate measure of degree to which changes in potential differed from changes in potential in background EEG). The multivariate measures were somewhat superior to the univariate measures in the ability to discriminate three stimulus intensities when used as the dependent measure of AER in 47 subjects, though differences between measures were small. At the same time the multivariate measures were somewhat inferior to the univariate measures in retest reliability. There were major discrepancies between the multivariate and univariate measures of AER for some individuals. Comparison of the two multivariate measures indicates that in these data from sleeping new-borns, use of background EEG as a baseline reference for the multivariate measure. Fjk, decreased its reliability without enhancing its ability to distinguish stimulus intensities.     

Electrical Conductivities of the Freshly Excised Cerebral Cortex in Epilepsy Surgery Patients; Correlation with Pathology, Seizure Duration, and Diffusion Tensor Imaging

Summary The electrical conductivities (σ) of freshly excised neocortex and subcortical white matter were studied in the frequency range of physiological relevance for EEG (5–1005 Hz) in 21 patients (ages 0.67 to 55 years) undergoing epilepsy neurosurgery. Surgical patients were classified as having cortical dysplasia (CD) or non-CD pathologies. Diffusion tensor imaging (DTI) for apparent diffusion coefficient (ADC) and fractional anisotropy (FA) was obtained in 9 patients. Results found that electrical conductivities in freshly excised neocortex vary significantly from patient to patient (σ = 0.0660–0.156 S/m). Cerebral cortex from CD patients had increased conductivities compared with non-CD cases. In addition, longer seizure durations positively correlated with conductivities for CD tissue, while they negatively correlated for non-CD tissue. DTI ADC eigenvalues inversely correlated with electrical conductivity in CD and non-CD tissue. These results in a small initial cohort indicate that electrical conductivity of freshly excised neocortex from epilepsy surgery patients varies as a consequence of clinical variables, such as underlying pathology and seizure duration, and inversely correlates with DTI ADC values. Understanding how disease affects cortical electrical conductivity and ways to non-invasively measure it, perhaps through DTI, could enhance the ability to localize EEG dipoles and other relevant information in the treatment of epilepsy surgery patients.     

Reproducibility and stability of neuromagnetic source imaging in primary somatosensory cortex

The present study investigated the test-retest reliability of magnetoencephalography (MEG) source localization of somatosensory evoked fields (SEFs) over an extended time period. Five healthy subjects were stimulated pneumatically at the first and fifth digit in two sessions spaced several months apart. At each location 400 stimuli were presented. The validation of the results was performed by overlay of the dipole localizations into the individual anatomic structure of the subjects' cortex by the use of magnetic resonance images (MRIs). The source localizations of the SEF component were found to be highly reproducible. The mean standard deviation of the dipole locations of the first digit was 1.55 mm in the x-, 1.55 mm in the y- and 3.49 mm in the z-direction. The mean standard deviation of the fifth digit was 3.69 mm in the x-, 4.27 mm in the y- and 6.60 mm in the z-direction. These results support the use of MEG recordings combined with MRI as an adequate method to define the organization of the human primary somatosensory cortex and provide a useful approach to the rapid detection of neuroplasticity.     

Intensity expectation modifies gustatory evoked potentials to sweet taste: Evidence of bidirectional assimilation in early perceptual processing

Expectations can affect subjective sensory and hedonic ratings of tastes, but it is unclear whether they also shape sensory experience at a perceptual level. The neural correlates of the taste‐expectancy relationship were explored through EEG analysis. Using a trial‐by‐trial cueing paradigm, lingual delivery of 0.05 M or 0.3 M sucrose solutions was preceded by congruent or incongruent visual cues designed to promote anticipation of either a low‐sweet or high‐sweet solution. When participants were cued to expect low‐sweet, but received high‐sweet (incongruent cue), intensity ratings for high‐sweet decreased. Likewise, expectation of high‐sweet increased intensity ratings of low‐sweet solutions. Taste‐dependent, right central‐parietal gustatory ERPs were detected, with greater P1 (associated with greater right insula activation) and P2 peak amplitudes for high‐sweet tastes. Valid cue‐taste pairings led to specific reduced right‐lateralized N400 responses (associated with an attenuation in right insula activation) compared with invalid cue‐taste pairings. Finally, P1 amplitudes following invalidly cued low‐sweet tastes closely matched those generated by expected high‐sweet tastes, and P1 amplitudes for invalidly cued high‐sweet tastes resembled those generated by low‐sweet tastes. We conclude that, as well as modifying subjective ratings toward the anticipated intensity level, expectations affect cortical activity in a top‐down manner to induce bidirectional assimilation in the early perceptual processing of sweet taste and modulate N400 ERP components not previously associated with gustatory stimulation.     

Localization of Nitric Oxide Synthase-containing Neurons in the Bat Visual Cortex and Co-localization with Calcium-binding Proteins

Microchiroptera (microbats) is a suborder of bats thought to have degenerated vision. However, many recent studies have shown that they have visual ability. In this study, we labeled neuronal nitric oxide synthase (nNOS)—the synthesizing enzyme of the gaseous non-synaptic neurotransmitter nitric oxide—and co-localized it with calbindin D28K (CB), calretinin (CR), and parvalbumin (PV) in the visual cortex of the greater horseshoe bat (Rhinolophus ferrumequinum, a species of microbats). nNOS-immunoreactive (IR) neurons were found in all layers of the visual cortex. Intensely labeled neurons were most common in layer IV, and weakly labeled neurons were most common in layer VI. Majority of the nNOS-IR neurons were round- or oval-type neurons; no pyramidal-type neurons were found. None of these neurons co-localized with CB, CR, or PV. However, the synthesis of nitric oxide in the bat visual cortex by nNOS does not depend on CB, CR, or PV.