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Cortical traceability analysis of ERP
2022-06-27 20:56:00 【Yueying Technology】
Cortical source analysis of EEG signals has become an important tool for brain activity analysis . The purpose of source analysis is to reconstruct the cortical generator of EEG signals on the scalp ( Source ). The quality of source reconstruction depends on the accuracy of the forward problem , It also depends on the accuracy of the inverse problem . When the appropriate imaging modality is used to describe the head geometry , Through the sensor position on the scalp 3D Map to determine the precise electrode position , And determine the true conductivity value for each tissue type of the head model , Accurate positive solutions can be obtained . Together, these parameters help define a real head model . ad locum , We describe the necessary steps to reconstruct the cortical generator of EEG signals recorded on the scalp . We provide an event-related potential (ERPs) Example of source reconstruction , In a 6 Facial processing tasks performed by infants aged six months . We discussed using different ERP Measures adjustments required for source analysis . The proposed method can be applied to study different cognitive tasks of subjects of different ages .
1 introduction
Electroencephalogram (EEG) Cortical source analysis of signals has become an important neuroimaging tool to locate brain functional responses throughout the life cycle . Functional magnetic resonance imaging (fMRI) It is the most widely used technique for locating brain functional responses in cognitive tasks , It has recently been used in pediatric population studies . lately , The researchers used functional magnetic resonance imaging (fMRI) To study conscious infants and young children performing visual tasks 、 Attention and neural responses to statistical learning tasks . These promising first applications fMRI The protocol is still affected by high wear rate and small sample size . On the other hand , Late childhood and older participants had higher scan success rates .
Thanks to advances in recording procedures and signal analysis , EEG is one of the most important neuroimaging methods in infant research . Its high temporal resolution allows EEG Capture dynamic neural responses . Besides ,EEG The high time resolution of can capture millisecond events , And not suffer fMRI Depends on a slower hemodynamic response (~ second ) The limitation of . Identify the nerve generators that record activity on the scalp by applying source analysis calculations , Can overcome EEG The characteristics of poor spatial resolution . Cortical source analysis uses head scalp recordings , And infer the source of the signal in the cortex by calculation . therefore , Functional activities provided by EEG signals ( Such as changes in amplitude and latency )、fMRI The obtained structural anatomy and advanced data processing and analysis methods can determine the source analysis method for studying brain development . Source localization analysis can be divided into dipole method or distributed source method . The equivalent current dipole method is based on the assumption that EEG can be simulated as a single or multiple discrete dipoles , Whose position is defined a priori . This method has been applied to the study of cognitive processes , Including the developmental changes of late auditory evoked potentials in children and adults . In another study , The dipole source model is used to study 6 The detection mechanism of auditory changes in infants aged 6 months . The negative and positive peaks produced during voice recognition were located in the supratemporal and frontal regions . In a similar study , The researchers evaluated 6 The time course of brain activity in a 6-month-old baby while processing voice information . It turns out that , Infants' reactions are similar to those of adults , But the activation time is different . Compared with temporal activity , Early activation of the anterior cingulate cortex may depend on differences in maturation of the limbic and cortical regions , This has an important impact on the distribution of speech functional responses .
The distributed source modeling method estimates the spatial distribution of neural current in a defined source space , The source space is usually confined to the cortical surface . This analysis calculates the current density of brain regions that together produce scalp activity patterns . Some studies have implemented distributed source method for EEG recording of children , Designed to investigate a wide range of cognitive tasks . Source analysis can be used as a method to evaluate the functional connectivity of EEG . The functional connection of EEG is usually in the scalp - Completed at the electrode level , The correlation observed between different electrodes is shown by phase coupling . The functional connection with EEG can be completed at the source level . This reveals connections between cortical sources rather than scalp locations .
All in all , These results show how source analysis is widely used in cognitive tasks , It also provides further information about the functional organization of the brain . A reminder of the use of source analysis in the pediatric population is , Need a real head model , To reflect the accurate representation of the individual head . The gold standard in this regard is to use structural NMR imaging from individual participants to construct the basic properties of source analysis . This method can explain individual differences in head geometry , This may have a greater impact on the child sample than on the adult sample , Because the growth rate of the head and brain is different when they are young . for example , The relationship between the brain and the skull is very different between adults and infants . After full-term birth, the skull is separated by fontanel and suture for several months . This feature of the human skull contributes to the growth of the brain , And affect the conduction characteristics of the compartment . Infant and toddler corticospinal fluid (CSF) The increase in helps to change the brain - Scalp distance , It has an important impact on the source signal strength . Besides , If a real individual head model is used , The inverse solution becomes more accurate , Especially in areas that cannot be well described by spherical models ( Such as temporal lobe ), And including clinical samples with brain structural changes ( Such as pathological changes 、 Tumor ) In the research of . Development researchers should take additional precautions against procedures for electrode location . EEG networks were placed between different young participants , Therefore, the use of standard electrode montages should be avoided .
In this paper , We outline the reconstruction process of high-density EEG cortical generator , Some solutions for development researchers are discussed . We use a 6 Month old participants and 12 A distributed source analysis was conducted on the data of a representative infant aged 6 months . This source analysis method will be used for P400 ERP Study on amplitude value of components . We will discuss alternative methods of source analysis when single structure MRI or digital electrode locations are not available . Last , We will briefly discuss the non -... - of functional activities ERP measures ( Such as frequency band 、 Connectivity ) Make adjustments required for source analysis .
2 Overview of source analysis methods
Changes in EEG potentials recorded on the scalp are caused by postsynaptic potentials that occur during cortical activity . The potential change on the scalp is the sum of electrical activities from various sources , Therefore, there is no real relationship between the electrode position on the scalp and the underlying cortical source . Cortical source analysis has developed into a quantitative tool , To determine the generators of cortical activity (s). Once the forward model components are calculated , An inverse space filter can be constructed to solve the inverse problem . The inverse model utilizes an inverse spatial filter to record EEG of scalp /ERP Multiply , The pattern of producing the source of brain endothelium . This step generates many voxels from the smallest electrode group . This process is defined as an inverse problem , The solution is undetermined . The spatial inverse calculation is limited to the theoretical distribution by selecting parameters , Solved this problem .
We use the distributed source analysis method , There is no need to assume the number of dual poles ; Source space is created using realistic head models , The model comes from MRI ; Anti constraint adoption eLORETA Method . The matrix diagram of forward and inverse calculation is shown in Figure 1 Shown . chart 2 Shows a visual representation of the source analysis method , You can follow these steps to summarize :
1)EEG and MRI Preprocessing . If you can't do precise MRI choice , You can choose an alternative to a single volume .
2) Create... For electrode position 3D coordinate , And with MRI Head volume co registration .
3) The finite element method or the boundary element method are used MRI Head volume segmentation .
4) When the source space is limited to gray matter, the pilot field matrix is calculated .
5) Application of precise low resolution brain electromagnetic imaging (eLORETA) The algorithm solves the inverse problem .
6) Source estimation and voxel power calculation .
7)ROI Application of spatial filter in statistical analysis of source values .
chart 1 positive ( On ) And reverse ( Next ) Model diagram . From left to right of the previous line , These elements correspond to EEG/ERP Scalp records 、 Electrode position 、 Participants' heads MRI Derived geometry and conductivity 、 Source space and source generator represented by current density diagram .
chart 2 Schematic diagram of the proposed source analysis method .
2.1 EEG and MRI preprocessing
Obtaining the data with the smallest artifact of each participant from the original EEG signal is the key to accurately reconstruct the source . The segmented EEG records and the average ERP Data sets can be used as input to the source analysis process . It is beyond the scope of current work to discuss EEG preconditioning in developmental research . Other details can be found elsewhere . The reliable nerve generator can be reconstructed by EEG signal , Noise from non brain sources ( Such as eye artifact ) Be minimized . Besides , The high-density EEG system will allow the development of a wider range of source generators , Taking into account the individual differences in scalp distribution . We have provided evidence in previous studies , It indicates that high-density EEG recording provides a representation of scalp electrical potential that montage can not capture . therefore , High density is recommended ( for example ,64 Channel or higher ) And distributed montage to realize source location technology . Establishing a real head model is an important step to accurately locate the scalp EEG distribution nerve generator . Adult anatomy and parameters cannot be directly applied to participants in pediatric studies . Single MRI The use of scanning overcomes the positioning errors caused by the use of adult head models to analyze the sources of children's EEG data .
The gold standard for source analysis headforms is for individual participants MRI. However ,fMRI The acquisition of is challenging , Especially for very young participants , Therefore, the possibility of obtaining high-quality images is limited . When an individual MRI cannot be used , There are two ways to get a real head model . First , It can be provided free of charge MRI database ( for example , Neurodevelopment MRI database ). This solution needs to collect the exact size of the participant's head , Find the ideal match in a set of MRI . We recommend getting multiple head measurements , In order to describe the complex head shape as accurately as possible . The schematic diagram of head measurement is shown in Figure 3 in . The circumference of the head can be accurately measured with a flexible non stretching tape and a head caliper 、 Front back distance and left-right distance . second , individual MRI The alternative is to use an age appropriate average template . The MRI template was constructed using data obtained from multiple subjects . For developmental research , Select the MRI The template includes participants in the investigated age range , To capture the appropriate features of the developing brain is crucial . therefore , Even if there is no participant's head measurement data , The average template with appropriate age can also be used as an alternative to head model construction MRI.
chart 3 Schematic diagram of head measurement , To select a near size MRI. The head width and length can be measured by using a caliper ( Top panel ) obtain . Head circumference ( The yellow line ), Sagittal ( Orange Line ), Crown ( Red thread ), And the front ( The blue line ) And the rear ( The green line ) The half circumference of can be measured with flexible tape .
chart 4 With a single MRI The position of the jointly bonded electrodes 3D Render view ( Lower right panel ). Participants' specific electrode positions can be placed from the network ( Red box and electrode ) Or a digital representation placed from all electrodes ( Green frame and electrode ) determine .
2.2 Electrode position
Electrode position and MRI Accurate volume registration is an important requirement for realistic head models . Due to the special difference of the head shape , Especially in infancy and childhood , The average position of the electrodes does not reflect the actual mesh position . therefore , A positioning system should be implemented to determine the analytical position of the electrode on the scalp . Many devices use different methods on the head of participants 3D Positioning electrode in space , Such as electromagnetic fields or pictures . Without a positioning system , Images of participants' mesh layouts can be used to locate electrodes in three-dimensional space . The position of several electrodes around the head can be used as a reference mark for reconstructing the position of all electrodes . We suggest that 128 The tunnel Hydrocel Geodesic sensor network (HGSN) Upper use 5 Datum marks , Corresponding to vertices respectively 、 middle ear 、 Small wings and left and right ears , Respectively Cz、17、75、57 and 100 An electrode . A photograph of the placement net can be taken to locate the reference mark on the subject's head , And manually place it in MRI On . The coordinates of the reference point are used to reconstruct the position of the remaining electrodes , And jointly register to MRI Head . We provide one for use in a single MRI An example of a picture marked with a reference ( chart 4).
2.3 Head model construction
The positive solution of source analysis is obtained by introducing the field matrix . According to the source model 、 The lead field matrix is calculated from the head model and the electrode placement position ( chart 1). The head form describes the head geometry and conductivity values of the compartment under consideration . Personalized head models come from the participants MRI Accurate segmentation and use of age appropriate conductivity values . In order to determine the influence of the head model on the source location accuracy , Some simulation studies have been carried out . The main conclusion is , Compared with the simplified head model ( For example, three chambers are concentric ), A real head form should be preferred , And at least model 5 Two chambers .
When determining conductivity values of different compartments simulated by source location , Other factors should also be considered . The composition of different head media is different , It will lead to age-related changes in tissue conductivity . There is a high variability in the estimation of cranial bone conductivity in adults and young children , The conductivity of other tissues is relatively consistent . The cranial chamber has important changes due to its progressive ossification . Besides , The cranial ventricular conductivity in infancy is not uniform on its surface , until 2 It was not completely closed until about years old . Several methods have been implemented to estimate the optimal conductivity value for young children .
When important developmental structural changes occur , The influence of the inaccuracy of the conductivity modeling of the head interstitial compartment on the localization of electrical activity needs to be further studied . Each medium in the finite element model can be represented by hexahedral or tetrahedral meshes . In the study of adult participants , Both hexahedral and tetrahedral meshes perform well , And shows the smallest difference in source analysis accuracy . And directly by MRI The hexahedral model generated by the segmentation process is compared with , Tetrahedralization process is more complex and time-consuming . ad locum , We distinguished in the headform 10 There are three different partitions ( The scalp 、 skull 、 Dura mater 、 muscle 、 eyes 、 nasal cavity 、 cerebrospinal fluid 、GM、WM And unmyelinated axons ), The source results of two grid types are reported . The hexahedral and tetrahedral meshes of the head model are shown in the figure 5 Shown .
chart 5 Single MRI Split output hexahedron ( panel a) And tetrahedron ( panel b) The grid represents .
2.4 Guided field and inverse solution
The potential of the electrode is calculated according to the headform configuration . It can be explained that the measured signal is reconstructed in the source analysis . The inverse problem is caused by the ill conditioned relationship between the electrode and the inverse source model , That is, very few electrodes record activity from multiple sources . There are many suggested inverse method procedures ( for example , Minimum norm estimation ). We use the distributed source analysis method , There is no need to assume the number of dipoles . Source space is created using realistic head models , The model comes from MRI .
2.5 Source estimation
Source activation is represented by a vector consisting of three parameters in each voxel in the source space , Each dimension in the space has a parameter . then , By calculating the three-dimensional vector of each voxel position ( This is called CDR Method ) Size , Reduces the current density reconstruction to a single value . In this way , Whatever the polarity of the EEG signal , High values in the original EEG data will lead to CDR High value . perhaps , Singular value decomposition (SVD) It can be used as a projection method . And CDR The difference is , The reconstructed source retains the polarity of EEG activity .SVD Methods are considered to be available in ERP Provide robust results in source analysis , It has advantages in time-frequency decomposition and connectivity analysis .
3 Materials and methods
3.1 The subjects had experiments
collect 1 example 6 The baby is... Months old (M, 186 God ) The data of . Reported 5 A representative 12 A month old baby (1 Men , Age is 376.4 God ) The aggregate result of . All babies are full-term ( At least pregnant 38 Zhou , Birth weight at least 2500 g ), Healthy at birth , There are no known developmental abnormalities . Participants were predominantly white and of medium socio-economic status . According to the ethical approval of the institutional review board of the University of South Carolina , With informed parental consent . All babies passively look at vertical and inverted faces and houses ,500 Random rendering in milliseconds , There is no replacement . A variable test interval was used for the stimulus sequence , When the baby looks away from the screen , Sesame Street videos are presented as attention grabbing elements . Head measurements were performed before the EEG task . stay EEG At the end of the mission , adopt GPS Take pictures and electrode positions ( See the picture 4).
3.2 Magnetic resonance data acquisition and preprocessing
The subject's MRI The volume is in Siemens Magnetom Prisma 3.0 T Separate acquisition on the scanner , use 20 Channel head coil . use MP-RAGE agreement , Use the following parameters to get the full header t1 Weighted images : Repeat time (TR) = 3000 ms, Echo time (TE) = 4.44 ms, Flip angle (FA) = 9◦, View (FoV) = 192 × 192 Pixels , Voxel size = 1.0 × 1.0 × 1.0 mm. The total collection time is 9 min 36 s. Use continuous 4 min and 32 s Of SPC Sequence acquisition t2 Weighted images (TR = 4500 ms, TE = 13 ms, FoV = 192 ×192 pixels, Voxel size = 1.0 ×1.0 ×2.5 mm). take MRI Divided into 10 There are different types of organizations , A realistic head model is obtained ( chart 5). The conductivity values for each tissue type are as follows : Scalp 0.35 S/m, skull 0.0132 S/m, cerebrospinal fluid 1.79 S/m, WM 0.2 S/m, GM 0.33 S/m, Dura mater 0.33 S/m, muscle 0.35 S/m, eyes 0.5 S/m, nasal cavity 0.0048 S/m.
Define the anatomy according to the anatomical stereotactic atlas ROI. These maps are used to define 17 Anatomical regions related to the neurogenesis hypothesis of infant facial processing . Selected ROI Of 3D The rendered representation is shown in the figure 6 Shown .
chart 6 Different maps . Each atlas is registered in 6 A month old MRI in . Select the occipitotemporal and parietal regions of interest .
4 result
chart 7 Shows a typical 6 The parietal lobe and parietal occipital passage of infants aged 6 months ERP Line graph .P400 About... After the stimulation attack 400-600 Between milliseconds , On the right electrode ( for example P10 and PO10) The amplitude on the is large .、 A similar scalp map shows greater facial response and right side activity than the house .
Calculate the estimated source at each point of the source grid , Limit here GM And eyes . Then apply the inverse solution to the data , And calculate at each point of the grid CDR and SVD value . The dimension of the generated three-dimensional matrix is determined by the number of points in the grid 、 Condition number and time point definition .
chart 7 A typical 6 The parietal lobe and parietal occipital passage of infants aged 6 months ERP Line graph . Of the whole clip ERP The activity is plotted as a function of stimulus type ( Top ). The scalp atlas shows all types of stimulation P400 Peak activity of components ( That is, after the stimulation starts 432 ms).
A specific set of ROI Source activity applied to rebuild , And used for further analysis .ROI Set can be from an existing stereoscopic atlas or fMRI In the task, the subjects' functional responses are created . chart 8 With 3D Report in the form of drawing P400 Distribution activity results of peak to vertical response . The line graph reports that the ROI Source activity under four experimental conditions . The activities of superior temporal sulcus and posterior cingulate gyrus are related to ERP The reaction pattern is similar . Reconstructed P400 Of ERP There seems to be a specific reversal effect at the peak . By using... That is close to the size of the participant's head MRI And age appropriate MRI Templates , We got a similar solution . The results of hexahedral mesh calculation are shown in Figure 9.
In the figure 10 in , We compared the source activity of the posterior cingulate gyrus obtained from three different head models . The reaction patterns of the three solutions are similar . The results of self MRI and short-range MRI were more similar than those of age-matched MRI average data . However , Only self NMR and near NMR solutions seem to be sensitive to inversion effects . These results need to be further explored in the study of larger sample size . All in all , A source analysis program using a head model tailored to the participant's head characteristics and electrode position will provide a more accurate solution . For all that , Source solutions obtained with age appropriate headforms are considered more appropriate than adult headforms . Last , We use age appropriate MRI Hexahedral mesh of template and head model , Apply source analyzers to a small group of 12 Of the month old subjects . Apply positive and negative solutions at the subject level , And average the results among the participants . chart 11 It describes ERP and CDR Aggregate results of values .
chart 8 In a single t1 weighting MRI Draw on P400 ERP Peak response activity to upright faces .(a) Use two grid types of reports ( Hexahedron , tetrahedron ) And source measurement (CDR, SVD) Single head model reconstruction source activity .(b) The line diagram depicts the source activity during the whole test under four experimental conditions .
chart 9 (a) Near MRI headform ( Left panel ) And the average template headform ( Right panel ) Line graph of source analysis results and 3D rendering .(b) 3D The rendered image shows that the reconstruction source is close to or at an appropriate age t1 weighting MRI Drawn on P400 Peak surface response .(c) The graph shows the source activity under four experimental conditions .
chart 10 The posterior cingulated gyrus of the subjects CDR Activities and MRI type ( chart a) And facial orientation ( chart b) of . Be careful ,y The axis scale is set to maximize each distribution .400 - 600 ms The average amplitudes between are plotted as oriented functions . stay P400 Within the time window , Whether it's me MRI Or close up MRI, The face reversal effect seems to be more obvious .
chart 11 from 12 Data collected from a small sample of subjects older than months . The top panel reports the characteristics of the parietal and occipital channels under four experimental conditions ERP reaction , And upright faces P400 Peak scalp atlas . The bottom panel shows four ROI Medium CDR Activity as a function of stimulus type , as well as P400 The voxel source diagram of the vertical plane at the peak 3D present .
5 Discuss
This paper summarizes the process of EEG source analysis , Special reference is made to the adjustments required for such analysis using data obtained from the pediatric population . The key of EEG reconstruction is to obtain accurate electrode position map and realistic head model . Digital map of all electrode positions ( namely GPS Solution ) Is to get all the electrodes 3D The most accurate method of coordinates , This is a procedure acceptable to very young babies . The digital coordinates of several reference electrodes can be used to reconstruct the position of the remaining positions . When there is no digitized electrode or reference position , Trained researchers can use participants' EEG networks to place pictures , Manually place the reference electrode at MRI On . We recommend taking pictures from all sides of the head at the end of EEG recording . According to adult studies , Personalized electrode placement is better than standard electrode placement , To reduce the source location error .
The establishment of head model is another key step of source analysis . The use of realistic head models helps minimize source localization errors , Regardless of the age group investigated . However , children MRI and MRI The lack of templates led development researchers to use adult head models . This method increases the anatomical difference of positioning error , Participants of different ages . So , It's crucial , Take accurate head measurements during the experiment . When head measurements are not available , Age appropriate MRI Templates . All participants used the same head model and source space to calculate the nerve generator of scalp EEG activity . therefore , Potential individual differences may be overlooked , Resulting in an increase in positioning error . However , The small positioning error caused by using the average template to construct the head model can be achieved through the pre-defined anatomy ROI Internal average source activity to compensate . Our comparison with a representative participant shows that , Age appropriate head models pass through themselves MRI、 A little distance MRI or MRI The form of the template produces a similar source activation pattern in the posterior cingulate gyrus . Further research is necessary , To better quantify the positioning errors resulting from the three alternative headform solutions .
All in all , The gold standard method of source analysis utilizes participants MRI Digital map of middle electrode position and personalized head model . Other methods can be used to overcome the problem that each participant obtains the electrode position and MRI The limitation of . Images using a neuronavigation system or mesh layout , The electrode coordinate map can be reconstructed from the coordinates of several electrode markers . You can use... That is close to the size of the participant's head MRI Or age appropriate MRI Templates to create head and source models .
reference :Cortical Source Analysis of Event-Related Potentials: A Developmental Approach
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