title: EEG and MEG coherence: measures of functional connectivity at distinct spatial scales of neocortical dynamics.
abstract: We contrasted coherence estimates obtained with EEG, Laplacian, and MEG measures of synaptic activity using simulations with head models and simultaneous recordings of EEG and MEG. EEG coherence is often used to assess functional connectivity in human cortex. However, moderate to large EEG coherence can also arise simply by the volume conduction of current through the tissues of the head. We estimated this effect using simulated brain sources and a model of head tissues (cerebrospinal fluid (CSF), skull, and scalp) derived from MRI. We found that volume conduction can elevate EEG coherence at all frequencies for moderately separated (<10 cm) electrodes; a smaller levation is observed with widely separated (>20 cm) electrodes. This volume conduction effect was readily observed in experimental EEG at high frequencies (40-50 Hz). Cortical sources generating spontaneous EEG in this band are apparently uncorrelated. In contrast, lower frequency EEG coherence appears to result from a mixture of volume conduction effects and genuine source coherence. Surface Laplacian EEG methods minimize the effect of volume conduction on coherence estimates by emphasizing sources at smaller spatial scales than unprocessed potentials (EEG). MEG coherence estimates are inflated at all frequencies by the field spread across the large distance between sources and sensors. This effect is most apparent at sensors separated by less than 15 cm in tangential directions along a surface passing through the sensors. In comparison to long-range (>20 cm) volume conduction effects in EEG, widely spaced MEG sensors show smaller field-spread effects, which is a potentially significant advantage. However, MEG coherence estimates reflect fewer sources at a smaller scale than EEG coherence and may only partially overlap EEG coherence. EEG, Laplacian, and MEG coherence emphasize different spatial scales and orientations of sources.
authors: Department of Cognitive Sciences, University of California, Irvine, CA 92697-5100, United States. r.srinivasan@uci.edu
authors: Ramesh
authors: R
authors: Srinivasan
authors: William R
authors: WR
authors: Winter
authors: Jian
authors: J
authors: Ding
authors: Paul L
authors: PL
authors: Nunez
mesh_terms: D000465:Algorithms; D016477:Artifacts; D001931:Brain Mapping / Q000379:methods*; D003198:Computer Simulation; D003344:Cortical Synchronization; D004566:Electrodes / Q000592:standards; D004569:Electroencephalography / Q000379:methods* / Q000592:standards; D005071:Evoked Potentials / Q000502:physiology; D007839:Functional Laterality / Q000502:physiology; D006801:Humans; D007091:Image Processing, Computer-Assisted / Q000379:methods; D008279:Magnetic Resonance Imaging / Q000379:methods; D015225:Magnetoencephalography / Q000379:methods* / Q000592:standards; D008959:Models, Neurological; D015233:Models, Statistical; D019579:Neocortex / Q000033:anatomy & histology / Q000502:physiology*; D009431:Neural Conduction / Q000502:physiology; D009434:Neural Pathways / Q000033:anatomy & histology / Q000502:physiology*; D012535:Scalp / Q000033:anatomy & histology / Q000502:physiology; D012815:Signal Processing, Computer-Assisted; D012886:Skull / Q000033:anatomy & histology / Q000502:physiology; D013269:Stochastic Processes; D013346:Subarachnoid Space / Q000033:anatomy & histology / Q000502:physiology
references: Clin Neurophysiol. 1999 Mar;110(3):469-86
references: 10363771
references: IEEE Trans Biomed Eng. 1998 Jul;45(7):814-26
references: 9644890
references: Clin Neurophysiol. 1999 Aug;110(8):1351-62
references: 10454270
references: J Physiol Paris. 2006 Jan;99(1):29-36
references: 16054348
references: Brain Topogr. 2006 Spring;18(3):167-87
references: 16544207
references: Cereb Cortex. 2006 May;16(5):597-608
references: 16107587
references: Clin Neurophysiol. 2006 Nov;117(11):2424-35
references: 16996303
references: Prog Brain Res. 2006;159:29-42
references: 17071222
references: Stat Med. 2007 Sep 20;26(21):3946-57
references: 17607723
references: Stat Med. 2007 Sep 20;26(21):3911-26
references: 17671957
references: Clin Neurophysiol. 1999 Nov;110(11):1842-57
references: 10576479
references: Neuron. 1999 Sep;24(1):49-65, 111-25
references: 10677026
references: Clin Neurophysiol. 2000 Feb;111(2):246-58
references: 10680559
references: Brain Topogr. 2000 Winter;13(2):79-96
references: 11154104
references: Behav Brain Sci. 2000 Jun;23(3):371-98; discussion 399-437
references: 11301576
references: Hum Brain Mapp. 2001 Jul;13(3):125-64
references: 11376500
references: Neuroreport. 2003 Jan 20;14(1):67-71
references: 12544833
references: Brain Topogr. 2004 Summer;16(4):249-54
references: 15379222
references: Electroencephalogr Clin Neurophysiol. 1986 Jan;63(1):75-82
references: 2416539
references: IEEE Trans Biomed Eng. 1987 Apr;34(4):283-8
references: 3504202
references: Electroencephalogr Clin Neurophysiol. 1989 Feb;72(2):184-7
references: 2464490
references: Brain Topogr. 1989 Spring;1(3):199-215
references: 2641263
references: Brain Topogr. 1991 Spring;3(3):365-71
references: 1878284
references: Brain Topogr. 1991 Winter;4(2):151-68
references: 1793689
references: IEEE Trans Biomed Eng. 1993 Feb;40(2):145-53
references: 8319965
references: Brain Topogr. 1993 Summer;5(4):361-4
references: 8357708
references: Electroencephalogr Clin Neurophysiol. 1995 Sep;95(3):161-77
references: 7555907
references: Proc Natl Acad Sci U S A. 1995 Dec 5;92(25):11568-72
references: 8524805
references: Brain Topogr. 1996 Summer;8(4):355-66
references: 8813415
references: Neurosci Lett. 1997 Jan 31;222(2):103-6
references: 9111739
references: Electroencephalogr Clin Neurophysiol. 1997 Nov;103(5):499-515
references: 9402881
references: J Neurosci. 1999 Jul 1;19(13):5435-48
references: 10377353