The major goals of noninvasive studies of the human visual cortex are: to increase knowledge of the functional organization of cortical visual pathways; and to develop noninvasive clinical tests for the assessment of cortical function. Noninvasive techniques suitable for studies of the structure and function of human visual cortex include magnetic resonance imaging (MRI), positron emission tomography (PET), single photon emission tomography (SPECT), scalp recorded event-related potentials (ERPs), and event-related magnetic fields (ERFs). The primary challenge faced by noninvasive functional measures is to optimize the spatial and temporal resolution of the measurement and analytic techniques in order to effectively characterize the spatial and temporal variations in patterns of neuronal activity. In this paper we review the use of neuromagnetic techniques for this purpose. 8 refs., 3 figs

Aine, C. J.

George, J. S.

Maclin, E. L. (Veterans Administration Medical Center, Albuquerque, NM (USA). Center for Magnetoencephalography)

Supek, S. (Los Alamos National Lab., NM (USA))

UNT Digital Library

II IJA-UR -9o'3634¢LA-UR--90-3634DE91 001856Los Alamos National Laboratory _s operated by the Unwerslty of Cahtorn,a tor the Unlled States Department of Energy under col_lracT W-7405-ENG-36i i iii ii i imm|l, ]t,, USINC NE ROMAGNI.:,'ITICTECiINI(_UI{,_TITLE NONINVASTIVE STUDIES OF lltJ_b\I'_ VISUAL COR ':X I.!AUTHOR(S): C.J. Aine, J. S. George, S. Supek, P-6E. L. Maclin, VA Medical Center, Albuquerque, NMSUBMITTEDTO: Optical Society of Amer.-Noninvasive Assessment of the Visual System,February 4-7, 1991DISCLAIMER! ,! iThis report was prepared as an account of work sponsored by an agency of the United States , . ,Government. Neither the United States Government nor any agency/hereof, nor any of their _ ; ''/ t J /?employees, makes any warrant)', express or implied, or' assumes any legal liability or responsi- NO V , . .bility for the accuracy, completeness, or usefulness of any informatiov., apparatus, product, or 0 r:q tU_er_ra"'/process disclose, d, or represents that its use would not infringe privately owned rights. Refer-ence herein to any specific commercial product, process, or service by trade name, trademark,manufacturer, or otherwise does not necessarily constitute or imply its endorsement, recom- ,.mendation, or favoring by the United States Government or any agency thereof. The viewsand opinions of authors expressed herein do not necessarily state or reflect those of theUnited States Government or any agency thereof.By acceptance of Th=s art,cle the pubhsher recogrnzes thai the U S Government rein,ns a nonexcluslve, royalty-free hcense to publish or reproducethe pubhsned form Of th_S contr,buhon, or tO allow others tO do so. (or U S Government purposesThe Los Alamos Nat=onal LaboraTory requeSTS thai the pubhsher _dentdy th_s arhcle as work performed under the auspmes of the U S Department of EnergyLosAlamos NationalLaboratory, ew Mexico 87545""s, No.26_._,e, DIS-IF,'tL:_UF_©i"' (-i- }'b!).::-_I:.)OC';L,,_'.,4{iLNIL "ltNoninvasive Studies of Human Visual Cortex Using Neuromagnetic TechniquesC.J. Aine, J.S. George, S. Supek, and E.L. Maclin*Physics Division, Mail Stop M715Los Alamos National Laboratory, Los Alamos, NM 87545*Center for Magnetoencephalography, VA Medical CenterAlbuquerque, NM 87108IntroductionThe major goals of noninvasive studies of the human visual cortex are: (1) to increaseknowledge of the functional organization of cortical visual pathways; and (2) to developnoninvasive clinical tests for the assessment of cortical function. Noninvasive techniquessuitable for studies of the structure and function of human visual cortex include magneticresonance imaging (MRI), positron emission tomography (PET), single photon emissiontomography (SPECT), scalp recorded event-related potentials (ERPs), and event-relatedmagnetic fields (ERFs). The primary challenge faced by noninvasive functional measures is tooptimize the spatial and temporal resolution of the measurement and analytic techniques inorder to effectively characterize the spatial and temporal variations in patterns of neuronalactivity. In this paper we review the use of neuromagnetic techniques for this purpose.MethodsActivation nf human visual cortex by suitable visual stimuli produces magnetic fields thatcan be recorded outside the head using magnetic detectors comprised of sensor coils coupled tosuperconducting quantum interference devices (SQUIDs). Depending upon the specific coilconfiguration, such detectors act as first- or second-order gradiometers which produce avoltage output proportional to the first or second spatial derivative of the magnetic fieldoriented normal to the surface of the head. Neuromagnetic fields associated with corticalactivity typically range from 10-12 to 10-14 tesla (T), compared to urban background noiseof up to 10-6 T, and the earth's magnetic field of approximately 5 x 10-5 T. A neuromagneticmeasurement system is illustrated schematically in figure 1.Neuromagnetic measurements result in a spatial- TOEL_mO.C.Stemporal data matrix (magnetic field strengths at each of {_w_T time points for each of N recording sites). Determining _ _ _ "rthe pattern of neuronal currents in the brain that give UOUOHEUUM /.__rise to an observed magnetic field pattern outside the headis termed the neuromagnetic inverse problem. Because agiven magnetic field on the surface of the head could be dueto any number of possible source configurations, the _GN_..nCREU_S-..._ _.jnve sehasnoun,ueo,u,,on   rox,ma,esolutions are possible, however, if assumptions are made F,a.o_Pregarding tile shape and conductivity of the head and the "_--Jnumber and configuration of neuronal currentsresponsible for the surface distributions (Sarvas,1987): Using this approach numerical minimization' VISUtechniques can be used to determine the best.fittingsource or sources, subject to the assumptions of aparticular model. This approach to the inverse problembecomes more attractive when solutions are furtherconstrained by applying knowledge of brain physiologyandanatomical structure to the source modeling procedures. Figure 1. Experimental Setup.The most commonly used source model for both magnetic and electrical data is a single pointcurrent dipole which represents the centroid of synchronous activity of a population of neurons(Kaufman et al., 1984; Wood, 1982). This model is applied to the field distribution at a singlelatency and yields the location, orientation, and strength of the current dipole that bestaccounts for the observed data in a least-squares sense. Such best-fitting current dipoles areequivalent in the sense that the magnetic field 1. 7-SENSOR ARRAYgenerated by a single current dipole is used asa simplified representation of fields generated Sinusoidal Grating in Right Visual Fieldby more complex source configurations.Multiple dipole models can be fitted usingsimilar techniques when single dipole models .-. _ -._._ 20oare inadequate to account for the data. More E 110advanced models under development by a _ 2o-70number of investigators attempt to account for _ .,6othe entire spatial-temporal data matrix using ,_ .2s0a small number of spatially fixed dipoles withtime-varying magnitudes.Figure 2 illustrates the steps involved in ,=o _70deriving equivalent current dipoles (ECDs) Lalency(msec)2. CONTOUR PLOT 3. LEAST SQUARES CODEMagnetic Field Distribution- 100 msec TheoreticalModel ResidualC ., o _,C 6 e 6 ',.$ -ItD 4 4> =O.Q il= o-| .-_1_1 -| -4 -1 -t -I 0 ! | -6 -I, -4 -,t -1 -I tO I ! O II Q :1 | I 0 I z •cm left/rightof inlon4. MAGNETIC RESONANCE IMAGING (MRI)MidsagittalView HorizontalViewFigure 2. Generalprocedures for dataanalysis. See text forexplanation.from magnetic field distributions. Part 1 displays magnetic data from a single placement of a7-channel hexagonal sensor array. During a given experiment, the 7-sensor array is moved toseveral locations over the surface of the head and the experimental conditions are repeated foreach position of the array until the desired surface area is completely mapped. Field amplitudesare measured relative to a prestimulus baseline. Iso-amplitude maps, as shown in Part 2,are then contructed at individual latencies by interpolation and are displayed as contour orpseudocolor maps. Each contour line represents a region of equal magnetic field strength(positive values indicate magnetic flux out of the head, negative values into the head). Oneplacement of the 7-sensor array is indicated by dots in the upper left of the contour map. Inthis field map the inion (the bony protrusion at the back of the head) is at x=0, y=0 (indicatedby + on the map). Note the existence of a positive peak at x=-4.5, y=6.0 and a negative peak atx=0, y=2.0.When the field maps demonstrate the existence of two primary extrema (i.e., a single pair ofpositive and negative peaks), the location, orientation, and strength of the best-fitting singlecurrent dipole are calculated using a nonlinear least-squares algorithm. Part 3 illustratesthe magnetic field generated by the best-fitting ECD for the empirical data shown in part 2,together with a map of the residual magnetic field strength unaccounted for by the single dipolemodel. This ECD is shownas an arrow on the contour map. In this case the ECD accounted for94% of the total variance of the original data. Two and 3-dipole models are applied to the dataif there is evidence of three or more extrema in the field distributions and/or the residual fieldmaps contain a dipole-like field pattern. A comparison of single versus multiple dipole modelscan be made by examining reduced Chi-square (Xr2) values associated with each model; valuesbetween 1.0 and 1.5 are interpreted as adequate fits to the data (e.g., Bevington, 1969). MonteCarlo simulations are performed to assess the stability of the model solutions with re.spect tonoise and to determine whether source parameters differ significantly as a function ofexperimental conditions (Medvick et al., 1990).1 cpd Right Visual Field -- 110 msEmpirical Fields Theoreticals Di:)ole 1 Dipole 2'° : . .C:_ ' ............ __i_!i:'_;:_i:i_iii:i!ii_:::::._!!:.:::i!::!_i i_i;_:i;::::i:i _ili_:::.i:ii:i'i:i;: !! ii:i i: #i:::-_i_' -_ ?o , ©!iii i!ii....., ....:_!:,iiiii:::/_i:_:,:.iiiii:::i::,ili'.......ii , :: ,ili,(:i :,iiiii_ili!ii::i:_i, , :, "."_." ':_: :!ii :i:iiii :_ii_ii_iiii!:_!i'•*Dmll 0"-i .- ' 0-_o -8 -¢, -* -_' o t _ o I • _, o t Io II • _i 4_ o ?:i'L:.!i;iii_ ii.o (.,,.) ' * *. ,,.,.._o o _,o _ _, ,, o ;' ,o a ._, * _ o _ * " • _o _ -_. .* 2 o 2 * -_- _o"J"--_" -_, -. -;' o ; *cm left/right of inionFigure 3. Empirical and 2-dipole theoretical distributions (first two columns)for twosubjects show at least two sources at 1I0 msec when a I cpd grating was centered 7° along thehorizontal meridian of the right visual field. Theoretical distributions are shown in columns 3and 4 for the component dipoles (Dipole I and Dipole 2) obtained from the :;)-dipole model."BIn Part 4 the coordinate system of the magnetic measurements and ECD model is_ransformed into that of MRI scans to permit determination of the locations of ECDs relative toanatomical structures (George et al., 1990). By _adiological convention, MRI sections aredisplayed as though viewed from in front of or underneath the head so that the left hemisphereis displayed on the right side of the MRI section. Coordinate reconciliation is achieved byattaching oil-containing capsules to the head at designated reference locations to provideprecise common fiducial marks in the two coordinate systems.Field distributions characterized by a single dipole-like configuration are often observed indistributions reflecting early cortical activity (e.g., 70-100 ms) in response to visualstimulation. Later activity (> 100 ms) most often appears to be a composite of multiplesources with different time-courses. Figure :3 displays such an example for two subjects. Atleast two ECDs can be fitted at 110 ms for both subjects when a 1 cpd grating was presented inthe right visual field. These models were considered adequate since the Xr2 values for the2-dipole model ranged from 1.13-1.5. The percent of variance accounted for by the 2-dipolemodels ranged from 75-77%. In comparison, Xr2 values for the single-dipole models rangedfrom 1,80-1.82 and the percent of variance ranged from 44-67%. A 3-dipole model was alsoconsidered adequate for GM's data (Xr2=1.0, percent of variance=77) which modeled anadditional right hemisphere ECD (see the empirical fields at x=l.0, y=0). A similar righthemisphere ECD has been identified in CA's data at 120 ms. Although the field distributions forthe two subjects appear ,quite different, the ECD solutions are more similar between subjects.One ECD is located along the midline (x-0) and the other is located lateral to the midline ECD inthe left hemisphere (x---5). The major difference between the ECDs across these subjects isthe location along the y axis, which may be attributable to differences in cortical geometrybetwe,_,n subjects (e.g., the relation of the calcarine fissure to the inion). Radically differentfield distributions can result from slight differences in orientation of two or more dipoles.ConclusionThe primary challenge for noninvasive functional measurements is the spatial and temporalresolution of multiple sources. This paper has described ways in which neuromagnetictechniques have been applied toward this end. When complex field patterns are reduced tocomponent sources, similarities across subjects are more apparent than in the fielddistributions themselves. By combining neuromagnetic measurements with manipulation ofvisual stimuli, it should be possible to characterize the normal and pathological function ofcortical visual pathways,ReferencesBevington, P.R. 1969: Data Reduction and Error Analysis for the Physica! Sciences. New York:McGraw-Hill.Fox, P.T., Miezin, F.M., AIIman, J.M., Van Essen, D.C. and Raichle, M.E. 1987: J. Neurosci.7:913-922.George, J.S., Jackson, P.S., Ranken, D.M. and Flynn, E.R. 1990: In S.J. Williamson, M. Hoke, G.Stroink, an-: M. Kotani (eds.). Advances in Biomaanetisrn, New York: Plenum, pp. 737-740.Kaufman, L., Okada, Y., Tripp, J. and Weinberg, H. 1984: Ann. N.Y. Acad. Sci., 425:722-436.Maclin, E.L., Okada, Y.C., Kaufman, L. and Williamson, S.J. 1983: II Nuovo Cimento.2:410-419.Medvick, P.A., Lewis, P.S., Aine, C. and Flynn, E.R. 1990: In S.J. Williamson, M. Hoke, G.Stroink, and M. Kotani (eds.). Advances in Biomaanetism., New York: Plenum, pp. 543-546.Sarvas, J. 1987: Phys. Med. Biol., 32:11-22.Wood, C.C. 1982: Ann. N.Y. Acad. Sci., 388:139-155.i t_

Noninvasive studies of human visual cortex using neuromagnetic techniques

https://digital.library.unt.edu/ark:/67531/metadc1210282/m2/1/high_res_d/6437678.pdf

Noninvasive studies of human visual cortex using neuromagnetic techniques

Abstract

Similar works

Full text

Available Versions

UNT Digital Library