Studying the dynamics of visual perception using a dipole model

E. S. Mikhailova,a) M. A. Kulikov, A. V. Slavutskaya, and I. A. Shevelev*

Institute of Higher Neural Activity and Neurophysiology, Russian Academy of Sciences, Moscow

(Submitted February 1, 2011)Opticheskiı̆ Zhurnal 78, 34–41 (December 2011)

A study of the encoding of the basic attributes of an image in the human visual cortex by meansof moving dipoles has shown for the first time that, in the 50–300-ms interval after the stimulus,equivalent current dipoles of induced-potential waves lasting about 27 ms are displacedpredominantly along curved trajectories. At 110–120 ms from the beginning of the stimulus,there is an abrupt displacement of the dipole from a lateral to a medial position. Two zones ofpreferred localization of the dipoles are detected in the lateral and medial regions of the visualcortex, whose coordinates coincide with the beginning and end of the trajectories, while the sizevaries as a function of the phase of the potential. The resulting data are important for estimatingthe dynamics and kinetics of the processing of the attributes of an image in the visual cortex ofthe human brain. c© 2011 Optical Society of America.

I. P. Pavlov once said concerning the dynamic studyof the spatial distribution of the functions of the brain that“. . . future achievements of science will make it possibleto see the motion over the cerebral cortex of the mainneural processes of excitation and inhibition.” If one turns tothe history of the appearance of methods for the functionalmapping of the brain, the first method (in the 1950s) waselectroencephaloscopy, based on multichannel (up to 100points) abduction of biopotentials from the brain surface.1

The mapping of the brain using the criterion of its electricalactivity or characteristic magnetic field currently occupiesan important place among methods of qualitatively andquantitatively evaluating the dynamics and kinetics of thefunctions of the brain.

This communication presents the results of brain mappingusing an indicator of the dynamic localization or displacementof the equivalent dipole sources of induced-potential (IP)waves of the human brain. The localization of the IP sources isbased on solving the inverse problem of electrostatics: to usethe data of multichannel recording of the brain biopotentials tocalculate the three-dimensional position, strength, and dipolevector that creates the potential distribution on the humanscalp that best coincides with the experimental distribution.The method of unfixed sources in the brain (“dipole tracing”)in this case can detect shifting or moving dipoles,2,3 and thisallows it to be regarded as an adequate method for studying thedynamics of brain activation. The method of “fixed” dipoles4

has become the predominant one, although it excludes theirpossible kinetics.

The specific task of this paper was to study the dynamicthree-dimensional localization of the dipole sources of thecomponents of the visual IP when the images of simple visualstimuli—strips and crosses, which are the basic attributesof most objects—are being observed. The indicators of thetrajectories of the displacements of IP dipoles over the brainwere analyzed: their shape, localization, and extent in time andspace.

∗ Deceased

TECHNIQUE

The subjects. The study was carried out on eighteenhealthy subjects (18–21 years old) with normal vision. Ethicalagreement of the subjects to participate in the experiments wasobtained in all cases.

The recording of the electroencephalogram (EEG). Wecarried out 34-channel recording of the EEG of the brainwith a signal-quantization frequency of 1000 Hz/channel (theNeocortex-Pro system, made by Neurobotics, Russia). Theright ear electrode was inert. The resistance of the electrodesdid not exceed 5 k�. The frequency band of the amplifiers waslimited at 100 Hz from above and 0.1 Hz from below, with aslope of the response of 12 dB per octave.

Visual stimulation. At the time of the study, the subject satin an armchair in a darkened room with background luminance6 cd/m2. The distance from the eye to the high-resolutionmonitor screen (Mitsubishi Diamond Pro 2070SB) was 70 cm.Images consisting either of 45 horizontal or vertical strips orof crosses consisting of these strips were presented on thescreen in random order. The diagonal angular size of the entireimage is 18.8◦, while that of a single element of it (cross orstrip) is 0.6◦. The mean optical densities of the stimuli wereequalized. The stimulus was exhibited for 100 ms, while theinterstimulus interval was varied within the limits 5 ± 1 s.In all, 600 stimuli were presented in the experiment—200 foreach type of image (horizontal and vertical strips and crosses).The study, including rest periods, lasted about an hour.

Instructions: “Various images, at which you must simplylook, will appear in front of you on the screen. A warningsignal will sound before an image appears. Concentrate on thecenter of the screen, where the cross is, and do not blink.”

Preliminary selection of the data. After the experiment,sections with artifacts (electromyography, motion of the eyes)were eliminated from the continuous recording of the EEG.The IP was averaged over 40–45 sections of the recordingwith a duration of 600 ms: 100 ms before and 500 ms after thebeginning of the exposure of the image. For each subject, four

790 J. Opt. Technol. 78 (12), December 2011 1070-9762/2011/120790-07/$15.00 c© 2011 Optical Society of America 790

FIG. 1. Shift of the ECD localization of the average visual IP on a set ofcross-shaped stimuli in the occipital cortex of the right hemisphere of thebrain of subject I.R. The results of a calculation of the localization with astep of 1 ms (dark points) are superimposed on a horizontal MRT section ofa model of the subject’s brain. The start of the trajectory is indicated by anarrow.

averaged IPs were thus analyzed for each of the three types ofstimuli, twelve averaged IPs in all. The wave composition ofthe IP is shown in Fig. 2(c).

Estimating the localization of the current dipoles. Thetwo-dipole three-dimensional localization of the equivalentcurrent dipole (ECD) on a three-layer spherical model ofthe head was calculated with a step of 1 ms in theNEOCORTEX-PRO program. The dipolarity factor was set at0.95. This paper shows data on the localization of one of thetwo ECDs—the caudal dipole in the regions of the striatal andextrastriatal visual cortex, in the interval from 50 to 300 msfrom the beginning of the stimulation.

Analysis of the displacement trajectories of the dipoleover the brain. Three independent experts visually analyzedthe displacement trajectory of the dipoles in all eighteensubjects, using the graphs obtained in the NEOCORTEX-PRO

program. In this case, sections with no visible breaks, on whichthe displacement of the dipole either maintained its directionor changed smoothly, were discriminated from the overalltrajectory of motion of the dipole. The types of trajectorieswere qualitatively classified, and the times (from the beginningof the stimulus) at which they began and ended and theirduration were estimated. The results of the NEOCORTEX-PRO

program were then used to determine the X coordinate(back-to-front direction), the Y coordinate (the left-to-rightdirection), and the Z coordinate (the bottom-to-top direction)for each of the points of the trajectory corresponding to thedipole’s position at a definite instant. The displacement of thedipole in three-dimensional space in 1 ms was estimated as1R =

√1X2 +1Y2 +1Z2, where 1X, 1Y , and 1Z are the

displacements along the corresponding axes. When analyzingthe motion of the dipoles, the main attention is paid to theirdisplacements in the x and y directions, since the displacementalong z was insignificant. Besides the visual criterion, anumerical criterion of the end of the trajectory was used: It

was considered the end when there was an abrupt (greater than20 mm) displacement of the dipole along one of the axes.

To evaluate the kinetics of the process, the probability (thefrequency of cases) was studied that a dipole fell into onediscrete element (a 1-cm2 pixel) on the maps of horizontal,frontal, or sagittal “sections” of the computer model of thebrain. Each two-dimensional map consisted of 121 pixels(11 × 11). Dipoles that had reliability less than 0.95 wereexcluded from the data.

RESULTS OF THE INVESTIGATION

The main results can be reduced to the following:1. When an induced potential is developed in the human

brain because of an image of horizontal and vertical strips andcrosses that consist of these strips, a successive displacementover the brain of equivalent current dipoles of IP waves isdetected by solving the inverse problem of an EEG with astep of 1 ms in the two-dipole spherical model of the head.Figure 1 shows an example of the successive displacement ofan IP dipole in the 88–95-ms period after the beginning of thestimulus (a set of crosses) in subject I.P. on a horizontal sectionof an individual magnetic-resonance tomogram (MRT).1) Itcan be seen that the dipoles are localized at this time in theoccipital cortex of the right hemisphere and, being displacedforward and medially, form a short trajectory that is close tolinear. Below in the text, we shall call the line that describesthe successive positions of the dipole in the three-dimensionalspace of the brain with a step of 1 ms the ECD trajectory.

2. The visual analysis carried out by three independentexperts on a selection of 260 ECD displacement trajectoriesin the occipital cortex showed that the dipole moves predomi-nantly along a curved trajectory (75.8% of the cases), while itsduration is characterized by insignificant variability betweenindividuals: from 23 to 29 ms, averaging 27.43 ± 1.3 ms.The connection of the starting time and the ending time of thetrajectory is very tight (the correlation coefficient is r = 0.95with p < 0.0005) and is close to linear (the regression equationis y = 24.6 + 1.0x). This makes it possible to assume thatthe duration of the trajectories corresponding to the earlier andlater IP waves are fairly constant on the average and are notreliably associated with the phase of the induced potential. Ananalysis of the coordinates of the beginning and end of thetrajectories of the caudal dipole2) (X and Y) showed that theyare predominantly located in the occipital region of the righthemisphere.

3. When studying the kinetics of the dipoles, aphenomenon is detected that we called a “jump” of the ECD(Fig. 2). It is determined visually as a rapid displacement ofthe dipole from the dorsolateral region of the right hemisphereto the medial position (Figs. 2(a) and 2(b)) but is observedonly when going from the first to the second trajectory on therising section of the P1 wave (Fig. 2(c)). The beginning timeof this jump, averaged over the group, was 110.2±2.3 ms (theend of the first trajectory) and the end (the beginning of thesecond trajectory) was 119.3 ± 2.5 ms. This displacement isreliable only along the y axis (36.1± 4.5 mm, p < 0.003) andis insignificant along the x and z axes. The value of 1R thatreflects the displacement of the dipole in three-dimensionalspace in the time of the jump is significant and significantly

791 J. Opt. Technol. 78 (12), December 2011 Mikhailova et al. 791

C1

P1

N1

P3

t, ms

t, ms

t, ms

1

1

2

2

–4

–2

0

2

4

100

80

60

40

20

050 100 150

1

0.99

0.98

0.97

0.96

0.9550 100 150

1000 200 300 400

(c)

(d)

(e)

(a)

(b)

ΔR, mm

k, rel. units

IP, μV

FIG. 2. Jump of the coordinates of the dipoles between the first (1) and second (2) trajectories of their displacement over the brain. (a) and (b) are examples of thejump in ECD localization in subject T.I. on two projections of the brain in a spherical model of the head [(a) horizontal and (b) frontal projections]; (c) exampleof IP with an indication of the successive C1, P1, N1, P3 waves; (d) histogram of the size 1R of the jumps; (e) graph of the dipolarity factor k.

greater (Fig. 2(d)) than in the time of the trajectory (on theaverage, 43.7 ± 2.8 mm versus 2.8 ± 0.1 mm). A sharpreduction of the dipolarity factor, coinciding with the jump,can be seen in Fig. 2(e).

4. When the set of coordinates of the “moving” dipolesof the occipital cortex are analyzed, preferred “visiting” zonesare detected that alternate with zones of reduced probabilityof their appearance. Figure 3 shows data that characterize thefrequency of appearance of a dipole in the 1-cm2 pixels ofthe horizontal (xy) plane in a time of 50–300 ms for the IPon the horizontal and vertical planes. Segments (a) and (b)show the individual data for subject D.T., and graphs (c) and(d) show three-dimensional histograms constructed from thegroup data. It can be seen in maps (a) and (b) that there are twodistinct “visiting zones” in the occipital part of this subject’sbrain. The center of one of them is located along the mid-lineof the head or in the neighborhood of it (the value of coordinateY is about zero) and 40–50 mm more caudal than the interauralzero line. The second visiting zone is more lateral—in the righthemisphere, with mean coordinates of the center X = −50 to−60 mm, Y = 50–60 mm. It can be seen that a wide visitingregion is visible around both zones, connecting them like apedestal. Two distinct visiting zones are also seen on the groupdata of (c) and (d) in three-dimensional reliefs, in which thenumber n of cases in which the ECD falls into one pixel oranother is plotted along the vertical axis.

We have traced the variation of the location of zones“visited” by the dipoles in the successive phases of the IPcorresponding to the C1 wave, the falling and rising parts ofthe N1 wave, and its transition to the P3 wave (Fig. 2(c)). Itcan be seen from the group data (Fig. 4(a)) that two preferredvisiting zones are formed even in the first period of the analysis(50–90 ms, the C1 wave). The central occipital zone is moredistinct, while the second zone occurs 4–7 cm to the right ofit. The pattern does not substantially change in the next period(time sections 90–130 ms and 135–260 ms, the falling andrising phases of P1 and N1), even though the zones becomemore local (Figs. 4(b) and 4(c)). At the last stage (sections262–300 ms, wave P3), the lateral zone approaches the mainzone by being displaced medially (Fig. 4(d)).

The evaluation of the nodal points of the trajectories doesnot exhaust the information that can be obtained from thedistribution of the dipoles on the plane. It is not sufficient toknow how often a dipole falls into one section of the brainor the other. It is interesting to clarify whether this happensmore often than can be expected when it is randomly displacedover the brain. This would make it possible to estimatewhether the motion of the dipoles has some “structure.” Inorder to check this assumption, the expected mean frequencyof the dipole falling into any pixel was computed on theassumption that there is no structure, and the expected andobserved distributions of the activity were compared, using

792 J. Opt. Technol. 78 (12), December 2011 Mikhailova et al. 792

10

6

2

–2

–6

–10

350 350

250 250

150 150

50 50–60 –80–20 –4020 0

60 8040

100 120

–80–40–120

–120

–80

–4000 40

40

80

80

120

–10

–8

–6

–4

–2

0

2

4

6

8

10

n n

–10 –6 –2 0 2 6 10 –10 –8 –6 –4 –2 0 2 4 6 6 10y, cm y, cm

y, mmy, mm

x, mm

x, mm

x, c

m

(a) (b)

(c) (d)

Num

ber

of c

ases

Num

ber

of c

ases

FIG. 3. Individual maps of the occipital part of the brain in a horizontal plane, constructed according to the criterion of the number of (1 cm2) pixels of the map“visited” by a displaced dipole, when stimulated by horizontal (a) and vertical (b) strips. The x axis is the back-to-front direction (the negative values correspondto the caudal, and the positive to rostral sections of the brain), y is the left-to-right direction (the negative values correspond to the left and the positive to theright hemisphere). The number of times that a dipole falls into a (1 cm2) pixel of the map is shown by the degree of darkening. The maps are interpolated by themethod of distance-weighted least squares. (c) and (d) are the mean grouped data. The z axis shows the number n of times that a dipole falls into a (1 cm2) pixelof the map.

the χ2 criterion. A reliable difference between them wasdetected (p < 0.001), and this made it possible to confirm thehypothesis that the displacement of the dipoles was random.The distribution of the differences between the actual andmathematically expected number of dipoles in various sectionsof the horizontal map of the brain is shown in Fig. 5, using thedata of subject D.T. It can be seen on what sections of the brainmore frequent (1) or, conversely, less frequent (2) localizationof the dipole is observed than follows from the hypothesis ofequal probability that it will fall into any point of the visualcortex. Thus, two distinct diagonally oriented strips where theappearance of the dipole is preferred and two zones where theprobability of its appearance is relatively reduced showed upon the map.

DISCUSSION OF THE RESULTS

The dipole data and direct estimates of the activation ofthe brain structures coincide. Because the goal of our workwas to trace the IP dipoles in order to estimate the activationof the occipital sections of the human brain under visualstimulation, the question arises of whether the method that weselected to solve this problem is adequate and reliable. Dipolarsources of IP waves of the human brain with patterned visualstimulation have been statically studied in many papers.5–7

The study of the localization of fixed dipoles in combinationwith MRT methods and functional MRT made it possible

to establish that the sources of the early IP waves arelocated in the primary visual cortex and in the extrastriatalregions.6 The results of such papers correspond to the classicalrepresentation of retinotopy in the region of the primary visualcortex of the human brain. It is especially important that it wasshown in the human brain that estimates of the localization ofa “moving” dipole during MRT monitoring coincide with thelocalization of the charges from deep electrodes.8

Comparison of the results of our studies with the literaturedata. It must be pointed out that, in virtually all publicationsdevoted to the tracking of dipoles, the authors localized thedipoles in the static regime without evaluating their dynamicsand kinetics, even though this effect is overlooked in theillustrations of a number of articles. Thus, in Ref. 9, thelocalization of the dipoles of the visual IP on local flashesin different parts of the field of view were studied with astep of 10 ms. It was shown that there are differences in theactivation sequence of three dipolar sources, depending on theeccentricity of the stimulus and its significance for the subject.Our data on the displacement direction of the dipoles coincidewith the information that we found in Ref. 6. These dynamicsare clearly seen in the table presented by the authors and in oneof the figures, although they are not discussed in the article.The tabular data given in the same article are evidence thatthe dipole is successively displaced along the x axis from thecaudal to the rostral sections of the brain as the IP waves

793 J. Opt. Technol. 78 (12), December 2011 Mikhailova et al. 793

20

–9 –9

–7 –7

–5 –5

–3 –3

–1 –1

1 1

7

5

3

7

5

3

9 9

–9 –9

–6 –6–4 –4–2 –20 02 24 46 68 8

9 9

7 7

5 5

3 3

1 1

–1 –1

–3 –3

–5 –5

–7 –7

–6–6 –4–4 –2–2 00 22 44 66 88

(a) (b)

(c) (d)

x, c

mx,

cm

y, cm y, cm

FIG. 4. Dynamics of the zones of most frequent activation of the brain in subject D.T. during various periods of exposure of the horizontal strips. (a) 50–90, (b)90–130, (c) 135–260, and (d) 262–300 ms.

develop. Actually, the source of the early component of theIP, with a peak at 90 ms, is localized in the primary visualcortex, and the source of the early phase of the P1 wave (apeak around 100 ms) is in the dorsal extrastriatal cortex of themiddle occipital convolution, while the source of the late phaseof P1 (peak around 140 ms) is in the ventral extrastriatal cortexof the fusiform gyrus.6

The data obtained in our work show that the com-paratively early (50–90 ms) discrimination of such simpleattributes of the image shape as strips and their crossings isassociated with preferential activation of the central part ofthe occipital lobes of the human brain, as well as zones inthe right hemisphere. This corresponds to the data showingthat the dipoles of the IP waves during the time interval ofinterest are in most cases localized in the region of the sagittalline, i.e., the calcarine fissure in the occipital cortex of theright hemisphere—the dorsal extrastriatal cortex of the middleoccipital convolution, as well as in the ventral extrastriatalcortex of the fusiform gyrus.6

What does the phenomenon of the shift of the dipolarsources of the IP over the brain indicate? It is tempting tointerpret the shift over the brain of “unfixed” dipolar sources

of the IP waves detected in this paper as a reflection of anactual displacement over the brain of the sections where it isactive. We see no physiological, biophysical, or mathematicalprohibitions against such an interpretation. However, it cannotbe neglected that this shift can be simulated in a number ofcases by redistributing the power of static activation sites. Itmust be noted that the actual potential field on the scalp isaccurately modelled only by concentrated dipolar sources. Ifa source is diffuse, it is called not an equivalent dipole butan optimal dipole. The estimate of where it is localized canbe distorted because of the low electrical conductivity of thebones of the skull. Therefore, criteria were developed earlierfor estimating the reliability of the localization of the ECDs.3

The “jump” of the coordinates of the dipole. The natureof the jump that we discovered is unclear. It may be associatedwith the appearance of a new powerful activation site of thebrain. At the same time, the abrupt decrease of the dipolarityfactor at the time of the jump may be a reflection of a transitionfrom generation of a postsynaptic potential (PSP) in the cortexto pulsed transmission of a signal along the axons of thecortical neurons, and this, as is well known, makes virtuallyno contribution to the total potentials of the brain.

794 J. Opt. Technol. 78 (12), December 2011 Mikhailova et al. 794

120

80

40

0

–40

–80

1

2 1 2

10

8

6

4

2

0

–2

–4

–6

–8

–10

y, cm

x, c

m

0 2 4 6 8 10–10 –8 –6 –4 –2

FIG. 5. Distribution of differences between the actual and mathematicallyexpected number of dipoles in different sections of the horizontal map ofthe brain. The degree of darkening of the map characterizes the differencebetween the number of cases of actual and mathematically expected times adipole visits a pixel of the map. Two preferred zones for the appearance ofa dipole are visible (1), and two relatively forbidden bands (2), where theprobability of the appearance of a dipole is reduced.

The stable appearance of the jump at the leading edgeof the P1 wave when it makes a transition to an N1 wavedeserves attention. The differences in the localization ofthe sources of the early and late parts of the P1 waveare described in Ref. 2. The jump may be functionallyassociated with the complication of the reprocessing of visualinformation coincident with the beginning of the N1 wave,10

its identification, and the switching on of the symmetricalregions of the left hemisphere.

The possible functional significance of the phenomenastudied here. When tracing the IP dipole, we found that itmoves predominantly along curved trajectories in the occipitallobes of the brain. It seemed natural to assume that therepeated entries of the trajectory of the dipole detected in thispaper into some small section of the brain are proportional toits activity. We conventionally called this effect the frequencywith which the section of the brain is “visited” by the displaceddipole. The distribution of such frequencies in a volume or ona plane can characterize the relative involvement of varioussections of the brain in generating the IP.

How can the existence of the nodal points that wedetected—local sections of the brain that are preferentiallyvisited by the moving dipoles—be explained? We comparedthese data with information on the localization of the initialand final points of the ECD trajectories. It turned out that theseestimates coincide with the localization of the visiting zonesand are evidence of a certain delay of the dipoles in two localzones of the brain at the beginning and ending periods of theirdisplacement. As far as the pedestal that connects the mostactive visiting zones to the maps of the dipole distribution isconcerned, it is natural to assume that it is a mapping of asuperposition of the typical displacements of the dipoles fromthe initial to the final point of their trajectory. The significanceof this process presumably can consist of scanning the field

of view11 and/or the functioning of a feedback loop in thevisual cortex.12 Further studies are unquestionably needed fora deeper understanding of the mechanisms and a more specificdiscussion of the functional significance of the activationkinetics of the brain, reflected by the displacement of dipolarsources of the IP over it.

The method of step-by-step localization of the currentdipoles of the IP waves thus describes the dynamicdisplacement topography of the activation zone inside thecerebral cortex, which determines the potential distributionover the cortex and its variation in time. It has the undoubtedadvantage of high temporal resolution, and consequently thepossibility of tracing in time the displacement trajectory ofthe preferred activation zone. In combination with mappingmethods, this method makes it possible to obtain completeinformation concerning the operation of the living brain andthe distribution of its functions in time and space.

CONCLUSIONS

1. The successive displacement of equivalent currentdipoles of IP waves over the brain has been detected duringthe development of the induced potential of the human brainon an image of horizontal and vertical strips and crossescomposed of these strips in a two-dipole spherical model ofthe head. A dipole localized in the occipital cortex movespredominantly along a curved trajectory, the length of whichis comparatively standard and is associated neither with thephase of the induced potential nor with the type of stimulus.

2. Between the first and second ECD trajectories, thereis a typical jump of their coordinates (in 85% of the cases),which manifests itself in a short-term abrupt and reliabledisplacement of the dipole from a lateral position into a medialposition.

3. The moving dipoles have two distinct nodal pointsthat they predominantly “visit” in the occipital cortex, 5 cmmore caudal than the mid-line of the head. The first of theseis located around the mid-line of the head, while the secondis located in the right hemisphere at a distance of 6–7 cmfrom the first. Estimated from the dipole data, the activationof the brain at the nodal points varies somewhat in pattern andprominence as the IP develops.

a)Email: esmikhailova@mail.ru1)Magnetic-resonance tomography of the subject’s head was carried out

in the Tomikon S50 tomograph (made by Bruker, Germany) at theCenter for Magnetic-Resonance Tomography and Spectroscopy of theI. M. Lomonosov Moscow State University. The Polhemus Isotrak IIelectromagnetic digitizer (USA) and the NEOCORTEX-PRO program wereused to combine the EEG localization data and the MRT data.

2)Here and below, “caudal” means located behind.

1M. N. Livanov and V. M. Anan’ev, Electrical Encephaloscopy (Medgiz,Moscow, 1960).

2B. He, T. Musha, Y. Okamoto, and S. Homma, “Electric dipole tracingin the human brain by means of the boundary-element method and itsaccuracy,” IEEE Trans. Biomed. Eng. 34, 406 (1987).

3T. Musha and S. Homma, “Do optimal dipoles obtained by thedipole-tracing method always suggest true source locations?” BrainTopogr. 3, 143 (1990).

4M. Scherg, “Fundamentals of dipole source analysis,” in Auditory EvokedMagnetic Fields and Electric Potentials, eds., F. Grandori, M. Hoke, andG. L. Romani (Karger, 1990), pp. 40–69.

795 J. Opt. Technol. 78 (12), December 2011 Mikhailova et al. 795

5E. S. Mikhaı̆lova, A. V. Slavutskaya, V. A. Konyshev, Yu. A. Pirogov,N. V. Anisimov, and I. A. Shevelev, “Localization of the P1 dipole wave ofthe visual induced potential of the human brain,” Dok. Ross. Akad. Nauk409, No. 5, 1 (2006).

6F. Di Russo, A. Martı́nez, M. I. Sereno, S. Pitzalis, and S. Hillyard,“Cortical sources of the early components of the visual evoked potential,”Hum. Brain Mapp. 15, No. 2, 95 (2002).

7C. J. Stok, H. J. Spekreijse, M. J. Peters, H. B. Boom, and F. H. Lopesda Silva, “A comparative EEG/MEG equivalent-dipole study of thepattern-onset visual response,” EEG Clin. Neurophysiol. 41S, 34 (1990).

8K. Whittingstall, G. Stroink, and M. Schmidt, “Evaluating the spatialrelationship of event-related potential and functional MRI sources in theprimary visual cortex,” Hum. Brain Mapp. 28, No. 2, 134 (2007).

9C. J. Aine, S. Supek, and J. S. George, “Temporal dynamics ofvisual-evoked neuromagnetic sources: effects of stimulus parameters andselective attention,” Int. J. Neurosci. 80, No. 1–4, 79 (1995).

10E. K. Vogel and S. J. Luck, “The visual N1 component as an index of adiscrimination process,” Psychophysiology 37, 190 (2000).

11E. D. Bark, I. A. Shevelev, M. A. Kulikov, V. M. Kamenkovich, andL. N. Pokazan’eva, “Trajectories over the human brain of a dipole sourceof background alpha activity,” Zh. Vyssh. Nerv. Deyat. 55, 336 (2005).

12A. Angelucci and P. C. Bressloff, “Contribution of feedforward, lateraland feedback connections to the classical receptive field center andextra-classical receptive field surround of primate V1 neurons,” Prog. BrainRes. 154, 93 (2006).

796 J. Opt. Technol. 78 (12), December 2011 Mikhailova et al. 796