Activation of Human V5 Complex and Rolandic Regionsin Association with Moving Visual Stimuli

M.A. Uusitalo,* V. Virsu,† S. Salenius,* R. Nasanen,‡ and R. Hari**Brain Research Unit, Low Temperature Laboratory, Helsinki University of Technology, P.O. Box 2200, FIN-02015 HUT, Finland;

†Department of Psychology, P.O. Box 4, FIN-00014 University of Helsinki, Finland; and ‡Department ofOptometry and Vision Sciences, University of Wales, College of Cardiff, P.O. Box 905, Cardiff CF1 3XF, United Kingdom

Received September 3, 1996

We recorded magnetoencephalographic responsesfrom seven healthy humans during the presentation ofstationary and rotating radial gratings. Rotations last-ing 1 s evoked movement-specific sustained activity inthe parieto-occipitotemporal border area, in agree-ment with the activation of the V5 complex specializedfor the analysis of movement. The source areas of themovement-specific sustained fields were transientlyactive 100–130 ms after the onsets of both rotating andstationary stimuli, suggesting that movement-relatedcortical areas respond to any transient changes in thevisual environment. Transients were evoked also inother brain areas 60–200ms after onsets of both stimuli.Four subjects displayed additional motion-related sus-tained activity in the rolandic region. Sustained activ-ity continued after the stimulus movement in severalsubjects during perception of the movement afteref-fect. The transient activity may evoke visual attentionwhile sustained activity of the V5 complex may berelated to the conscious perception ofmovement. r 1997

Academic Press

INTRODUCTION

The human extrastriate visual cortex contains sev-eral areas that are dominantly sensitive to certainattributes of visual stimuli. Evidence for an area special-ized for the processing of visual motion, area V5 (alsoknown as MT), was first provided by lesion data (Zihl etal., 1983) and later especially by functional imagingstudies (Zeki et al., 1991; Probst et al., 1993; Watson etal., 1993; Tootell et al., 1995b; Tootell and Taylor, 1995;Anderson et al., 1996).The special electrophysiological property of neurons

in monkey V5/MT is their selectivity to the direction oflinear movement (Dubner and Zeki, 1971; Albright andStoner, 1995). Responses to moving stimuli are moregeneral in MT than in V1 cells, because some MT cellsrespond to the direction of movement independently ofthe orientation of parts in the stimulus (Albright,

1992). Inhibition of MT neurons by stimuli moving innonpreferred directions also distinguishes them fromV1 cells (Rodman andAlbright, 1987).Other movement-related areas neighbor area MT in

the monkey. One of these is area MST in the medialsuperior temporal sulcus adjacent and anterior to areaMT. The MST neurons respond optimally to, e.g.,expanding, contracting, or rotating patterns, and theyhave much larger receptive fields and prefer morecomplex movements than MT cells (Andersen et al.,1990). The hypothetical homologue of MST (Zeki andShipp, 1988; Corbetta et al., 1990; Cheng et al., 1995;Tootell et al., 1996), also called V5A, and several othermotion-related cortical areas have been identified inhumans, like area KO processing the kinetic contoursdefined by motion differences (Orban et al., 1995), ananterior motion area activated in addition to V5 byexpanding/contracting visual motion stimuli (Ahlfors etal., 1995), and a superior parieto-occipital area, SPO,responding to coherent motion (Tootell et al., 1996). Theinterplay between all these areas is still poorly under-stood. The cessation of movement in one directioncauses an illusory perception of movement in theopposite direction, so-called movement aftereffect(MAE). The human V5 seems to be activated alsoduring MAE (Tootell et al., 1995a).In the present study, we investigated the activation

of the human cerebral cortex in relation to visualrotatory motion and its perception using a whole-scalpneuromagnetometer. Neuromagnetic recordings (for areview, see Hamalainen et al., 1993) provide a reason-able spatial resolution and an excellent temporal reso-lution. Consequently, transient and sustained activa-tions can be differentiated, which is not possible withPET or fMRI. The stimuli were stationary and rotatingradial gratings, like the wheels of a wagon. The rota-tory movement was easily recognized, and both stimuliproduced a strong enough activation to be recordedwith magnetoencephalography (MEG). We preferredrotatory motion to expanding/contracting patterns be-

NEUROIMAGE 5, 241–250 (1997)ARTICLE NO. NI970266

241 1053-8119/97 $25.00Copyright r 1997 by Academic Press

All rights of reproduction in any form reserved.

cause it produced MAEs of shorter duration and thusallowed us to repeat the stimuli rapidly and collect asufficient number of responses for averaging in areasonable time.A conference abstract of part of this study has been

published (Uusitalo et al., 1995).

MATERIALS AND METHODS

Subjects and Recordings

We recorded cerebral magnetic fields from sevenhealthy right-handed subjects (mean age 27 years,range 23–33; two females and five males). All hadnormal or corrected-to-normal visual acuity. The experi-ments were undertaken with the understanding andinformed consent of each subject. The subjects weresitting in a magnetically shielded room with the headleaning against the lower end of a helmet-shapedwhole-scalp neuromagnetometer Neuromag-122 (Aho-nen et al., 1993). Using 122 sensors at 61 locationsaround the scalp, this instrument measures tangentialderivatives ≠Bz/≠x and ≠Bz/≠y of the magnetic fieldcomponentBz perpendicular to the surface of the sensorarray. The planar sensors sense the strongest signalsdirectly above local cortical currents. Because onlycurrents tangential to the surface can be detectedoutside a sphere (a commonly used volume conductormodel for the head), and because the net neuronalcurrents are orthogonal to the cortical surface, theneuromagnetic signals mainly reflect activity of thefissural cortex (Hamalainen et al., 1993). Activity of theconvexial cortex may be detected when the local skullsurface deviates sufficiently from a spherical surface, orthe net current is tilted with respect to the surface andthus has a detectable tangential component.The recording passbandwas 0.03–90Hz (3-dB points;

high-pass roll-off 35 dB/decade, low-pass over 80 dB/decade), and the signals were digitized at 0.3 kHz.Horizontal and vertical electro-oculograms (EOGs) weresimultaneously recorded to monitor eye movementsand blinks. Responses coinciding with EOG deflectionsexceeding 150 µV were rejected.The position of the headwith respect to the neuromag-

netometer was found by recording magnetic signalsproduced by currents driven into three coils attached tothe scalp. The positions of the coils with respect tolandmarks on the skull (right and left preauricularpoints and nasion) were measured with a 3-D digitizer(Isotrak 3S10002; Polhemus Navigation Sciences, Col-chester, VT). This information was used to superimposesources of brain signals on T1-weighted magnetic reso-nance images (MRIs) obtained with a 1.0- or 1.5-TSiemens device for all the subjects; the slice thicknesswas 1.2–1.5 mm.

Stimuli and Experimental Conditions

The stimuli were produced by a Macintosh Quadra840AV computer and transferred with a video projector(Sony LCD Data Projector VCL-350QM) into the mag-netically shielded room, onto a white cardboard in frontof the subject. During the recordings, the screen wasthe only light source in the room, and the subjectsviewed it binocularly with natural pupils at a distanceof about 50 cm. The average luminance of the stimulusequaled that of the background and was constantlyabout 20 cd/m2. The subjects stayed in the room at least5 min before the experiment started.The stimuli, shown in Fig. 1, were black-and-white

radial gratings with a diameter of 6°. The luminance ofthe pattern changed sinusoidally along concentric circleswith 45°/cycle; the whole grating thus contained eightcycles. The display luminance was approximately lin-ear and it was coded with 8 bits. Thus, 256 differentluminance levels were used to generate stimuli of 100%contrast and 26 for 10% contrast. To avoid spatialaliasing in themiddle of the stimulus, the 0.2° diametercenter part of the grating was replaced by a blackfixation dot surrounded by a narrow ring of backgroundgray. The rotating grating turned clockwise by 1.9° onceevery 14 ms, resulting in an angular rotation velocity of140°/s; the flicker frequency at any image point wasthus 3.1 Hz. In a centrally fixated radial grating thespatial frequency decreases linearly with eccentricity,and the decrease may compensate the decreasing reso-lution of the retina at increasing eccentricities (Virsuand Hari, 1996). Our stimulus, like all motion stimuligenerated on a computer or TV screen, or in the movies,contained no continuous motion, but this should notaffect the results (Purves et al., 1996).In the two subsequent sessions, shown in Fig. 1,

stationary and rotating gratings, aiming at the determi-nation of movement-specificity of responses, appearedonce every 3 s for 1 s on a gray background, whichcontained a black fixation spot in the center. Contrastsof 100% and 10%were tested in different runs.After theexperiment, the subjects were interviewed on whatthey had perceived.

FIG. 1. Experimental conditions. In different sessions, stationaryand rotating gratings appeared once every 3 s for 1 s on a graybackground which contained a black fixation spot in the center. Theexact stimulus duration was 0.97 s.

242 UUSITALO ET AL.

Data Analysis

More than 100 responses were averaged under everycondition. The reproducibility of the signals was exam-ined by averaging every second response separately.Only channels containing reproducible signals weretaken into account in later analyses; typically less than10% of the channels were rejected. The averaged sig-nals were digitally low-pass filtered at 40 Hz, and thusthe passband of all signals shown is 0.03–40 Hz.Response amplitudes were measured with respect tothe mean signal level during 450 ms before the stimu-lus.Sources of evoked responses were modeled by equiva-

lent current dipoles (ECDs) in a spherical volumeconductor; the direction of the current dipole indicatesthe direction of the net intracellular current flow in thesource area. The sphere was determined according tothe local curvature of the posterior part of the head,measured from individual MR images. Single dipoleswere identified on the basis of signals from a limitednumber of sensors around a local signal maximum.After this, the remaining unexplained part of therecorded signals was extracted with a Signal–SpaceProjection method (Uusitalo and Ilmoniemi, 1997), andnew sources were identified when possible. A multidi-pole model with all four to seven dipoles was then usedto estimate the time behavior of all active cortical areason the basis of the original signals. Typically, sourcesidentified on the basis of responses to stationary androtating stimuli were similar in location, and the samemultidipole model was used to explain signals evokedby both stimuli. For details of the source analysismethods, see Hamalainen et al. (1993).A two-tailed t test for paired differences was used in

the statistical evaluation of the data.

RESULTS

Responses and Source Locations of One Subject

Figure 2 presents the distribution of responses torotating and stationary gratings in subject S1. Thereplicability of signals was good as shown by the inset.The onsets of both stimuli were followed by transientdeflections, with peak latencies at 65–250 ms at differ-ent loci, strongest at posterior parts of the head. Forrotating stimuli, the transients were followed by strongsustained fields (SFs) which ended with an off response,peaking 180 ms after the offset of rotation. The mostanterior sensors showed no signals exceeding the pre-stimulus noise level. As the inset shows, the onsettransients to both gratings were similar, but the sus-tained fieldwas considerably smaller during the station-ary than during the rotatory stimulus. The subjectreported perceiving a weak MAE, with the fixationmark rotating in the direction opposite to the stimulus

for less than the 2-s pause between the stimuli. Afterthe rotating stimulus disappeared, a slowly decayingMEG signal lasted several hundred milliseconds, coin-ciding with the beginning of the MAE.To specify the brain areas that produced the re-

sponses in Fig. 2, altogether six current dipoles wereidentified at different moments of time. These sourceswere necessary to explain 80–90% of the variance of allthe whole-head data during both transients and sus-tained signals. The dipoles are presented in Fig. 3 withthe estimated source waveforms. Dipoles 1, 2, and 4accounted for transient responses to stationary androtating stimuli, dipoles 2–5 for sustained activityduring the rotating stimulus, and dipole 6 for signalsduring MAE. Dipole 1 was situated in the medialposterior occipital lobe, with peak in activity at 65 mswith the rotating and at 80 ms with the stationarystimulus. Dipole 2, peaking at 110 ms for the rotatingand 130ms for the stationary stimulus, was close to themidline occipitoparietal junction. The SF started 400ms after the onset of the rotating stimulus, and thesignal returned back to the baseline 450 ms after thestimulus offset.Dipoles 3–5 accounted for the motion-related sus-

tained activity. Dipole 3 accounted for the SF over theright posterolateral occipital lobe, reached full activa-tion at 250–300 ms, and decreased to 0 about 100 msafter the rotation. It also accounted for transient activa-tion at 130 ms.Dipole 4 accounted for the SF on the leftparieto-occipitotemporal junction. It also reached fullactivation at 250–300 ms and decreased to 0 about 100ms after the rotation. It accounted for a transient at 110ms. Dipole 5 in the left lower rolandic region was muchweaker than the simultaneously active dipoles 3 and 4and was thus localized on the basis of the signals fromwhich the contribution of the other dipoles was sub-tracted by the Signal–Space Projection method (Uus-italo and Ilmoniemi, 1997). It reached full activation at250 ms and decreased to 0 about 150 ms after therotation. The right-hemisphere anterior signals (seeFig. 2) were explained by dipole 3.Dipole 6was situatednear the inferior brain surface, close to the borderbetween the occipital and the temporal lobes, and itaccounted for activity from 80 to 800 ms after the offsetof the rotation. The good reproducibility of sourceidentification is illustrated by the two ECDs superim-posed on each brain slice in Fig. 3, determined frommeasurements separated by 2 weeks.

Results for All Subjects

Figure 4 shows the responses of each subject over thetwo hemispheres from the sensor that displayed thelargest difference between the posterolateral responsesto stationary and rotating stimuli. The stimuli (100%contrast) evoked in all subjects transient onset andoffset responses. The average SF level during the

243CORTICAL ACTIVATION AND VISUAL MOTION

rotatory stimulus (from 300 to 1000 ms) differed statis-tically significantly from that elicited by the stationarystimulus over both hemispheres (P , 0.01;mean 6 SEM difference 5 12.1 6 2.9 fT/cm on the leftand 17.0 6 4.3 fT/cm on the right). During stationarysignals, the mean SF amplitude did not differ signifi-cantly from 0. Thus, these SFs seemed to be movementspecific.In general, stimulus onsets were followed by tran-

sient activations peaking in midline posterior regionsat 60–120 ms, near the parieto-occipital sulcus at80–170 ms, near the lateral regions (that later pro-duced the SFs) at 100–130 ms, and in the remainingright and left extrastriate areas at 100–200 ms. Stimu-lus offsets also evoked transient responses. Depending

on the subject, four to six ECDs were needed to accountfor the onset transients. The latencies were shorterwith 100% than with 10% contrast for both stationaryand moving stimuli (P , 0.01; differences 14 6 3 and21 6 6 ms, respectively). Moreover, the latencies wereshorter for moving than for stationary stimuli, both for10 and 100% contrasts (P , 0.01; differences 13 6 2and 17 6 1 ms).Figure 5 shows for the posterolateral SFs the source

locations and the source strengths as a function of time;the waveforms were derived from multidipole models.Signals of subjects S6 and S7 were too weak for sourcelocalization. The sources indicated SFs significantlydiffering from 0 during the rotatory (mean 12.7 6 1.9nAm; P , 0.001) but not during the stationary stimu-

FIG. 2. Whole-scalp magnetic responses of subject S1 to rotating (solid lines) and stationary (dashed lines) gratings of 100% contrast.Sensors are organized in pairs, measuring both the latitudinal (top one of each pair) and the longitudinal (bottom) gradients of the normalcomponent of the magnetic field, as indicated by the graphs near the lower right sensor pair. Three pairs of signals were omitted because ofslow drifts during the measurement. The view is from the top, as indicated by the graph on the lower left. Thus, for example, the uppermostsignals have been recorded above the frontal cortex and those on the left above the left temporal cortex. The inset on the lower right showsenlarged signals from the right lateral region, with replications superimposed and peak latencies marked in milliseconds. The shading showsthe signals originating from the rolandic source, as explained in the text. The vertical bars show the onset and offset of the stimuli. Note thatalthough the largest signals suggest the approximate location of the source currents underneath, no conclusive decisions about the activebrain areas should be made on the basis of the signal distribution without detailed source analysis.

244 UUSITALO ET AL.

lus. All sources, except one, were in the parieto-occipitotemporal border area, the depth varying fromsubject to subject. The exception, the right-hemispheresource of S1, was more posterior. The sustained signalsbegan 200–300 ms after the onset of the rotation andended 0.1–1 s after the offset. SFs were on average1.8 6 0.3 times stronger for stimuli with 100% thanwith 10% contrast (nine hemispheres; P , 0.05).As shown in Fig. 5, the source areas of the posterolat-

eral SFs were typically transiently active 100–130 msafter the onsets of both stimuli. Transients exceedingthe prestimulus noise level were clear in the sourcewaveforms of six of the nine hemispheres. The tran-sients were 1.3 6 0.1 (six hemispheres; NS) timesstronger for stimuli with 100% than with 10% contrast.In four subjects, the sources localized on the basis of

the posterolateral SFs did not account for all sustained

activity, and a more anterior source had to be added tothe model to explain 40–50% of the remaining varianceat least at some latency. Figure 6 shows the locationsand estimated amplitudes of these sources, which werewithin 615 mm from the central sulcus; in threesubjects the location corresponded to the hand and inone to the face somatomotor representation area. Thecentral sulcus was identified on the basis of conven-tional recordings of somatosensory evoked fields (Hari,1993) and on 3-D MRI surface reconstruction of thesubjects’ brains. As shown in Fig. 6, the rolandic SFselicited by the rotating stimuli began about 250 msafter the stimulus onset in all subjects, and ended150–250 ms after the stimulus offset in subjects S1 andS7, whereas subjects S2 and S4 indicated prolongedactivity lasting over 1 s.Figure 7 compares the coordinates of the dipoles of

Figs. 5 and 6 with those activated by linear motion inanother study (six subjects, three in common with thepresent study; Uusitalo et al., 1996). Except for theright-side sources of subjects S1 and S2, the V5 complexsources were 3.8–6.7 cm above the plane defined by thepreauricular points and the nasion, 2.1–3.4 cm poste-rior to the preauricular points, and 2.8–5.1 cm lateralfrom themidline. These sources were anterior to sourcesactivated by linear motion; the mean difference was 16mm in both hemispheres (P , 0.05 on the left andP , 0.1 on the right; two-tailed t test for group differ-ences). No statistical differences were observed in otherdirections. In each subject, the rolandic sources wereabout 3 cm anterior and 3 cm superior to sources in theV5 complex.

Movement Aftereffect

Four subjects (S1, S2, S3, and S4) indicated slowlydecaying sustained activity within 400 ms after theoffset of rotation, i.e., during the perception of MAE(Fig. 4). In subjects S2, S3, and S4 similar postrota-tional SFs were observed in the source waveforms(Fig. 5). The stationary stimuli seemed to produceclearly less sustained activity 200–500 ms after thestimulus; this difference reached statistical signifi-cance (P , 0.05) on the right. As shown above (Fig. 3),subject S1 displayed strong activity after the move-ment in the ventral occipitotemporal border area (di-pole 6), in a site different from the movement-relatedSFs generated during rotation.

DISCUSSION

Source Locations and Methodological Considerations

We found sustained motion-related activity in theparieto-occipitotemporal border area, close to the V5complex region, and additional activity in the rolandicregion. The rolandic activation is surprising, but not

FIG. 3. (Left) The locations for dipoles 1–6 of subject S1 superim-posed on his coronal, sagittal, and horizontal magnetic resonanceimages. The source locations and orientations determined from twomeasurements, separated by 2 weeks, are indicated by the whitecircles and their tails; the signal strengths are shown for a dipole atthe middle location. The white lines show how the slices are situatedwith respect to each other. (Right) The strengths of the dipoles (innanoampere · meters) for stationary (upper trace) and rotatory (lowertrace) stimuli. Peak latencies are indicated in milliseconds.

245CORTICAL ACTIVATION AND VISUAL MOTION

totally without antecedents. Lounasmaa et al. (1984)observed MEG signals originating close to the primarymotor face representation area in association withchanging velocity of a moving grating. One possibilityis that the rolandic activity reflects automatic move-ment preparation in association with visual motion.However, since we did not notify any concomitantchanges in the level of the somatomotor rhythms,checked in all subjects in the 7- to 14- and 15- to 25-Hzbands, this finding has to be interpreted with caution.Some of the V5 complex sources seemed to be in the

white matter although MEG is supposed to mainlyrecord cortical activity. One possibility for such deepsource locations would be an extended cortical activa-tion, or two separate source areas, for example, due tosimultaneous activation of linear motion detectors inV5/MT and rotatory motion detectors in V5A/MST.Modeling such active regions (the whole V5 complex)with a single dipole would result in an erroneously deepdipole location. The overlapping subdivisions of the V5complex are difficult to differentiate even by means offMRI (Howard et al., 1996).

Support for MEG signals arising from both the V5and the V5A areas comes from our recent MEG studywith linear motion (Uusitalo et al., 1996), in which thesources were on the average 1.6 cmmore posterior thanin the present study (cf. Fig. 7). Also PET and fMRIevidence supports the more posterior location of V5than of V5A (Zeki et al., 1993; Tootell et al., 1996). Aftertransformation into Talairach stereotaxic space, the V5complex sources of the present study were on average 1cm superior and 0.8–1.6 cm anterior to PET activationareas to linear motion reported by Watson et al. (1993),thereby also supporting the more anterior location ofV5A than of V5. The deep sources of two subjects (S1,S2) might represent a movement-related area in theventral occipitotemporal border area, which has beenshown to be active during perception of optic flow andillusory rotation (Zeki et al., 1993; de Jong et al., 1994).Other motion-related areas (Dupont et al., 1994; Ahl-fors et al., 1995; Cheng et al., 1995; Howard et al., 1996;Tootell et al., 1996) were not identified in the presentstudy, although some of these areasmight have contrib-uted to the variability of our source locations.

FIG. 4. Responses to stationary (dashed line) and rotatory (solid line) stimuli of 100% contrast for all subjects from one posterolateralsensor on each hemisphere. Stimulus onsets and offsets are indicated by vertical lines. The horizontal gray bars on the time scale refer to timeintervals used to quantify signal amplitudes (see text).

246 UUSITALO ET AL.

Interindividual Variability

According to previous studies (Probst et al., 1993;Watson et al., 1993; Tootell et al., 1995b) we expectedour stimuli to activate the V5 complex bilaterally, butonly unilateral activation was seen in one subject, andtwo subjects had no reliable V5 complex signals at all.One possibility is that the activity was not detectablewith MEG due to the radial orientation of the sourcecurrents (Hamalainen et al., 1993). On the other hand,the opposite movement on the different sides of arotating grating might have decreased neural activa-tion due to inhibition within the same receptive field.The receptive fields of many MT and MST cells of themacaque monkey are large and bilateral, covering thecentral parts of the visual field completely (Desimone

and Ungerleider, 1986; Raiguel et al., 1995), and thehuman V5 cortex also seems to contain cells withreceptive fields extending more than 20° into theipsilateral visual field (Tootell et al., 1995b).The interindividual variability in the location and

number of sources (four to six) needed to account fortransient responses probably reflects variability incortical gyrification and MEG’s preference for tangen-tial currents. Differences in visual processing betweensubjects, evident for example in a picture-naming study(Salmelin et al., 1994), cannot be ruled out either.Considerable variability in the location of the humanV5 has been emphasized in previous MEG recordings(Patzwahl et al., 1996) and PET studies (Watson et al.,1993).

FIG. 5. The locations of the V5 complex sources, superimposed on horizontal magnetic resonance images for subjects S1–S5, and thecorresponding source strengths as a function of time for rotating (solid lines) and stationary (dashed lines) gratings, with the differenceshadowed. The vertical bars show stimulus onsets and offsets. The contrasts (c 100% or c 10%) are indicated for each source. The transientonset responses at 100–130 ms are indicated by small dots.

247CORTICAL ACTIVATION AND VISUAL MOTION

Functional Significance of Sustained vs Transient Activity

We observed significantly stronger SFs in the V5complex for rotating than for stationary gratings. Be-cause a continuous movement was perceived through-out the presentation of the stimuli, it is probable thatthe sustained activity is related to the conscious percep-tion of movement, in agreement with previous studies(Salzman et al., 1990; Tootell et al., 1995a; Zeki, 1995).Zeki et al. (1993) and Tootell et al. (1995a) have shownthat the same areas are active during perceptions ofnormal movement, illusory motion, and MAE. OurMAE data, although sparse, are in line with thisconjecture in general but suggest that MAE-relatedactivity can also occur in brain regions other than thosegenerating the motion-specific SFs (cf. Fig. 3).Sustained activity in the V5 complex seems to be

related to the actual perception of movement, whereastransient signals may have an attentional role assuggested by Anderson et al. (1996). The sources in theV5 complex were transiently active from 100 to 130 msafter stimulus onset for both rotatory and stationarystimuli, suggesting that movement-related cortical ar-

eas respond to any transient changes in the visualenvironment. We were not able to show movementspecificity in the transient responses whichwere evokedby the onsets and offsets of both stimuli. They alsooccurred in many other than movement-related brainareas. Consequently, the early transient components ofa response evoked by a stimulus may reflect a general-ized sensory reaction that precedes and is necessary fora more specific and detailed analysis in a smallersubset of cortical neurons and areas (Ungerleider,1995).

Contribution from Magno- and Parvocellular Pathways

At the retinal and geniculate levels, the neurons ofthe parvocellular pathway produce more sustainedresponses to long-lasting stimuli than do neurons of themagnocellular pathway, but at the cortical level thetransient or sustained nature of the responses does notnecessarily differentiate M and P cells (Maunsell andGibson, 1992). However, since the M cells saturate atlower stimulus contrasts than P cells, the significantlystronger movement-specific SFs of the V5 complex for

FIG. 6. The locations and amplitudes of the rolandic sources, superimposed on coronal, sagittal, and horizontal magnetic resonanceimages of the four subjects. The contrast was 100%. For other details, see Fig. 5.

248 UUSITALO ET AL.

the 100% than the 10% contrast suggest a contributionof the P pathway to movement-specific responses.The increase of contrast did not have a significant

effect on the amplitudes of transients, in agreementwith their M cell origin (cf. Anderson et al., 1996).Furthermore, the magnocellular signals arrive earlierto cortex than the parvocellular signals do (Maunselland Gibson, 1992; Nowak et al., 1995). Thus, ourobservation of 10–20 ms earlier transients for rotatingthan for stationary gratings agrees with the notionthat M cells play an important role in movementspecificity.

ACKNOWLEDGMENTS

The study was supported by the Academy of Finland, the MagnusEhrnrooth Foundation, the Vilho, Yrjo, and Kalle Vaisala Founda-tion, the Helsingin Sanomat Centenary Foundation, and the Gyllen-berg Foundation. We thank Mika Seppa and Kimmo Uutela for helpin producing the stimuli and an anonymous referee for very usefulcomments. The magnetic resonance images were obtained at theDepartment of Radiology, University of Helsinki.

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