Retinotopy and color sensitivity in human visual cortical area V8

Transcription

1 Results COLOR- VERSUS LUMINANCE-VARYING STIMULI First, we compred the ctivity produced y color vritions to tht produced y vritions in luminnce, in the sme surticles Retinotopy nd color sensitivity in humn visul corticl re V8 Nouchine Hdjikhni, Arthur K. Liu, Anders M. Dle, Ptrick Cvngh nd Roger B. H. Tootell Nucler Mgnetic Resonnce Center, Msschusetts Generl Hospitl, th Street, Chrlestown, Msschusetts 02129, USA Correspondence should e ddressed to N.H. (nouchine@nmr.mgh.hrvrd.edu) Prior studies suggest the presence of color-selective re in the inferior occipitl-temporl region of humn visul cortex. It hs een proposed tht this humn re is homologous to mcque re V4, which is rguly color selective, ut this hs never een tested directly. To test this model, we compred the loction of the humn color-selective region to the retinotopic re oundries in the sme sujects, using functionl mgnetic resonnce imging (fmri), corticl flttening nd retinotopic mpping techniques. The humn color-selective region did not mtch the loction of re V4 (neither its dorsl nor ventrl sudivisions), s extrpolted from mcque mps. Insted this region coincides with new retinotopic re tht we cll V8, which includes distinct representtion of the fove nd oth upper nd lower visul fields. We lso tested the response to stimuli tht produce color fterimges nd found tht these stimuli, like rel colors, cused preferentil ctivtion of V8 ut not V4. In Old World primtes such s mcque monkeys nd humns, visul informtion out color is processed in ntomiclly segregted columns, lyers, chnnels or res. It is importnt to know to wht extent color is processed in seprte versus convergent visul informtion pthwys, ecuse the dded dimension of color is so rich in visul informtion. For exmple, we cn discriminte out fifteen hundred different levels of luminnce 1, wheres we cn mke severl million discrimintions if we lso consider vritions in color 2. It is likely tht this glut of color informtion is incorported into the leled lines of the neurl rchitecture in some orgnized wy. In mcque monkeys, n ntomicl segregtion etween chromtic-opponent versus chromtic-opponent cells hs een reported s erly s the lterl geniculte nucleus. Color-specific ntomicl segregtion hs lso een descried in primry (V1) nd secondry (V2) visul cortex. In V1, prominent popultions of color-selective cells hve een reported in specific lyers 3 5 nd in the cytochromeoxidse los 4 6, though the ltter clim hs een disputed 7,8. Similr (nd eqully controversil) clims hve een mde out the prominence of color-opponent cells in the thin stripes in re V2, to which the V1 los project (ref. 9,10, ut see 11). However, the most prominent controversy out the ntomicl segregtion of color-selective neurons occurs t higher level, in corticl re V4. According to different reports, high percentge of color-selective cells is either present or sent 16 in the lrgest nd est-studied portion of tht re, dorsl V4 (V4d). A high percentge of color-selective cells hs not een reported in the smller, ventrl sudivision of V4 (V4v). More recent evidence suggests tht rin mechnisms criticl for color selectivity re locted not in mcque V4, ut rther in res nterior to it (ref , Vnduffel et l. Soc. Neurosci. Astr. 23, 845, 1997). This controversy out color selectivity in V4 hs now een extended to humn visul cortex. Bsed on humn neuroimging studies, smll ptch of color-selective ctivity ner the middle of the collterl sulcus hs een nmed V4 (ref ). This choice of nme presupposes tht (1) n re homologous to mcque V4 exists in humns, (2) V4 is color-selective, nd (3) this region in or ner the collterl sulcus is the mcque V4 homolog. However, in humns, the loction of this color-selective region hs not yet een compred with the mp of retinotopic res, to see whether color selectivity is relly is in retinotopiclly defined humn re V4. Furthermore, the degree of color selectivity in mcque V4 is itself controversil This issue is not just of cdemic interest. In n intriguing clinicl syndrome ( chromtopsi ), humn ptients report tht the visul world ecomes colorless following dmge to corticl region tht pprently includes this color-selective re in the collterl sulcus This suggests tht the conscious percept of color involves tht re, lthough it is known tht physicl wvelength-dependent differences re coded throughout prior levels of the visul system s well. If we cn define etter which re this is in humns, we cn lern something out where conscious perceptions of color rise. Accurte locliztion in humns should lso mke it possile to study the homologous re in mcques using more incisive (ut invsive) clssicl neuroiologicl techniques. We hve ttempted to clrify these issues in humns using functionl mgnetic resonnce imging (fmri). Technicl detils were similr to those descried elsewhere 26, except tht here we mnipulted the color content of the visul stimuli. We lso used high-field MRI scnner nd other improvements to sustntilly increse the sensitivity of the retinotopic mps (Methods). nture neuroscience volume 1 no 3 july

2 rticles Fig. 1. Topogrphy of color-selective ctivity in humn visul cortex. (, ) The inferior, inflted cortex, with posterior to the left nd nterior to the right, in two sujects. (c, d) The posterior portion of cortex in fully flttened formt, nother view of the sme dt shown in () nd (), respectively. In ll pnels, gyri from the originl rin re shown s light gry nd sulci s drk gry. The fundus of the collterl sulcus (cs) is indicted y the dshed lck line. The orders of previously c d descried retinotopic res (V1, V2, V3, VP, V3A, nd V4v) re indicted in white (horizontl meridins, solid lines, upper verticl meridins, dotted lines, lower verticl meridins, dshed lines). Typiclly, color-vrying stimuli produced reltively higher ctivtion in the fovel representtion of V1 nd often V2 nd V3/VP nd distinctive ptch of color-selective ctivtion pproximtely midwy in the collterl sulcus. When present, the ltter ptch ws lwys locted just nterior to the horizontl meridin representtion mrking the nterior order of re V4v, rther thn within V4v. jects (Fig. 1). Our stimuli consisted of slowly moving sinusoidl rdil grtings ( pinwheels ) of low sptil frequency, defined y either color or luminnce contrst (Methods). As shown erlier in V1 nd V2 (ref. 27), we found tht oth color- nd luminnce-vrying stimuli produced roust ctivtion in mny res of visul cortex, when compred with uniform gry field (dt not shown). Here we focused on those loctions where the color stimuli produced more ctivtion thn the luminnce stimuli. In the clssiclly retinotopic visul res (V1, V2, V3/VP, V3A nd V4v), we found prominent color-selective ctivtion in the representtions of the fove (center of gze) ut not in peripherl representtions (Fig. 1). A fovel color is hs not een reported in previous imging studies, perhps ecuse it is more ovious in our flttened mps. However, such fovel color is is consistent with the well known predominnce of cone photoreceptors, nd the corresponding sence of rods, in the fove of the retin. A similr fovel color is is found in routine clinicl perimetry nd in numerous psychophysicl studies. In 25 of 26 hemispheres (13 sujects) tested, we found n dditionl region tht responded preferentilly to color, locted midwy long the length of the collterl sulcus. Bsed on the ntomicl loction nd the nture of the functionl comprison used here, this collterl color-selective ptch ppers equivlent to the previously reported re involved in chromtopsi 20,22, which hs een proposed s the humn homologue of mcque re V4 (refs 20 22, 28 30). However, when we compred the loction of tht collterl color-selective ptch to the retinotopic orders in the sme sujects, we found tht the color-selective ptch ws consistently locted just eyond the most nterior retinotopic re defined previously, re V4v. Erlier reports 26,31 33 suggested tht humn V4v is qurter-field representtion of the contrlterl upper visul field. The more sensitive retinotopic mpping Fig. 2. Retinotopic fetures of re V8 y fmri mpping. ( c) Retinotopy of polr ngle in the inferior row of corticl res, from three flttened hemispheres. From left to right, ech pnel shows the representtions of the contrlterl upper qurter field (red through lue or vice vers; see pseudocolor logo) in inferior V1, then inferior V2, then VP, then V4v. To the right of (nterior to) V4v is the distinctive hlf-field representtion comprising V8 (green through lue through red, from upper to lower in this figure). (d) Retinotopic representtion of eccentricity (the other dimension in polr spce), from the sme hemisphere shown in (c). The representtions c d of centrl-through-more-peripherl eccentricities re coded in red-through-luethrough-green, respectively (see pseudocolor logo, ottom right). The representtions of the center of gze re indicted with n sterisk. Are V8 hs its own representtion of the fove, quite distinct (nd 3.5 cm) from the fovel representtion in djcent re V4v. 236 nture neuroscience volume 1 no 3 july 1998

3 rticles c d Fig. 3. Detiled retinotopy of the polr ngle representtion, from the sme hemisphere shown in Fig. 2. This figure shows the pek fmri response (noisy white lines) corresponding to polr ngle grdients of pproximtely 20 o, superimposed on the stndrd pseudocolor rendering of res V1, V2, VP nd V4v (drwn in ). To the right of ech pnel is logo indicting the specific polr ngle (white line) stimulted. The complete contrlterl visul field is represented in V8, from the lower visul field ( nd ), cross the horizontl meridin (c e) to the upper visul field (f h). Note in (e h) tht the upper visul field representtion in V8 cn clerly e distinguished from, nd is mirror symmetric to, tht in djcent V4v. methods used here confirmed tht V4v represents just tht qurter field, with its fovel representtion locted superiorly longside tht of djcent res V3/VP (Figs 2 nd 3). The improved retinotopic methods lso reveled dditionl retinotopic fetures nterior to V4v. Tken together, these fetures indicte the presence of n dditionl retinotopic mp, comprising previously undifferentited corticl re tht we cll V8. This continues the nming scheme egun y Zeki nd collegues, who identified res V1 through V ,21,22,34. (We lso identified n re V7, which is representtion of the contrlterl lower visul field nterior to humn V3A.) Are V8 hs unique polr ngle retinotopy nd distinctive fovel representtion. This contrsts significntly with the three extrstrite representtions posterior to V8 (V4v, VP, nd the inferior wing of V2), ll of which re qurter-field representtions of the contrlterl upper visul field. Although the polrngle retinotopy in V8 includes n dditionl representtion of this qurter field, it lso extends further to include lower-field representtion s well (Figs 2 nd 3). These three extrstrite visul res (V4v, VP nd inferior V2) lso shre contiguous representtion of the fove, t the top (superior) end of this row of res (Fig. 2d). However, the fovel representtion in V8 is not prt of this contiguous fovel nd; insted it is locted out 3.5 centimeters wy long the corticl surfce, t the nterior order of V8 (Fig. 2d). e f g h In conventionl Tlirch coordintes, fovel V4v (s defined retinotopiclly in this study) ws centered t ± 32, -87, -16, wheres fovel V8 ws centered t ± 33, -65, -14. When we verged the Tlirch coordintes of the color-selective re V4 descried in previous studies ( ± 26, -67, -9), we found tht it ws out twice s close to the loction of our retinotopiclly defined V8, compred to our retinotopiclly defined V4v. This supports ll the other evidence tht the colorselective ctivity is locted in re V8, rther thn in V4. In ech hemisphere, V8 comprises continuous representtion of the entire contrlterl hlf of the visul field. Although the uthors did not ttempt polr-coordinte retinotopy or flttened mpping, prior study lso concluded tht this humn color-selective region included representtion of upper nd lower visul fields 22. Together with V8 in the opposing hemisphere, the entire visul field is thus represented. Such complete representtion of the visul field would e pproprite in n re processing higher-order, lrge-field color informtion. HUMAN VERSUS MACAQUE MAPS Becuse so much of the historicl controversy out corticl color processing rose in studies of mcque monkey, it is nturl to wonder which re in mcque corresponds to re V8 in humns. To clrify the topogrphic reltionship of the humn nd mcque mps, we first verged together Fig. 4. Comprison of the polr ngle retinotopy in humn visul cortex, reltive to tht reported in mcque monkeys. In oth species, visul cortex is shown in flttened formt, with visul re oundries nd polr ngle continu s in Fig. 2 c. Are MT is shown in gry. In mcque, dorsl re V4 is lso indicted (V4d). The retinotopy of V8 is similr to tht reported in re TEO, in tht oth res re locted immeditely djcent to re V4v. However, the two res differ in overll shpe, nd the retinotopy of V8 is rotted pproximtely 90 o reltive to tht reported in TEO. nture neuroscience volume 1 no 3 july

4 rticles Alternting colors Constnt colors Fixtion Fixtion Fixtion % MIRI isgnl % MIRI signl Color Fixtion Time (s) Time (s) Fig. 5. The time course of V8 ctivity is relted to the perception of color fterimges. () Time course from ll voxels in retinotopiclly defined V8 tht responded (p< ) to the colored stimuli, reltive to the initil presenttion of the uniform gry stimulus, verged cross 16 MR scns, showing the response to oth the ctul nd the illusory color stimuli. Epochs in which sujects viewed uniform grey field, or n illusory fterimge on gry ckground, re indicted with white ckground; epochs when the sujects viewed colored stimuli (lternting- or constnt-colored) re indicted with gry ckground. After the color stimulus, sujects viewed uniform gry field, or n illusory fterimge on gry ckground. During color fterimge, the fmri response ws quite prolonged, consistent with the time course of the percept of the illusory colors. () The MR fterimge is shown more directly y sutrcting signl during constnt color nd susequent gry period from lternting color nd susequent grey period. six of the most roust humn retinotopic mps, using digitl morphing techniques (Fig. 4) s descried 26. These verged mps could then e compred to the mp of mcque retinotopy, s estimted from single-unit mpping 35 (Fig. 4). This comprison suggests tht humn re V8 shows some retinotopic nd topogrphic similrity to mcque re TEO. Furthermore, TEO nd/or more nterior res hve ppered strongly color selective in recent studies of mcque visul cortex (refs 17 19, 28, Vnduffel et l. Soc Neurosci. Astr. 23, 845, 1997). However, the retinotopic similrity etween V8 nd TEO is fr from exct (Fig. 4). Furthermore, other investigtors hve proposed different re oundries in this region of mcque cortex Unfortuntely, those lterntive models of the mcque mps re even less similr to the empiricl humn mps in this region of cortex. Thus it is not cler which mcque re is homologous to humn re V8. However, the flt mps in Fig. 4 do mke it cler tht this humn collterl color re is not topogrphiclly similr to mcque V4 (neither dorsl nor ventrl sudivisions). Humn V8 is lso topogrphiclly inconsistent with the loction of sudivisions proposed in mcque dorsl V4, such s V4t (ref. 39) nd V4A (e.g. ref. 14). COLOR AFTERIMAGES Another wy to ssess functionl selectivity is y mesuring the fmri responses during visul ftereffects, rther thn during the fmri effects produced y the visul stimuli themselves. In other dimensions such s motion 40 nd orienttion 41, such indirect ftereffects hve ironiclly proven to e functionlly more selective thn the effects themselves. Here we tested whether illusory color would lso ctivte V8, s did rel color stimuli. If one stres for time t sturted color, then looks wy t uniform gry field, one sees n illusory percept of the complementry color. Unlike motion or orienttion ftereffects, these negtive color fterimges re thought to rise primrily in the retin 42,43. However, they lso presumly trigger ctivity t higher levels, s would rel stimulus tht ws similrly stilized on the retin 29,44. To test for the presence of fmri responses to these illusory colors, we produced such color fterimges in the MR scnner, long with control stimuli tht were very similr ut did not produce color fterimges. Figure 5 shows the time course of fmri ctivity produced y these stimuli in re V8. As expected, the colored stimuli produced roust fmri ctivity in V8, whether lternting or constnt. However, only the constnt-colored stimuli produced perceptul color fterimge nd corresponding fmri ftereffect in V8 during susequent viewing of the uniform gry field (Fig. 5). The lternting-colored stimuli produced neither perceptul color fterimge nor prolongtion of the norml fmri return to seline during the susequent viewing period (Fig. 5). The durtion of the isolted fmri color ftereffect ws prolonged, consistent with the prolonged durtion of the illusory color percept (Fig. 5). Overll, this strongly suggests tht these fmri responses were relted to the processing of the illusory colors. One unexpected finding ws tht the stimuli with lternting colors produced slightly more ctivity thn the stimuli in which color remined constnt (Fig. 5). This my reflect the fct tht the hues in the constnt-colored stimuli ecome progressively less sturted (less densely colored) with time ecuse of chromtic dpttion 45. Essentilly one egins to see the mixture of the color fterimge nd the ctul color, Becuse these two colors re complementry, they produce less sturted, more wshed-out color. This decresed fmri response to the decresingly sturted colors supports the other evidence tht V8 is involved in color perception. Findings similr to our fmri fterimges in V8 were reported from scttered voxels in single-slice imging through nery posterior fusiform gyrus 29, ut the loction of those voxels ws not loclized to ny specific corticl re. However, 238 nture neuroscience volume 1 no 3 july 1998

5 rticles Fig. 6. The perception of color fterimges produces reltively higher ctivtion in corticl re V8, compred with other corticl res. The ctivtion shown here represents ll regions tht responded significntly more (p ) during viewing of the uniform gry stimulus following the constnt color stimulus, compred with viewing of the sme gry stimulus following the lternting color stimulus. none of these dt ddress the possiility tht similr fmri fterimges occur nonspecificlly throughout much wider res of visul cortex. This could rise from retinl color processing tht is trnsmitted pssively to cortex, or from glolly incresed ttention when viewing the color fterimges. To test this, we renlyzed our dt to find the res in n ctivity mp tht respond differentilly during the presence of the visul fterimge (Fig. 6). At lower levels of significnce thn shown here, numer of dditionl visul res (e.g. V1, V2) do respond more to color fterimges. However, t the more strict significnce threshold used in Fig. 6, the ctivity in V8 ws more prominent thn in ny other re. In prticulr, the wider res of fovel color selective ctivities including res such s V1, V2, etc., were reltively less prominent in the fterimge test, compred to the direct comprisons of color versus luminnce (see Fig. 1c nd Fig. 6, showing the sme hemisphere). Discussion The retinotopic mps mke it cler tht n dditionl re (V8) exists eyond those res descried previously in humn visul cortex. Are V8 is retinotopiclly distinct from the previously descried re V4v, sed on t lest four different criteri. First, V4v nd V8 hve seprte fovel representtions, pproximtely 3.5 cm prt long the corticl surfce. Second, V4v nd V8 ech include seprte representtions of the upper visul field, seprted from ech other y representtion of the horizontl meridin. Third, V8 differs from V4v in its glol functionl properties, including ut not limited to color sensitivity. Fourth, the nture of the retinotopy in V8 is different from tht in V4v. The direct comprisons etween color- nd luminnce-vrying stimuli (Fig. 1) indicte tht re V8 responds slightly etter thn neighoring corticl regions to colored stimuli. However, we lso found tht the color-vrying stimuli produce preferentil ctivtion in the fovel representtion of ll retinotopic res. Thus, V8 my seem especilly ised for color stimuli merely ecuse its fovel representtion sets it topogrphiclly prt from the conjoined fovel color responses of its neighors (Figs 2 nd 3). This is reltively trivil explntion for the color selectivity reported erlier, ut we cnnot rule it out completely. This ide is further supported y the fct tht re V8 responds t resonle levels to wider vriety of visul stimuli. However, other evidence rgues tht V8 is involved in wvelength-dependent processing nd perhps in the conscious perception of color itself. The roust nd selective response to illusory colors (Figs 5 nd 6) strongly supports this ide. Also, the ntomicl colocliztion of V8 compred with the previous clinicl dt mkes it likely tht re V8 is dmged in chromtopsic ptients Wht do these humn dt tell us out mcque visul cortex? This question is constrined y severl fctors. The humn dt re sed on clinicl nd neuroimging dt, wheres the mcque dt re derived from different techniques (e.g., single units, lesions nd DG imging), which could conceivly produce different results. Also, there my e significnt iologicl differences etween the corticl orgniztion of color sensitivity in humns compred with mcques. As we lern more out humn nd mcque visul cortex, the numer of differences etween these species re incresing correspondingly 26, Despite these cvets, the dt suggest tht the re of mcque cortex tht is homologous to the humn chromtopsi re should e locted in or nterior to TEO, rther thn in V4. This is supported y dt from mcque s well s the present dt from humn V8. Methods GENERAL PROCEDURES. Except for modifictions descried elow, the methods in this study re similr to those descried 26. Informed written consent ws otined for ech suject prior to the scnning session, nd ll procedures were pproved y Msschusetts Generl Hospitl Humn Studies Protocol numers nd Norml humn sujects, with (or corrected to) emmetropic vision, were scnned in Generl Electric mgnetic resonnce (MR) scnners retrofitted with ANMR echo-plnr imging. Most scns were cquired in high-field (3 T) scnner, ut some erly scns were cquired in scnner of conventionl (1.5 T) field strength. Bsed on signl-to-noise rtios otined during otherwise comprle conditions, four functionl scns t 1.5 T were found to e pproximtely equl to one functionl scn t 3 T, so this ws the rtio used to equte dt cquired from the two scnners. Hed motion ws minimized y using ite rs with deep, individully molded dentl impressions. The suject s tsk in ll experiments ws to fixte the center of ech type of visul stimulus throughout the period of scn cquisition. MR imges were cquired using custom-uilt qudrture surfce coil, shped to fit the posterior portion of the hed. MR slices were 3-4 mm thick, with n in-plne resolution of 3.1 x 3.1 mm, oriented pproximtely perpendiculr to the clcrine fissure. Ech scn took either 4 min 16 s (color-versus-luminnce nd color fterimge scns) or 8 min 32 s (retinotopy), using TR of either two or four seconds, respectively. Ech scn included 2,048 imges, comprised of 128 imges per slice in 16 contiguous slices. Improved retinotopic mps were otined from 32 sujects (79 scns polr ngle, 79 scns eccentricity, 323,584 imges totl). Among them, 13 sujects were lso tested for color-versus-luminnce (112 scns, 229,376 imges totl). Of these, five sujects were tested extensively for color fterimges (100 scns; 204,800 imges totl). In most sujects, dditionl scns were done to clrify the loction of re MT nd other visul res. nture neuroscience volume 1 no 3 july

6 rticles VISUAL STIMULI. The gol of the first color-relted experiment (Fig. 1) ws to mp the MR ctivity produced y color- versus luminncevrying stimuli throughout visul cortex, using conventionl psychophysicl stimuli. Prior to scnning, the equiluminnce vlues for different color comintions (red-cyn, CIE x nd y coordintes 0.645, nd 0.185, respectively, or green-purple, CIE x nd y coordintes 0.277, nd 0.351, 0.220, respectively) were mesured for ech suject, outside the scnner. Equiluminnce ws mesured using motion-null test, with the sme stimulus projector (NEC model MT 800), lens nd color softwre used susequently in the MR experiments. In the first experiment, oth color- nd luminnce-vrying stimuli were produced using slowly moving (0.5 Hz) sinusoidl rdil grtings ( pinwheels ) of low sptil frequency (3 cycles per revolution). The grtings vried either in chromtic luminnce (mximum, 95% luminnce contrst) or in equiluminnt color (t mximum ville sturtions of the disply device within constrints of pproximtely 140 cd per m 2 men luminnce nd pproximtely white men chromticity), in direct lterntion, in 16-second epochs, using 16 epochs per scn. In second experiment (Figs 5 nd 6), color fterimges were produced y showing sujects colored dpttion ptterns. Sujects dpted to two generl types of stimuli; one produced pronounced color fterimge when sujects susequently viewed uniform gry field, ut very similr control stimulus did not produce such color fterimge. There were five epochs in ech scn, presented in the following order: (1) uniform gry, (2) lternting-color (control dpttion), (3) uniform gry, (4) constnt-color (experimentl dpttion) nd (5) uniform gry. All epochs were 48-s long, except tht the finl fixtion period ws prolonged 16 s to revel the finl trces of the MR fterimge. In the nlysis for Fig. 5, the lst 16 s in tht finl epoch ws truncted to mtch the durtion of ll other epochs. Both the constnt- nd the lternting-colored ptterns were comprised of complementry colors (red-cyn or green-purple), sptilly rrnged in opposed qurter-fields (i.e. two-cycle-per-revolution polr squre wves), kin to interleved ow ties. All hues were presented t equl luminnce, sed on motion-null tests in ech suject. Following dpttion to the constnt colors, sujects initilly experienced prominent color fterimge ginst the uniform gry ckground, which fded over tens of seconds. The fterimge ws retinotopiclly similr to the dpttion stimulus, ut of complementry color. As controls, we presented stimuli equivlent to the constnt colors, except tht the colors lternted etween color nd complementry color, reversing every 2 s. The ltter condition produced no perceptul color fterimges during the susequent epoch of uniform gry stimultion. Stimuli for retinotopic mpping were slowly moving, phse-encoded thin rys or rings comprised of counterphsing lck nd white checks, scled ccording to polr coordintes, similr to those descried 26,31,32,50. However, to produce the most informtive retinotopic mps possile, severl stimulus modifictions nd new procedures were implemented. First, ll retinotopic mesurements were mde in the 3 T scnner. This incresed the MR mplitudes y fctor ner four, nd the physiologicl signl-to-noise rtio y fctor ner two. Second, we signl-verged the informtion from 4 12 scns (8,192 24,576 MR imges) of polr ngle or eccentricity. Dt were lso comined from different slice prescriptions on the sme corticl surfce, to reduce intervoxel lising. Third, the retinotopic stimuli were incresed in extent oth fovelly nd peripherlly, to extend from 0.2 through This ctivted correspondingly more surfce in ech corticl re. Fourth, the visul stimuli were presented using new LCD projector of higher sptil resolution (800 x 600), using etter optics thn previously (perture lens, ypssing shielding screen, etc.). Fifth, the retinotopic stimuli vried in color s well s luminnce, to etter ctivte ny color-selective cells in the region. The sum of ll these mnipultions produced very roust retinotopic mps. DATA ANALYSIS. Dt from two-condition experiments (e.g., color-versusluminnce comprisons) nd phse-encoded retinotopic experiments were initilly nlyzed y doing fst Fourier trnsform on the MR time course from ech voxel. Sttisticl significnce ws clculted y converting the Fourier mgnitude of the response to n f-sttistic. The phse of the signl t the stimulus frequency ws used to trck stimulus loction in the cse of retinotopic stimuli, nd to distinguish etween positive- or negtivegoing MR fluctutions in the cse of two-condition stimulus comprisons. Scns compring more thn two stimuli (e.g., the color fterimge dt) were nlyzed y selective verging of two conditions. This ws followed y sttisticl comprison using t-test of the difference of the first seconds following onset of the next epoch (here stimulus offset). For topogrphic clrity, ll dt were nlyzed nd displyed in corticl surfce formt, s descried 36,43,44. This mde it possile to extrct the MR time courses from voxels in specific corticl res, which were defined in the sme sujects. The specific res smpled were V1, V2, V3/VP, V3A, V4v, MT+, nd V8. Are MT+ ws defined on the sis of dditionl scns compring moving nd sttionry stimuli 26,40. All other res were sed on retinotopic criteri. For ese of comprison, ll hemispheres re shown in right hemisphere formt. Aove minimum threshold, the sttisticl significnce of the displyed pseudocolor rnge hs een normlized ccording to the overll sensitivity of ech suject, s descried elsewhere. 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