Is The Motion System Relatively Spared In Amblyopia .
Vol. 36, No. 1, pp. 181 190, 1996Copyright 1995 Elsevier Science LtdPrinted in Great Britain. All rights reserved0042-6989/96 9.50 0.00Vision Res.Pergamon0042-6989(95)00055-0Is the Motion System Relatively Spared inAmblyopia? Evidence from Cortical EvokedResponsesZUZANA KUBOVA,*? MIROSLAV KUBA,* JOSEF JURAN,: COLIN BLAKEMORE§Received 10 June 1994; in revised form 29 November 1994; in final form 21 February 1995Visual evoked potentials (VEPs) produced by pattern reversal were compared with those elicited byonset of motion in 37 amblyopic children (20 with anisometropic amblyopia, seven with strabismicamblyopia and 10 with both anisometropia and strabismus). The amplitudes and peak latencies of themain P1 peak in the pattern-reversal VEP and of the motion-specific N2 peak in the motion-onset VEPthrough the amblyopic eye were compared with those through the normal fellow eye. Regardless ofthe type of amblyopia, the amplitude of the pattern-reversal VEP for full-field stimulation wassignificantly smaller and its latency significantly longer through the amblyopic eye (P 0.001). Incontrast, neither the amplitudes nor the latencies of the N2 motion-onset VEPs differed significantlybetween amblyopic and non-amblyopic eyes. For pattern-reversal VEPs through the amblyopic eyes,the extent to which amplitude was reduced and latency prolonged correlated well with the reductionof visual acuity, whereas the amplitudes and latencies of motion-onset VEPs did not vary with visualacuity. Even for stimuli restricted to the central visual field (5 or 2 deg diameter) or to the peripheralfield (excluding the central 5 deg), motion-onset responses were indistinguishable through the two eyes,while pattern-reversal responses always differed significantly in amplitude. These results suggest thatthe source of motion-onset VEPs (probably an extrastriate motion-sensitive area) is less affected inamblyopia than that of pattern-reversal VEPs (probably the striate cortex). The motion pathway,presumably deriving mainly from the magnocellular layers of the lateral geniculate nucleus, may berelatively spared in amblyopia.Visual evoked potentialsPattern reversal Motion onsetAmblyopia Anisometropia Motion system HumanINTRODUCTIONMagnocellular pathwayStrabismus(1992), who studied responses evoked by moving stimuliof various velocities, argued that the P1 peak is specifically related to pattern offset. This component occursmainly for stimuli of high temporal frequency (themultiple of velocity and spatial frequency), causingblurring of pattern at the beginning of motion, and/orwhen the duration of movement is long and the interstimulus interval short (causing adaptation to themotion itself).The main element of the VEP associated with theonset of steady linear motion appears to be a laternegative component (N2) with a peak latency ofabout 160-200 msec (Yokoyama, Matsunaga, Yonekura& Shinzato, 1979; G6pfert, Miiller, Markwardt &Schlykowa, 1983; Kuba & Kubovfi, 1992; Bach &Ullrich, 1994; Kubovfi, Kuba, Spekreijse & Blakemore,1995). A similar negative component, though usuallyof smaller amplitude, is often also seen in VEPs forpattern reversal.The deficit in visual acuity that characterizes developmental amblyopia is associated with a reduction inamplitude and an increase in latency of pattern-reversalSudden reversal of the contrast of a pattern (counterphase modulation) elicits a characteristic visual evokedpotential (VEP), dominated by a positive component(Pj) with a peak latency of 100 msec or so, localized toa dipole source in the striate cortex (Maier, Dagnelie,Spekreijse & van Dijk, 1987). Spekreijse, Dagnelie,Maier and Regan (1985), who considered that the typicalresponse to the onset of motion was a positive peak witha latency of about 120 msec, suggested that the patternreversal VEP might be a mixture of responses to theonset and offset of motion associated with the abruptdisplacement of a pattern. However, Kuba and Kubovfi*Departments o f Physiology and Pathophysiology, Medical Faculty ofCharles University, Simkova 870, 500 38 Hradec Krfilov6, CzechRepublic.tTo whom all correspondence should be addressed.: Department of Ophthalmology, Medical Faculty of Charles University, Simkova 870, 500 38 Hradec Krfi.lov6, Czech Republic.§University Laboratory o f Physiology, Parks Road, Oxford OX1 3PT,England.181
182ZUZANA KUBOV, . et al.VEPs through the amblyopic eye (e.g. Arden & Barnard,1979; Wanger & Persson, 1980; Mayeles & Mulholand,1980; Sokol 1983, 1986; Levi & Manny, 1986; Odom,1991). Thus the neural generator of pattern-reversalVEPs may be the substrate of visual acuity. It is generally thought that the detection of fine spatial detail(as well as chromatic vision) depends primarily onsignals from so-called P cells in the parvocellular layersof the LGN, which, like the P retinal ganglion cells thatprovide their input, have relatively small receptive fieldcentres, are usually chromatically coded and have lowcontrast sensitivity. It is conceivable, then, that neuronalmechanisms at some point in the parvocellular pathwayare compromised in amblyopia.On the other hand, the perception of image motion ismore likely to depend on signals from M cells, in themagnocellular layers of the LGN, which have slightlylarger receptive field centres, with higher sensitivity tocontrast, and are not obviously chromatically selective(see DeYoe & Van Essen, 1988; Lennie, Trevarthen, VanEssen & W issle; 1990; Zeki, 1990). In macaques, themagnocellular system projects, principally via layer 4b ofthe striate cortex, to subdivisions of area V2 and to areaM T (or V5) in the superior temporal sulcus (Maunsell &Newsome, 1987; Newsome & Par6, 1988; Livingstone &Hubel, 1988). While both M and P systems undoubtedlycontribute to both the dorsal and the ventral corticalprocessing streams (Merigan & Maunsell, 1993), it isgenerally accepted that areas of the dorsal stream,feeding into the parietal cortex, are selectively concernedwith the analysis of image motion and are dominated bythe M system (Maunsell & Newsome, 1987; Newsome &Par6, 1988).Functional imaging of the human brain suggests thata region anterior and lateral to the calcarine sulcus,which may be homologous to area MT or V5 in themacaque cortex, is specifically activated by movingstimuli (Mora, Carman & Allman, 1989; Corbetta,Miezin, Dobmeyer, Shulman & Petersen, 1991; Zeki,Watson, Lueck, Friston, Kennard & Frackowiak,1991; Watson, Myers, Frackowiak, Hajnal, Woods,Mazziotta, Shipp & Zeki, 1993). Motion-related VEPsignals, as well as the small N2 component often seen forpattern reversal, probably originate from this extrastriate motion area (Probst, Plendl, Paulus, Wist & Scherg,1993; Spekreijse, Gilhuijs, Kubovfi & Van Dijk, inpreparation).Now there is evidence that some aspects of theperception of motion are relatively less impaired inamblyopia than the perception of fine spatial detail(e.g. Hess & Anderson, 1993; Hess, Howell & Kitchin,1978; Hess, France & Tulanay-Keesey, 1981; Levi, Klein& Aitsebaomo, 1984; Rentschler, Hilz & Brettel, 1981).In a preliminary study, Kubov t and Kuba (1992) reported that motion-onset VEPs did not differ betweenthe amblyopic and normal fellow eyes of five adultamblyopes. Here we have extended those experiments bycomparing pattern-reversal and motion-onset VEPs fordefined classes of amblyopic children, correlating theresults with the deficit in visual acuity and examiningresponses from different parts of the visual field, in anattempt to see whether the neural mechanism associatedwith the processing of motion is indeed relatively sparedin amblyopia.METHODSSubjectsIn the main experiments, VEPs for full-field stimulation were recorded from 30 amblyopic children (ninegirls and 21 boys) of 6-14 yr of age. Snellen acuity wasmeasured at 4 m (with refractive errors corrected) usingconventional Landolt C charts (NAS NRC Report,1980). The acuity of the amblyopic eyes ranged from20/50 to 20/200, while that of the fellow eye was always20/20 (or slightly better).Children of this relatively late age were chosen becauseevoked responses are generally more variable in youngerchildren. In 14 cases the amblyopia was associated withanisometropia, in six with strabismus (always esotropic)and in 10 with both strabismus and anisometropia. Allthe children had a history of occlusion or CAM therapy(Campbell, Hess, Watson & Banks, 1978; Peregrin,Sverfik, Kuba, Vit & Juran, 1987), which had been onlypartially successful in restoring visual acuity. In 18children fixation of the amblyopic eye was central butunstable, while in 12 cases it was consistently parafovealor peripheral.In a further seven amblyopic children (four boys,three girls from 7 to 12 yr old; six anisometropic, onestrabismic), responses were studied not only with fullfield stimulation but also with stimuli restricted to theperipheral visual field and to the central 5 and 2 deg.Recording and analysis"All recordings were performed in a sound-attenuated,electromagnetically shielded chamber with a backgroundluminance of 1 cd/m z. The subject was seated in acomfortable dental chair with a neck support to reducemuscle artefacts. A dark fixation point of 15 min arcdiameter was placed in the centre of the stimulus field:the subjects were instructed not to follow the moving orreversing pattern with their eyes (and the absence ofobvious tracking eye movements was verified occasionally by means of electro-oculography). Stimulation wasalways monocular, with optimal refraction: the othereye was patched. All measurements for each individualsubject (left eye and right eye; pattern reversal andmotion onset) were always completed in a single recording session without changing electrode placements.For the bulk of the experiments, involving full-fieldstimulation, performed on 30 amblyopic children, thepatterned stimuli (square-wave, black and white checkerboards with an element size of 35 min arc mean luminance 15 cd/m - and contrast 0.9) were back-projected viaa mirror on to a 20 deg diameter circular field. Mirrormovement was produced by an optical scanner (GeneralScanning Inc., U.S.A.) controlled by square-wave orramp signals.
PATTERN A N D MOTION VEPs IN AMBLYOPIAFor pattern-reversal VEPs we used a reversal rate ofI Hz (2 reversals/sec) and we carefully adjusted theamplitude of displacement to be equal to the width of asingle check. The frequency response of the scanner wassuch that the nominal square-wave displacement wascompleted in 2 msec. Just as with pattern reversal generated by television techniques, the whole array appearedeither to flicker or to undergo stepwise displacement inany one of the four principal directions.For motion-onset VEPs, the checks moved horizontally rightwards at a velocity of 6 deg/sec for 200 msecperiods, with interstimulus intervals of 1 sec duration,during which the pattern was stationary. This regimewas selected to minimize motion adaptation but to keepthe sessions to a tolerable length.VEPs were recorded in the bipolar lead Oz-Cz and inthree unipolar leads with the electrodes placed at Oz and5 cm to the right and left (these electrodes were designated OR and Oc). Linked earlobes served as reference. After amplification (Tektronix AM 502) in the0.1-100 Hz band, 100 epochs of 400 msec duration wereaveraged with a sampling rate of 500 Hz on a PDP-11/03microcomputer or an IBM compatible 386 PC computerwith a 12-bit A/D converter (Data Translation).For a further seven children checkerboards with4 0 m i n a r c checks, of contrast and mean luminanceidentical to those in the main experiments, were generated on a computer monitor (ViewSonic 21; 100frames/sec; total display size 30 x 40deg) under computer control (IBM compatible 486 PC). In these experiments, responses to full-field stimulation were comparedwith those for stimuli restricted to the central 5 and 2 degof the field, and with responses from the peripheral fieldalone (excluding the central 5 deg). In all conditions, thefixation point appeared in the middle of the display. Formotion-onset stimulation using this television display,the pattern was displaced at a velocity of 5 deg/sec andthe direction of displacement varied randomly from trialto trial (left, right, up or down). Otherwise the stimulusconditions were the same as for the experiments withprojected stimuli.183motion-onset VEPs for stimulation of the normal andamblyopic eyes of six children, selected to be representative of the entire group. Although there was somevariability between individuals in the overall amplitudeand general form of the signals, especially the latercomponents, the major early peaks described in Fig. 1could always be distinguished. Comparison of responsesthrough the two eyes shows that in every case thepattern-reversal (P ) VEP was clearly reduced in amplitude through the amblyopic eye and it was usuallysomewhat delayed in latency, while the motion-onset(N2) VEP did not differ consistently between the eyes.Note that a small positive peak at about 110-120 msec,probably equivalent to the pattern-offset P component(Kubovfi et al., 1995), is also discernible in the VEPs tomotion onset through the normal eyes. The reduction orvirtual absence of this peak for the amblyopic eye ispresumably responsible for the broadening of the N2peak through that eye.For the whole set of amblyopic children (n 30),Table 1 gives mean values of latency and amplitude ofpattern-reversal (P ) and motion-onset (N2) VEPsthrough non-amblyopic and amblyopic eyes, and average interocular differences calculated from individualmeasurements for each child.While the pattern-reversal VEPs had significantlylonger latencies and reduced amplitudes through theamblyopic eye (P 0.001 for both), motion-onset VEPswere always very similar in both amplitude and latencythrough the two eyes of individual subjects. This wastrue for all three subgroups of patients, classified according to the origin of their amblyopia (i.e. anisometropia,strabismus and anisometropia combined with strabismus). These three subgroups could not be distinguishedin any of the parameters estimated (Kruskal-Wallisnon-parametric test).Correlation of the deficits in pattern-reversal VEP withthe loss of acuityWe were interested to know whether the deficits inpattern VEPs correlated with the depth of amblyopia.Even among the 11 children with relatively mildRESULTSFigure 1 shows representative pattern-reversal andmotion-onset VEPs for stimulation of the normal eye ofan amblyopic child, with the major peaks designated.For pattern-reversal VEPs we determined the latency ofthe first positive peak, Pj, and its amplitude [measuredas (NjPI P N2)/2] for the Oz-Cz lead.For motion-onset VEPs we measured the latency andamplitude of the major negative peak, N2 (the mostdistinct and constant peak of such VEPs). This motiononset VEP [measured as (PIN2 N2P2)/2] often differsin amplitude between the two sides of the brain (Kuba& Kubovfi, 1992), being clearly larger over the righthemisphere in about 50% of cases and over the left inabout 30%. The parameters of the N 2 peak given beloware always for the channel with the largest amplitude.Figure 2 shows examples of pattern-reversal andPA'I-I-ERN-REVERSALVEPMOTION - ONSETP,VEPPp, N 1"5 v'100msecOz - CzN,OR- A,. FIGURE 1. Typical examples of the pattern-reversal VEP (with itsmain positivepeak, P ) and the motion-onset VEP (with its dominantN2peak) for stimulation through the normal eye of an amblyopicchild.The recording leads are indicated.
184ZUZANA KUBOV,& et al.110Pattern - reverse[ VEPNotion-onsetAmbtyopic eyeVEPNormal eyeAmblyopic eye(20/160)125148155165162114{20/63)1850 Z - CZ- AI 5100 msecFIGURE 2. Pattern-reversal and motion-onset VEPs through each eye for six of the amblyopic children, selected to berepresentative of the variation in the general patterns of response among the entire group. Each row of recordings was takenfrom one child in a single recording session. Latencies in msec are indicated above the P peaks of the pattern-reversal VEPsand below the N 2 peaks in the motion-onset responses. Pattern VEPs all show a sharp positivity (P ) at about 110 msec. Itsamplitude varies considerably from subject to subject but in every case it is considerably larger through the normal than throughthe amblyopic eye. The visual acuity of the amblyopic eye is indicated above the pattern VEP for each child. The amplitudesand the latencies of the N 2 peaks of the motion-onset VEPs also vary somewhat from child to child but are very similar throughthe two eyes in every individual case. Note that, for several of the children, the N 2 component appears broader through theamblyopic than the normal eye. This was the case in two-thirds of our subjects and it is probably due to the reduction in thepreceding small positive peak, seen in the responses through the normal eyes, which may be equivalent to the P pattern-offsetcomponent (Kubovfi et al., 1995).a m b l y o p i a ( v i s u a l a c u i t y o f 20/50 o r 20/63) t h e m e a nvalues of latency and amplitude of the pattern-reversalV E P s differed significantly b e t w e e n the two eyes( P 0.001; K r u s k a l l - W a l l i s test). N e v e r t h e l e s s , t h e a b n o r m a l i t i e s in t h e P c o m p o n e n t in t h e p a t t e r n - r e v e r s a lr e s p o n s e w e r e , o n a v e r a g e , e v e n m o r e p r o n o u n c e d in t h echildren with more severe amblyopia. Interestingly, noc l e a r P p e a k c o u l d b e d e t e c t e d in m o t i o n - o n s e t V E P sf r o m t h o s e a m b l y o p i c eyes w i t h p a r t i c u l a r l y p o o r v i s u a la c u i t y ( b e l o w 20/125).TABLE 1. Mean latency and amplitude ( 1 SD) of pattern-reversal and motiononset VEPs for non-amblyopic and amblyopic eyes in all the children (n 30)Non-amblyopiceyeAmblyopic eyeI nteroculardifference117.7 8.8 msec10.9 4.4 ttV12.2 7.7 msec6.9 4- 4.6 pV158.2 8.4 msec7.3 2.3/ V7.2 5.5 msec1.8 2.0 #VPattern-reversal V E P sLatencyAmplitude105.5 4.5 msec17.9 6.0/ VMotion-onset VEPsLatencyAmplitude157.7 9.1 msec8.2 2.9 #VNote that the interocular differences were calculated as the means of the differencesin individual children. Since these differences were not always consistent indirection, especially for motion VEPs, these means are not equal to the differencesbetween the pooled means for each set of eyes.
P A T T E R N A N D M O T I O N VEPs I N A M B L Y O P I A140A [pV]30/(a) iPattern-reversal VEP185(b)Pattern-reversal VEPL [ms] 130. ;120110100unI20/20 20/50 I 20/80i20/63,20/125 2 a20/1009020/200-- o '200(d)15 i0160102oh25zo;1. ohooVAMotion-onset VEP1808'20/100--L [ms]M o t i o n - o n s e t VEP2o o20/63(c)6!0 ;". i ! 1405e02 oVA20 A [ V]T"20 : zo;so ' 20 0 '20/6320/100 2s20 16320 00VA12020}20zo so ' 20/ 020/6320}12S20 1e320/10020 00VAF I G U R E 3. The g r a p h s plot the amplitude, A (a, c) and latency, L (b, d) of pattern-reversal (a, b) and motion-onset VEPs(c, d), as a function of visual acuity (VA) (on a linear scale for the decimal value of acuity). D a t a f r o m the n o r m a l eyes areplotted as open circles, above a visual acuity of 20/20, those from the amblyopic eyes as solid circles. Linear regression linesare plotted. A l t h o u g h there is considerable scatter of values at each acuity value, especially for amplitude, the amplitude ofthe pattern V E P clearly tends to decrease and its latency to increase with decreasing visual acuity (P 0.001 for both), butthere are no obvious changes in the motion VEP.The scatter diagrams in Fig. 3 plot the amplitudes andlatencies of pattern-reversal [Fig. 3(a, b)] and motionVEPs [Fig. 3(c, d)] as a function of visual acuity for all30 children. The open circles represent results throughthe normal eyes, all of which had corrected visual acuityof 20/20 (or slightly better). The solid circles show datafor the amblyopic eyes. Linear regression lines (calculated from decimal values of acuity) are shown. At eachacuity level there is considerable variation from eye toeye in the absolute amplitude of signals and some scatterof latencies (cf. Fig. 2). However, despite this variability,the average amplitude of pattern VEPs clearly decreasedand the latency increased progressively with decreasingvisual acuity (correlation coefficients 0.57 and 0.65 respectively; P 0.001 for both). These trends for patternVEPs were evident for all three classes of amblyopes(anisometropic, strabismic and mixed). In contrast,neither the amplitude nor the latency of the N2 motiononset VEPs showed any obvious variation with visualacuity.We also considered the results in relation to thepattern of fixation of the amblyopic eye. The amplitudesand latencies of pattern VEPs and also the latencies ofmotion VEPs did not differ between the amblyopic eyeswith central fixation (n 18) and those with eccentricfixation (n 12). Curiously, motion-onset VEPs were,on average, slightly larger for amblyopic eyes withparafoveal or peripheral fixation than for those eyeswith central fixation, and this difference just reachedstatistical significance (P 0.05).Responses from central and peripheral visual fieldIn seven additional amblyopic children (six anisometropic and one strabismic), whose visual acuitythrough their amblyopic eyes was in the range of 20/80to 20/200, we recorded pattern VEPs and motion VEPsnot only with full-field checkerboard stimulation(30 40 deg; generated on the television display) butalso with the pattern limited to a circular central patchof 5 or 2 deg diameter, centred on the fixation point. Inaddition, responses were recorded for peripheral stimulation alone, produced by covering the central 5 degdiameter of the screen with a mask. In every conditionthe child was instructed to hold fixation on the point inthe centre of the screen.Typical results for one anisometropic subject areshown in Fig. 4. The amplitude of the Pj pattern-reversalcomponent through the normal eye was clearly dependent on the area of the field stimulated. The responseappears to originate disproportionately from the macular visual field since occlusion of only the central5 deg roughly halved its amplitude, while it remainedclearly detectable, with about one-third of the fullfield amplitude, even for stimulation of the central2 deg alone. Under all conditions the response was
186ZUZANA KUBOV, et al.@PATTERNREVERSALNon- blyo c eAmblyopic eye / . 10 pV1200msecMOTIONONSETNon-amblyopic eyeAmblyoplc eyeFIGURE 4. Typical examples of pattern-reversal and motion-onset VEPs, through the two eyes of one child, with stimulationrestricted to various parts of the visual field. Visual acuity was 20/20 through the normal eye and 20/125 in the amblyopiceye. For these experiments the checkerboards were generated on a computer monitor of 30 x 40 deg (see Methods). The fourstimulus conditions are illustrated schematically (not to scale) above the traces: peripheral stimulation alone, with the central5 deg diameter occluded; full-field stimulation; central stimulation with a 5 deg diameter patch; foveal stimulation with a 2 degpatch. The Pt peak in the pattern-reversal VEPs and the N 2 component in the motion-onset responses are indicated by smalldots above or below the records. Note that the pattern response is consistently smaller through the amblyopic eye for allstimulus configurations. While covering the central field has no effect on the motion-onset response, stimulation of the centralfield alone produces a clear motion signal. Indeed even for foveal (2 deg diameter) stimulation there is a distinct N 2component.Moreover, the motion response is indistinguishable in amplitude between the two eyes under all conditions.consistently smaller (about half the amplitude) throughthe amblyopic eye c o m p a r e d with the normal.The N2 c o m p o n e n t o f the response to m o t i o n onsetwas also detectable for all stimulus areas, t h o u g h itsbehaviour across these stimulus conditions was quitedifferent from that o f the pattern VEP. Covering thecentral 5 deg o f the stimulus had no effect on the N2amplitude. A l t h o u g h the N2 c o m p o n e n t became smallerwith decreasing field size, its decline in amplitude wasless dramatic than for the P pattern VEP. M o s t important, the similarity in amplitude t h r o u g h the two eyes o fthe N 2 c o m p o n e n t for m o t i o n onset with full-field stimulation was maintained at all field configurations. Evenwith a stimulus patch restricted to the central 2 deg, N2m o t i o n - o n s e t responses were very similar t h r o u g h thetwo eyes (except for the broadening o f the responset h r o u g h the amblyopic eye, presumed to be associatedwith attenuation o f the initial small P c o m p o n e n t ; seeabove).The results were similar for all seven children testedand the results are pooled in Fig. 5 in the form o fhistograms plotting the mean amplitude o f P and N2c o m p o n e n t s in the pattern and m o t i o n VEPs respectively, t h r o u g h the n o r m a l and amblyopic eyes, underthese four stimulus conditions. F o r every field configuration the pattern response was significantly smallert h r o u g h the amblyopic eye than t h r o u g h the normal eye(P 0.001), while there were no significant differencesbetween the eyes in the amplitude o f the motion-onsetresponse, even for foveal stimulation alone.DISCUSSIONO u r findings fully confirm the results o f n u m e r o u sprevious studies in showing that the amplitude of them a j o r positive c o m p o n e n t of the pattern-reversal VEP isreduced and its latency increased t h r o u g h amblyopiceyes c o m p a r e d with normal eyes (e.g. Arden & Barnard,1979; W a n g e r & Persson, 1980; Mayeles & Mulholand,1980; Sokol, 1983, 1986; Levi & M a n n y , 1986; O d o m ,1991). Furthermore, these abnormalities clearly correlatewith the severity o f amblyopia (Fig. 3).Neural correlates o f amblyopiaThe acuity deficit in amblyopia could be due to neuralunder-sampling, neural "'blurring" or positional uncertainty in the representation o f the fine detail o f the imagein the visual p a t h w a y (e.g. Levi & Klein, 1986; Hess,Field & Watt, 1990). In m o n k e y s reared with one eyeclosed (e.g. Baker, Grigg & Von N o o r d e n , 1974; LeVay,Wiesel & Hubel, 1980; Swindale, Vital-Durand &Blakemore, 1981 ; see Blakemore, 1988) or even unilaterally defocused (Movshon, Eggers, Gizzi, Hendrickson,Kiorpes & Boothe, 1987), the ocular d o m i n a n c e o f
P A T T E R N A N D M O T I O N VEPs IN A M B L Y O P I A187UII20A[ V]PATTERN15REVERSAL10T10MOTIONONSETm'ltAEvliF I G U R E 5. Pooled results for all seven children in which responses were measured with stimulation restricted to different partsof the visual field. The stimulus configurations are illustrated schematically as in Fig. 4. The histograms plot the mean (andSD) amplitude (A) through the non-amblyopic eyes (solid blocks) and the amblyopic eyes (open blocks), under these differentstimulation conditions, for pattern-reversal responses and motion-onset signals. In every, condition the amplitude of the patternresponse differs significantly between the two eyes (P 0.001) while there are no significant differences in the motion-onsetresponses through the two eyes with any stimulus configuration.individual neurons in the striate cortex becomes biasedin favour of the normal eye, with only a small proportionof cells responding through the amblyopic eye. Presumably such a gross loss of input could result inundersampling of the image, hence contributing to theletter-acuity deficit and the reduction in amplitude of thePt component of the pattern-reversal VEP, which almostcertainly derives from the striate cortex (Maier et al.,1987).In addition to these dramatic changes in ocular dominance, the spatial characteristics of the receptive fieldsof individual neurons in the striate cortex are degradedthrough the amblyopic eye. In monkeys made amblyopicby deprivation of vision (Blakemore & Vital-Durand,1984; Blakemore, 1990) or by unilateral defocus(Movshon et al., 1987), striate cells driven through theamblyopic eye have lower spatial resolution and contrastsensitivity for grating stimuli than those driven throughthe normal eye. Such neural "blurring" at the level of thestriate cortex could also presumably contribute to thedecrease in amplitude of the pattern-reversal VEP, because neurons produce smaller responses for stimuli ofany particular spatial frequency and contrast throughthe amblyopic than through the normal eye.Foveal and peripheral-field contributionsreversal VEPsto pattern-It is well known that responses from the fovea, whichis of course normally specialized for fine spatial resolution, contribute disproportionately to the patternreversal VEP (Blumhardt, Barrett, Halliday & Kriss,1989). In the present study the Pt component for full-field stimulation (through normal eyes) was reduced inamplitude by about 50% if either the central 5 deg wasmasked or the stimulus was restricted to the central 5 deg(Figs 4 and 5), which implies approximate additivity ofpattern-reversal signals from central and peripheral field.However, this result shows that the amplitude was farfrom linearly related to the area of field stimulated.Spatial summation was non-linear even within thecentral 5 deg, since a reduction in field area by 84%(from 5 to 2 deg diameter) only halved the amplitude.Most important for the present study, the deficit in thepattern response through the amblyopic eye was evidentfor all field sizes and configurations. Even for peripheralstimulation alone, with the central 5 deg covered, the P component was significantly smaller through the amblyopic eye (Fig. 5). This is somewhat surprising in view ofthe finding that peripheral vision is often less dramatically affected in amblyopia than is central vision (Hess,1978; Hess & Howell, 1978; Hess & Pointer, 1985). Hess,Campbell and Zimmern (1980) reported that visi
ated on a computer monitor (ViewSonic 21; 100 frames/sec; total display size 30 x 40deg) under com- puter control (IBM compatible 486 PC). In these exper- iments, responses to full-field stimulation were compared with those for stimuli restricted to the central 5 and 2 deg of th
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Chính Văn.- Còn đức Thế tôn thì tuệ giác cực kỳ trong sạch 8: hiện hành bất nhị 9, đạt đến vô tướng 10, đứng vào chỗ đứng của các đức Thế tôn 11, thể hiện tính bình đẳng của các Ngài, đến chỗ không còn chướng ngại 12, giáo pháp không thể khuynh đảo, tâm thức không bị cản trở, cái được
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Le genou de Lucy. Odile Jacob. 1999. Coppens Y. Pré-textes. L’homme préhistorique en morceaux. Eds Odile Jacob. 2011. Costentin J., Delaveau P. Café, thé, chocolat, les bons effets sur le cerveau et pour le corps. Editions Odile Jacob. 2010. Crawford M., Marsh D. The driving force : food in human evolution and the future.
