Miniature Eye Movements Enhance Fine Spatial Details

Transcription

1 Boston University OpenBU Cognitive & Neurl Systems CAS/CNS Technicl Reports Miniture Eye Movements Enhnce Fine Sptil Detils Rucci, Michele Boston University Center for Adptive Systems nd Deprtment of Cognitive nd Neurl Systems Boston University

2 Miniture eye movements enhnce fine sptil detil Michele Rucci, Rmon Lovin, Mrtin Poletti, nd Frizio Sntini Deprtment of Cognitive nd Neurl Systems Boston University 677 Becon Street Boston, MA USA Sumitted: Octoer, 2006 CAS/CNS Technicl Report Copyright 2006 Permission to copy without fee ll or prt of this mteril is grnted provided tht: 1. The copies re not mde or distriuted for direct commercil dvntge; 2. the report title, uthor, document numer, nd relese dte pper, nd notice is given tht copying is y permission of the BOSTON UNIVERSITY CENTER FOR ADAPTIVE SYSTEMS AND DEPARTMENT OF COGNITIVE AND NEURAL SYSTEMS. To copy otherwise, or to repulish, requires fee nd / or specil permission.

3 Brod noise ndwidth Lrge stimuli (low frequency) (high frequency) Vol June 2007 doi:.38/nture05866 LETTERS Miniture eye movements enhnce fine sptil detil Michele Rucci 1, Rmon Iovin 1, Mrtin Poletti 1 & Frizio Sntini 1 Our eyes re constntly in motion. Even during visul fixtion, smll eye movements continully jitter the loction of gze 1 4.Itis known tht visul percepts tend to fde when retinl imge motion is eliminted in the lortory 5 9. However, it hs long een deted whether, during nturl viewing, fixtionl eye movements hve functions in ddition to preventing the visul scene from fding 7. In this study, we nlysed the influence in humns of fixtionl eye movements on the discrimintion of grtings msked y noise tht hs power spectrum similr to tht of nturl imges. Using new method of retinl imge stiliztion 18, we selectively eliminted the motion of the retinl imge tht normlly occurs during the intersccdic intervls of visul fixtion. Here we show tht fixtionl eye movements improve discrimintion of high sptil frequency stimuli, ut not of low sptil frequency stimuli. This improvement origintes from the temporl modultions introduced y fixtionl eye movements in the visul input to the retin, which emphsize the high sptil frequency hrmonics of the stimulus. In nturl visul world dominted y low sptil frequencies, fixtionl eye movements pper to constitute n effective smpling strtegy y which the visul system enhnces the processing of sptil detil. It ws originlly hypothesized tht fixtionl eye movements might contriute to the perception of fine sptil detils, proposl lter refined into the dynmic theories of visul cuity 113. Clssicl experiments tht eliminted retinl imge motion did not support these theories 7, These pioneering experiments, however, suffered from significnt technologicl limittions. Most notly, the devices used to stilize imges on the retin did not llow selective stiliztion during periods of visul fixtion etween sccdes, s would hve een necessry to study fixtionl eye movements in their nturl context Insted, ll trils with stilized vision hd to e run in uninterrupted locks while the suject mintined fixtion highly unnturl condition tht unvoidly led to visul ftigue nd fding. In this study, we exmined the influence of fixtionl eye movements on the discrimintion of trgets t different sptil frequencies (grting spcings). We compred discrimintion performnces mesured in two conditions: with nd without the retinl imge motion produced y fixtionl eye movements. To overcome the limittions of previous experiments, we developed new retinl stiliztion technique sed on rel-time processing of eye-movement signls 18. Like previous stiliztion methods, this technique does not gurntee perfect elimintion of retinl imge motion; however, unlike previous methods, it comines high qulity of stiliztion with experimentl flexiility (see Supplementry Informtion). This flexiility enled us to disply nd selectively stilize the stimulus fter sccde, method tht isoltes the norml fixtionl motion of the eye. It lso llowed us to rndomly lternte etween trils with retinl stiliztion nd trils with norml retinl motion, procedure tht prevents visul ftigue nd llows rigorous comprison of the two conditions, nd to ssess the ccurcy of retinl stiliztion independently from the suject s own judgement, development tht llows inexperienced nd nive sujects to prticipte in experiments. In forced-choice tsk, sujects reported whether grting ws tilted y 45u clockwise or nticlockwise. Two seprte experiments investigted the discrimintion of the stimuli shown in Fig. 1. In c Percentge correct 85 Low frequency 75 High frequency 65 Unstilized Stilized d Fixtion cross 1.5 s 0.25 s Gze position Cued loction Sccde Stimulus (stilized or unstilized) Vrile Time 1.0 s 1.3 s High frequency Msk Low frequency Unstilized Stilized SF RM SF RM Figure 1 Impct of retinl stiliztion., Exmples of stimuli., Experimentl procedure. The grey rrows nd dots represent the suject s eye movements nd the centre of gze, respectively. c, Men suject performnce (N 5 6). For every suject, in ech condition, percentges were evluted over minimum of trils. Individul performnce differences etween stilized nd unstilized conditions were ll significnt in experiment one nd were sttisticlly indistinguishle in experiment two (P, 0.05; one-tiled z-test). d, Results of control experiments. The lrge stimulus ws 5.6 u. For the rod ndwidth control, the frequency nd of noise ws 04 cycles per degree. For oth sujects, performnce dropped under stiliztion using grting with 11 cycles per degree (high frequency, experiment one) (P, 0.05; one-tiled z-test), ut not using grting with 4 cycles per degree (low frequency, experiment two). The numers of trils re indicted on ech r. In oth c nd d, error rs represent 95% confidence intervls. SF, RM, sujects. 1 Deprtment of Cognitive nd Neurl Systems, Boston University, Boston, Msschusetts 02215, USA. 851

4 LETTERS NATURE Vol June 2007 experiment one, the stimulus ws high-frequency grting pertured y noise t low sptil frequencies. In experiment two, the frequency nds of the grting nd the noise were reversed: the grting ws t low sptil frequencies nd the noise ws t high frequencies. In oth cses, the power of the noise ws inversely proportionl to the squre of the sptil frequency, similr to the power spectrum of nturl imges 26. As shown in Fig. 1c, when high-frequency grtings were used, men percentges of orienttion discrimintion dropped y more thn 16% in the presence of retinl stiliztion, chnge tht ws highly significnt (experiment one: t , P, 0.01; one-tiled pired t-test). By contrst, the retinl imge motion produced y fixtionl eye movements did not improve performnce with lowfrequency grtings (experiment two: t 5 0.2, P. 0.05). Thus, fixtionl eye movements improved discrimintion of the orienttion of high-frequency grting msked y low-frequency noise ut did not help with low-frequency grting msked y high-frequency noise. This result is surprising ecuse it contrdicts trditionl views of the influence of fixtionl eye movements on vision. Indeed, the pronounced reduction in contrst sensitivity t low sptil frequencies mesured y previous experiments with prolonged retinl stiliztion predicts more significnt drop in performnce with lowfrequency thn with high-frequency grtings. Control experiments exmined the roustness of this result. As shown y Fig. 1d, the sme pttern of results ws otined in first control experiment, which used lrger stimulus, nd in second experiment, in which the noise ndwidth ws rodened to overlp the frequency of the grting. Furthermore, eneficil effect of fixtionl eye movements ws lso found in the contrst thresholds the lowest detectle contrst of grting mesured t high sptil frequencies (Fig. 2). We concluded tht the high-frequency discrimintion impirment oserved under retinl stiliztion ws not ffected y the precise size of the stimulus or y the ndwidth of low-frequency noise. To estlish further evidence for the cusl reltionship etween fixtionl modultions of luminnce nd performnce, we eliminted retinl imge motion on selected xis while leving motion on the orthogonl xis unltered. As shown in Fig. 3, discrimintion ws impired when retinl imge motion ws restricted to the xis prllel to the grting ut ws norml when motion occurred on the orthogonl xis. These results re consistent with the informtionl content of the modultions of luminnce introduced y fixtionl eye movements. These modultions only convey informtion out the pttern of noise during motion prllel to the grting, ut provide mximl informtion out the grting when motion occurs on the xis orthogonl to the grting. For one suject, we lso rtificilly reconstructed the visul input signls resulting from fixtionl eye movements y moving n otherwise stilized stimulus. Pssive exposure to the fixtionl motion of the retinl imge, otined y motion of the stimulus insted of the eye, ws sufficient for reestlishing norml level of performnce. These results confirm Contrst Sptil frequency (cycles per degree) Figure 2 Contrst thresholds. Men levels of Michelson contrst 6 s.d. for two sujects:, suject AR nd, suject GD. Consistent with the results of Fig. 1, the two conditions norml retinl imge motion (filled tringles) nd retinl stiliztion (open circles) only produced different thresholds t high sptil frequencies. 852 tht the discrimintion impirment shown in Fig. 1 ws cused y the sence of the retinl imge motion produced y fixtionl eye movements. To understnd the mechnisms y which fixtionl eye movements improve the discrimintion of high sptil frequency stimuli, we nlysed the frequency contents of the sptiotemporl signls entering the eye in the unstilized (stimuli fixed t loction on the screen) nd stilized (stimuli moved with the eye to cncel the retinl motion resulting from fixtionl eye movements) conditions of experiments 1 nd 2 (the signls I U (x, t) nd I S (x, t) received t time t y receptor locted t position x on the retin, see Fig. 4). Under retinl stiliztion, I S (x, t) did not chnge with time, nd its power ws confined to the sptil frequency plne t zero temporl frequency. In contrst, in the unstilized condition, the motion of the eye spred the sptil power of the stimulus cross temporl frequencies wy from zero frequency. As shown y Fig. 4, the extent of this temporl spreding ws not uniform throughout the sptil frequency plne; it incresed with sptil frequency. This dependence on sptil frequency occurred ecuse the verge chnge in luminnce produced y smll displcement of grting increses with the grting frequency. This effect hd different impct on the visul input signls of the two experiments. Figure 4c compres the power spectrum of the stimulus with the sptil frequency distriution of the power in I U, which, s consequence of oculomotor ctivity, left the temporl zero frequency plne nd ecme ville t non-zero temporl frequencies. This dynmic power is identicl to the power of the chnge in luminnce tht occurs s result of the motion of the eye the signl ~I U ðx, tþin eqution (2). With the stimuli of experiment one, these input modultions exhiited oth ttenution of the low-frequency noise nd Prllel Orthogonl Time Prllel Orthogonl c Percentge correct Suject RM Un. St. Orth. Pr. Suject SF Un. St. Orth. Pr. Rec. Figure 3 Controlled retinl imge motion., Selective stiliztion of the stimulus on single xis enles control of the signl-to-noise rtio of the visul input signls resulting from fixtionl eye movements., The fixtionl modultions of luminnce experienced y retinl receptor (the circle in ) convey no informtion out the grting when motion occurs on the xis prllel to the grting (Pr.) nd convey mximl informtion when motion occurs on the xis orthogonl to the grting (Orth.). c, Men percentges of correct discrimintion 6 s.e.m. for two sujects. Suject SF ws lso tested in n dditionl condition (Rec.), in which we reconstructed the fixtionl motion of the retinl imge under retinl stiliztion. In this condition, recorded eye-movement trjectory ws superimposed on the motion of n otherwise stilized stimulus. In ech tril, this trjectory ws rndomly selected from the pool of ll previously recorded unstilized trils tht did not include fixtionl sccdes. Significnt differences from complete retinl stiliztion (St.) nd from norml retinl imge motion (Un.) re indicted y nd, respectively (P, 0.05; one-tiled z-tests). The numers of trils re indicted on ech r.

5 NATURE Vol June 2007 LETTERS (high frequency) Unstilized Stilized (low frequency) Unstilized Stilized T 0 T 1 I I 0 T 2 1 I 2 I U (x,y,t) I S (x,y,t) Time I 0 I 0 I 1 I 0 Sptil frequency (cycles per degree) c Power stilized (sttic) 3 3 I 2 Unstilized enhncement of the power of the high-frequency grting. Tht is, the signl-to-noise-rtio (SNR, the rtio of the power of the grting to the power of the noise) of ~I U ws much higher thn tht of the stilized input SNR ~I U SNR IS ~21. In contrst, in the cse of the low-frequency grting of experiment two, fixtionl modultions enhnced the noise reltive to the grting. In this cse, the SNR of ~I U ws lower thn tht of the stimulus SNR ~I U SNR IS ~0:33. Therefore, fixtionl eye movements introduced temporl modultions tht enhnced the grting with respect to the noise in experiment one ut not in experiment two. On verge, the SNR of the chnges in luminnce cused y oculomotor ctivity ws more thn 30 times lrger with high-frequency grtings thn it ws with low-frequency grtings (Fig. 4d). This effect cn e seen in the Supplementry Movies, which reconstruct the input to the retin. The result of Fig. 4d cn e explined on the sis of the sptil chrcteristics of fixtionl instility. Becuse of the smll scle of fixtionl eye movements, it is possile to use Tylor expnsion to pproximte the visul input: I U ðx, tþ<sðxþzjðþ t : +SðxÞz 1 2 jðþh t Sj T ðþ t where S(x) is the stimulus luminnce, H S its Hessin, nd the vector j(t) represents the eye trjectory during the presenttion of the stimulus. Eqution (1) enles n nlyticl pproximtion of the frequency content of the dynmic chnge in luminnce tht occurs s result of the motion of the eye. It predicts tht the totl power of ~I U t given sptil frequency juj is proportionl to R SS (juj)juj 2, where R SS is the power spectrum of the stimulus. Tht is, for smll eye movements, the totl power tht ecomes ville t non-zero temporl frequency is given y the power spectrum of the stimulus multiplied y juj 2, term tht enhnces high sptil frequencies nd I 0 Stilized ~ I U I S Sptil frequency (cycles per degree) Power unstilized (dynmic) Temporl frequency (Hz) Figure 4 Influence of fixtionl eye movements on visul input., For every tril in experiments one nd two, movie ws generted to reconstruct the sptiotemporl input to the retin during the period of stimulus presenttion. In the movie I U (x,y,t) of n unstilized tril, the frme t time T i ws n imge I i centred t the current loction of gze. In the movie I S (x,y,t) of stilized tril, ech frme ws the sme imge I 0. Exmples of these re included s Supplementry Movies., c, Power spectr of the signls entering the eye of suject MD. The full spce time spectr re shown in. c, Sptil spectrl densities, fter integrtion over temporl frequency, d SNR dynmic/snr sttic ð1þ e Stimulus Sptil frequency FEM u 2 u 2 u u Dynmic power Sptil frequency of I S nd of the chnges in luminnce cused y oculomotor ctivity (~I U in eqution (2)). d, Rtio etween the SNRs of ~I U nd I S in the two experiments. Brs represent men 6 s.d. cross ll sujects. e, Theoreticl explntion of the impct of fixtionl eye movements. Fixtionl instility cts s filter (FEM) tht enhnces high sptil frequencies in the temporl modultions of luminnce present in the input to the retin. The totl power t sptil frequency u of these input modultions (dynmic power) is pproximtely equl to the power of the stimulus multiplied y u 2. ttenutes low frequencies. As illustrted in Fig. 4e, this term mplifies the grting reltive to the noise in experiment one nd the noise reltive to the grting in experiment two. Our results show tht vision is impired t high sptil frequencies in the sence of fixtionl eye movements. This finding is consistent with the sptil frequency dependence of the temporl modultions resulting from fixtionl eye movements. Neurons in the erly visul system re sensitive to these input modultions As with the stimuli of experiment one, nturl visul environments possess sustntil power t low sptil frequencies. Our results indicte tht smpling visul informtion y mens of jittering fixtion is n effective strtegy for nlysing nturl scenes, fcilitting the processing of sptil detil in the fce of otherwise overwhelming lowfrequency power. METHODS SUMMARY Sujects reported the orienttion (645u) of grting msked y noise. In experiment one, grting with 11 cycles per degree ws pertured y low sptil frequency noise (low-pss cutoff frequency f c 5 5 cycles per degree). In experiment two, the stimulus ws grting with 4 cycles per degree overlpped y high sptil frequency noise (high-pss f c 5 cycles per degree). Stimuli were displyed t the onset of fixtion fter sccde towrds rndomly cued loction. Stimuli were either fixed t loction on the screen (unstilized condition) or moved with the eye to cncel the retinl motion resulting from fixtionl eye movements (stilized condition). In oth experiments, the contrst of the grtings ws individully djusted for ech suject so tht performnce levels in the unstilized condition were pproximtely 75%. Contrst thresholds were evluted under conditions similr to those of experiments one nd two using n dptive trcking method 30, oth in the presence nd in the sence of the retinl imge motion originting from fixtionl eye movements. 853

6 LETTERS NATURE Vol June 2007 In the experiment reported in Fig. 3, the tsk, stimuli nd procedure were identicl to those of experiment one, except tht trils rndomly lternted mong ll conditions. Rndom selections of the stiliztion xis nd the stimulus ensured lck of correltion etween the direction of retinl imge motion nd the grting orienttion. Power spectr were evluted using the Welch periodogrm technique. In the unstilized condition, the input signl I U (x,t) cn e expressed s: I U ðx, tþ~sðxzjðþ t Þ~I U ðxþz~i U ðx, tþ ð2þ where I U ðxþ is the men luminnce received y the receptor over the period of stimulus presenttion. Figure 4 shows the spectr of I S, I U nd ~I U. I S nd I U possessed equl totl power ut differed in their spectrl distriutions. Full Methods nd ny ssocited references re ville in the online version of the pper t Received 5 Mrch; ccepted 18 April Rtliff, F. & Riggs, L. A. Involuntry motions of the eye during monoculr fixtion. J. Exp. Psychol. 40, 687 (19). 2. Ditchurn, R. W. Eye movements in reltion to retinl ction. Opt. Act 1, (1955). 3. Steinmn, R. M., Hddd, G. M., Skvenski, A. A. & Wymn, D. Miniture eye movement. Science 181, (1973). 4. Murkmi, I. & Cvngh, P. A jitter fter-effect revels motion-sed stiliztion of vision. Nture 395, 798 (1998). 5. Ditchurn, R. W. & Ginsorg, B. L. Vision with stilized retinl imge. Nture 1, (1952). 6. Riggs, L. A. & Rtliff, F. The effects of countercting the norml movements of the eye. J. Opt. Soc. Am. 42, (1952). 7. Riggs, L. A., Rtliff, F., Cornsweet, J. C. & Cornsweet, T. N. The disppernce of stedily fixted visul test ojects. J. Opt. Soc. Am. 43, 495 (1953). 8. Yrus, A. L. Eye Movements nd Vision (Plenum, New York, 1967). 9. Mrtinez-Conde, S., Mcknik, S. L., Troncoso, X. G. & Dyr, T. A. Microsccdes counterct fding during fixtion. Neuron 49, (2006).. Hering, E. Üer die Grenzen der Sehschärfe. Berichte der Königlichen Sächsischen Gesellshft der Wissenschften. Mth. Phys. Klsse 20, (1899). 11. Averill, H. I. & Weymouth, F. W. Visul perception nd the retinl mosic, II. The influence of eye movements on the displcement threshold. J. Comp. Psychol. 5, (1925). 12. Mrshll, W. H. & Tlot, S. A. in Biologicl Symposi Visul Mechnisms. Vol. 7 (ed. Kluver, H.) (Cttel, Lncster, Pennsylvni, 1942). 13. Arend, L. E. Sptil differentil nd integrl opertions in humn vision: implictions of stilized retinl imge fding. Psychol. Rev., (1973). 14. Ahissr, E. & Arieli, A. Figuring spce y time. Neuron 32, (2001). 15. Greschner, M., Bongrd, M., Rujn, P. & Ammermüller, J. Retinl gnglion cell synchroniztion y fixtionl eye movements improves feture estimtion. Nture Neurosci. 5, (2002). 16. Ölveczky, B. P., Bccus, S. A. & Meister, M. Segregtion of oject nd ckground motion in the retin. Nture 423, (2003). 17. Rucci, M. & Csile, A. Fixtionl instility nd nturl imge sttistics: implictions for erly visul representtions. Network Comp. Neur. Syst. 16, (2005). 18. Sntini, F., Redner, G., Iovin, R. & Rucci, M. EyeRIS: A generl-purpose system for eye movement contingent disply control. Behv. Res. Methods (in the press). 19. Koenderink, J. J. Contrst enhncement nd the negtive fterimge. J. Opt. Soc. Am. A 62, (1972). 20. Kelly, D. H. Motion nd vision. I. Stilized imges of sttionry grtings. J. Opt. Soc. Am. 69, (1979). 21. Tuluny-Keesey, Ü. Fding of stilized retinl imges. J. Opt. Soc. Am. 72, (1982). 22. Tuluny-Keesey, Ü. & Jones, R. M. The effect of micromovements of the eye nd exposure durtion on contrst sensitivity. Vision Res. 16, (1976). 23. Steinmn, R. M., Cunitz, R. J., Timerlke, G. T. & Hermn, M. Voluntry control of microsccdes during mintined monoculr fixtion. Science 155, (1967). 24. Steinmn, R. M. & Collewijn, H. Binoculr retinl imge motion during ctive hed rottion. Vision Res. 20, (19). 25. Kpoul, Z. A., Roinson, D. A. & Hin, T. C. Motion of the eye immeditely fter sccde. Exp. Brin Res. 61, (1986). 26. Field, D. J. Reltions etween the sttistics of nturl imges nd the response properties of corticl cells. J. Opt. Soc. Am. A 4, (1987). 27. Leopold, D. A. & Logothetis, N. K. Microsccdes differentilly modulte neurl ctivity in the strite nd extrstrite visul cortex. Exp. Brin Res. 123, (1998). 28. Mrtinez-Conde, S., Mcknik, S. L. & Huel, D. H. Microsccdic eye movements nd firing of single cells in the strite cortex of mcque monkeys. Nture Neurosci. 3, (2000). 29. Snodderly, D. M., Kgn, I. & Gur, M. Selective ctivtion of visul cortex neurons y fixtionl eye movements: implictions for neurl coding. Vis. Neurosci. 18, (2001). 30. Mcmilln, N. A. & Creelmn, C. D. Detection Theory A User s Guide 2nd edn (L.Erlum Assocites, London, 2005). Supplementry Informtion is linked to the online version of the pper t Acknowledgements We thnk E. Ahissr, G. Desordes, W. S. Geisler, K. J. Nielsen, E. L. Schwrtz, D. M. Snodderly nd J. D. Victor for help. This work ws supported y grnts from the Ntionl Institute of Helth nd the Ntionl Science Foundtion to M.R. Author Informtion Reprints nd permissions informtion is ville t The uthors declre no competing finncil interests. Correspondence nd requests for mterils should e ddressed to M.R. (rucci@cns.u.edu). 854

7 doi:.38/nture05866 METHODS Sujects. Five nive sujects nd one experienced oserver, ll with norml vision, prticipted in the experiments. Informed consent ws otined from ll sujects in ccordnce with the procedures pproved y the Boston University Chrles River Cmpus Institutionl Review Bord. Apprtus nd stimuli. To overcome the technicl limittions of previous studies of retinl stiliztion, the experiments of this study used EyeRIS (ref. 18), custom-developed system sed on digitl signl processor, which llows flexile gze-contingent disply control on cthode-ry tue (CRT) monitor. This system processes eye movements in rel time nd gurntees refresh of the stimulus with mximum dely equl to the time required to render two frmes on the CRT disply ( ms t 200 Hz, typicl dely 7.5 ms). Joint nlysis of the performnce of EyeRIS nd eye-movement dt shows tht the error of retinl stiliztion in the experiments ws smller thn 19 (see Supplementry Methods). Such high qulity of retinl stiliztion ws possile ecuse of the rief durtion of stimulus presenttion nd the smll mplitude of fixtionl eye movements. Stimuli were designed on the sis of the predictions of eqution (1) to enhnce the impct of fixtionl eye movements on visul input signls. s nd noise fields were linerly superimposed, nd the resulting ptterns were weighted y two-dimensionl gussin envelope so tht the visile re covered pproximtely 309. Such smll stimulus could e seen with single fixtion without the need for mcroscopic eye movements. Stimuli were displyed on grey ckground of uniform luminnce equl to the men luminnce of the stimulus (21 cd m 2 ). To compenste for individul differences in contrst sensitivity, contrst levels were djusted for ech suject in preliminry experimentl session, so tht discrimintion percentges were round 75% in the presence of the normlly moving retinl imge. Michelson contrst levels rnged from 4% to 9%. Stimuli were displyed on fst phosphor CRT monitor (Iiym HM204DT) t resolution of pixels nd verticl refresh rte of 200 Hz. Movements of the hed were prevented y use of ite r with customized dentl imprint nd hedrest. Eye position ws mesured with Genertion 6 DPI eyetrcker (Fourwrd Technologies, Inc.). Procedure. Stimuli were displyed for 1 s t the centre of the visul field. To study the norml motion of the retinl imge tht occurs when periods of fixtion re seprted y sccdes, stimuli were presented fter the suject performed sccde from the centre of the screen to rndomly cued loction t 1.5u of visul eccentricity. A rel-time routine running on EyeRIS displyed the stimulus s soon s it detected the onset of fixtion within 309 of the cued loction. Offline nlysis of eye-movement trces showed tht ctivtion of the disply occurred with n verge dely of 12 ms fter the onset of fixtion, s evluted y humn expert. In ech tril, the grting hd equl proility of eing tilted y 645u nd ws followed y high-energy msk for 1.3 s. Sujects reported the orienttion of the grting using joypd fter disppernce of the msk. Trils lternted rndomly etween the two conditions of norml retinl motion nd stiliztion. In the stilized condition, the stimulus ws ctively trnslted on the screen under rel-time computer control to compenste for the suject s eye movements nd lwys ppered to e immoile t the centre of the fove. Sujects did not report stimulus fding nd were often unle to tell whether or not tril ws stilized. Trils with sccdes lrger thn 309, or in which the gze exceeded 309-rdius window centred on the stimulus, were discrded from dt nlysis. Contrst thresholds were evluted in the sme conditions s experiments one nd two. The grting contrst ws chnged t ech tril following 3-up-1-down rule, so tht percentges of correct discrimintion settled round 79%. To enle fine chnges etween trils, these experiments were conducted t lower monitor contrst setting with ckground luminnce of 6 cd m 2. Every few trils, n imge rndomly extrcted from dtse of nturl scenes ws presented for 2 s to ensure tht fding did not ffect the results. Ech suject underwent minimum of trils until thresholds settled on stedy vlues. The vlues shown in Fig. 2 re men 6 s.d. evluted over the lst ten trils.