Saccadic eye movements cause compression of time as well as space

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1 25 Nture Pulishing Group Sccdic eye movements cuse compression of time s well s spce M Concett Morrone 1, John Ross 2 & Dvid Burr 3,4 There is now considerle evidence tht spce is compressed when stimuli re flshed shortly efore or fter the onset of sccdic eye movement. Here we report tht short intervls of time etween two successive perisccdic visul (ut not uditory) stimuli re lso underestimted, indicting compression of perceived time. We were even more surprised tht in criticl intervl efore sccdes, perceived temporl order is consistently reversed. The very similr time courses of sptil nd temporl compression suggest tht oth re medited y common neurl mechnism, proly relted to the predictive shifts tht occur in receptive fields of mny visul res t the time of sccdes. We continully mke llistic eye movements, clled sccdes, in order to redirect the fove to ojects of interest. Although this strtegy is highly efficient in llowing the visul system to nlyze wide field of view t high resolution, it poses serious prolems for perception y chnging the mpping of externl spce onto the retin. In norml viewing, sttionry ojects do not pper to shift their positions in spce with eye movements, implying tht continul corrective chnges re mde to neurl frmes of reference. But the corrections re not entirely successful. Experiments show tht stimuli flshed riefly just efore or erly in sccde re seen in flse positions (for review, see ref. 1). Two components of error hve een identified: shift in the direction of the sccde 2,3 nd shrinkge of distnces etween stimuli, compressing them towrd the sccde s trget 4 6.Thecompressionis primrily one-dimensionl, prllel to the sccdic pth 5,7. The shift hs een ttriuted to mismtch etween ctul eye position during sccdes nd prediction of position from n internl corollry dischrge signl. But such mismtch fils to explin compression: tht remins mystery. However, it hs een suggested 1 tht compression my e linked to the nticiptory shift of the receptive fields of neurons in mny corticl res, prticulrly the lterl intrprietl re (LIP), tht ecome trnsiently crniotopic from well efore until well fter sccdes, up to the next intention to move the eye 8 1. LIP neurons hve recently een implicted in encoding temporl durtion s well s sptil distnce 11,12. If their perisccdic ctivity is responsile for the perceived compression of spce, we my well expect tht the perception of intervls of time will lso e ltered when sccdes re imminent or in progress. Here we show tht time, like spce, is indeed strongly compressed during sccdes, following similr dynmics s sptil compression. Furthermore, during criticl durtion just efore sccde initition, temporl order of riefly presented stimuli is consistently reversed. RESULTS Temporl compression We sked oservers to compre the time intervl etween two pirs of extended horizontl rs while they mde lrge horizontl sccdes (Fig. 1). The first pir ws test stimulus, with the intervl etween rs fixed t 1 ms, presented t unpredictly vrying times reltive to the sccde. The second pir ws proe stimulus of vrile intervl, presented 2 s fter the test (see smple results for two sujects in Fig. 2). The dt for intervls well efore sccdic onset re well fit y cumultive Gussin curve with men of 1 ms (corresponding to the point of sujective equlity (PSE)) nd s.d. of round 4 ms (corresponding to the precision of the mtch). The dt for stimuli presented to sttionry eyes just efore sccdic onset re lso well fit y cumultive Gussin, ut its men (nd hence PSE) is not 1 ms, ut 5 ms. This suggests tht sujective time hs een compressed y fctor of two. Results for stimuli presented perisccdiclly to region ner the sccdic trget, where ttention is known to e directed well efore sccdes egin 13,14, were very similr to those with lrge peripherl trgets remote from the sccdic trget (Fig. 2). We oserved compression over wide rnge of sccde sizes (3.5 to 451), with only modest vrition of mgnitude for the very short sccdes, even though sccde durtion vried from 3 to 12 ms over tht rnge (Fig. 2). The compression of time ws specific to visul stimuli nd to sccdes. We did not oserve compression for uditory clicks presented just efore sccdes, nor for visul stimuli presented just efore link (known to cuse sccde-like visul suppression 15,16 ). Compring PSEs nd judgment precision s function of test presenttion time shows tht the compression of time ws gretest when the test rs were presented ner the strt of sccdes nd extended over period of some 3 ms round sccdic onset (Fig. 3). Compression of time is ssocited with n improvement in 1 Fcoltà di Psicologi, Università Vit-Slute Sn Rffele, Vi Olgettin 58, Milno 2132, Itly. 2 School of Psychology, The University of Western Austrli, Perth, Western Austrli, Austrli. 3 Istituto di Neuroscienze del CNR, Vi Moruzzi 1, Pis 561, Itly. 4 Diprtimento di Psicologi, Università di Firenze, Vi S. Nicolò 89, Firenze, Itly. Correspondence should e ddressed to D.B. (dve@in.cnr.it). Pulished online 19 June 25; doi:1.138/nn VOLUME 8 [ NUMBER 7 [ JULY 25 NATURE NEUROSCIENCE

2 25 Nture Pulishing Group 5 precision, oth reltive to erly presenttion nd to presenttion during fixtion (Fig. 3). Plotting precision ginst perceived durtion for sccdic conditions for 1 ms tests nd lso for fixtion with vrile tests (Fig. 3) shows this reltionship more clerly. In oth cses precision ws directly proportionl to perceived durtion, yielding Weer frctions round 3%. Temporl inversion Oservers were next sked to judge the temporl order in which pir of horizontl rs hd een presented. The order nd temporl seprtion of the rs nd the time of presenttion reltive to the sccde were ll vried unpredictly. Figure 4, shows for two oservers the proportion correct s function of stimulus presenttion time, for three rnges of r seprtion. When r seprtion ws lrge (76 2 ms), temporl order ws lmost lwys judged correctly, with smll drop in ccurcy for ins just efore sccdes. However, for shorter seprtions (2 44 ms nd ms), rs presented just efore sccdes were seen in reversed order, with very high proility. When presented t times remote from sccdic onset, order judgments within this rnge of r seprtions were correct with high proility. Test Proe 5 1, 1,5 2, Time (ms) Figure 1 Schemtic illustrtion of the experimentl setup. () Impression of the screen, with fixtion nd trget spots nd upper nd lower r stimuli (which never ctully ppered simultneously). Both stimuli were rief (8 ms), 6 61 horizontl, ner-equiluminnt green rs, one 181 ove nd one 181 elow screen center. Color nd seprtion were chosen to minimize the senstion of motion (nd sujects reported no motion perception). () Schemtic sccde out 5 ms in durtion, eginning t time zero (y definition). The lower trce illustrtes the timing of the test nd proe stimuli for the perceived durtion experiment. The pirs of ticks illustrte the presenttion time of upper nd lower rs (order rndomized). The test rs were lwys seprted y 1 ms, the proe seprtion vrile. A closer exmintion of reversl of the perception of time is shown in Figure 4c,d. A plot of dt for stimuli presented 1 ms or more fter sccde (Fig. 4c) produces conventionl psychometric function, vrying monotoniclly with r seprtion. The psychometric function for dt within the rnge of 7 to 3 ms (Fig. 4d) is fr from conventionl: it is triphsic, nd for r seprtions within the rnge 75 ms, it runs in the opposite direction, s if time hd reversed. Yet despite the reversl of time, oservers did not see rs s simultneous t some nonzero seprtions. Figure 4e,f shows signed mgnitude estimtes of perceived r seprtion for suset of the dt of Figure 4c,d.For stimuli presented fter the sccde (Fig. 4e), the estimtes were ner veridicl, ut when they were centered within the criticl intervl of 3 to 7 ms, durtion ws underestimted, consistent with compression of time, nd perceived order ws often inverted for stimulus seprtions less thn 1 ms. Note tht for some seprtions (round 7 ms), stimuli were sometimes reversed nd sometimes not (leding to 5% performnce in Fig. 4d), ut the mgnitude of the estimtion never pproched zero, which would imply perceived simultneity. DISCUSSION Our results show tht t the time of sccdes, strong compression of time ccompnies the well-known shift nd compression of spce 2 6, suggesting common cuse for these phenomen. Both the temporl nd sptil effects occur within pproximtely the sme sccdic epoch, from shortly efore to shortly fter sccdes, nd oth re mximl just t the strt of sccdes. As eye movements nd ttention re known to e tightly relted, it is worthwhile to consider the possile role of ttention in temporl compression. Attention is known to influence perceived durtion nd lso temporl order: slient, ttention-gring stimuli hve longer pprent durtion nd cn e seen to precede less-ttended stimuli tht were presented erlier The extended horizontl rs of this study presumly engge ttention mechnisms, especilly s sujects Proportion 'longer' Apprent durtion (ms) 1 5 Sccde durtion (ms) Clicks Blinks Proe durtion (ms) Sccde size (deg) 1 5 Figure 2 Compression of time during sccdes. () Psychometric functions showing proportion of trils where two sujects (one uthor,, nd one nive, ) judged the temporl seprtion of the proe to e longer thn tht of the test while they mde lrge (31) horizontl sccdes. The intervl etween test rs ws fixed t 1 ms, wheres tht of the proe ws vried s indicted y the sciss. Open squres refer to presenttions when the temporl center of the test pir fell etween 4 nd 2 ms efore sccdic onset, nd the open circles when it fell in the intervl of 1 to 5 ms. Filled tringles show results when stimuli were short verticl rs (6 151) ner the sccdic trget, 1 to 5 ms efore sccdic onset. All dt were fit with cumultive Gussin functions to derive the PSE (given y the men) nd the precision (given y the s.d.). () Apprent durtion of 1-ms doule-r perisccdic stimulus ( 1 to 5 ms) s function of sccdic mplitude (filled circles:, open circles: ). Upper sciss: durtion of sccdes clculted from Crpenter s 4 formul d ¼ 2.2A +2ms(d, durtion;a, sccdic mplitude), which greed well with our own mesurements. Right pnel shows results for two controls: the link condition (when stimuli fell 1 to 5 ms efore the onset of voluntry link) nd the pprent temporl seprtion of clerly udile clicks (4-ms durtion, 1-ms seprtion, 1 to 5 ms efore sccdic onset). Verticl rs: s.e.m., ootstrp method. NATURE NEUROSCIENCE VOLUME 8 [ NUMBER 7 [ JULY

3 25 Nture Pulishing Group PSE or precision (ms) Precision (ms) PSE Precision Stimulus presenttion time (ms) Fixtion Sccde Perceived durtion (ms) hd to ttend to them in order to judge their durtion or temporl order. However, s ll rs were of equl size, durtion, rightness nd eccentricity, it is unlikely tht some were more ttention-gring thn others. As sccdes cuse shifts in sptil ttention towrd the sccdic trget 13,14, it my e rgued tht the effects reported here re due to reduced sptil ttention for stimuli not t sccdic trget, ut s compression lso occurred for stimuli ner sccdic trget (Fig. 2), this seems unlikely. There is lso evidence for generl dmping of ttention t the time of sccdes 25 27, time when informtion is lest relile 28. As perceived durtion vries with ttention, this inttention could reduce the pprent durtion of stimuli presented t the time of sccdes. However, lthough inttention cn reduce perceived durtion, the reported effects re smll (out 12%) nd re ssocited with n increse in vriility in judgment 18 : our results (Fig. 3) show decrese in perceived durtion of out 5%, together with decrese in vriility (implying incresed precision) in temporl judgments. The temporl inversion is even more difficult to ccount for y ttention, s the prediction runs in the wrong direction. The inversion ws mximl when the stimulus pir ws centered 5 ms efore sccdic onset, so the second r occurred ner sccdic onset. If the system were less ttentive t sccdic onset, the second r should hve less ttentionl weight 22 nd therefore should e pushed even further Figure 4 Temporl inversion. (,) Proportion of correct judgments of temporl order for two oservers (verging top first with ottom first ) s function of stimulus presenttion time (the verge time of the pir from sccdic onset), for three levels of seprtion of stimulus pirs. (c,d) Two psychometric functions from the dt of Figure 3 (suject ) for presenttion times more thn 1 ms fter sccdic onset (c) nd for criticl perisccdic intervl 7 to 3 ms (d). Ech curve incorportes out 15 trils. (e,f) Mgnitude estimtions of perceived durtion for suset of the dt in c,d (61 points for e, 81 for f). For presenttion times 1 ms fter sccdic onset (e), the judgments were ner veridicl (slope of liner fit of ll points.93, r ¼.98, solid lck line). During the intervl 7 to 3 ms (f), mgnitudes were underestimted nd consistently reversed (non-inverted dt indicted y solid lck line: slope.47, r ¼.9; inverted dt y solid gry line: slope.8, r ¼.82). For clrity, the inverted dt points of oth grphs re shown s open squres Figure 3 Compression nd incresed precision of time during sccdes. () Point of sujective equlity (PSE) nd precision s function of stimulus presenttion time (men time of the two rs reltive to sccdic onset). The dt were clculted from psychometric functions like those of Figure 1, with 5 8 trils per point. The upper solid horizontl line nd the lower horizontl dotted line show estimtes of PSE nd precision during fixtion. Verticl rs refer to s.e.m. (ootstrp method), nd horizontl rs to the width of the verging in. () Precision of temporl judgment s function of perceived durtion, for test stimuli of 1 ms presented t vrious times reltive to sccdic onset (open tringles, from Fig. 2) nd for tests of 5, 75 nd 1 ms during fixtion (gry dimonds). In ll cses, precision is proportionl to perceived durtion, suggesting Weer reltionship. The dshed gry nd lck lines show the est liner regressions (constrined to pss through zero) for the sccdic nd fixtion dt, respectively. The slopes of these fits give n estimte of the Weer frction:.28 nd.25, respectively, for sccdic nd fixtion for, nd.4 nd.35 for ck in temporl priority, rther thn jumping the cue to invert perceived order. Although we cnnot exclude completely the role of ttention in this study, its precise role is fr from ovious t this stge. It is unlikely tht the chnges in perceived durtion result from lowlevel processing of the stimulus. The use of widely seprted equiluminnt stimuli minimized the impression of pprent motion (nd indeed, no sujects reported the senstion of motion). Sccdes chnge neither the mplitude nor the dynmics of the temporl impulse-response function for equiluminnt stimuli 29. For luminnce-modulted stimuli, the impulse response ecomes slightly fster (consistent with decrese in contrst gin), ut the difference in pek response is only out 8 ms, fr shorter thn needed to ccount for the results reported here. Proportion correct c Proportion 'top first' e Estimted seprtion (ms) D.B ms ms 76 2 ms Stimulus presenttion time (ms) d f Stimulus seprtion (ms) VOLUME 8 [ NUMBER 7 [ JULY 25 NATURE NEUROSCIENCE

4 25 Nture Pulishing Group Previous studies 3 hve shown tht sccdes nd other voluntry movements cn cuse time distortions, incresing the pprent durtion of stimuli following sccdes. This effect, termed chronostsis, is thought to compenste perceptully for the time lost during sccdic suppression. Chronostsis my e relted in some wy to the compression nd inversion effects reported here, ut the connection is not ovious: chronostsis shows strong dependency on sccdic size nd durtion, is neither tightly linked to the sccdic time course nor restricted to sccdes 31,32, nd lso occurs in the uditory domin 33. The temporl compression reported here does not occur with links or uditory clicks, follows precise time course nd is lmost independent of sccdic size. Chronostsis is thought to compenste for the time lost during sccdes, with lengthening of post-sccdic time ( cut nd stitch model), ut we find no lengthening of pprent durtion s lte s 5 ms fter the sccde; the time lost y compression during sccdes is not recovered. Perhps chronostsis is more relted to perceptul compenstions for movement intentions 34 nd ttention. The effects oserved here were specific for visul stimuli nd for sccdes, implicting timing mechnism for visul stimuli tht is modulted y sccde-relted neurl circuitry. Neurons in prietl corticl re LIP hve een implicted oth in nticiptory rempping round the time of sccdes 8 nd lso in encoding rief temporl durtions in ehviorlly relevnt loctions 11,12. Although the effect of sccdes on the temporl encoding of these neurons hs not yet een mesured, it is conceivle tht it chnges mrkedly round the time of sccdes, s these neurons shift receptive fields. Our results suggest tht the chnge is tntmount to slowdown of the neurl clock on which judgments of time depend. Not only were temporl intervls for stimuli presented ner sccdes seen s shorter, ut the precision of the temporl judgment ws higher, with the Weer frction remining constnt s function of perceived rther thn ctul durtion (Fig. 3). This is to e expected if the precision of the judgment is determined y neurl noise tht vries inversely with the numer of ticks of n internl clock 35. If the clock slows momentrily round the time of sccdes, fewer clock ticks will occur during stimulus presenttion, so the temporl mtch should e less noisy. It is worth emphsizing tht temporl judgments re one of the very few tsks tht ctully improve during sccdes, nd y significnt mount (see ref. 36). The slowing of neurl clock t the time of sccdes does not in itself explin temporl reversl. However, the reversl cn e explined within this frmework y incorporting the notion of retrospective perception 37 or postdiction 38,39, tht hs een pplied successfully to descrie temporl phenomen such s the flsh-lg effect. If ech of the two successive rs is leled independently nd then referred ckwrd in time y N clock ticks (to compenste for delys in neurl processing), the second r could e pushed ckwrd eyond the first if the clock were ticking more slowly when the second r ws presented. Sccdes, common though they re, hve effects tht cn escpe notice in norml viewing ecuse they re prtilly nullified y n nticiptory shift of sptil frme of reference. Our results mke cler tht temporl compression is found only with sccdes, not with links, nd, like sptil compression, occurs just when the predictive rempping of receptive fields tht shifts frmes of reference is in progress. Perhps the sptil nd temporl distortions tht we find for trnsient stimuli re consequence of these fst, ut not instntneous, shifts; they my e fst enough to cuse oth diltion of clock time nd compression of sptil metric. METHODS Stimuli nd procedure. Oservers fixted 11 lck disk 151 left of center of n otherwise uniformly red screen (CEI x ¼.55, y ¼.4, men luminnce 17 cd/m 2 ) of n NEC monitor running t 25 Hz nd sutending from the viewing distnce of 3 cm (Fig. 1). On wrning, the fixtion spot disppered nd lck 11 sccdic trget ppered 151 right of center, to which the suject sccded (with the individully stereotypicl ltency of out 16 ms). After rndomly vrile dely from sccdic trget presenttion, pir of horizontl green rs (ner-equiluminnt; 6 81: CEI x ¼.26, y ¼.63, men luminnce 17 cd/m 2 ) were displyed t the top nd ottom of the screen, ech for 8 ms (two frmes) with onset synchrony of 1 ms. The eccentricity, wide seprtion nd color where successfully chosen to minimize the senstion of motion. Two seconds lter comprison pir ppered, whose onset synchrony vried rndomly from 8 to 2 ms. Sujects reported verlly which stimulus pir ppered longer. The experimenter recorded the responses fter verifying tht the sccde hd een correctly executed (eye movements were monitored with n HVS SP15 limus eye trcker t 1, Hz); otherwise the tril ws orted. Dt were nlyzed y n offline progrm tht inned nd verged responses ccording to their presenttion time reltive to sccdic onset. Mgnitude estimtion. In the second experiment, the suject ws required to report which of the two rs ppered first. Presenttion order, r seprtion durtion nd disply ltency fter sccde trget were ll vried rndomly from tril to tril. For the ltter hlf of the trils, sujects were required to report oth the pprent order of the rs nd the pprent seprtion. After their response, stimulus pir with the reported seprtion nd order ws presented (during post-sccdic fixtion); sujects either confirmed their estimte or revised it. They were given extensive trining on fixtion efore the experiment nd, in prctice, rrely needed to revise their estimtes. Control experiments. Three control experiments were performed for compression. In one, the stimuli were short verticl, equiluminnt rs (6 151) strddling the sccdic trget, one ove nd one elow ut overlpping y 61. In nother control condition sujects were required to link on cue. As efore, the stimuli were presented just efore the link (mesured y the eye-trcker). Finlly, we mesured pprent temporl seprtion of uditory clicks. The procedure ws otherwise the sme s tht descried ove, except the rs were replced with pir of 4 ms clerly udile clicks. Four sujects were used in this experiment, two nive nd two uthors (ll of whom provided informed written consent). Complete dt is reported only for nive oserver, nd uthors nd D.B. ACKNOWLEDGMENTS We cknowledge relevnt previous work in our lortory y M.R. Dimond. This reserch ws funded y grnts from the Austrlin Ntionl Helth nd Medicl Reserch Council nd the Austrlin Reserch Committee nd y the Itlin Ministry of Eduction, Universities nd Reserch (Progetti di Ricerc di Interesse Nzionle). COMPETING INTERESTS STATEMENT The uthors declre tht they hve no competing finncil interests. Received 22 Ferury; ccepted 23 My 25 Pulished online t 1. Ross, J., Morrone, M.C., Golderg, M.E. & Burr, D.C. Chnges in visul perception t the time of sccdes. Trends Neurosci. 24, (21). 2. Mtin, L. in Hndook of Sensory Physiology vol. VII/4: Visul Psychophysics (eds. Jmeson, D. & Hurvich, L.M.) (Springer-Verlg, Berlin, 1972). 3. Hond, H. Perceptul locliztion of visul stimuli flshed during sccdes. Percept. Psychophys. 45, (1989). 4. Ross, J., Morrone, M.C. & Burr, D.C. Compression of visul spce efore sccdes. Nture 386, (1997). 5. Morrone, M.C., Ross, J. & Burr, D.C. Apprent position of visul trgets during rel nd simulted sccdic eye movements. J. Neurosci. 17, (1997). 6. 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