The effect of orientation in binocular contour rivalry of real images and afterimages*

Transcription

1 Perceptin & Psychphysics 1974, Vl. 15, N.2, The effect f rientatin in bincular cntur rivalry f real images and afterimages* N.J. WADE University fdundee, Dundee DDI 4HN, Sctland Bincular rivalry was investigated using gratings f different rientatins in three experiments. N cnsistent effects f rientatin were fund fr predminance measures f rivalry between real images. Rivalrus afterimages, n the ther hand, did exhibit rientatin selectivity: vertical gratings were visible fr lnger than were 45-deg gratings. This effect was cmpared t the similar rientatin selectivity fund fr mncular bservatin f grating afterimages. Cmparisns f bincular rivalry between real images and afterimages were made in terms f the frequency distributins f the dminance perids. Cnturs f different rientatins presented t crrespnding areas f each eye underg phases f apparent dminance and suppressin. This is referred t as bincular rivalry. Fr example, if a vertical line is presented t ne eye and a hrizntal line t the ther, rivalry will take place at their intersectin such that alternately ne line and then the ther will appear t be unbrken. Wilen a grating rather than a single line is presented t each eye, dminance is indexed by the visibility f the cnturs f ne rientatin alne. In this sense, gratings are mre suitable stimuli fr the investigatin f cntur rivalry, as dminance is readily discriminable; with single lines, a wide variety f fragmentatins can ccur, and dminance may be difficult t determine (see Cgan & Gldstein, 1972). An attendant feature f rivalry between gratings is that cmpsites, in which different rientatins dminate in different parts f the field, are als visible fr sme f the time. The measures mst cmmnly emplyed in rivalry experiments are the rate falternatin and the verall r percentage duratin fr which ne stimulus is dminant (predminance). The predminance f a given stimulus can be influenced by features such as the amunt f cntur, the cntrast at the cntur, and its clarity (see Levelt, 1968). The purpse fthe present experiments was t determine whether r nt cntur rientatin culd influence predminance in bincular rivalry. The first experiment examined rivalry between a vertical (0 deg) grating and a variety finclined nes; the gratings were bserved as real images during cntinuus sterescpic viewing. Tw further experiments cmpared rivalry between 0- and 45-deg gratings, viewed as prlnged afterimages and als as real images. Bincular afterimages, being perfectly stabilized, are nt influenced by eye mvements during bservatin. *This research was supprted by a grant frm the Science Research Cuncil. I wish t thank Kirsteen Greig fr her assistance in the cllectin and analysis f the data. I am grateful t J. Crmack fr cnducting the ptical examinatins f the Ss in Experiment III. Mrever, mncular afterimages f gratings are affected by rientatin (Wade, 1973), and the experiments sught t determine whether similar effects ccurred during bincular rivalry. EXPERIMENT I Almst all previus experiments n bincular cntur rivalry have used lines that were rthgnal. In this experiment, a range f rientatins was emplyed. One eye was presented with a vertical grating and the ther with lin inclined ne, the rientatin f which culd be varied between 15 and 90 deg t the vertical. The gratings were in the frm f circles s that they culd be cmpletely superimpsed fr all the rientatins used. The verall duratins fr which each fthe gratings in a given rivalry cnfiguratin were visible in their entirety (i.e., their predminance) were measured. Subjects Methd Twenty Ss frm an intrductry curse in psychlgy participated in the experiment. Nne f the Ss were practiced in bserving bincular rivalry. All had nrmal r crrected visin. Apparatus The stimulus cnfiguratins cnsisted f square-wave gratings f 8.9 cycles/deg spatial frequency and a cntrast f 0.7. They were in the frm f circles subtending IY deg at the eye. The gratings were viewed in a sterescpe, and the cards bearing the cnfiguratins culd be adjusted t achieve superimpsitin. Tw micrswitches, perated by S's right and left hands, were cnnected t timers which recrded the cumulative duratin f switch depressins. Prcedure All rivalry cnfiguratins cnsisted f a vertical grating presented t ne eye and an inclined ne t the ther. The inclined gratings were riented at 15,,45, 60, 75, r 90 deg clckwise f the vertical. Each member f the six pairs f rivalrus stimuli was presented t each eye t balance any effects f eye dminance. The task f S was t depress ne switch when the vertical grating alne was visible and the ther switch when nly the inclined grating culd be seen. The switches were 227

2 228 WADE Mean SD Table 1 Mean Predminance Duratins (Secnds) fr Vertical and Inclined Gratings Viewed fr 100 Sec Under Cnditins f Bincular Rivalry, Tgether With the Standard Deviatins Grating Orientatin (Degrees) depressed fr the duratin f visibility f these categries, and neither switch was depressed when cmpsites were apparent. A practice trial was given t ensure that the instructins had been understd. There were 12 trials each f 100 sec duratin, with an interval between them f 60 sec, during which S's eyes remained clsed. The rder f presentatin f the rivalrus pairs was randmized. Results and Discussin The data fr a given pair f rivalrus stimuli were averaged ver eyes t balance any effects due t eye dminance. The mean verall duratins fr which each member f a rivalrus pair was visible (predminance) are shwn in Table I, tgether with the standard deviatins. Fr all cnfiguratins, the duratin fr which either grating was predminant was apprximately 60 sec f the 100-sec inspectin perid. The remaining time wuld have cmprised cmpsites. The mean duratins are very similar fr rivalry between O-deg gratings and thse frm t 75 deg. The O-deg grating was visible fr slightly lnger than the 90-deg and much shrter than the 15-deg grating. Six t tests were cmputed fr the differences between the mean visibility duratins f the vertical and inclined gratings fr each rivalry cnfiguratin. The differences failed t reach statistical significance in every case (p >.05), indicating that a vertical grating is visible fr abut the same duratin as an inclined ne in rivalry with it, irrespective f its rientatin. Cntur rientatin des nt seem t influence predminance in bincular rivalry. The lnger visibility f the l Sdeg grating relative t the vertical ne may have been due t sme degree f central fusin in additin t slight cyclfusinal rtatin f the eyes (see Kertesz & Jnes, 1970). The fused percept wuld have appeared midway between the tw rientatins, i.e., nnvertical, and this culd have inflated the 15-deg predminance measure. This effect was nt prduced by all Ss, and the individual variatins were large. The nly cnditin favring the vertical cnturs invlved rivalry with a hrizntal grating. Such a difference might be expected frm the pattern f nncnjugate eye mvements that ccur during rivalry, since these are mainly in a hrizntal directin (Kaufman, 1963). That is, a small hrizntal eye mvement wuld displace the cnturs f a vertical grating nt new retinal areas, whereas a hrizntal grating wuld be displaced such that nly the ends f the cntur wuld prject nt new retinal areas. Hwever, the vertical grating was nt predminant fr significantly lnger than the hrizntal in this experiment. EXPERIMENT II The effects f cntur displacements ver the eyes can be eliminated by bincular stabilizatin f the rivalrus stimuli. This was dne in the present experiment by means f prlnged afterimages, and the pattern f rivalry was cmpared with that fr equivalent real image stimuli. Any differences that emerge frm these patterns might be related t the effects f eye mvements in bincular rivalry. Prlnged afterimages fgratings viewed mncularly exhibit rientatin selectivity: vertical afterimages remain visible in their entirety fr lnger than d thse at 45 deg (Wade, 1973). The same rientatins were chsen fr study in this experiment in rder t determine whether r nt such rientatin selectivity als btained in bincular rivalry. The tempral characteristics f rivalrus O and 45-deg gratings viewed as afterimages and real images were examined. The predminances and frequencies f visibility f each grating were measured under bth cnditins. In additin, the visibility f cmpsites was recrded in the same manner as was dminance. Subjects Methd Twenty-fur Ss, graduate and undergraduate members f the University f Dundee, tk part in the experiment. All had nrmal r crrected visin. Apparatus The gratings were viewed thrugh a mdified prism sterescpe in frnt f which was a chinrest and frehead supprt fr cntrlling S's head psitin. The gratings we~e lcated 28.5 in. frm S's eyes and were frmed frm apertures In black masking cards. The apertures crrespnded t square-wave gratings f 2 cycles/deg spatial frequency; they were in the frm f a circle with an angular substense f 3 deg. The gratings were either 0 r 45 deg, and they culd be lcated n crrespnding retinal areas by means f adjustable biprisms, Three switches were psitined in frnt f S; these activa ted timers and separate channels f a Rustrak event recrder. Afterimages were generated by the discharge f a Multiblitz Reprt flash gun, with an utput f 120 J in 2 rnsec. The real images were prduced by transilluminatin thrugh the apertures, with a luminance f apprximately 20 fl. Since the

3 EFFECT OF ORIENTATION IN BINOCULARCONTOUR RIVALRY 229 Table 2 Mean Predminance Duratins (SecAda) and Frequencies f VisllJility fr the 6- and 4S-Deg Gratings and fr Cmpsites Under Cnditinsf Rivalry Between Ileal Images and Afterimages, Tgether With the Standard Deviatins Visibility Categry Duratin Frequency Mean SD Mean SD Real Images Afterimages Deg Cmpsite 45 Deg ODeg Cmpsite 45 Deg gratings were in the frm f a circle and the apertures prvided the nly surce f light in the therwise dark rm, superimpsitin f the real images was readily maintained withut additinal aids t bincular alignment. Prcedure A brief descriptin and demnstratin f bincular rivalry was given t S, fllwed by the instructins regarding switch depressins. The task invlved depressing the right switch fr the visibility f vertical lines alne, the left ne fr the 45-deg lines, and the center switch fr the cmpsites. Fur trials with the 0 and 45-deg rivalrus gratings were given, tw with afterimages and tw with real images, in rder t balance the rientatins ver eyes. The rder f presentatin f the trials was cunterbalanced ver Ss, with the cnstraint that the tw afterimage and the tw real image rivalry cnditins be successive. In the afterimage cnditin, S was given a practice trial and then dark-adapted fr 10 min befre the first afterimage trial was started. Fllwing cmpletin f the IIISt trial, a further 5-min dark adaptatin was given befre the secnd afterimages were generated. A dim light illuminated the stimuli prir t the flash t ensure that the gratings were superimpsed. The afterimages were bserved with the eyes clsed and in darkness. Fr rivalry between real images, S aligned the gratings befre cmmencement f a trial, and they remained in that psitin thrughut the trial. The inspectin perid was 100 sec fr all cnditins. Results and Discussin The predminance duratins and frequencies were averaged ver eyes t reduce any eye dminance effects. The means and standard deviatins fr all measures are given in Table 2. The ttal duratin f visibility f any categry was abut 93 sec fr real images and 87 sec fr afterimages. In the case f the frmer, it is pssible that the latencies invlved in s many respnses (48 n average) cntributed t the 7-sec difference between the visibility and the bservatin perids. Fr afterimages, there were fewer respnses (27 n average), and s this is unlikely t have been the main factr. Hwever, bth afterimages were nted t disappear ccasinally during the inspectin perid, and a similar effect has been nted in rivalry between ptically stabilized patterns (Ditchburn & Pritchard, 1960); furthermre, the afterimages had faded cmpletely fr sme Ss befre terminatin f the 100-sec inspectin perid. These factrs render a direct cmparisn between rivalry under the tw cnditins difficult. The striking differences are that rivalry is much slwer with afterimages: the average perid f dminance (predminance duratin/frequency) is in the rder f 4 sec fr afterimages and abut 2 sec fr real images. Breese (1899) nted a similar effect fr rivalry between much weaker afterimages than thse generated here. Mrever, cmpsites are visible fr cnsiderably shrter verall duratins with afterimages than with real images. Grating rientatin appeared t have a differential effect n rivalry under the tw cnditins: the 45-deg grating was visible fr slightly lnger with real images, whereas the O-deg grating was predminant fr lnger with rivalry between afterimages. The data fr each rivalry cnditin were analyzed separately. Fr real image rivalry, neither the duratin nr the frequency measures f the three visibility categries differed significantly (p >.05). In the afterimage cnditin, the duratins f visibility differed significantly [F(2,42) = 69.27, p <.001]. Pst hc cmparisns using the methd devised by Rdger (1965) indicated that the difference between the 0- and 45-deg means was significant (p <.05), and als that the cmpsite duratin differed Significantly frm the average f the 0 and 45-deg duratins (p <.001). The frequency measure yielded verall significance [F{2,42) = 18.06, P <.001), and this was due mainly t the significant cmparisn between the cmpsite frequency and that fr the average f the 0- and 45-deg frequencies (p <.001). The results with real images supprt thse f Experiment I in yielding n effect f rientatin n predminance in bincular rivalry. Mrever, the duratin values crrespnd quite clsely with thse f the previus experiment; in bth, predminance f either rientatin accunted fr abnt 60% f the inspectin perid. Rivalrus afterimages, n the ther hand, exhibit rientatin selectivity-the O-deg grating predminates fr significantly lnger than the 45-deg grating. This effect crrespnds t the selectivity fr mncularly viewedgrating afterimages (Wade, 1973). They als yield very shrt duratins f cmpsites relative t the predminance f either rivalry cnfiguratin alne. EXPERIMENT III This experiment was cncerned with the same general prblem as the previus ne, but a smaller number f Ss was tested intensively. In additin t examining cntur rivalry between gratings viewed as real images and

4 2 WADE Table 3 Mean Predminance Duratins (Secnds) and Frequencies f Visibility f 0- and 4S-Deg Gratings and Cmpsites fr Rivalry f Real Images and Afterimages* Real Images Rivalry Afterimages Deg Cmp 45 Deg Deg Cmp 45 Deg Duratin M Frequency S S M Whle Mncular Afterimages Partial Deg 45 Deg Deg 45 Deg Duratin M Frequency S M "The whle and partial duratins and frequencies fr mncular afterimage visibility are als presented. afterimages, the mncular visibilities f afterimages were als studied. By presenting each rientatin singly t each eye, it is pssible t determine whether r nt the degree f rientatin selectivity evident fr mncular afterimages is equivalent t that prduced in rivalry. Subjects Methd Fur Ss, tw male and tw female, were tested in the experiment. They were well practiced in bserving bincular rivalry. Tw Ss were emmetrpic in ne eye and had slight uncrrected mypia in the ther; ne was emmetrpic in bth eyes, and the furth had crrected mypia in ne eye while the ther was emmetrpic. Nne f the Ss had any evidence f astigmatism. Apparatus The apparatus was the same as that used in Experiment II. The gratings had the same verall angular subtense (3 deg), but the spatial frequency was 4 cycles/deg. The rientatins f the gratings were 0 and 45 deg. In the mncular cnditins, ne field f the sterescpe was blanked ut. The luminance f the gratings fr the real image rivalry cnditin was again 20 fl. Prcedure Each 5 was given eight sessins fr afterimages and eight fr rivalry between real images. The afterimage sessins cnsisted f tw bincular rivalry trials and fur mncular trials in which each rientatin was presented t each eye. The rivalry trials fllwed the same prcedure as thse in Experiment II, except that the bservatin perid was 120 sec. The predminance duratins and frequencies fr each grating were recrded as well as thse fr cmpsites. Fr the mncular trials, S was instructed t depress ne switch when the afterimage was visible in its entirety and anther switch when it was nly partially visible thrughut the 120-sec inspectin perid. The rder f presentatin f the six trials was balanced s that the bincular pairs were either first r last and the eye used in the mncular cnditins was alternated. A IO-min dark-adaptatin perid was given befre the first trial, and the intertrial interval was 5 min. Fr rivalry between real images, S was presented with tw trials nly per sessin, and respnded in terms f the visibility f either grating r a cmpsite. The inspectin perid was als 120 sec. Results and Discussin The data fr each grating rientatin were averaged ver eyes, and the results fr each S are given in Table 3, tgether with the means. During bincular rivalry, the ttal visibility duratin f either grating r a cmpsite was abut 117 sec fr real images and 111 sec fr afterimages. In bth cnditins, these visibility duratins ccupied a larger prprtin f the inspectin perid than thse in Experiment II. The mncular afterimages were visible in either whle r partial frm fr abut 110 sec fr the, O-deg and 104 sec fr the 45-deg gratings. The average dminance perids fr rivalrus afterimages were abut 4 sec, which is in gd agreement with Experiment II. In fact, in bth experiments, the dminance perids fr the O-deg afterimages were lnger than thse fr the 45-deg afterimages. The dminance perids f the gratings in real image rivalry were abut 1 sec. The results fr rivalry between real images crrespnd clsely with thse f the previus tw experiments: predminance duratin was nt significantly influenced by grating rientatin (p>.05). The values, when cnverted t percentages t equate the inspectin perids, are very similar t thse f the previus experiment. The frequencies f visibility were nt affected by rientatin (p >.05), althugh these values are much higher than in Experiment II. This was due, mst prbably, t. the difference in the spatial frequencies f the gratings used in the experiments. That is, small eye mvements during bservatin wuld be likely t crss mre cntur bundaries with the higher than with the lwer spatial frequency gratings. It shuld be nted that all Ss reprted sme difficulty in fllwing the rapid pattern f changes in the present experiment.

5 EFFECT OF ORIENTATION IN BINOCULAR CONTOUR RIVALRY 231 The results fr rivalry between afterimages prvide general supprt fr thse f Experiment II, althugh the variatin between the Ss was quite large. The mean predminance f the O-deg grating differed significantly frm that fr the 45-deg visibility [t(3) = 3.19, P <.05]. The rientatin selectivity was abut 10% cmpared t 6% in Experiment II. The duratins f visibility f cmpsites were even mre variable, and did nt differ significantly frm the average visibility f the tw gratings (p >.05). The differences between the frequencies f visibility were nt significant (p >.05). The mncular afterimages exhibited rientatin selectivity: the O-deg grating was visible in whle frm fr significantly lnger than the 45-deg grating [t(3) = 16.68, P <.001]. Cntrariwise, the partial visibility f the 45-deg grating was significantly lnger than that fr the Odeg grating [t(3) = 5.75, P <.05]. Neither f these effects was accmpanied by frequency differences (p >.05). These results are similar t thse reprted previusly (Wade, 1973). Therefre, rientatin selectivity has been shwn t perate bth fr mncular grating afterimages and in their rivalrus cmbinatin. Mrever, the relatinships between the predminance f the 0- and 45 deg gratings in rivalry and in mncular bservatin are very similar. If the ratis f the 0- t the 45-deg duratins are calculated fr each cnditin, the average values fr bth are abut 1.3. This indicates that a similar degree f rientatin selectivity is perating fr mncular bservatin and in bincular rivalry. GENERAL DISCUSSION Bincular cntur rivalry between gratings viewed as real images is nt influenced by rientatin. A vertical grating remains visible fr a similar perid t an inclined ne. Rivalry between afterimages, n the ther hand, exhibits rientatin selectivity favring the vertical rientatin. That is, a vertical grating afterimage predminates fr significantly lnger than an inclined ne. The e...tent f this selectivity crrespnds t that fr the mncular visibility f similar afterimages (Experiment III). The difference in the effect f rientatin n rivalry between real images and between afterimages might be related t the ccurrence f small eye mvements in the frmer, but it is by n means clear hw this masking wuld have been prduced. Nncnjugate eye mvements have a greater hrizntal than vertical amplitude (Kaufman, 1963), and such wuld appear t favr the visibility f vertical cnturs during nrmal sterescpic bservatin. The shrter average dminance perids fr rivalry between real images than between afterimages can be mre readily interpreted, due t the mvement f the images ver the eyes in the frmer. The mean dminance perids (predminance/frequency) were abut 4 sec in each experiment using afterimages, but between 1 and 2 sec fr real images. These values crrespnd t thse LEFT EYE RIGHT EYE REAL IMAGES 45' 100r r6 0 AFTERIMAGES sr...iii. r~:' SOf...II.L r.~ ~~ s ~ lot (f r.6 r 6 "l.ill.. r.rf SOf ~ 0~50~ DURATION (SET TO COMMON BASE) Fig. I. Frequency distributins f dminance perids fr Experiment II under au rivalry cnditins. The data fr each S were divided by their wn mean duratin s that the verau mean fr all distributins is btained by Blake, Fx, and McIntyre (1971) and Fx and Herrmann (1967) fr ring-disk rivalry invlving afterimages and real images, respectively. In rder t investigate the stchastic prperties f rivalry, they examined the dminance perids f their alternatin data by means f a runs test, autcrrelatin, and the frequency distributin. The stimuli they used exhibited virtually n cmpsites. The ccurrence and measurement f cmpsites in the present investigatin precludes the use f the first tw tests, but it is pssible t plt the frequency distributins f the dminance perids in the varius cnditins. Fllwing the prcedure emplyed by bth Levelt (1968) and Fx and Herrmann (1967), the individual dminance perids were measured fr each S, and these were then divided by the mean duratin fr that S, resulting in an verall mean value f This prcedure was fllwed fr each cnditin in Experiments II and III, and the resulting frequency distributins are shwn in Figs. 1 and 2. The usefulness f fitting a specific functin t these distributins can be questined. Nnetheless, it is

6 232 WADE ~Z LEFT \.0 EYE z REAL 0" 400 ' 4 '0 ZO ~. «lo ' 4 ~O '0 20 u.. 0,~. ' 2 '~~ '0 20 IMAGES RIGHT EYE,r...: 5 "2 AFTERIMAGES.. '0 20 r!,.3 '~~, 0 20 DURATION (ssr TO COMMON BAse) Fig. 2. Frequency distributins f dminance perids fr au rivalry cnditins in Experiment III. The frequency scale fr the afterimage cnditins has been dubled relative t that fr real images. finterest t nte that all the distributins are psitively skewed and cnfrm t the general shape f the gamma functin, which functin als apprximated the distributins f Levelt's (1968) and Fx's (Blake et al, 1971; Fx & Herrmann, 1967) data. Furthermre, the r parameter f the gamma distributins fr rivalry between real images are in clse crrespndence with thse reprted previusly. There are, hwever, sme revealing differences in the r values fr rivalry between afterimages and real images: r is smaller fr the afterimage than fr the real image cnditins. In Experiment II, the average r fr the real image distributins was 6 while that fr the afterimages was 5. In 'Experiment III, where mre extensive data were derived frm a smaller number f Ss s that the estimates f r were mre precise, the crrespnding values were 4 and 2. Since the r parameter can be interpreted as an index f the number f cmpnent events cmprising a respnse (Restle, 1961), then clearly mre such cmpnents wuld be invlved in rivalry between real images than between afterimages. These wuld mst prbably include the eye mvements that ccur under nrmal sterescpic bservatin. The influence that incrpratin f the cmpsite categry has had n the abve pints remains unknwn, but this issue might be reslved by examining rivalry between shrt single lines viewed either as real images r as afterimages. The rientatin selectivity that has been demnstrated fr mncular afterimages and rivalrus afterimages crrespnds t a wide variety f similar perceptual effects (see Appelle, 1972). The ccurrence f such selectivity in bincular rivalry further supprts the crtical lcalizatin f these effects. The absence f any effects f rientatin during rivalry between real images might have been due t the pattern f small eye mvements during bservatin. These might als have been invlved in the rapid changes in dminance fr real images, and in the high prprtin f cmpsite visibility. Afterimages alternated mre slwly and cmpsites cmprised a small prprtin f the ttal visibility duratin. REFERENCES Appelle, S. Perceptin and discriminatin as a functin f stimulus rientatin. Psychlgical Bulletin, 1972, 78, Blake, R. R., Fx, R., & Mcintyre, C. Stchastic prperties f stabilized-image bincular rivalry alternatins. Jurnal f Experimental Psychlgy, 1971,88, Breese, B. B. On inhibitin. Psychlgical Mngraphs, 1899,3, N.1. Cgan, R., & Gldstein, A. G. Reprting f fragmentatins in the bincular rivalry f cnturs. American Jurnal f Psychlgy, 1972,85, Ditchburn, R. W., & Pritchard, R. M. Bincular visin with tw stabilized retinal images. Quarterly Jurnal f Experimental Psychlgy, 1960, 12, Fx, R., & Herrmann, J. Stchastic prperties f bincular rivalry alternatins. Perceptin & Psychphysics, 1967, 2, Kaufman, L. On the spread f suppressin and bincular rivalry. Visin Research, 1963,3, Kertesz, A. E., & Jnes, R. W. Human cyclfusinal respnse. Visin Research, 1970, 10, Levelt, W. J. M. On bincularrivalry. The Hague: Mutn, Restle, F. Psychlgy f judgment and chice: A theretical essay. New Yrk: Wiley, Rdger, R. S. Intermediate statistics. Sydney: University Cperative Bkshp, Wade, N. J. Orientatin and spatial frequency effects n linear afterimages. Perceptin & Psychphysics, 1973, 13, (Received fr publicatin March 7, 1973; revisin received August 27, 1973.)