Perception at the blind spot and similarity grouping

Transcription

1 Pereption & Psyhophysis 1984, 36 (2), Pereption at the blind spot and similarity groping N. KAWABATA EletrotehnialLaboratory, Niiharign, Ibaraki, Japan Pereptal ompletion at the blind spot was investigated by sing patterns with a vertial blank stripe. The pereptal ompletion orred aross the vertial blank stripe in the test pattern, whih was presented at the blind spot when a similarity groping based on differenes in the property of elements onstitting the vertial blank stripe pattern was formed. There was no interation between similarity grops in the filling-in proess. The reslts obtained here indiate that pereptal ompletion is generated after similarity groping, and that similarity grops are independent of eah other in pereptal ompletion. The reslts seem to sggest that filling-in is losely related to a segregative proess based on stimls differenes. It is well known that a stimlation pattern that is displayed aross the blind spot is ompleted in the visal information handling proess and is pereived as a omplete pattern. Information proessing in pereptal ompletion reqires that ertain onditions be flfilled, for example, the presene of a pair of stimli faing eah other aross the blind spot (Kawabata, 1982). However, pereptal ompletion annot be desribed by a linear smmation of filling-in patterns, inferred from the loal pereptal ompletion (Kawabata, 1983). Pereptal ompletion at the blind spot is affeted by the global property ofthe stimls presented arond the blind spot: it is inhibited by the presene of a gap in the stimls pattern (Kawabata, 1983; I.e Grand, 1967). If the gap fails to inhibit ompletion, it means that ertain information proessing has been done before sh an inhibitory proess takes plae. It has been sggested that information proessing, sh as pereptal groping, takes plae before attention foses (Bek, 1972). Similarity is one of the most important of the Gestalt priniples in pereptal groping. Similarity groping is proded by the similarity of figral elements. Figral variables involved in proding a similarity groping inlde slope of elements, element density, distane between elements, sbjetive brightness, et. Therefore, when the property of the elements onstitting a part of the pattern differs from that of the elements in the other part of the pattern, similarity groping ors. Then, it is expeted that elements with a different property will be pereptally segregated from the other elements. This may affet the formation of edges and the gap in the pattern. If the ompletion is affeted by the similarity groping, it is expeted that the ompletion will or after the similarity groping ors. If generation of the ompletion depends on the The athor's mailing address is: Eletrotehnial Laboratory, 1-14 Umezono, Sakramra, Niiharign, Ibaraki 35, Japan. similarity groping, then ompletion at the blind spot will give s a measre for looking at pereptal groping. On the other hand, if ompletion is not inflened by the similarity groping, the property of the edges and gap inhibiting the ompletion wold be more general than that of the pereptal groping. In this paper, pereptal ompletion at the blind spot is stdied from the viewpoint of similarity groping, by hanging sh properties of figral omponents as orientation and density ofthe line segments. GENERALMETHOD Stimlation patterns were displayed on a graphi display (21-in. CRT with P31 phosphor). The sbjet sat in a darkened room, 5 em from a CRT display, with his or her fae fixed in a hinrest. The sbjet fixated, with right eye alone, a target presented on the CRT display. The sbjet triggered presentation of the test pattern while fixating the target. The test pattern omposed of brilliant lines approximately 2 min wide, was displayed for.5 se. The target was a spot 5. min in diameter. The lminane of the line was 8.2 d/m", and that of the bakgrond, approximately.1 d/m 2 The blind spot was mapped by sing a flashing spot. In a rn of trials, the order of test pattern presentation was randomized. Usally 3 measrements were made for one test pattern for eah sbjet. For sbjets with normal or orreted-to-normal aity partiipated in the experiments. Eah experiment/sed three sbjets. Two of the sbjets were tested in all experiments. The instrtions iii Experiment 1 were: "I will show yo a series of test patterns. Eah of the test patterns yo will see is a striped retangle traversed by a vertial blank stripe and a horizontal patterned stripe [the sbjet was shown the pattern). The task I will ask of yo is to jdge whether the horizontal patterned stripe is seen as onneted or separated. The onnetion of the horizontal patterned stripes may be blrred. If two areas of the horizontal patterned stripe are bridged by ontinos blrring, it shold be jdged that the two areas are onneted. A point, presented on the left side of the CRT, is the fixation point. Before eah trial, yo mst fixate the point with the right eye alone, and when ready yo an trigger the presentation of the test pattern by pressing the swith." In order to familiarize the sbjet with the stimli, the entire set was shown in the training session. The instrtions in Experiments 24 were essentially the same as in Experiment 1, exept 151 Copyright 1984 Psyhonorni Soiety, In.

2 -- --_._,-_._- 152 KAWABATA that the examples presented were hosen from the patterns sed in Experiments 2-4, respetively, and the sbjet was asked to jdge whether the stripes, presented at a entral horizontal zone in whih the horizontal patterned stripes had been displayed in the previos experiment, were seen as onneted or separated. The width of the vertial blank stripe in the test pattern, that is, the gap length, sed for these experiments was seleted in sh a way that the horizontal ompletion might or stably when only the horizontal zone in the test pattern was presented arond the blind spot. (AJ (8) litij]]j ==-=, T I 1 EXPERIMENT 1 Pereptal ompletion ors aross the blind spot when a stimlation pattern is presented arond it. However, a vertial blank stripe rnning throgh the stimlation pattern inhibits ompletion if the length and width of the vertial blank stripe are appropriate (Kawabata, 1983). Sh a vertial blank stripe pattern may be bonded on eah side by two retanglar areas striped either vertially or horizontally, as shown in Figre 1. If the vertial line length in this figreis shorter and nder the vertial diameter of the blind spot, pereptal ompletion ors aross the blind spot. Now, let s form another test pattern by inserting a entral horizontal zone striped in an orthogonally different orientation from that in the bilateral retangles, as shown in Figre 2, and present it so that the two stripes ross eah other at the blind spot. In these patterns, vertial edges and a vertial gap between them may be formed globally. If the vertial edges and the gap inhibit pereptal ompletion aross the blind spot horizontally, a vertial blank stripe shold be pereived over a horizontal patterned stripe. On the other hand, if a horizontal patterned stripe rossing the vertial blank stripe is visible, it means that the prolivity of the vertial edges to inhibit ompletion is not operating and the horizontal ompletion ors independently of the srronding pattern. In this experiment, a series of test patterns was presented in whih only the line spaings of the stripe in the horizontal (A) (8) -- ==-======-,. Figre 1. Examples of patterns made p of parallel lines with a vertial blank stripe. Pattern A is omposed of vertial parallel lines, and Pattern B of horizontal ones. Pereptal ompletion aross the blind spot horizontally is deteriorated when the vertial blank stripe pattern is presented arond the blind spot. A dotted line shows shematially the relative position of the blind spot when the pattern is presented. I _ Figre 2. Test patterns with the orientation of parallel lines at a entral horizontal zone orthogonal to ones in for retangles. Vertial lie and lis were 1.29 and 6.88 deg, respetively. Horizontal length lih was 6.88 deg. Width lig of the vertial blank stripe was seleted to allow a better fit to the individal blind spot. The range of lig sed in the experiments was deg. The interspae of the parallel lines was.43 deg. zone were hanged. The sbjets were asked whether or not they saw a horizontal patterned stripe rossing the vertial blank stripe. Reslts and Disssion The experimental reslts are given in Figre 3. A horizontal zone, omposed of parallel lines of different orientation from that in the srrondings, was pereived as onneted withot any relation to line spaing, althogh there were some individal differenes among sbjets. As a ontrol, the striped patterns in the horizontal zone were presented at the same eentriity as, bt otside, the blind spot. In this ondition, however, ompletion did not or. Therefore, it an be said that the experimental reslts were de to ompletion at the blind spot. That is, the pereptal ompletion orred aross the blind spot horizontally and was not deteriorated by the vertial blank stripe in the test pattern. In other words, the vertial blank stripe shown in Figre 2 did not inhibit pereptal ompletion aross it at the blind spot. This sggests that pereption of the vertial edges and gap is not properly formed and that the horizontal filling-in orred independently of the for retangles in the stimls. EXPERIMENT 2 In Experiment 1, the orientation of parallel lines in the horizontal zone of the test pattern was orthogonally different from that in the for retangles. It was thoght that a differene in orientation of the parallel lines might make the pereptal ompletion possible. Therefore, test patterns with parallel lines in the horizontal zone were omposed with the same orientations as those in the for retangles bt with different line spaings (Figre 4). If vertial edges and gap globally formed in sh a test pattern simply inhibit the pereptal ompletion aross them, the filling-in proess wold fail to make ompletion aross the vertial blank stripe. If ompletion is pereived aross the blind spot horizontally, it means either that vertial edges and gap are not properly

3 (A) :: " 6 '-' <.> on C 4-2 '-'... -, x--x- "' \I \ '" / \ " x,?-.. \ p... x-4. 'o-' '"''-..4;/ x " " " ---- A H ---)(--- KM N K ( B) PERCEPTION AT BLIND SPOT _... ;e::::::. ::::==e. '----- x, X / "''x---.x I< I 'x :: 8 1/ a. " <.> > 6 on A H : --x-- KM a. :.: NK o o Line spaing ldegl Li ne spaing ldeg) Figre 3. Pereptal ompletion aross the blind spot horizontally for the test patterns shown in Figre 2. (A) gives the reslts for the pattern with the for retangles made p of vertial parallel lines, and (B) gives those for the pattern with horizontal lines. The absissa shows the line spaing of the parallel lines, and the ordinate denotes the probability of pereption. The width of the vertial blank stripe was 5.16 deg for Sbjets A.H. and K.M.; it was 3.44 deg for Sbjet N.K. formed or that the horizontal ompletion ors independently of the vertial blank stripe. This experiment presented, in random order, a series of test patterns in whih only line spaing in the horizontal zone was varied. Exept for orientation, the line spaing in the for retangles and the sizes of the horizontal zone and the for retangles were the same as those of Figre 2. Reslts and Disssion The experimental reslts are shown in Figre 5. When the line spaing in the horizontal zone was similar to that in the for retangles, the filling-in aross the vertial blank stripe at the blind spot did not or and a vertial gap was pereived shoving aside the horizontal patterned stripe, reslting in the deterioration of the (A) Ii I1 I 1 I I1111I1111 I!I!! I I If" 'I'll Figre 4. Test patterns in whih the line spaing at the horizontal zone is different from that in the for retangles. (A) is omposed of vertial parallel lines, and (B) is omposed of horizontal parauel lines. The sizes of the horizontal zone and of the for retangles were the same as those in Figre 2. (8) horizontal pereptal ompletion. When the line spaing in the horizontal zone differed from that in the for retangles, a horizontal patterned stripe rossing the vertial blank stripe was pereived, that is, the horizontal filling-in was not deteriorated. With.43-deg line spaing in the for retangles, the line spaing of.43 ±.43 deg in the horizontal zone was regarded as idential and inhibited pereptal ompletion in the ase of patterns omposed of vertial parallel lines, althogh there were some individal differenes among sbjets. In ase of horizontally oriented patterns, the range was.43 ±.9 deg. It shold be noted that there was no signifiant differene in pereptal ompletion between vertially and horizontally oriented test patterns, exept for the tendeny of a slightly broader range of line spaing 1 the latter ase to inhibit pereptal ompletion'[na) =.29, p >.7]. It is known that similarity groping ors withot a ompliated pereptal proess. A differene in perepts at the blind spot de to the differene in line density may be based on the formation of similarity groping. That is, when a test pattern is displayed, similarity groping ors first. Then one of the following ors, depending on line spaing in the horizontal zone of the test pattern: (1) When the line spaing in the horizontal zone is similar to that in the for retangles, the vertial blank stripe is formed and then the fillingin aross it is inhibited. Ths, the horizontal pereptal ompletion is deteriorated. (2) When the line spaing in the horizontal zone is different from that in the for retangles, the horizontal zone is segregated from the

4 154 KAWABATA ( A) ( B) 1 -: a a_ 1..,o,.-...x> 8 8 /x " r-: C E.x I / - a. ' r I' ' a. \ \ 6 \ 6 \ C \ x \ \ 4 -- A H 4 \ ---- A H Vl a '" a \ --x-- - KM --x-- KM. 2 NK N K U CL CL Ratio of line spaing Ratio of line spaing Figre 5. Pereptal ompletion aross the blind spot horizontally for the test patterns shown in Figre 4. (A) gives the reslts for the pattern made p of vertial parallel lines, and (B) gives those for the pattern with horizontal lines. The absissa shows the ratio of line spaings at the horizontal zone to those in the srronding regions. When the line spaing at the horizontal zone was similar to that at the for retangles, the ompletion was deteriorated. for retangles in the information handling proess for the similarity groping. Ths, the horizontal ompletion ors aross the vertial blank stripe at the blind spot independently of the for retangles. The differene in the line spaing that was effetive enogh to ase horizontal ompletion was smaller in the pattern omposed of vertial parallel lines than in the horizontally oriented one. In the vertial pattern, remarkable sbjetive ontors appear at a bondary between the horizontal zone and the for retangles, sine, bease of the differene in the line spaing, the parallel lines in the horizontal zone and in the for retangles are not aligned. Ths, the horizontal zone is easily separated from the for retangles, and the parallel lines in the horizontal zone are groped. On the other hand, in the pattern with horizontal parallel lines, the formation of the sbjetive ontors is weaker at the bondary between the horizontal zone and the for retangles, in ontrast with the ase of the vertial parallel lines, sine there is no parallel-line termination array at the bondary. Ths, the formation ofseparated groping may be diffilt for the pattern in whih the differene in the line spaings is less distint. EXPERIMENT 3 In the previos experiments, the line spaing and the orientation of the parallel lines at the horizontal zone of the test pattern onstitted experimental variables in a series of test patterns, and the pereptal ompletion at the blind spot was investigated. Sine the line spaing in the horizontal zone was different from that in the for retangles in the test pattern, the horizontal zone and srronding of the test pattern might have been separated into different grops on the bases of both the sbjetive ontors at the bondaries and the differene in the line spaing. In order to stdy the effet of the sbjetive ontors, the test patterns sed inlded ones with parallel lines in the horizontal zone that had spaings and orientation that were the same as, bt naligned with, those in the srrondings. An example of the test patterns sed is shown in Figre 6. The instrtions to the sbjets were the same as in the previos experiments. In experiments that had sed a series of test patterns that differed only in phases in alignment, the reslts had fltated extensively inasmh as the sbjets' responses were greatly affeted by their pereptions of the pattern presented first. Therefore, the test patterns involving phase differenes were randomly mixed with test patterns sed in Experiments I and 2. In experiments sing this series of test patterns, anhor phenomena in the sbjets' responses disappeared, and the responses old be regarded as independent of eah other I111 I.!.!..,1 111, I I 'I! 11I111 iii Figre 6. Test pattern with a 18-deg phase differene between the parallel lines at the horizontal zone and those in the for retangles. The sizes of the horizontal zone and the for retangles were the same as those of Figre 2. I

5 PERCEPTION AT BLIND SPOT = '- 8 o. "' 6 o o'" o -. U ' Q A H --x-- KM - NK o.2 :4.6.8 Phase differene Figre 7. Pereptal ompletion aross the blind spot horizontally for the test patterns with phase differenes. Pereptal ompletion orred aross the blind spot for vertial blank stripe patterns with phase differenes in parallel lines. The absissa shows the deviation in phase of the parallel lines at the horizontal zone from those at the for retangles. The vale indiates that both parallel lines are in phase, and the vale.5 shows that both are ompletely ot of phase. The ordinate indiates the probability of pereption. Reslts and Disssion Experimental reslts obtained in this way are shown in Figre 7. Horizontal pereptal ompletion orred aross the vertial blank stripe at the blind spot for the patterns in whih the phase of the parallel lines in the horizontal zone was different from that in the for retangles, althogh the ompletion was not as strong as that in the previos ases. That is, the horizontal ompletion was not deteriorated by the vertial blank stripe shown in Figre 6. This indiates that the sbjetive ontors based on the phase differene in the parallel lines separate them into different grops, and then the ompletion ors independently in eah grop. This sggests that groping may depend on the line spaing differene, bt that the sbjetive ontors also playa role in pereptal groping. EXPERIMENT 4 Is pereptal ompletion at the blind spot always inhibited by the presene of a blank stripe, even when the test pattern ontains no loal differene in the property of pitre omponents? Let s onsider the pereptal filling-in proess by introding a time lag in the presentation of a part of the test pattern. The test patterns sed were ones in whih, bease of the vertial blank stripe, the horizontal ompletion did not or aross the blind spot. One of the test patterns, shown in Figre 8, with an exlded horizontal zone enlosed by broken lines was displayed first. After a random time-interval delay, the stripe at the horizontal zone was added, and the fll pattern, inlding the exlded zone, was presented for 5 mse. If the ompletion is not affeted by a time lag in the presentation, then the filling-in will not or horizontally aross the blind spot. If the time lag indes the ompletion, it means that the ontinos vertial blank stripe is not pereived. It was expeted that the pereptal ompletion might be inhibited at the end of the fll-pattern display time in the seond step, sine the ompletion does not or for the independent presentation of the fll pattern. The ompletion may also be inflened by eye movements. Therefore, the jdgment regarding the orrene of ompletion shold be made as qikly as possible after the seond presentation. The instrtions to the sbjets were: "The first presentation of for retangles will be followed after a ertain time lag by a seond one of fll pattern. Yor task is to jdge whether the horizontal stripes are seen as onneted or separated at the time they are displayed." The sizes of the horizontal zone and for retangles were the same as those of the previos patterns. The line spaing in the test pattern was.43 deg. The display time for the initial exposre, that is, the time lag, was -18 mse. Reslts and Disssion Experimental reslts obtained nder these experimental onditions are shown in Figre 9. The horizontal pereptal ompletion orred aross the vertial blank stripe at the blind spot for the time delay longer than 5 mse. For the time delay shorter than 3 mse, the ompletion did not or. That is, two kinds of pereptal ompletion orred, depending on the time delay between two presentations. For a shorter time delay, the pattern seems to be dealt with as if both parts were presented at the same time. Ths, the vertial edges and gap are formed at the blind spot, deteriorating the horizontal ompletion. For a time lag longer than 5 mse, the horizontal zone displayed later seems to be segregated as a grop rather than inorporated into the for retangles presented earlier. Then the horizontal (Ai 1 I '111 1 I I Figre 8. Test patterns sed for Experiment 4. First, for retangles, exept for a horizontal zone enlosed by a dotted line, were displayed. After a random time-interval delay, the horizontal zone was added and both parts were presented for 5 mse, The sizes of the horizontal zone and the for retangles were the same as those of the previos patterns. The dotted line is presented only for the onveniene of explanation and did not appear in the test patterns sed for the experiments. (8)

6 156 KAWABATA ( A) ( B) 1 1 x... _x"'" r- 1'-,,\ p-...-d-->t:s :... :."--..."".»:... 8 '" 1\-../ r->,/ \ / ::: 8 I j->/ '" x :7 - a. x"",x/ \ /' " " I ' 6 6 v // '" : : : : 4 '" I A H '" 4 - A H I I //...x...,,p - '" -- : I --x-- : KM --x-- KM - - N K NK U... a. CL Differene in presentation (m se) Differene in presentation (rn se) Figre 9. Pereptal ompletion aross the blind spot horizontally for the test patterns shown in Figre 8. (A) and (8) give the reslts for test patterns A and 8 of Figre 8, respetively. The absissa indiates the time lag in presentation of the horizontal zone after the for retangles were displayed; the ordinate indiates the probability of pereptal ompletion. zone is dealt with independently of the for retangles in the filling-in proess, and ths the horizontal pereptal ompletion ors. After the experiment, when asked abot the riteria sed in making their deisions, the sbjets reported that they had responded on the basis of sessiveness. EXPERIMENT 5 The ompletion aross the vertial blank stripe orred depending on whether or not a similarity groping was formed on the basis of the omponent property differene. To onfirm this point of view additionally, the rate at whih the horizontal patterned stripe was distingished pereptally from the lines above and below it was measred by varying the horizontalstripe line spaings. If the rate of the orrene of horizontal ompletion was similar to that of the pereptal disrimination, it wold mean that the orrene of pereptal ompletion an be predited from the rating of the pereptal disrimination. On the other hand, the rating of the orrene of ompletion provides a means for looking at pereptal groping. The test patterns sed for the present experiment were the same as those sed in the previos experiments. The instrtions to the sbjets were the same as those in the previos experiments, exept that they were asked to jdge whether or not the stripe pattern in the horizontal zone was different from that in the srrondings. Reslts and Disssion For the test patterns shown in Figre 2, the stripe pattern in the horizontal zone old be distingished learly from that in the srrondings. Experimental reslts with the test patterns shown in Figre 4 are illstrated in Figre 1. The reslts obtained from the disrimination experiment are in agreement with those of the pereptal ompletion experiment as shown in Figre 5. Experimental reslts with the test patterns involving phase differene, as shown in Figre 6, are given in Figre, As an be seen in Figres 7 and 11, the reslts of the two experiments indiate a similar tendeny, althogh the high and low ratings are more learly separated in the latter than in the former experiment. Groping in the horizontal zone is weaker in the patterns involving a phase differene in the horizontal zone than in the patterns shown in Figre 4, sine there is no differene in the line spaings between the horizontal zone and the srrondings. Therefore, the jdgment in the disrimination experiment might be made based only on phase differene rather than groping. On the other hand, the orrene of ompletion depends on the groping. Hene, the differene in ratings between them may be de to variation in jdgment. The reslts obtained here, however, indiate that the orrene of ompletion an be predited from the rating in the disrimination; on the other hand, pereptal groping an be investigated from the orrene of ompletion. GENERAL DISCUSSION Pereptal ompletion does not or aross edges or a gap between them when a stimlation pattern that inldes a blank stripe is displayed arond the blind spot. The experimental reslts obtained here, however, indiate that pereptal ompletion does or aross

7 PERCEPTION AT BLIND SPOT 157 (A) 1 (B) :*;:, V--'il x 8 8,. 6 E, V) V) " 4, AM " ' - E AM ' --x-- - RS ' - --x-- RS 2 NK 2 NK ::: ::: o o Ratio of line spaing Raf io of line spaing 1.8 Figre 1. Rating of disrimination of the stripe at the horizontal zone from lines above and below in the test pattern shown in Figre 4. (A) gives the reslts for the pattern made p of vertial parallel lines; (B) gives those for the pattern with horizontal lines. a vertial blank stripe when the property of the pitre omponents bordering the blind spot differs from that of the remaining parts. In the experimental reslts, there were some deviations among sbjets. These may be de to differenes in jdgment. In the jdgment of ompletion, a blrred pattern rossing the vertial blank 1, 8 -, 6 I E..... '" " '- AH ' --x-- RS C' ' 2 - NK Cl:: o.2 Phose.4.6 differene.8 Figre 11. Rating of disrimination of the stripe at the horizontal zone from lines above and below in the test pattern shown in Figre 6. The absissa shows the deviation of the parallel lines at the horizontal zone from the ones at the for retangles. The vale indiates that both parallel lines are in phase, and the vale.5 indiates that both are ompletely ot of phase. stripe was allowed to be regarded as a onneted pattern. Hene, it is possible that jdgments may differ from individal to individal, depending on the degree of blrring. However, sh deviations wold not affet the signifiane of the reslts, sine the range of variation in the ompletion was abot 8%-1%. Now, variations in fixation de to eye movements may be expeted to affet experimental reslts. The effet of variations in fixation on the ompletion is of a sppressive natre, even if the eye moves dring presentation of the stimls. Therefore, the horizontal ompletion is not signifiantly affeted by the eye movements, if any. In Experiment 2, when line spaings in the horizontal zone and for retangles were sffiiently different, the horizontal ompletion orred. Ths, variations in fixation based on eye movements do not affet the ompletion. When line spaings in two areas are similar, the horizontal ompletion do not or. This may be de to variations in fixation dring the experiments. As reported by Kawabata (1983), however, this is not de to eye movements. In that previos experiment, eye movements were heked by sing a test pattern in whih a portion to be displayed at the blind spot was deleted. That is, if the eye moves while the pattern is being presented, the stimls pattern will not be pereptally ompleted and onavities at the edges of the test pattern will be pereived. In those experiments in whih line spaings in the horizontal zone were idential to the for retangles, two vertial edges were observed in the filled-in pattern. Therefore, it an be said that inhibitory effets on the ompletion reported here are not de to eye movements either. However, it is possible that eye movements might have depressed some estimates of the

8 158 KAWABATA ompletion perentage. In a series of experiments in whih the alignment of parallel lines and the onset of the presentation at the horizontal zone deviate from those of the for retangles, the irmstanes are the same. Therefore, it an be said that the pereptal ompletion reported here was not signifiantly affeted by eye movements even if they orred. In the test patterns sed here, the blank ompletion was in the vertial diretion and the pattern ompletion was in the horizontal one. This might have affeted the reslts, bease the blind area is approximately oval and its vertial diameter is somewhat longer than the horizontal diameter. However, the vertial and horizontal lengths of the test patterns were sffiiently longer than those of the blind area to derease the effet de to a differene between the vertial and horizontal sizes of the blind area. Therefore, the ompletion wold not be signifiantly affeted by them. Now, the pereptal ompletion orred aross the vertial blank stripe when orientation or line spaing of parallel lines in the horizontal zone was hanged or when a temporal differene in the presentation of a part of the stimls pattern was introded. These reslts sggest the existene of a proess that detets differenes in orientation and in line spaing of parallel lines before the fjlling-in proess takes plae. The ompletion ors aross the vertial blank stripe if sh differenes are deteted in the pattern. It has been known that similarity groping is formed on the basis of the similarity of orientation and density of omponents, sh as line segments onstitting a figre (Olson & Attneave, 197). Sh similarity groping is thoght to be formed on the basis of the response of the visal system to stimls differenes before attention foses (Bek, 1972). In the test patterns sed here, no dobt, information for the formation of similarity groping wold be proessed in a manner that depended on the properties of the figral omponents. Ifsimilarity groping is formed after the filling-in proess, the pereptal ompletion mst or withot any relation to the similarity groping. The experimental reslts, however, indiate that the ompletion is greatly inflened by mixing omponents of different orientation or density in a part of the stimls pattern. This sggests that the ompletion is remarkably affeted by the similarity groping. In other words, when a stimlation pattern is presented, the information for similarity groping is proessed on the basis of the similarity in omponents of the pattern, and sbseqently the information for the pereptal ompletion is proessed. Now, the reslts also show that interation between similarity grops, segregated on the basis of differenes in the property of omponents, is absent or very weak in the filling-in proess. That is, the pereptal ompletion depends on whether or not eah grop satisfies some onditions. This may sggest that both the vertial blank stripe and the horizontal patterned stripe are filled in aross the blind spot when the horizontal zone with different omponents is segregated on the basis of similarity from the stimls pattern ontaining the vertial blank stripe. The reslts of Experiment 3 indiate that the sbjetive ontors, based on the phase differene in alignment, also playa role in similarity groping, althogh it is not so powerfl. This spports the viewpoint that in the patterns shown in Figre 4, similarity groping is formed both on the basis of a differene in line spaings between the horizontal zone and the for retangles and on the basis of the sbjetive ontors at the bondaries between them. Ths, the sharp tning in the deterioration of ompletion observed in Figre SA may also be explained by the differenes in both line spaings and sbjetive ontors. In Experiment 4, the reslts show that the pereptal ompletion orred aross the vertial blank stripe if a part of the pattern was displayed with an appropriate delay. This sggests that similarity groping is formed throgh temporal differenes in the presentation of the test pattern. Now, similarity groping is a segregative proess based on the response of the visal system to stimls differenes before attention foses. The experimental reslts obtained here indiate that information proessing for pereptal ompletion at the blind spot is done after sh a segregative proess takes plae and is proessed independently in eah similarity grop. This seems to sggest that the fjlling-in is losely related to the segregative proess based on stimls differenes. It is pointed ot that the segregative proess may be a level of orientation analysis (Jlesz, 1981). Ths, the present stdy sggests that the ompletion proess mst be at a level higher than the level of orientation analysis, sine the pereptal ompletion ors after similarity groping. Cmming and Friend (198) sggest that, on the basis of the magnitde of a tilt aftereffet, the fillingin proess mst be aomplished at a higher level in the visal system. The reslts here are in agreement with the reslts obtained from the tilt aftereffet. REFERENCES BECK, J. (1972). Similarity groping and peripheral disriminability nder nertainty. Amerian Jornal ofpsyhology, 85, CUMMING, G., & FRIEND, H. (198). Pereption at the blind spot and tilt aftereffet. Pereption, 9, JULESZ, B. (1981). Textons, the elements of textre pereption, and their interation. Natre, 29, KAWABATA, N. (1982). Visal information proessing at the blind spot. Pereptal and Motor Skills, 55, KAWABATA, N. (1983). Global interations in pereptal ompletion at the blind spot. Vision Researh, 23, LE GRAND, Y. (1967). Form and spae vision. Bloomington: Indiana University Press. OLSON, R. K., & ATTNEAVE, F. (197). What variables prode similarity groping? Amerian Jornal ofpsyhology, 83, (Mansript reeived Febrary 17, 1984; revision aepted for pbliation Jne 14, 1984.)