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Visual Mislocalization in Moving Background and Saccadic Eye Movement Conditions
VISUAL MISLOCALIZATION IN M O V I N G B A C K G R O U N D A N D SACCADIC EYE M O V E M E N T CONDITIONS Hitoshi Honda Department of Psychology, Faculty of Humanities, Niigata University, Niigata 950-21, Japan
Abstract There is a possibility that visual mislocalization of targets flashed at the time of a saccadic eye movement is due almost entirely to the shift of the retinal image of the background. To clarify this matter, errors in target localization were analyzed in both saccadic eye movement and moving background conditions. In the latter condition, subject kept fixating and the visual background made a saccadic movement. In both conditions, a horizontal luminous scale was used as the background and the subject reported the position on the scale that the target appeared to occupy. Large localization errors were shown in both conditions. However, the pattern of error for the moving background condition was distinctively different from that for the saccadic eye movement condition, suggesting that the shift of the background image is not sufficient to explain the localization error in the saccadic eye movement condition,
Keywords saccade, visual stability, image displacement, signal, copy.
Introduction A visual stimulus flashed at the time near a saccadic eye m o v e m e n t is perceived at a different position from its actual position in space. This p h e n o m e n o n is observed when visual stimuli are presented on an illuminated structured background (Bischof & Kramer, 1968; Honda, 1993; Mateeff, 1978; O'Regan, 1984) as well as when they are presented in the dark (Honda, 1989, 1990, 1991; Matin, Matin, Pearce, 1969; Matin, Matin & Pola, 1970) In the former condition, the retinal image of the background scene rapidly shifts on the retina with the m o v e m e n t of the eye. Therefore, there is a possibility that the mislocalization of visual stimuli observed in the illuminated background conditions is irrelevant to the eye movement itself, but rather is produced by a complex retinal event generated by quick movements of retinal images contingent upon the eye movement (O'Regan, 1984). In 1965, Sperling and Speelman reported that the position of a line was mislocalized when it was flashed at a time near the rapid displacement of the background scene (Sperling & Speelman, 1965). Their finding was later confirmed by MacKay (1970). In MacKay's experiment, as was the case in Sperling and Speelman's, a scale pattern was moved horizontally in the visual field, and a small flash stimulus was presented at various points in time before, during, or after the displacement of the scale pattern. The subjects were asked
Eye Movement Research/J.M. Findlay et al. (Editors) 9 1995 Elsevier Science B.V. All rights reserved.
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to report the position on the scale that the flash appeared to occupy. According to MacKay, mislocalization was confined chiefly to flashes presented in the 50 msec or so before and during displacement. From this observation, together with the finding by Sperling and Speelman, MacKay argued that the oculomotor system need not be implicated in order to account for mislocalization during saccadic eye movement. O'Regan (1984) also presented a similar interpretation. He argued that mislocalization is caused by the difference between the foveal and the peripheral position sense, and by the smearing of the retinal image of the background. This kind of explanation is very persuasive because the time course of mislocalization in the moving background condition reported by MacKay seems very similar to that reported for saccadic eye movement conditions by Mateeff (1978). However, it is not conclusive. This is because, in the MacKay and Sperling and Speelman experiments, target stimuli for visual localization were always presented at the midpoint of a moving scale used as a background scene. The subjects were asked to fixate the midpoint of the scale (in the MacKay experiment) or the left side of the scale (in the Sperling and Speelman experiment), and then the scale moved horizontally. If we focus on the target position on the retina, we can easily understand that these experimental situations corresponded to the saccadic eye movement conditions in which a target was always presented at the position of the original fixation point (MacKay's experiment) or at the midpoint between the fixation point and the goal of the saccade (Sperling and Speelman's experiment). However, it should be noted here that, in the saccadic eye movement condition, the size and the direction of mislocalization largely depend on the actual position of the target. Honda (1993), for example, investigated the accuracy in judging the position of targets briefly presented at a time of a saccade at various positions scattered two-dimensionally on a dimly illuminated structured background, and demonstrated that the spatio-temporal pattern of errors in judging the target position largely varied with the actual target position on the illuminated background. For example, when a target was presented at a position beyond the saccade destination, mislocalization occurred exclusively in the direction opposite to the saccade. On the other hand, when a target was presented on the opposite side to the saccade destination across the original fixation point, errors in the saccade direction were dominant. Therefore, if it is true that mislocalization in the saccadic eye movement condition is caused by retinal image displacements which are not necessarily contingent upon saccadic eye movements, then it will be expected that, irrespective of the target position in space, the pattern of errors in target localization will be almost the same between the moving background and the saccadic eye movement conditions. The present study was conducted to directly examine this prediction, and to explore the mechanism responsible for producing mislocalization in the saccadic eye movement condition.
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Methods Saccadic eye m o v e m e n t condition
A subject was seated with the head fixed by a chin- and forehead rest. Horizontal movements of the right eye were monitored by a photo-electric method. The subject's right eye was illuminated by an i.r. light-emitting diode (Toshiba, TLN103), and the reflected light from the two positions of the lower limbus (iris-sclera boundaries in 4 o'clock and 8 o'clock positions) was collected by a phototransistor (Toshiba, ~ 1 ) , and horizontal eye movements were monitored by recording the difference between the two phototransistor outputs. A horizontal luminous scale with divisions was used as a background scene (Figure 1). Its image [white (12 c d / m 2) on a dark ground (5cd/m2)] was rearprojected on a screen (50cm x 75cm) placed 57cm from the subject's eye. On each trial, a buzzer warning signal was given, and then a fixation point (red LED, 0.3 deg in dia, 20 c d / m 2) was presented at the zero scale division. The duration of the fixation point varied randomly from trial to trial between 1.0 and 2.0 sec. The subject was asked to binocularly keep watching the fixation point. At the offset of the fixation point, a small visual cue stimulus for saccades was presented for 20msec, at the position of 8 deg right of the fixation point, i.e., at the eight scale division The visual cue consisted of two vertically arranged rectangular red LEDs (0.1 deg x 0.3 deg, 18 cd/m2), the distance between the center of the LEDs being 0.4 deg. The subject was asked to make a horizontal saccade (primary saccade) toward the visual cue. Because the duration of the visual cue was short (20 msec), it disappeared before the beginning of the primary saccade. At various points in time before, during, or after the primary saccade, a vertical rectangular visual stimulus (0.3 deg x 1.5 deg), which was illuminated by an electronic flash tube (Nisshin, HD-100), was presented at the -4, +4, or +12 scale division, and used as a target for visual localization. The subject verbally reported the scale division on which he had seen the target. For example, he reported "minus five", "nine point five", or "I did not see". The fixation point and the visual cue for saccade were set on a black board placed at a different position from the screen, and seen by the subject through a hail-mirror set before the subject's eye. By this method, these stimuli were presented as an optical image on the background scene. To present the target during or after the saccade, the output from the eye movement monitor apparatus was fed into a differential circuit that triggered the electronic flash tube. Targets before the saccade were presented by pre-setting a shorter time interval than a normal saccade latency (200 msec) between the target and the visual cue for eliciting the saccade. In addition to the saccade condition described above, localization was also examined in a condition in which the target was presented when the eye remained still. In this control condition, either the fixation point or the cue for saccade was presented for 1.8 sec, and the subject was asked to keep watching these stimuli. Just after the offset of these stimuli, a flash target was presented. The subject made a saccade to the target, and reported its apparent position.
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Figure 1. A background scene used in this experiment. The horizontal length of the scale (from the -8 to the +16 scale division) was 24 o when it was projected on the screen.
Moving background condition In this condition, the background scene was rapidly displaced by moving a mirror placed between the screen and the projector. The mirror was mounted on a galvanometer, and moved by a computer. As was the case in the saccadic eye movement condition, the subject was asked to keep watching the fixation point. Two hundred msec after the offset of the fixation point, the background scene was rapidly displaced horizontally 8 deg to the left. (The 200msec interval was chosen as an equivalence to normal saccade latencies.) The duration of the displacement was 30 msec, approximately equal to the average saccade duration observed in the saccadic eye movement condition. At various point in time before, during, or after the beginning of the background displacement, a target was flashed at the -4, +4, or +12 scale division. The subject verbally reported the scale division on which the target appeared. Localization accuracy was examined also in a condition in which the background scene did not move. In this control condition, the target was flashed while the subject was watching the fixation point.
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Subjects and procedure Four subjects participated in this experiment. Subject HH was the author and the remaining three subjects were male university students. Each served as a subject for 6 days, 3 days for the saccadic eye movement conditions and 3 days for the moving background condition. On each day, the target's position was restricted to one of the three positions (-4, +4, or +12 on the scale). In the saccadic eye movement condition, eight sessions of the experimental (saccade) condition and one session of the control condition were conducted on each day. Each session consisted of 17 experimental trials or 14 control trials. The timing of the target's presentation was randomized within each session. In the moving background condition, a total of 136 trials, divided into eight sessions, were conducted on each day. In each session, 16 experimental (moving background) trials and one control trial were conducted in random order. The timing of the target's presentation also was randomized within each session.
Results Saccadic eye movement condition Primary saccade The subject's eye movement was analyzed by a high-speed digital storage scope (Iwatsu, DS-6121A). In the saccadic eye movement condition, the subjects sometimes failed in making a saccade in the way required. That is, they made saccades with extremely short (<50msec) or long (>300msec) latencies. In these cases, the target was not presented. The frequencies (percentage) of these expected or delayed saccade responses varied with the subjects ranging from 2.4% (subject HH) to 14.2% (subject HU). These trials were excluded from the following data analysis. When a target was presented immediately after the presentation of the visual cue for eliciting a primary saccade, the eye sometimes moved directly to the target. In this case, therefore, a saccade to the visual cue did not occur. This type of response was observed in about 10% of the trials. In the remaining trials, the expected primary saccades of about 8 deg were observed. The means of the latency and the duration of the primary saccades were about 200 msec and 32 msec, respectively.
Visual Localization Figure 2 shows the localization errors, i.e., the discrepancies between the actual target position and the perceptually judged position, as a function of the time interval between the onset of the primary saccade and the occurrence of the target. It is evident that the time course of the error largely varied with the position at which targets were actually presented. When targets were presented at a position opposite to the saccade direction (i.e., on the -4 scale division), all subjects showed a localization error in the saccade direction. The localization error was shown when the target was presented just before or during the primary saccade. When targets were presented at a position located between
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the original fixation point and the saccade destination (ie., on the +4 scale division), the error was relatively small. At this position, small errors in the saccade direction were shown when the target was presented immediately before the saccade onset, while errors in the direction opposite to the saccade direction were observed when the target was presented at the end of the saccade. Finally, when targets were presented at a position beyond the saccade destination (i.e., on the +12 scale division), all subjects except subject AT showed errors only in the direction opposite to the saccade when the target was presented at the end of the saccade. Subject AT exceptionally showed small errors in the saccade direction as well as errors in the direction opposite to the saccade. In summary, the results in the saccadic eye movement condition were approximately the same as those I previously reported elsewhere (Honda, 1993), in which it was demonstrated that localization errors were largely dependent upon the actual target's position in the illuminated visual field.
Moving background condition Eye movements Eye movement recordings showed that all subjects kept watching the position of the original fixation point during the trials. Usually, the eye moved toward the apparent target position more than 300msec (in subjects AT and HH) or 200msec after the movement of the background (in subjects NN and HU).
Visual localization Figure 3 shows the results obtained in the moving background condition. When targets were presented on the -4 scale division, errors in both saccade direction and in the opposite direction to the saccade were observed. However, in two subjects, MM and HU, the errors in the saccade direction were small or almost absent. When targets were presented on the +4 scale division, all subjects showed a bipolar pattern of mislocalization consisting of errors in the saccade direction and ones in the direction opposite to saccade. Similar results were shown for the targets presented on the 12 scale division. In this case, however, the size of the error remarkably increased. In three subjects, AT, MN, and HU, the error in the saccade direction appeared about 100msec before the saccade onset.
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Figure 2. The time course of mislocalization in the saccadic eye movement condition. For each subject, results are shown separately for each actual target position (-4, +4, and +12). The abscissa indicates the time interval (msec) between saccade onset and target presentation. A minus sign in the abscissa shows that targets were presented before the saccade onset. The ordinate indicates the size of mislocalization (deg). A plus sign in the ordinate shows mislocalization in the saccade direction (rightward), and a minus sign mislocalization in the direction opposite to the saccade (leftward). Each dot in the figure represents the average error (and the SD) of about 5-25 trials. The error curves were fitted by eye based on the average errors (dots). Open circles indicate the results on control trials in which the subjects kept watching the fixation point (left circle) or the visual cue for saccade (right circle). Vertical dotted lines indicate the mean duration of the saccades.
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Figure 3. Time course of mislocalization in the moving background condition. Notations are the same as in Figure 3 except the following. The abscissa indicates the time interval between the beginning of background movements and the target presentation. Each dot indicates the average of 8 trials. Open circles in the figure indicate the results in the control condition in which the background did not move. Vertical dotted lines show the movement time of the background. Discussion
Comparison of saccadic eye movement and moving background conditions In this study, it was assumed that the retinal event was the same between the two experimental conditions. If the localization error observed in the two conditions was exclusively generated by a retinal event, i.e., rapid
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displacements of retinal images produced either by saccadic eye movements or by movements of the background scene, then it should be expected that the pattern of errors was the same between the two conditions. However, this was not the case. The results clearly showed that the pattern of errors was not the same between the two conditions. The discrepancy was predominant in particular when the target was presented on the -4 or the +12 scale division. Therefore, it is impossible to explain that errors in the two conditions were generated by the same mechanism: a complex retinal event produced by a rapid displacement of retinal images. It should be noted here that some authors suggested that a pattern of errors similar to one reported for saccadic eye movement conditions was observed also in moving background conditions (O'Regan, 1984; Sperling & Speelman, 1965). However, their finding is not conclusive. In Sperling and Speelman's study, targets were presented exclusively at the position between the original and the final fixation points (i.e., on the 4 scale division in the present study). As shown in Figures 2 and 3, at this target's position, the shape of error curves was about the same between saccadic eye movement and the moving background conditions. Therefore, Sperling and Speelman's experiment should be replicated using other target positions than they employed in their pioneering In O'Regan's (1984) study, targets were presented only during the displacement of the background scene, and never presented before or after the displacement. Therefore, the exact time course of localization error is not clear, and it is hard to make a comparison between the moving background and the saccade conditions. Origins of localization error As described above, the pattern of localization error shown in the saccadic eye moment condition was substantially different from that in the moving background condition. Does this mean that there is a possibility that mislocalization observed in the each of two conditions was caused by a distinctively different mechanism? Before considering this question, it seems necessary to examine the explanations proposed so far in the earlier studies. As regards mislocalization in moving background conditions, MacKay (1970) explained this as follows. "The location of a flashed image to its background is determined by an interaction between the neural signals generated by each, which interaction takes an appreciable time to complete. If during this time the retinal image of the background shifts to a new position, the integrative process will have two different background signals to cope with, each making its own contribution to the total weight of evidence respect to flash location." For example, "the later the flash comes, before the moment of transition, the greater will be the weight attached to the new scale-position as compared with the old." (MacKay, 1970, p.732) Thus, the apparent position of a flash changes as a function of the timing of flash presentation relative to the background's displacement. Furthermore, another important implication of the MacKay's argument is that the position of the flash is determined by the background scene before and after its shift, not by the background during displacement. The fact that the image of the background during displacement is irrelevant to mislocalization was established by Sperling (1990). In one of his experiments,
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the background scene was distinguished during its displacement. Even in this condition without image smearing on the retina, mislocalization very similar to the one in normal moving-background conditions was observed. The idea that mislocalization is produced by the successive appearance of the background in two different positions seems plausible because subjects usually report that they cannot perceive the scale during its shift owing to its smearing. However, this explanation has a serious problem. If this explanation is correct, then it is expected that the pattern of errors will be the same irrespective of the position at which flashes are presented. Strictly speaking, this was not the case in the present study although the basic shape of error curves seems approximately the same. Next, why were the flashes mislocalized when they were presented at the time of saccadic eye movements? In this condition, too, a rapid displacement of images occurred on the retina. In addition, neural activities of the oculomotor system for generating a saccade were involved. Mislocalization occurs also when targets are presented in the dark (Honda, 1989, 1991; Kennard, Hartman, Kraft, Glaser, 1971; Matin et al., 1969, 1970) as well as when they were presented on an illuminated background. In the case of experiments conducted in the dark, there is almost no retinal event such as image displacements of the background scene. Therefore, the mislocalization in the dark seems to be caused primarily by a sluggish activity of the extraretinal eye position signal (EEPS), resulting in a failure in completely canceling the shift of images of flash targets on the retina by the EEPS. Then, why was a flash mislocalized when it was presented on an illuminated background on the occasion of saccade generation? Honda (1993) examined the accuracy of localization of flash targets presented at the time of a saccade at various positions scattered two-dimensionally on a dimly illuminated structured background, and argued that localization error is primarily produced by a sluggish activity of the EEPS, and that the error is partially corrected by visual cues from the illuminated background and modified by the subject's selective inattention to image displacements. In addition, it was assumed that the rapid displacement of the retinal image of the background has no substantial role in producing mislocalization. According to this explanation, it is expected that, in the moving background condition without saccadic eye movements, there will be shown no or, if any, small localization errors, because it was speculated that the primary factor for mislocalization is the sluggish activity of the EEPS. However, the present study did not support this prediction; mislocalization was larger in the moving background condition than in the saccadic eye movement condition!
Tentative explanation Why did the subject mislocalize the flash when it was presented on an illuminated background in the saccadic eye movement condition? It should be noted here that, in both moving background and saccadic eye movement conditions, the image of the background rapidly moved on the retina. That is, the retinal event was the same between the two conditions, and the only difference was the involvement of neural activities of the oculomotor system (EEPS) in the saccadic eye movement condition. This leads us to speculate that
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the discrepancy in localization errors between the two conditions was produced as a result of these additional neural activities. The present study showed that localization errors were in general larger in the moving background than in the saccadic eye movement conditions. From this finding, we can suppose that, in both conditions, mislocalization was primarily caused by retinal image displacements and that, in the saccadic eye movement condition, the mislocalization was corrected by an involvement of the EEPS. In other words, saccadic eye movements reduced the size of localization error. Although this explanation sounds reasonable, it has some difficulties. As shown in Figures 2 an 3, when a flash was presented at the -4 scale division, the error in the minus direction observed in the moving background condition disappeared in the saccadic eye movement condition. On the other hand, at the target position of the +12 scale division, the large error in the plus direction observed in the moving background condition was not shown in the saccadic eye movement condition. In short, reduction of error by the EEPS was dependent upon the position where the flash was presented. Although it is evident that mislocalization was reduced in the saccadic eye movement condition, it is not clear why the corrective effect of the EEPS depended on the target position. This is because, if the EEPS has a corrective effect on localization error, the effect should be observed in the same way irrespective of the target position. To resolve this problem, an additional explanation is necessary. One possible explanation is that a saccadic eye movement generates some kind of cognitive bias such as attentional bias (Bridgeman, 1983) which depends on the spatiotemporal aspects of the saccade, and that this bias produces the corrective effect of the EEPS which works differently from position to position in the visual field. According to this idea, the results of the present study are explained as follows. At the time immediately before the saccade onset and during the first half of the saccade, the corrective effect appears at the positions near or beyond the saccade's destination, but not at the position located in the direction opposite to the saccade. This is because the visual system enhances the efficiency of its cognitive function in a part of the visual field to which the eye gets after the saccade, and in this area localization is not so much influenced by the retinal event produced by rapid image displacements. On the other hand, at the end of the saccade, the corrective effect appears only at the position near the original fixation point. This is because the enhanced cognitive function is brought back to the original fixation point from the saccade destination, or it spreads out all over the visual field. (the latter possibility comes from the fact that, when a flash was presented at the time near the end of the saccade, the reduction of errors was observed also at the position beyond the saccade destination, i.e., on the +12 scale division.) The explanation described above also has a serious problem which should be answered. My earlier studies (Honda, 1993) showed that mislocalization was larger when a flash was presented in the dark than when it was presented on an illuminated background, and further it was suggested that the large mislocalization in the dark was primarily caused by a sluggish activity of the EEPS. This earlier finding is not consistent with the assumption proposed here that the EEPS corrects the error produced by retinal image displacements. In
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other words, w h y does the EEPS, which generates large localization errors in the dark, reduce the errors w h e n a flash target is presented on an illuminated background? In conclusion, the present study d e m o n s t r a t e d that the shape of the error curves in the saccadic eye m o v e m e n t condition was not the s a m e as that in the m o v i n g b a c k g r o u n d condition. This finding casts d o u b t on the idea that mislocalization d u r i n g saccadic eye m o v e m e n t s is primarily caused by retinal i m a g e displacements which are not necessarily contingent u p o n the m o v e m e n t of the eye. It was a r g u e d that some kind of additional a s s u m p t i o n such as saccade-contingent cognitive bias is necessary to successfully explain the discrepancy b e t w e e n the two conditions. (This research was s u p p o r t e d by a 1992 Grant-in-Aid for Scientific Research from the Ministry of Education, Science and Culture to the author.)
References Bischof, N. & Kramer, E. (1968). Untersuchung und Uberlegungen zur Richitungswahmehmung bei Willkurlichen sakkadischen Augenbewegungen. Psychologische Forschung, 32, 185-218. Bridgeman, B. (1983). Mechanisms of space constancy. In Hein, A. & Jeannerod, M. (eds.), Spatially Oriented Behavior (pp. 263-279). New York: Springer. Honda, H. (1989). Perceptual localization of visual stimuli flashed during saccades. Perception and Psychophysics, 45,162-174. Honda, H. (1990). Eye movement to a visual stimulus flashed before, during, or after a saccade. In Jeannerod, M. (ed.), Attention and Performance (Vol. 13, pp. 567-582). HiUsdale: LEA. Honda, H. (1991). The time courses of visual mislocalization and of extra-retinal eye position signals at the time of vertical saccades. Vision Research, 31, 1915-1921. Honda, H. (1993). Saccade-contingent displacement of the apparent position of visual stimuli flashed on a dimly illuminated structured background. Vision Research, 33, 709-716. Kennard, D.W., Hartman, tLW., Kraft, D., & Glaser. G. H. (1971). Brief conceptual (nonreal) events during eye movement. BiologicalPsychiatry, 3, 205-215. Mackay, D. M.. (1970). Mislocation of test flashes during saccadic image displacements. Nature, 227, 731-733. Mateeff, S. (1978). Saccadic eye movements and localization of visual stimuli. Perception and Psychophysics, 24, 215-224. Matin, L., Matin, E., & Pearce, D. G. (1969). Visual perception of direction when voluntary saccades occur: I. Relation of visual direction of a fixation target extinguished before a saccade to a flash presented during the saccade. Perception and Psychophysics, 5, 65-80. Matin, L., Matin, E., & Pola, J. (1970). Visual perception of direction when voluntary saccades occur: II. Relation of visual direction of a fixation target extinguished before a saccade to a subsequent test flash presented before the saccade. Perception and Psychophysics, 8, 9-14. O'Regan, J. K. (1984). Retinal versus extraretinal influences in flash localization during saccadic eye movements in the presence of a visible background. Perception and Psychophysics, 36, 1-14. Sperling, G. (1990). Comparison of perception in the moving and stationary eye. In Kowler, E. (ed.), Eye Movements and Their Role in Visual and Cognitive Processes (pp. 307-351). Amsterdam: Elsevier. Sperling, G. & Speelman, R. (1965). Visual spatial localization during object motion, apparent object motion, and image motion produced by eye movements. Journal of the Optical Society of America, 55, 1576
Abstract There is a possibility that visual mislocalization of targets flashed at the time of a saccadic eye movement is due almost entirely to the shift of the retinal image of the background. To clarify this matter, errors in target localization were analyzed in both saccadic eye movement and moving background conditions. In the latter condition, subject kept fixating and the visual background made a saccadic movement. In both conditions, a horizontal luminous scale was used as the background and the subject reported the position on the scale that the target appeared to occupy. Large localization errors were shown in both conditions. However, the pattern of error for the moving background condition was distinctively different from that for the saccadic eye movement condition, suggesting that the shift of the background image is not sufficient to explain the localization error in the saccadic eye movement condition,
Keywords saccade, visual stability, image displacement, signal, copy.
Introduction A visual stimulus flashed at the time near a saccadic eye m o v e m e n t is perceived at a different position from its actual position in space. This p h e n o m e n o n is observed when visual stimuli are presented on an illuminated structured background (Bischof & Kramer, 1968; Honda, 1993; Mateeff, 1978; O'Regan, 1984) as well as when they are presented in the dark (Honda, 1989, 1990, 1991; Matin, Matin, Pearce, 1969; Matin, Matin & Pola, 1970) In the former condition, the retinal image of the background scene rapidly shifts on the retina with the m o v e m e n t of the eye. Therefore, there is a possibility that the mislocalization of visual stimuli observed in the illuminated background conditions is irrelevant to the eye movement itself, but rather is produced by a complex retinal event generated by quick movements of retinal images contingent upon the eye movement (O'Regan, 1984). In 1965, Sperling and Speelman reported that the position of a line was mislocalized when it was flashed at a time near the rapid displacement of the background scene (Sperling & Speelman, 1965). Their finding was later confirmed by MacKay (1970). In MacKay's experiment, as was the case in Sperling and Speelman's, a scale pattern was moved horizontally in the visual field, and a small flash stimulus was presented at various points in time before, during, or after the displacement of the scale pattern. The subjects were asked
Eye Movement Research/J.M. Findlay et al. (Editors) 9 1995 Elsevier Science B.V. All rights reserved.
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to report the position on the scale that the flash appeared to occupy. According to MacKay, mislocalization was confined chiefly to flashes presented in the 50 msec or so before and during displacement. From this observation, together with the finding by Sperling and Speelman, MacKay argued that the oculomotor system need not be implicated in order to account for mislocalization during saccadic eye movement. O'Regan (1984) also presented a similar interpretation. He argued that mislocalization is caused by the difference between the foveal and the peripheral position sense, and by the smearing of the retinal image of the background. This kind of explanation is very persuasive because the time course of mislocalization in the moving background condition reported by MacKay seems very similar to that reported for saccadic eye movement conditions by Mateeff (1978). However, it is not conclusive. This is because, in the MacKay and Sperling and Speelman experiments, target stimuli for visual localization were always presented at the midpoint of a moving scale used as a background scene. The subjects were asked to fixate the midpoint of the scale (in the MacKay experiment) or the left side of the scale (in the Sperling and Speelman experiment), and then the scale moved horizontally. If we focus on the target position on the retina, we can easily understand that these experimental situations corresponded to the saccadic eye movement conditions in which a target was always presented at the position of the original fixation point (MacKay's experiment) or at the midpoint between the fixation point and the goal of the saccade (Sperling and Speelman's experiment). However, it should be noted here that, in the saccadic eye movement condition, the size and the direction of mislocalization largely depend on the actual position of the target. Honda (1993), for example, investigated the accuracy in judging the position of targets briefly presented at a time of a saccade at various positions scattered two-dimensionally on a dimly illuminated structured background, and demonstrated that the spatio-temporal pattern of errors in judging the target position largely varied with the actual target position on the illuminated background. For example, when a target was presented at a position beyond the saccade destination, mislocalization occurred exclusively in the direction opposite to the saccade. On the other hand, when a target was presented on the opposite side to the saccade destination across the original fixation point, errors in the saccade direction were dominant. Therefore, if it is true that mislocalization in the saccadic eye movement condition is caused by retinal image displacements which are not necessarily contingent upon saccadic eye movements, then it will be expected that, irrespective of the target position in space, the pattern of errors in target localization will be almost the same between the moving background and the saccadic eye movement conditions. The present study was conducted to directly examine this prediction, and to explore the mechanism responsible for producing mislocalization in the saccadic eye movement condition.
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Methods Saccadic eye m o v e m e n t condition
A subject was seated with the head fixed by a chin- and forehead rest. Horizontal movements of the right eye were monitored by a photo-electric method. The subject's right eye was illuminated by an i.r. light-emitting diode (Toshiba, TLN103), and the reflected light from the two positions of the lower limbus (iris-sclera boundaries in 4 o'clock and 8 o'clock positions) was collected by a phototransistor (Toshiba, ~ 1 ) , and horizontal eye movements were monitored by recording the difference between the two phototransistor outputs. A horizontal luminous scale with divisions was used as a background scene (Figure 1). Its image [white (12 c d / m 2) on a dark ground (5cd/m2)] was rearprojected on a screen (50cm x 75cm) placed 57cm from the subject's eye. On each trial, a buzzer warning signal was given, and then a fixation point (red LED, 0.3 deg in dia, 20 c d / m 2) was presented at the zero scale division. The duration of the fixation point varied randomly from trial to trial between 1.0 and 2.0 sec. The subject was asked to binocularly keep watching the fixation point. At the offset of the fixation point, a small visual cue stimulus for saccades was presented for 20msec, at the position of 8 deg right of the fixation point, i.e., at the eight scale division The visual cue consisted of two vertically arranged rectangular red LEDs (0.1 deg x 0.3 deg, 18 cd/m2), the distance between the center of the LEDs being 0.4 deg. The subject was asked to make a horizontal saccade (primary saccade) toward the visual cue. Because the duration of the visual cue was short (20 msec), it disappeared before the beginning of the primary saccade. At various points in time before, during, or after the primary saccade, a vertical rectangular visual stimulus (0.3 deg x 1.5 deg), which was illuminated by an electronic flash tube (Nisshin, HD-100), was presented at the -4, +4, or +12 scale division, and used as a target for visual localization. The subject verbally reported the scale division on which he had seen the target. For example, he reported "minus five", "nine point five", or "I did not see". The fixation point and the visual cue for saccade were set on a black board placed at a different position from the screen, and seen by the subject through a hail-mirror set before the subject's eye. By this method, these stimuli were presented as an optical image on the background scene. To present the target during or after the saccade, the output from the eye movement monitor apparatus was fed into a differential circuit that triggered the electronic flash tube. Targets before the saccade were presented by pre-setting a shorter time interval than a normal saccade latency (200 msec) between the target and the visual cue for eliciting the saccade. In addition to the saccade condition described above, localization was also examined in a condition in which the target was presented when the eye remained still. In this control condition, either the fixation point or the cue for saccade was presented for 1.8 sec, and the subject was asked to keep watching these stimuli. Just after the offset of these stimuli, a flash target was presented. The subject made a saccade to the target, and reported its apparent position.
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Figure 1. A background scene used in this experiment. The horizontal length of the scale (from the -8 to the +16 scale division) was 24 o when it was projected on the screen.
Moving background condition In this condition, the background scene was rapidly displaced by moving a mirror placed between the screen and the projector. The mirror was mounted on a galvanometer, and moved by a computer. As was the case in the saccadic eye movement condition, the subject was asked to keep watching the fixation point. Two hundred msec after the offset of the fixation point, the background scene was rapidly displaced horizontally 8 deg to the left. (The 200msec interval was chosen as an equivalence to normal saccade latencies.) The duration of the displacement was 30 msec, approximately equal to the average saccade duration observed in the saccadic eye movement condition. At various point in time before, during, or after the beginning of the background displacement, a target was flashed at the -4, +4, or +12 scale division. The subject verbally reported the scale division on which the target appeared. Localization accuracy was examined also in a condition in which the background scene did not move. In this control condition, the target was flashed while the subject was watching the fixation point.
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Subjects and procedure Four subjects participated in this experiment. Subject HH was the author and the remaining three subjects were male university students. Each served as a subject for 6 days, 3 days for the saccadic eye movement conditions and 3 days for the moving background condition. On each day, the target's position was restricted to one of the three positions (-4, +4, or +12 on the scale). In the saccadic eye movement condition, eight sessions of the experimental (saccade) condition and one session of the control condition were conducted on each day. Each session consisted of 17 experimental trials or 14 control trials. The timing of the target's presentation was randomized within each session. In the moving background condition, a total of 136 trials, divided into eight sessions, were conducted on each day. In each session, 16 experimental (moving background) trials and one control trial were conducted in random order. The timing of the target's presentation also was randomized within each session.
Results Saccadic eye movement condition Primary saccade The subject's eye movement was analyzed by a high-speed digital storage scope (Iwatsu, DS-6121A). In the saccadic eye movement condition, the subjects sometimes failed in making a saccade in the way required. That is, they made saccades with extremely short (<50msec) or long (>300msec) latencies. In these cases, the target was not presented. The frequencies (percentage) of these expected or delayed saccade responses varied with the subjects ranging from 2.4% (subject HH) to 14.2% (subject HU). These trials were excluded from the following data analysis. When a target was presented immediately after the presentation of the visual cue for eliciting a primary saccade, the eye sometimes moved directly to the target. In this case, therefore, a saccade to the visual cue did not occur. This type of response was observed in about 10% of the trials. In the remaining trials, the expected primary saccades of about 8 deg were observed. The means of the latency and the duration of the primary saccades were about 200 msec and 32 msec, respectively.
Visual Localization Figure 2 shows the localization errors, i.e., the discrepancies between the actual target position and the perceptually judged position, as a function of the time interval between the onset of the primary saccade and the occurrence of the target. It is evident that the time course of the error largely varied with the position at which targets were actually presented. When targets were presented at a position opposite to the saccade direction (i.e., on the -4 scale division), all subjects showed a localization error in the saccade direction. The localization error was shown when the target was presented just before or during the primary saccade. When targets were presented at a position located between
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the original fixation point and the saccade destination (ie., on the +4 scale division), the error was relatively small. At this position, small errors in the saccade direction were shown when the target was presented immediately before the saccade onset, while errors in the direction opposite to the saccade direction were observed when the target was presented at the end of the saccade. Finally, when targets were presented at a position beyond the saccade destination (i.e., on the +12 scale division), all subjects except subject AT showed errors only in the direction opposite to the saccade when the target was presented at the end of the saccade. Subject AT exceptionally showed small errors in the saccade direction as well as errors in the direction opposite to the saccade. In summary, the results in the saccadic eye movement condition were approximately the same as those I previously reported elsewhere (Honda, 1993), in which it was demonstrated that localization errors were largely dependent upon the actual target's position in the illuminated visual field.
Moving background condition Eye movements Eye movement recordings showed that all subjects kept watching the position of the original fixation point during the trials. Usually, the eye moved toward the apparent target position more than 300msec (in subjects AT and HH) or 200msec after the movement of the background (in subjects NN and HU).
Visual localization Figure 3 shows the results obtained in the moving background condition. When targets were presented on the -4 scale division, errors in both saccade direction and in the opposite direction to the saccade were observed. However, in two subjects, MM and HU, the errors in the saccade direction were small or almost absent. When targets were presented on the +4 scale division, all subjects showed a bipolar pattern of mislocalization consisting of errors in the saccade direction and ones in the direction opposite to saccade. Similar results were shown for the targets presented on the 12 scale division. In this case, however, the size of the error remarkably increased. In three subjects, AT, MN, and HU, the error in the saccade direction appeared about 100msec before the saccade onset.
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Figure 2. The time course of mislocalization in the saccadic eye movement condition. For each subject, results are shown separately for each actual target position (-4, +4, and +12). The abscissa indicates the time interval (msec) between saccade onset and target presentation. A minus sign in the abscissa shows that targets were presented before the saccade onset. The ordinate indicates the size of mislocalization (deg). A plus sign in the ordinate shows mislocalization in the saccade direction (rightward), and a minus sign mislocalization in the direction opposite to the saccade (leftward). Each dot in the figure represents the average error (and the SD) of about 5-25 trials. The error curves were fitted by eye based on the average errors (dots). Open circles indicate the results on control trials in which the subjects kept watching the fixation point (left circle) or the visual cue for saccade (right circle). Vertical dotted lines indicate the mean duration of the saccades.
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Figure 3. Time course of mislocalization in the moving background condition. Notations are the same as in Figure 3 except the following. The abscissa indicates the time interval between the beginning of background movements and the target presentation. Each dot indicates the average of 8 trials. Open circles in the figure indicate the results in the control condition in which the background did not move. Vertical dotted lines show the movement time of the background. Discussion
Comparison of saccadic eye movement and moving background conditions In this study, it was assumed that the retinal event was the same between the two experimental conditions. If the localization error observed in the two conditions was exclusively generated by a retinal event, i.e., rapid
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displacements of retinal images produced either by saccadic eye movements or by movements of the background scene, then it should be expected that the pattern of errors was the same between the two conditions. However, this was not the case. The results clearly showed that the pattern of errors was not the same between the two conditions. The discrepancy was predominant in particular when the target was presented on the -4 or the +12 scale division. Therefore, it is impossible to explain that errors in the two conditions were generated by the same mechanism: a complex retinal event produced by a rapid displacement of retinal images. It should be noted here that some authors suggested that a pattern of errors similar to one reported for saccadic eye movement conditions was observed also in moving background conditions (O'Regan, 1984; Sperling & Speelman, 1965). However, their finding is not conclusive. In Sperling and Speelman's study, targets were presented exclusively at the position between the original and the final fixation points (i.e., on the 4 scale division in the present study). As shown in Figures 2 and 3, at this target's position, the shape of error curves was about the same between saccadic eye movement and the moving background conditions. Therefore, Sperling and Speelman's experiment should be replicated using other target positions than they employed in their pioneering In O'Regan's (1984) study, targets were presented only during the displacement of the background scene, and never presented before or after the displacement. Therefore, the exact time course of localization error is not clear, and it is hard to make a comparison between the moving background and the saccade conditions. Origins of localization error As described above, the pattern of localization error shown in the saccadic eye moment condition was substantially different from that in the moving background condition. Does this mean that there is a possibility that mislocalization observed in the each of two conditions was caused by a distinctively different mechanism? Before considering this question, it seems necessary to examine the explanations proposed so far in the earlier studies. As regards mislocalization in moving background conditions, MacKay (1970) explained this as follows. "The location of a flashed image to its background is determined by an interaction between the neural signals generated by each, which interaction takes an appreciable time to complete. If during this time the retinal image of the background shifts to a new position, the integrative process will have two different background signals to cope with, each making its own contribution to the total weight of evidence respect to flash location." For example, "the later the flash comes, before the moment of transition, the greater will be the weight attached to the new scale-position as compared with the old." (MacKay, 1970, p.732) Thus, the apparent position of a flash changes as a function of the timing of flash presentation relative to the background's displacement. Furthermore, another important implication of the MacKay's argument is that the position of the flash is determined by the background scene before and after its shift, not by the background during displacement. The fact that the image of the background during displacement is irrelevant to mislocalization was established by Sperling (1990). In one of his experiments,
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the background scene was distinguished during its displacement. Even in this condition without image smearing on the retina, mislocalization very similar to the one in normal moving-background conditions was observed. The idea that mislocalization is produced by the successive appearance of the background in two different positions seems plausible because subjects usually report that they cannot perceive the scale during its shift owing to its smearing. However, this explanation has a serious problem. If this explanation is correct, then it is expected that the pattern of errors will be the same irrespective of the position at which flashes are presented. Strictly speaking, this was not the case in the present study although the basic shape of error curves seems approximately the same. Next, why were the flashes mislocalized when they were presented at the time of saccadic eye movements? In this condition, too, a rapid displacement of images occurred on the retina. In addition, neural activities of the oculomotor system for generating a saccade were involved. Mislocalization occurs also when targets are presented in the dark (Honda, 1989, 1991; Kennard, Hartman, Kraft, Glaser, 1971; Matin et al., 1969, 1970) as well as when they were presented on an illuminated background. In the case of experiments conducted in the dark, there is almost no retinal event such as image displacements of the background scene. Therefore, the mislocalization in the dark seems to be caused primarily by a sluggish activity of the extraretinal eye position signal (EEPS), resulting in a failure in completely canceling the shift of images of flash targets on the retina by the EEPS. Then, why was a flash mislocalized when it was presented on an illuminated background on the occasion of saccade generation? Honda (1993) examined the accuracy of localization of flash targets presented at the time of a saccade at various positions scattered two-dimensionally on a dimly illuminated structured background, and argued that localization error is primarily produced by a sluggish activity of the EEPS, and that the error is partially corrected by visual cues from the illuminated background and modified by the subject's selective inattention to image displacements. In addition, it was assumed that the rapid displacement of the retinal image of the background has no substantial role in producing mislocalization. According to this explanation, it is expected that, in the moving background condition without saccadic eye movements, there will be shown no or, if any, small localization errors, because it was speculated that the primary factor for mislocalization is the sluggish activity of the EEPS. However, the present study did not support this prediction; mislocalization was larger in the moving background condition than in the saccadic eye movement condition!
Tentative explanation Why did the subject mislocalize the flash when it was presented on an illuminated background in the saccadic eye movement condition? It should be noted here that, in both moving background and saccadic eye movement conditions, the image of the background rapidly moved on the retina. That is, the retinal event was the same between the two conditions, and the only difference was the involvement of neural activities of the oculomotor system (EEPS) in the saccadic eye movement condition. This leads us to speculate that
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the discrepancy in localization errors between the two conditions was produced as a result of these additional neural activities. The present study showed that localization errors were in general larger in the moving background than in the saccadic eye movement conditions. From this finding, we can suppose that, in both conditions, mislocalization was primarily caused by retinal image displacements and that, in the saccadic eye movement condition, the mislocalization was corrected by an involvement of the EEPS. In other words, saccadic eye movements reduced the size of localization error. Although this explanation sounds reasonable, it has some difficulties. As shown in Figures 2 an 3, when a flash was presented at the -4 scale division, the error in the minus direction observed in the moving background condition disappeared in the saccadic eye movement condition. On the other hand, at the target position of the +12 scale division, the large error in the plus direction observed in the moving background condition was not shown in the saccadic eye movement condition. In short, reduction of error by the EEPS was dependent upon the position where the flash was presented. Although it is evident that mislocalization was reduced in the saccadic eye movement condition, it is not clear why the corrective effect of the EEPS depended on the target position. This is because, if the EEPS has a corrective effect on localization error, the effect should be observed in the same way irrespective of the target position. To resolve this problem, an additional explanation is necessary. One possible explanation is that a saccadic eye movement generates some kind of cognitive bias such as attentional bias (Bridgeman, 1983) which depends on the spatiotemporal aspects of the saccade, and that this bias produces the corrective effect of the EEPS which works differently from position to position in the visual field. According to this idea, the results of the present study are explained as follows. At the time immediately before the saccade onset and during the first half of the saccade, the corrective effect appears at the positions near or beyond the saccade's destination, but not at the position located in the direction opposite to the saccade. This is because the visual system enhances the efficiency of its cognitive function in a part of the visual field to which the eye gets after the saccade, and in this area localization is not so much influenced by the retinal event produced by rapid image displacements. On the other hand, at the end of the saccade, the corrective effect appears only at the position near the original fixation point. This is because the enhanced cognitive function is brought back to the original fixation point from the saccade destination, or it spreads out all over the visual field. (the latter possibility comes from the fact that, when a flash was presented at the time near the end of the saccade, the reduction of errors was observed also at the position beyond the saccade destination, i.e., on the +12 scale division.) The explanation described above also has a serious problem which should be answered. My earlier studies (Honda, 1993) showed that mislocalization was larger when a flash was presented in the dark than when it was presented on an illuminated background, and further it was suggested that the large mislocalization in the dark was primarily caused by a sluggish activity of the EEPS. This earlier finding is not consistent with the assumption proposed here that the EEPS corrects the error produced by retinal image displacements. In
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other words, w h y does the EEPS, which generates large localization errors in the dark, reduce the errors w h e n a flash target is presented on an illuminated background? In conclusion, the present study d e m o n s t r a t e d that the shape of the error curves in the saccadic eye m o v e m e n t condition was not the s a m e as that in the m o v i n g b a c k g r o u n d condition. This finding casts d o u b t on the idea that mislocalization d u r i n g saccadic eye m o v e m e n t s is primarily caused by retinal i m a g e displacements which are not necessarily contingent u p o n the m o v e m e n t of the eye. It was a r g u e d that some kind of additional a s s u m p t i o n such as saccade-contingent cognitive bias is necessary to successfully explain the discrepancy b e t w e e n the two conditions. (This research was s u p p o r t e d by a 1992 Grant-in-Aid for Scientific Research from the Ministry of Education, Science and Culture to the author.)
References Bischof, N. & Kramer, E. (1968). Untersuchung und Uberlegungen zur Richitungswahmehmung bei Willkurlichen sakkadischen Augenbewegungen. Psychologische Forschung, 32, 185-218. Bridgeman, B. (1983). Mechanisms of space constancy. In Hein, A. & Jeannerod, M. (eds.), Spatially Oriented Behavior (pp. 263-279). New York: Springer. Honda, H. (1989). Perceptual localization of visual stimuli flashed during saccades. Perception and Psychophysics, 45,162-174. Honda, H. (1990). Eye movement to a visual stimulus flashed before, during, or after a saccade. In Jeannerod, M. (ed.), Attention and Performance (Vol. 13, pp. 567-582). HiUsdale: LEA. Honda, H. (1991). The time courses of visual mislocalization and of extra-retinal eye position signals at the time of vertical saccades. Vision Research, 31, 1915-1921. Honda, H. (1993). Saccade-contingent displacement of the apparent position of visual stimuli flashed on a dimly illuminated structured background. Vision Research, 33, 709-716. Kennard, D.W., Hartman, tLW., Kraft, D., & Glaser. G. H. (1971). Brief conceptual (nonreal) events during eye movement. BiologicalPsychiatry, 3, 205-215. Mackay, D. M.. (1970). Mislocation of test flashes during saccadic image displacements. Nature, 227, 731-733. Mateeff, S. (1978). Saccadic eye movements and localization of visual stimuli. Perception and Psychophysics, 24, 215-224. Matin, L., Matin, E., & Pearce, D. G. (1969). Visual perception of direction when voluntary saccades occur: I. Relation of visual direction of a fixation target extinguished before a saccade to a flash presented during the saccade. Perception and Psychophysics, 5, 65-80. Matin, L., Matin, E., & Pola, J. (1970). Visual perception of direction when voluntary saccades occur: II. Relation of visual direction of a fixation target extinguished before a saccade to a subsequent test flash presented before the saccade. Perception and Psychophysics, 8, 9-14. O'Regan, J. K. (1984). Retinal versus extraretinal influences in flash localization during saccadic eye movements in the presence of a visible background. Perception and Psychophysics, 36, 1-14. Sperling, G. (1990). Comparison of perception in the moving and stationary eye. In Kowler, E. (ed.), Eye Movements and Their Role in Visual and Cognitive Processes (pp. 307-351). Amsterdam: Elsevier. Sperling, G. & Speelman, R. (1965). Visual spatial localization during object motion, apparent object motion, and image motion produced by eye movements. Journal of the Optical Society of America, 55, 1576