Ординатура / Офтальмология / Английские материалы / Eye Movements A Window on Mind and Brain_Van Gompel_2007

weighted, suggesting that the spatial centre of gravity of the postsaccadic arrangement is not identical to the geometrical centre of the distractor pair.

To conclude, the results of Experiment 2 suggest that in the (ambiguous) case of two spatially separate postsaccadic distractors, the visual system uses, as a first approximation, the centre of gravity of both stimuli as the landmark for postsaccadic localisation – the induced mislocalisation is about the same as if a single distractor had appeared at this location.

3. Experiment 3

The previous experiment suggested that when no unequivocal mapping between preand postsaccadic distractor configuration is possible, it is the global postsaccadic stimulus configuration that determines the effective landmark position – under this condition, a centre of gravity of the postsaccadic arrangement is used. The question arises what would happen if an unambiguous mapping of the presaccadic distractor to the postsaccadic configuration would be made possible – would then only the postsaccadic stimulus that is congruent to the presaccadic distractor be taken as a landmark, while the other distractor would be discarded? Therefore, in the next experiment, again a combination of two distractors was presented after the saccade. Now, however, one appeared above and the other below the horizontal meridian. The two vertical positions of the distractors were easy to discriminate for the participant.

3.1. Method

The experimental procedure is illustrated in Figure 3a and b. In this experiment, the presaccadic distractor was presented with equal probability either above or below the target, at a vertical distance of 0 5 from the horizontal meridian. Like in Experiment 2, the single line was replaced at the onset of the saccade by two postsaccadic distractor lines that appeared simultaneously. The distractor lines again had a horizontal separation of 1 6 . Now, however, one of the two lines appeared above and the other below the horizontal meridian. The horizontal distance D of the geometrical centre of the distractor pair and the presaccadic target position was varied between −0 8, 0, and +0 8 . After a blanking interval of 100 ms the target reappeared at one of five distances (−0 8, −0 4, 0, +0 4, or +0 8 ), relative to the spatial centre of the postsaccadic distractor lines.

These different arrangements were presented with equal probability in each block, yielding 60 different conditions (2 presaccadic distractor arrangements × 2 postsaccadic vertical distractor arrangements × 3 distractor displacements × 5 relative target displacements). In each block, each condition was presented twice. Each participant performed 7 of these experimental blocks, distributed over three days (totalling 840 trials per participant). Six participants (all female, aged 22–25 years) took part in the experiment.

In this experiment, we expected that the distractor line that is congruent with the presaccadic vertical distractor position should be mainly considered for the postsaccadic

localisation. For example, in the example presented in Figure 3a, the less eccentric postsaccadic distractor appeared above the target. Consequently, in combination with an upper presaccadic distractor, this distractor line should be preferentially taken as the postsaccadic landmark, while the other distractor line should be largely neglected.

3.2. Results

In total, 5040 trials were run; of these, 187 trials (3.7%) were discarded due to failures of the eye-tracker to track the eye or too short or too long latencies.

The data analysis was similar to the previous experiments. We computed the psychometric functions for each of the different distractor conditions and from these the perceived target displacements, Tperc. The results are displayed in Figure 3c for the postsaccadic distractor configurations in which the distractor line appearing closer to fixation was presented above the horizontal meridian (and, accordingly, the line farther from fixation appeared below the meridian). Conversely, Figure 3d displays the data for the conditions where the closer distractor line appeared below the meridian. In both graphs, the data are plotted separately for the trials where the presaccadic distractor was shown above the target (upright triangles) and below the target (inverted triangles), respectively.

It becomes obvious that the target is again localised at a position in between both distractor lines, indicating that, in contrast to our expectation, both distractor elements are considered in postsaccadic localisation. As in the previous experiment, the visual system uses the centre of gravity of both stimuli as landmark for postsaccadic localisation. A twofactor ANOVA (vertical arrangement × distractor displacement D) revealed a significant main effect of D, F 1 5 = 170 31 p < 0 001, but a non-significant effect of the vertical arrangement, F 1 5 = 0 029 p = 0 1005, and a non-significant interaction, F 2 10 = 0 0004 p = 0 793.

4. Experiment 4

The previous experiment demonstrated that the (vertical) spatial position of the presaccadic distractor – above or below the target – has no effect on the postsaccadic landmark computation. This is even the case when only one of the two postsaccadic distractor lines appears at a (vertically) congruent position – even then both distractor lines are considered about equally in the postsaccadic localisation process. The question therefore arises whether the presaccadic spatial arrangement of target and distractor is considered at all in postsaccadic localisation. Alternatively, the visual system may simply tend to take the position of any localised object found after the saccade as the assumed target location. Some evidence for the latter assumptions came from a recent experiment studying the effect of an irrelevant object presented only after the saccade on target localisation (Deubel et al., 2002, second experiment). As in the present study, the target reappeared after a blanking period. It turned out that the location of the distractor, present when the eyes landed after the primary saccade, was indeed taken by the visual system as the

Ch. 9: How Postsaccadic Visual Structure Affects the Detection of Intrasaccadic Target Displacements 207

position of the (presaccadic) target. In order to test whether the presaccadic allocentric information is indeed stored across the saccade and whether it would affect the localisation performance, we varied, in the final experiment, the horizontal distance between target and distractor in the presaccadic display.

4.1. Method

The sequence of stimulus presentation was similar to that of Experiment 2, except that the presaccadic distractor now appeared either with a forward or a backward displacement pD of 0 4 with respect to the target position (Figure 4a). The postsaccadic distractor pair was again presented at various positions depicted by the distance D of the spatial centre of the distractor pair from the presaccadic target location. D was selected at random from −0 8, 0, and +0 8 . After a blanking interval of 100 ms the target reappeared at one of five distances (−0 8, −0 4, 0, +0 4, or +0 8 ), relative to the spatial centre of the postsaccadic distractors.

Altogether, this resulted in 30 different spatial arrangements of the distractor pair and the target (2 presaccadic distractor positions pD × 3 distractor displacements D × 5 relative target displacements). The different conditions were each presented six times per experimental block, in a random order. The participants performed four blocks in two separate sessions (totalling 720 trials per participant). Six paid participants (5 female, 1 male) participated in this experiment. Their age ranged from 20 to 25 years.

4.2. Results

Altogether, 4320 trials were run; of these, 112 trials (2.6%) were discarded due to failures of the eye-tracker to track the eye or too short or too long latencies.

Figure 4b displays the size of the perceived target displacement Tperc induced by the different distractor displacements D. The data are plotted separately for both presaccadic target/distractor arrangements (for pD = −0 4 and +0 4 , respectively) and fitted with first-order regression lines. The dashed line in the graph represents the regression line through the data points of Experiment 2, that is for pD = 0 . It is obvious from the graph that the presaccadic distractor position relative to the target affects postsaccadic target localisation. So, when the presaccadic target appeared behind the distractor (i.e., the distractor was presented closer to the fixation, pD = −0 4 , there was also a strong tendency to expect the reappearing target behind the centre of gravity of the postsaccadic arrangement. Vice versa, for the positive value of pD (here the presaccadic saccade target appeared in front of the single distractor line), the perceived target displacements are shifted to closer locations. Quantitatively, the spatial distance between both presaccadic distractor positions of 0 8 yields a postsaccadic difference of target localisation of about 0 5 , indicating that about 60% of the presaccadic target–distractor distance is reflected in postsaccadic localisation.

A two-way ANOVA with pD and D revealed a significant main effect of the presaccadic target–distractor displacement pD, F 1 5 = 73 758 p < 0 001, and a significant

Ch. 9: How Postsaccadic Visual Structure Affects the Detection of Intrasaccadic Target Displacements 209

experiment show that transsaccadic localisation is dependent on relational information from before the saccade. If available, relational information in the presaccadic scene is a major determinant of postsaccadic localisation.

5. General discussion

In general, the results of the experiments presented here are well in accordance with our previous findings (Deubel et al., 1998, 2002; Deubel, 2004). They clearly demonstrate that the spatial layout found by the visual system after the saccade forms a major source of information used to establish space constancy. Found objects serve as anchor points to determine the expectations where objects such as the blanked target should appear after a saccade. The results, however, also reveal a few quite unexpected findings which will hopefully deepen our understanding of the perceptual processes that provide visual stability across saccadic eye movements.

The results from our first experiment demonstrate that stimuli found at saccade end serve as landmarks, even if they appear at a considerable distance from their presaccadic position. More specifically, we found that the efficiency of the landmark was about constant as long as it appeared with 25% 1 6 of the saccade size. Unfortunately, our experiments did not extend this range farther to test the limits of the system. However, some of the data reported in Deubel et al. (1998, Experiment 2) suggest that the spatial range within which postsaccadic landmarks are effective may extend even to half of the size of the saccade. This implies that landmarks probably dominate whenever available, overruling the information based on efference copies that would signal a displacement.

In the second experiment, the visual system was confronted with two distractor lines instead of one in the postsaccadic display. Since the visual features of the postsaccadic distractor pair were identical to the presaccadic distractor, it was not possible to decide which of both corresponded to the presaccadic distractor. It turns out that under this condition the visual system neither uses extraretinal signals in combination with the stored position information for the postsaccadic localisation nor does it take one of the distractor lines as the preferred landmark. Rather, the effective landmark position results from a combination of both distractors, suggesting that the spatial centre of gravity of the postsaccadic arrangement is used for the target localisation. This is the case even in Experiment 3 where, due to the vertical spatial arrangement of the postsaccadic distractor pair, preand postsaccadic distractors can be easily related. Obviously, the mechanism which detects the landmark position after the end of a saccade is not particularly selective about the geometric characteristics of the stimuli. The spatial layout of the reference object is rather unimportant when searching for the postsaccadic reference.

The finding that postsaccadic localisation results from a centre of gravity of both distractors is reminiscent of the so-called “global effect” in saccadic eye-movement programming. The global effect relates to the observation that, when a fast saccade is performed to a configuration of a target and a distractor, the eyes often land at a position intermediate to both objects, considering relative target properties such as size

and brightness (Deubel, Wolf, & Hauske, 1984; Findlay, 1982). This suggests that the saccadic amplitude computation is based on stimulation integrated over a rather wide area of visual space and involves integration of the visual signals in a relatively “raw” form. It has been suggested that the physiological substrate of saccade averaging may be the superior colliculus (SC) in the midbrain which uses a distributed population code to represent visual and oculomotor direction. So, Robinson (1972) and others have shown that simultaneous stimulation of two separate locations in the SC will produce a saccade which is a vector average, much in line with the global effect. The SC is now also known to be part of an attentional network, to draw visual attention efficiently to visual onsets in the periphery (e.g., Kustov & Robinson, 1996; Krauzlis, Liston, & Carello, 2004). So, it may be assumed that the locus of visual attention finally provides the spatial pointer that is also responsible for transsaccadic localisation.

An important question addressed in Experiment 4 was whether the visual localisation system would at all use the relational information of target and distractors from before the saccade, stored across the eye movement, to generate an expectation of the postsaccadic target location. Alternatively, the visual system may simply tend to take the position of any localised object or of a configuration found after the saccade as the assumed target location. The results show that the latter is clearly not the case. In Experiment 4, presaccadic (horizontal) distractor–target distance was varied. The data clearly demonstrate that perceptual localisation was indeed strongly affected by the relative arrangement of target and distractor stored across the saccade. It can be concluded that presaccadic, relational information in the direction of the saccadic eye movement is a major determinant of postsaccadic localisation. Only if this information is missing, as in the case of a landmark appearing only after the saccade (Deubel et al., 2002), the postsaccadic landmark itself is taken as the anchor for the localisation process. These findings imply that quite accurate relational information about the relative positions of a few objects in the visual field is stored in a transsaccadic memory and used after a saccade. Independent evidence for a precise transsaccadic memory of relative spatial positions have been provided in studies on the effect of contextual cues on the transsaccadic coding of objects. So, Verfaillie and De Graef (2000) showed that displacements of a target that brought it towards another object were easier to detect than changes that moved the target away from another object. Currie, McConkie, Carlson-Radvansky and Irwin (2000) found that detection of a location change of a saccade target across an eye movement can be made on the basis of both the target’s change of absolute position, and a change in the spatial relations formed by the target and its neighbours. Carlson-Radvansky (1999) demonstrated that relational information in scenes composed of geometrical figures are encoded before the saccade and retained in a transsaccadic memory.

Our findings have some implications for theories of visual stability around saccades. A few theories of visual stability have emphasised the special role of the postsaccadic visual layout for perceptual stability across saccades (e.g., Currie et al., 2000; Deubel et al., 1996). Deubel et al. (1984) were the first to propose that a transsaccadic memory representation of the saccade target may serve to relocate visual objects across saccades. In more recent work, we (Deubel & Schneider, 1994; Deubel et al., 1996, 1998) developed a

Ch. 9: How Postsaccadic Visual Structure Affects the Detection of Intrasaccadic Target Displacements 211

“Reference Object Theory” which assumed that preand postsaccadic visual “snapshots” are linked by means of the visual target structure which is assumed by the visual system as being stable. The theory states that with each new fixation the visual system runs through a sequence of processing steps that starts with the selection of one object as the target for the next saccade. Particular features of the saccade target and a few surrounding objects are selected and stored in a transsaccadic memory to facilitate its re-identification at the start of the next fixation. When the eye has landed after the saccade, the visual system searches for the critical target features within a limited region around the landing site. If the target object is found, the relationship between its retinal location and its mental representation is compared in order to coordinate these two types of information. If the postsaccadic target localisation fails, however (e.g., because the intrasaccadic target shift was too large or the target was absent), the assumption of visual stability is abandoned. As a consequence, a target displacement is perceived.

The present findings allow the specification of the theory in two important aspects. The first specification concerns the finding of this investigation and also of our previous studies on the landmark effect that not just the saccade target, but also other (distractor) objects can serve as spatial references (Deubel et al., 2002; Deubel, 2004). Whether an object is defined as the target or as a distractor before the saccade seems to play little role in the postsaccadic determination of the reference object. More critical for the selection of a postsaccadic object as a reference is a temporal constraint, namely its presence right at the time when the eyes land. This demonstrates that temporal continuity of an object is more important even than selection as a saccade target in establishing a reference object. The second important specification is related to the here-established fact that, when more than just a single stimulus is present at saccade end, the spatial centre of gravity is taken as the landmark position. Obviously, the mechanism postulated in the theory that searches for the critical features is far from being selective. This may imply that the mapping of preand postsaccadic information is based on spatially crude, low spatial frequency information.

Acknowledgements

This work was supported by the Deutsche Forschungsgemeinschaft (Grant De336/2). We wish to thank Birgitt Assfalg for her valuable help in running the experiments, and Ben Tatler for very insightful comments on an earlier version of the paper.

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