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

Figure 9. (a) Two examples of correct prediction by the safe corridor identification model. The line represents the predicted travel path, which coincides with the observed travel path. The open circles on the line represent the locations where the model was updated to plan for the next path segment. (b) Two examples of unsuccessful path prediction (solid line), along with the actual travel path (dashed line).

This model was able to successfully predict 90% of the travel paths, significantly higher than the on-line model or the avoid-a-crowd model (F2 8 = 13 13 p < 0 001; Figure 7). Two successfully predicted paths are shown in Figure 9a. The 10% of paths that were not correctly predicted were different between participants. This indicates that it is individual variation and not the generalizability of the model that accounts for less than 100% success. Two examples of unsuccessful predictions are shown in Figure 9b. In both examples, the participant’s initial direction of travel was appropriate for the second

exit point, but the travel direction was corrected later, leading to a different path. It is possible that in these trials the participants did not fixate on the required exit point and initiated their travel based on incorrect information.

The weights assigned to the four factors that define the safe corridor to be taken clearly highlight that minimizing the number of direction changes alone does not predict the paths taken by individuals. The size of both the obstacle free areas and the corridor widths influence the selected path. In contrast to the avoid-a-crowd model, in this model there is on-line planning of path segments. The number of path segments depends on the environmental features: the more cluttered the environment, the greater the number of path segments (see Figure 9a). The gaze fixation data are consistent with the model: intermittent fixations between the goal and the path/pylon region enable participants to plan the next path segment and/or check if the planned path segment is safe and appropriate.

Conclusion

Gaze fixation data reveal how visual information is used for safe passage around obstacles during goal-directed locomotion. Behavioral data alone do not tell us what information is important and when it is used. While models of route selection are useful to formalize the putative strategies used for travel, accuracy of prediction alone is not enough to validate them. Gaze fixation data can provide support for the models and guide the development of new models. The current study is an initial attempt to relate gaze fixation data to routeselection strategies. The eyes are indeed windows into the visual-motor transformations that are essential for adaptive human locomotion.

Acknowledgements

S. Sebastian Tomescu is, at the time of writing, at Queen’s University. Supported by grants from NSERC, Canada and Office of Naval Research, USA. We acknowledge the detailed insight provided by Dr Roger Van Gompel.

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Abstract

How do magicians misdirect their audiences? We recorded eye movements as observers watched a magician perform a trick on a live one-to-one basis. All observers watched the trick twice. Half of the observers were informed in advance that they would be watching a trick; half were not. Observers tended to follow the magician’s gaze, particularly in the second half of the trick. Even informed observers were susceptible to the magician’s social cues for joint attention, following his gaze during the trick. While knowing that they would be watching a trick was not sufficient for observers to defeat the magician’s misdirection, watching the trick a second time was; all observers were able to describe how the magician made a cigarette disappear after viewing the trick a second time. Our findings not only demonstrate an everyday example of inattentional blindness, but also that social cues for joint attention provide the magician with a powerful means of misdirecting his audience successfully.

Picture this: you are at a magic show and the magician announces that he is going to make a donkey appear behind a curtain in the middle of the stage. He walks to the curtain, which is lying on the floor, and raises it above his head. A moment later he drops the curtain to the ground to reveal a real live donkey behind it, to the amazement of you and the rest of the audience. How can the magician have performed such an impressive trick?

Now let us watch the trick again to reveal its secret: when the magician raises the curtain above his head, his glamorous (and probably scantily clad) assistant walks across the front of the stage. As they do this, the magician replaces the curtain on the floor, walks to the side of the stage, collects a donkey and drags it over behind the curtain. The magician raises the curtain again and waits for his moment to drop it to reveal the ‘magically appeared’ donkey. You are somewhat less impressed. Surely this is not the same trick? Surprisingly, it is exactly as it was performed the first time. How could it be that you missed such an obvious act as the magician walking over to drag a donkey on stage?

Ridiculous though it may seem, this illustrates the way in which many magic tricks are performed: the magician diverts the observers by directing their attention to a distracting act, while at the same time performing what would be an otherwise obvious act (see Lamont & Wiseman, 1999). Magicians have for many years accomplished their misdirection by combining processes that psychologists have learnt about only recently: our tendency to look where others look – social attention – and our inability to spot rather obvious events under certain circumstances – change blindness and inattentional blindness.

The fallibility of our visual sense in detecting unexpected events has become the focus of particular interest in recent years. Observers can fail to notice what would seem otherwise to be a very large change to a complex scene provided that change is accompanied by a brief interruption to viewing (for a review, see Rensink, 2002). These changes can include changing the colour of an object, moving it to another position in the scene or removing it completely. Our surprising inability to detect seemingly obvious changes has become known as change blindness, and can be observed when changes are made during an eye movement (e.g., Grimes, 1996; McConkie & Currie, 1996), a blink (e.g., O’Regan, Deubel, Clark, & Rensink, 2000), or an artificial flicker of the image (Rensink, O’Regan, & Clark, 1995, 1997, 2000).

However, it is not only abrupt changes occurring during periods where the scene is occluded that can go unnoticed by observers. Unexpected events, lasting for several seconds, can occur in full view of an observer and yet not be detected. Professional pilots can fail to notice an aeroplane across the runway in a simulator and go on to land through it (Haines, 1991). Observers watching teams pass a basketball to each other can fail to notice a person in a gorilla suit (Simons & Chabris, 1999) or a woman carrying an umbrella (Neisser, 1967) walk through the midst of the players. Such failure to detect unexpected events that occur in full view has become known as inattentional blindness (see Mack & Rock, 1998), and is thought to arise because attention has been allocated to a particular task and subset of objects in the visual display.

An important concern in studies of change blindness and inattentional blindness has been to relate these phenomena to everyday vision and to consider their implications for how vision operates normally (see Most, Scholl, Clifford & Simons, 2005, for a similar position). Researchers have attempted to devise more realistic situations in which to explore these phenomena (e.g., Levin & Simons, 1997; Simons & Levin, 1998). However, even these situations remain somewhat removed from ‘normal’ experience. A more familiar situation in which these phenomena can be studied, and one that many of us have experienced, is magic. The misdirection employed by magicians in many of their tricks parallels inattentional blindness paradigms; it occurs seemingly in full view of the observer, yet is not noticed.

For the magician to be successful, the audience must be misdirected when the crucial part of the trick is performed. How does the magician do this? From an early age our attention is strongly influenced by other people. This tendency to attend to locations indicated by others is known as shared (or joint) attention (e.g., Tomasello, 1995, 1999). A particularly strong cue for shared attention is where somebody else is looking; we show a strong and somewhat automatic tendency to follow someone else’s gaze (e.g., Driver et al., 1999; Langton, Watt, & Bruce, 2000; Scaife & Bruner, 1975; Triesch, Teuscher, Deák, & Carlson, 2006). Indeed, under certain conditions people automatically imitate another person’s gaze (Ricciardelli, Bricolo, Aglioti, & Chelazzi, 2002). Perhaps this is how the magician misdirects us? If social cues strongly influence how we direct our attention, then doing something as simple as looking away from the act that the magician wishes to conceal, might be enough to make us attend to the wrong location and so miss an act that otherwise would have been obvious.

In this chapter we consider the details of how a magician (the second author, GK) achieves his misdirection when performing a simple magic trick. The way in which the trick was performed will now be described.

1. Our magic trick

We developed a trick in which the magician made a cigarette and lighter ‘disappear’. The trick was performed ‘live’ by the magician, in front of the observer, on a one-to-one basis.

Figure 1 shows the typical progress of the trick. Facing the observer, the magician asks if they would mind if he smokes. He looks at and reaches for the cigarette packet and removes a cigarette. While moving the cigarette towards his mouth with one hand (the ‘cigarette hand’), he turns to look at the lighter and reaches for this with his other hand (the ‘lighter hand’). The cigarette is deliberately placed in the mouth the wrong way around. The magician then brings the lighter up to attempt to light the cigarette. There then follows the first misdirection by the magician. ‘Noticing’ that the cigarette is the wrong way around, the magician turns his head to the side, removes the cigarette and turns it around to replace it. Throughout this manoeuvre the magician keeps his gaze fixed on the cigarette and the hand manipulating it. While performing this manipulation of the

cigarette, the magician lowers his other hand, holding the lighter, towards the tabletop and drops the lighter onto his knees. The magician then brings his (now empty) hand back up to his face, while turning his head back towards the observer. He attempts to light the (now correctly positioned) cigarette and makes a show of being surprised that the lighter is no longer in his hand. This surprise is used to accomplish the second misdirection of this trick. Upon ‘noticing’ that the lighter is no longer in his hand, the magician turns his head to the side, raises his (now empty) lighter hand and opens it while looking at it. At the same time, the magician lowers his other hand, which is holding the cigarette, towards the tabletop. When it is near the tabletop, the magician drops the cigarette onto his knees. When dropped, the cigarette is usually about 10–15 cm above the tabletop and is therefore dropped in full view of the observer, visible for about 140 ms of its drop (calculated from the video records of the trick – see below). Shortly after he opens his empty lighter hand, the magician brings the now empty cigarette hand back up toward his face and turns to look at it, feigning surprise that the cigarette has also ‘disappeared’. Finally, the magician turns back to face the observer (a video clip of this trick can be viewed online at http://www.dur.ac.uk/gustav.kuhn/.)

2. The present study

In a previous report, we considered whether the success of the magician’s misdirection at the time of the cigarette drop in the above trick was dependent upon an overt misdirection of the eyes or a covert misdirection of attention (Kuhn & Tatler, 2005). In general, observers failed to spot how the trick was performed when they watched it for the first time, but all spotted how it was done when they watched the magician perform the trick for a second time. Detecting the cigarette drop was not dependent upon where the observer was fixating as it dropped; on trials where the drop was spotted, observers were still often fixating the lighter hand (as would be predicted for a successful misdirection). This result implied that it was the deployment of covert visual attention rather than overt gaze that determined whether the observer detected the cigarette drop.

One question raised by the Kuhn and Tatler study is how the magician achieves his misdirection. By considering the relationship between the magician’s gaze and the observer’s gaze we can evaluate the role of social cues (shared or joint attention) during the trick and whether these form the basis for the magician’s success at misdirection. Each participant watched the magician perform the trick twice. This allowed us to consider whether different gaze strategies are employed to observe the trick for the second time. Performing the trick a second time also allows us to consider whether the observer can overcome the normal tendency for joint attention and not attend to the locations intended by the magician. We divided our participants into two groups: half were told before the experiment that they were about to watch a magic trick in which a cigarette and lighter would be made to disappear and that they should try to work out how this was done. The remaining observers did not know that they were about to watch a magic trick. Given that task can have such a profound effect upon where observers fixate when

viewing static scenes (e.g., Buswell, 1935; Yarbus, 1967), we expected prior knowledge to have a significant impact upon how observers viewed the performances. We expected uninformed observers to be more prone to the misdirection and thus follow the magician’s gaze more closely than informed observers. Our hypothesis for the combined effects of prior knowledge and repetition is that these two factors will interact such that informed observers are more able to ignore the magician’s social cues for joint attention and that both groups are less prone to this misdirection on the second performance of the trick.

3. Procedure

Twenty participants (mean age = 21 65 SD = 6 3) took part in this experiment; half of whom knew in advance that they were about to watch the trick, half of whom did not. Each observer saw the trick performed twice.

Eye movements of the observers were recorded while they watched the magic trick, using Land’s custom-built head-mounted eye-tracker (for details, see Land 1993; Land & Lee 1994). We defined seven possible gaze target regions: the magician’s face, the cigarette hand, the cigarette packet, the cigarette, the lighter hand, other positions on the magician’s body, and other locations in the scene (such as items on the table that were not related to the trick). Consecutive fixations within a single region were summed and treated as a single gaze duration. Data from two of the observers were discarded at this stage due to poor quality of the recorded videos. The direction of the magician’s gaze was determined from the participant’s eye-movement video record, by observing the magician’s head movements throughout the performances of the trick. This measure is crude, but can be used to achieve a reasonably good idea of what the magician is looking at throughout the trick.

We were interested in how both prior knowledge and repetition of the trick influenced viewing strategies. Because we did not want to confound our data with any strategic differences that might arise in association with detecting the cigarette drop, we excluded the data for the two observers who spotted the cigarette drop on the first performance of the trick. The others all spotted it on the second performance. Eight of these remaining observers were previously informed that they would be watching the trick and eight were uninformed.

Given that the trick was performed live to each observer, it is important to ensure that there is a reasonable degree of consistency in the way in which this trick was performed. We divided the trick up into a series of easily identifiable actions carried out by the magician (depicted in Figure 1) and recorded the time at which these happened throughout each performance. We found no interaction between performance number (the two performances of the trick) and the prior knowledge of the observers on the timings of the actions performed by the magician throughout the trick, F7 8 = 0 660 p = 0 701. Similarly, we found no interaction between performance number and the prior knowledge of the observers on the times at which the magician moved his gaze to each of the targets depicted in Figure 1 throughout the trick, F7 8 = 0 436 p = 0 866. Thus, there were no