Supporting Information:
A new technique (Manipulation of Overlapping Rivalrous Images by polarizing filters; MORI) was invented for presenting two different images on the same screen that can be viewed separately by two groups without them noticing the overlap. Two polarizing filters placed perpendicular to each other provide separate invisible channels from a pair of video projectors to two groups of viewers on a single screen. It can easily be used to create artificial conflicts among viewers. The basic principle of the presentation technique, details of the apparatus, and limitations will be presented along with a series of eyewitness experiments as sample applications of this technique.
The MORI technique uses the polarization properties of light. A polarizing filter allows only light of one polarization through. Therefore, once light passes through one of these filters, it cannot pass through another polarizing filter placed perpendicular to its polarization. Two images polarized in directions perpendicular to each other and projected on the same screen can be seen overlapped to the ordinary eyes, but if a viewer wears a pair of polarizing sunglasses, only one image is seen while the other image is filtered out.
Thirty undergraduate pairs observed two versions of movies of the same basic content with three non-conforming points secretly inserted using the MORI technique. They were asked to report individually on what they had seen just after the presentation. Then they were allowed to discuss the event they had just observed. Fifteen pairs of the participants were instructed to make a unified group report whereas the other fifteen pairs were instructed to discuss what they had seen and to report individually. A week later, participants were asked to report what they had seen the week before. Finally, they were asked whether they had noticed the fact that they had seen two different versions.
No participants reported that they noticed the presentation trick in the post-experimental interview. The fact that the participants did not suspect anything was also inferred from the discussion sessions; all groups ended up reaching an agreement even on the three conflicting points. The week-later reports also showed some evidence of their belief that all had watched the same video. Once they reached an agreement, those who changed their minds tended to report the distorted memory a week later again. Even those who changed their minds after discussion rated high on the confidence scale in their week-later reports. These evidential behaviors were observed among all participants. The results were replicated in subsequent experiments with varying eyewitness groupings and presentation conditions. The results of these experiments have provided evidence of the effectiveness of this technique with various projectors, different video materials, and viewers in different group sizes and ages.
The full version of this study is to appear in Behavior Research Methods, Instruments, and Computers, under the title "Surreptitiously projecting different movies to two subsets of viewers."
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