Presentation of the stimuli and recordings of responses were controlled by a Macintosh G3 computer. Stimuli were displayed on a 19-inch color monitor. The following experiments used the same apparatus. Stimuli (23° × 18°) were color photographs of common scenes (e.g. photographs of parks, shopping streets). Of the 100 images created, 99 were used. Three experimental conditions were employed. Under the normal condition, normal photographs were used. Under the jumble 6 condition, photographs were divided into six sections, and the six sections were rearranged, with only the section that included the change region remaining in its original position. Under the jumble 24 condition, photographs were divided into 24 sections, and the 24 sections were rearranged, with only the section which included the change remaining in its original position. Under all conditions, black lines were drawn along the boundary regions of the 24 sections. This condition was introduced to control for the effect of boundary lines under the jumble condition (See
Figure 1). Modified images were created by adding one change to the original images. The extent of change was limited so as not to exceed one section created by dividing the images into 24 sections. There were three types of changes made (color change, positional change, and absence of object). Modifications of images were made so that changes were clearly visible once participants noticed them. We avoided subtle changes as much as possible. The degree of interest was tested for every object being changed in 99 of the 100 scenes used in all three experiments plus one scene only in experiments 2 and 3. Interest was determined via an independent pilot experiment in which five naïve participants provided a brief verbal description of each scene. Following the example of
Rensink et al. (1997), central interests were defined as objects and areas mentioned by three or more observers. As a result, 17 percent of 100 objects turned out as of central interest, if we used a broad criterion. However, the change for objects of central interests was limited to a relatively small part, because the extent of change did not exceed one section created by dividing the images into 24 sections. For example, participants selected a motorbike to be of central interest, which occupied many parts of 24 sections in a scene, but the changing part was its rearview mirror, which no one selected. If we used a narrow criterion, any object would not reach the status of having central interest.