Effects of Similarity in Sustained Inattentional Blindness
This page contains some sample demonstrations from the following paper:
Most, S. B., Simons, D. J., Scholl, B. J., Jiminez, R., Clifford, E., & Chabris, C. F. (2001). How not to be seen: The contribution of similarity and selective ignoring to sustained inattentional blindness. Psychological Science, 12(1), 9-17.
The demonstrations are provided as Quicktime movies, which can be downloaded or viewed directly in most web-browsers. These movies are a bit large and choppy, but they should be sufficient to illustrate the basic conditions. If the movies seem too choppy or if the lines are not drawn smoothly, try downloading the movies and playing them off your local hard drive.  
In typical 'inattentional blindness' phenomena, observers fail to perceive objects in briefly flashed scenes under certain conditions. We have generalized and explored this phenomenon in a dynamic computer-controlled context, with displays containing independently and unpredictably moving white and black shapes. In these trials, the observer must count the number of times that the white shapes (but not the black shapes) bounce against the edges of the display. On the third trial, an 'unexpected event' (UE) occurs: a new shape suddenly enters and moves across the display for 5 seconds. Even when the UE has a novel shape, color, luminance, and type of trajectory, 30% of subjects completely fail to perceive it! (In contrast, all observers see the UE when not engaged in an attentionally demanding task.) This phenomenon underscores the importance of attentional selection as a 'gateway' to conscious perception. This method allows us to parametrically explore the visual features which capture attention and thus result in conscious perception.  
For example, in these experiments we have shown that featural similarity plays a decisive role: subjects are much more likely to notice the UE when it is featurally similar to the attended items, and much less likely to notice it when it is featurally similar to the ignored items (where these two factors are unconfounded).  
No UE Trial (560 KB)  
No UE Trial (Smaller 356 KB movie)  
UE = White Shape (1.5 MB)  
UE = White Shape (Smaller 380 KB movie)  
UE = Novel Red Shape (588 KB movie)  
UE = Novel Red Shape (Smaller 396 KB movie)