This occurs when a brief display of target plus mask is continued with the mask alone. In a real sense, the initial pattern (target plus mask) is masked by parts of itself (mask alone). We believe that this is an outcome of iterative reentrant processing in the visual system (see My Research in Vision and Visual Attention). We call it "object substitution" because it occurs when there is a mismatch between the reentrant visual representation and the ongoing lower-level activity produced by current sensory input. For a thorough discussion of this fascinating and counterintuitive phenomenon, see Di Lollo, Enns, & Rensink (1999) as well as Matt Tata's Web Site.

The mask need not be eye-catching (just four small dots will do), and it does not need to be close to the target.

For object substitution to occur, two conditions are required:

1. Attention must be distributed among many potential targets.
2. The mask must remain alone on the screen after the target has been turned off. Object Substitution Demo

Equally informative are situations in which object substitution does not occur:

1. No masking occurs if there is only one target in the display. Single Target Demo
2. No masking occurs if the target and the mask are turned off together. Simultaneous Offsets Demo