The human brain faces a fundamental challenge in visual processing: inferring whether input arises from a single object continuing and changing across time, versus multiple distinct objects, occupying the same location at different points

The human brain is continuously confronted with dynamic visual input, and from this it must infer whether input belongs to a single versus multiple object identities across time. Object substitution masking (OSM), in which perception of a target stimulus is impaired by a temporally-trailing four-dot mask, reflects a failure to segment the target and mask as discrete objects. According to Bouvier and Treisman (2010), OSM only occurs for targets that require binding multiple separate features (e.g., colour and orientation) in order to be identified. In contrast, a target that represents a unique feature is thought to be impervious to masking. Here, however, we show that a single orientation target (a Gabor) is susceptible to masking with an orientation-discrimination task, but only when the mask is similar in orientation to the target. That is, target-mask similarity, rather than target complexity determines masking. A re-examination of Bouvier and Treisman’s results show that they can be explained within this target-mask similarity perspective. This means that the similarity of two objects determines whether they will be integrated or segmented across time, rather than the complexity of one of the objects in isolation.