¶ … boundary extension phenomenon is likely one of many evolutionary adaptations that serve to improve perception of visual phenomena beyond the strict limits of what is actually being observed. Thus, interrupted visual data is filled in on the basis of experience or on the basis of expectations based on the visual data that is actually observable. It has been known for almost a century that infants develop this ability or expectation at a certain age and that this ability corresponds to curiosity when an object appears to appear or to disappear unexpectedly. Prior to this stage, there is no expectation at all. The simplest example is the manner in which rapidly presented still images in a motion picture consisting of sequences convey the perception of movement even though each individual photograph or drawing is actually a still image. It is highly unlikely that the boundary extension phenomenon is limited to the relationship between static objects in a visual field. In fact, it is quite likely that the same phenomenon, or analogous elements of it, also exist in other forms of visual recognition, such as in relation to recognizing the faces of strangers depicted in a photograph. More specifically, subjects might falsely recall having seen more of a face than they actually observed based on expectations consistent with features they did, in fact, observe. Obviously, this aspect of the boundary extension phenomenon would have tremendous importance in the use of witness testimony to identify criminal defendants. The hypothesis that the boundary extension phenomenon also applies to the accuracy of facial recall could be tested in several ways. Subjects could be presented with glimpses of only parts of faces such as eyes or hairlines or smiles and then tested to determine what other (unseen) features they assume would correspond to those features that they actually saw glimpses of.
