PURPOSE: A visual search for targets is facilitated when the target objects are on a di?erent depth plane than other masking objects cluttering the scene.
METHODS: The ability of observers to determine whether four letters, presented stereoscopically at four symmetrically-located positions on the fixation plane, were the same or di?erent, was assessed when the target letters were masked by other randomly positioned and oriented letters appearing on the same depth plane as the target letters, or in front, or behind it. Three additional control maskers, derived from the letter maskers, were also presented on the same three depth planes: 1) random-phase maskers (same spectral amplitude composition as the letter masker but with the phase spectrum randomized); 2) random-pixel maskers (the locations of the letter maskers’ pixel amplitudes were randomized); and 3) letter-fragment maskers (the same letters as in the letter masker but broken up into fragments).
RESULTS: Performance improved with target duration when the target-letter plane was in front of the letter-masker plane, but not when the target letters were on the same plane as the masker, or behind it. A comparison of the results for the four di?erent kinds of maskers indicated that maskers consisting of recognizable objects (letters or letter fragments) interfere more with search and comparison judgments than visual noise have the same spatial frequency profile and contrast. In addition, performance was poorer for letter maskers than for letter-masker fragments, suggesting that the letter maskers interfered more with performance than the letter-fragment maskers because of the lexical activity they elicit.
CONCLUSIONS: The degree of binocular unmasking is greater when the target appears to be in front of rather than behind the masker in the letter discrimination task. These results contrast with those that indicate that when the target is presented foveally (at the point of fixation) its detectability appears to be independent of whether it is perceived in front of or behind the masker. Hence pattern recognition but not target detection is sensitive to the perceived spatial relationships of maskers to targets. The fact that similar results were obtained for meaningful (whole letters) as for meaningless (letter fragments) maskers indicates that the interference from the masker occurs at a pre-lexical stage of processing.