UBC Theses and Dissertations
Selective attention in object substitution masking Tata, Matthew S.
Backward masking in visual perception occurs when the visibility of a brief target stimulus is decreased by interference from another visual object appearing at the same location. Of particular interest is a type of backward masking known as object substitution, which has been shown by Di Lollo, Enns, and Rensink (1999) to be modulated by factors that also influence visual attention. The results of three experiments are presented here that corroborate their findings. It is shown that object substitution increases with the number of distractor items in the display, decreases when the target location is validly pre-cued, and is eliminated when the target can be located rapidly in the display. A theory accounting for this phenomenon has been proposed by Di Lollo and colleagues. Implicit in the theory is the notion that the mask becomes the focus of object perception mechanisms in the brain. Recent advances in the understanding of the neural mechanisms mediating attentional selection of visual objects are consistent with this theory. It is argued on theoretical grounds that attentional selection of the mask is likely to be involved in object substitution. Finally, the results of three experiments are presented that lend support to this claim. It is shown that object substitution is eliminated by the presence of distractors that divert attention from the mask at the target location. Furthermore, a mask that precedes the target in the display sequence is shown to be ineffective unless its contours are hidden in the background during the preview period. This is consistent with the findings of recent investigations into the attentional capture phenomenon and suggests that the mask may capture attention during visual search for the target.
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