27th European Conference on Visual Perception location:Budapest, Hungary date:August 2004
When 4 dots surround an unattended target and their offset is delayed relative to the target's offset, the non-overlapping trailing dots act as a mask. This phenomenon is known as object substitution masking (OSM) (Enns and Di Lollo, 1997 Psychological Science 8 135 - 139), but so far had not been demonstrated for realistic objects. We used 26 ms exposures of circular arrays of 8 object line drawings, displayed at 4° eccentricity. During presentation, one object (target) was surrounded by 4 dots. After 26 ms, all objects disappeared, while the dots remained on for durations ranging from 0 ms (common offset) to 300 ms. Participants' task was to determine whether the target was oriented to the left or the right. In addition to trailing mask duration, we varied set size and target - distractor distance. Performance revealed an OSM effect: as trailing mask duration increased, discrimination of object orientation decreased. In addition, performance suffered when displays contained more distractors. This effect occurred even in the common offset condition, indicating that set size did not increase target - distractor confusion after display offset. Instead, it affected target discrimination before display offset, as suggested by a more adverse effect of closer distractors.