The visibility of a target pattern can be affected by the presence of an adjacent stimulus (flanker). Here, we showed that such lateral masking effect depends on both the phase and the position of the flanker. The task of the observers was to detect a 4 cy/deg vertical Gabor target with the presence of vertical flankers. The target was in sine phase related to the fixation. The flankers had either the same or the opposite phase as the target and located at either the collinear or the side position from the target. The distance between the target and the flanker varied from 2 to 7.5 wavelengths. We measured the target threshold at 75% correct level. Compared with the no-flanker condition, the same-phase collinear flankers and the opposite-phase side flankers significantly decreased target threshold while the other flankers had little effects. Hence the side flanker has an effect only if the bright region of the target is adjacent to the bight regions of the flanker, and the dark region of the target to the dark region of the flanker. This result is consistent with the notion of border ownership in which facilitation occurs when the target and the flankers signal the same border.
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