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Benjamin Lester, Paul Dassonville; Attentional filtering modulates the induced Roelofs effect, but shifts of attention do not cause it. Journal of Vision 2009;9(8):227. doi: 10.1167/9.8.227.
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When a visible frame is offset left or right from an observer's objective midline, the subjective midline is pulled toward the frame's center. Targets presented within this frame are then misperceived as being shifted in the opposite direction - an illusion known as the induced Roelofs effect (Bridgeman et al. 1997; Dassonville & Bala 2004). However, a large frame is not necessary to generate the effect - even a small peripheral square is sufficient, raising the possibility that the effect is driven by a shift of attention toward the center of the frame. As a first test of this hypothesis, we set out to determine whether the illusion would be affected by a manipulation known to affect attention; namely, the color-contingency effect. In Experiment 1, a target (defined by its color) was presented with 3 variously-colored distractors, along with a Roelofs-inducing frame that was either the same color as the target, or a distractor color. We found that the attentional filtering that was required to isolate the target from the distractors also modulated the magnitude of the Roelofs effect, with a larger effect when the frame and target were the same color. To more directly assess the relationship between shifts of attention and distortions of the apparent midline, Experiment 2 adopted a dual-task design that first drew the participant's attention to the left or right, then presented a near-midline target whose location was to be reported. If shifts of attention cause the midline distortion associated with the Roelofs effect, the participant's perception of target location should vary as a function of the location of the earlier attentional cue. Results indicate that this was not the case, suggesting that while attention can modulate the magnitude of the Roelofs effect, the effect is not directly caused by shifts of attention.
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