Spatial Asymmetries in the Flanker-Congruency Effect: How Flanker Orientation Biases Attentional Scanning

Place a target between distractors and the response changes depending on what those distractors say. When the flanking items match the target, people respond faster. When the flankers call for a different response, performance suffers. That much has been established for a long time. What earlier work with ordinary letters also found is that the interference is not evenly distributed across space: flankers sitting to the left of the target push the effect around more than flankers sitting to the right.

The experiments reported here were built to figure out where that left-side advantage actually comes from. The first one tested a simple manipulation. Keep the target as a normal letter, but mirror-reverse the flankers. When that was done, the usual left-side bias disappeared. So the asymmetry isn't fixed to a location in the visual field; something about the appearance of the flankers is driving it.

The second experiment let the orientation of the whole stimulus string change from one trial to the next, at random. Here the direction of the bias followed the orientation of the characters. Trials with normal letters produced the familiar lean toward the left-side flanker. Trials with mirrored letters pulled the other way, tending to produce a right-side bias instead.

A further detail sharpens the picture. These orientation-linked biases only showed up when the flanker orientation on a given trial repeated the orientation from the trial before. When orientation switched between trials, the bias did not appear. That pattern is hard to square with the idea that the asymmetry reflects how the target itself is processed, or some strategy observers adopt on the fly to select the relevant item.

Hommel reads the results differently. Processing flankers that have a particular orientation seems to leave the cognitive system set to scan the next set of flankers in a particular direction. The carryover from one trial to the next, and its dependence on repeated orientation, is what you would expect if orientation tunes the direction of attentional scanning rather than tagging a privileged side of space.