Determining priority between attentional and referential-coding sources of the Simon effect through optokinetic stimulation

The "Simon effect" is the performance advantage for spatially corresponding target-response ensembles that is observed when coding of target position is irrelevant for the selection of motor responses. The "attentional-shift" account of the Simon effect holds that it arises from the congruency between response location and the direction of the last shift of attention toward the target. The "referential-coding" account traces the origin of the Simon effect back to the congruency between the response location and the position of the target with respect to a spatial reference frame. We were able to contrast these two hypotheses using full-field horizontal optokinetic stimulation (OKS). It was shown that OKS moving in one horizontal direction drives covert orienting of attention toward the side of arrival of OKS, i.e. the "In-coming" side, which is opposed to the direction of OKS motion toward the "Out-going" side (Teramoto et al., 2004; Watanabe, 2001). We therefore asked healthy participants to discriminate between slow and fast velocities of leftward or rightward OKS. "Fast" and "slow" responses were associated to response buttons positioned in the left or right side of space. The "attentional-shift" account of the Simon effect predicts that response compatibility should be related to the direction of the attentional shift induced by OKS, i.e. in the direction opposite to OKS motion. By contrast, the "referential-coding" hypothesis predicts that response compatibility should be related to the direction of OKS displacement with respect to its starting position. We observed faster RTs when the response button was on the "In-coming" side of space, opposite to the direction of OKS motion. This result supports priority of attentionall over referential-coding factors in the genesis of the Simon effect. (C) 2009 Elsevier Ltd. All rights reserved.