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 ensemblesthat 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 responselocation and the direction of the last shift of attention toward the target. The “referential-coding” accounttraces the origin of the Simon effect back to the congruency between the response location and theposition of the target with respect to a spatial reference frame. We were able to contrast these twohypotheses using full-field horizontal optokinetic stimulation (OKS). It was shown that OKS moving inone horizontal direction drives covert orienting of attention toward the side of arrival of OKS, i.e. the “Incoming”side, which is opposed to the direction of OKS motion toward the “Out-going” side (Teramoto etal., 2004; Watanabe, 2001). We therefore asked healthy participants to discriminate between slow andfast velocities of leftward or rightward OKS. “Fast” and “slow” responses were associated to responsebuttons positioned in the left or right side of space. The “attentional-shift” account of the Simon effectpredicts that response compatibility should be related to the direction of the attentional shift induced byOKS, i.e. in the direction opposite to OKS motion. By contrast, the “referential-coding” hypothesis predictsthat response compatibility should be related to the direction of OKS displacement with respect to itsstarting position.Weobserved faster RTs when the response button was on the “In-coming” side of space,opposite to the direction ofOKSmotion. This result supports priority of attentional over referential-codingfactors in the genesis of the Simon effect.