Looking for the source of the simon effect: evidence of multiple codes.

The Simon effect (SE) usually is described as the performance advantage that results when a target and its associated response share the same spatial code, as opposed to when they do not, even when the target's spatial code is task-irrelevant. To some, this task-irrelevant code represents the location of the target with respect to the participant (Simon & Craft, 1970), whereas to others (Umiltà & Nicoletti, 1992) it represents the location of the target with respect to the locus of attention. By using a spatial cuing procedure, we simultaneously tested both of these hypotheses and found evidence that both types of codes produce independent SEs, therefore providing evidence that multiple spatial codes can simultaneously influence performance in a Simon task.