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Facilitation and interference occur at different stages of processing in the Simon paradigm

The aim of the present experiment was to investigate whether response facilitation and interference could be demonstrated in the Simon task by comparing the speed of response to relevant visual stimuli presented at the focus of attention (neutral condition) and to relevant stimuli presented to the left and right of the attentional focus (compatible and incompatible conditions). Recent models have emphasised the role of attentional refocusing in producing the Simon effect (Nicoletti and Umilta, 1994; Stoffer, 1991; Stoffer and Yakin, 1994; Umilta and Nicoletti, 1992). We also wanted to study the locus of possible facilitation and interference in the chain of information processing. This was done by measuring both reaction times and movement times. The results showed clear indications of the Simon facilitation and interference effects. The total response times for the compatible trials were shorter than for the neutral trials, and the response times for the incompatible trials were longer than for the neutral trials. The reaction times and movement times showed, however, another pattern of difference between the conditions. The reaction times were faster in the compatible than neutral trials, whereas incompatible trials did not result in lengthened reaction times. The opposite pattern was true for the movement times; there was no difference in movement times between the compatible and neutral stimulus conditions, but the movement times were longer for the incompatible than the neutral trials. This result suggests that the processes which produce performance facilitation occur in the stages preceding the motor initiation and prior to the processes causing response interference. In order to explain the observed facilitation and interference effects, it is proposed that the presentation of a peripheral reaction signal does not only re-orient visual attention, but is also accompanied by preparation of motor programmes for spatially selective movements. This hypothesis is in line with the suggested functions of the dorsal visual system in controlling visual attention orienting and mediating visual information for the motor systems (Goodale and Milner, 1992).