Response selection in a four-choice reaction time task: Evidence for a two-step selection

In a bimanual four-choice reaction time (RT) task where the compatibility o f the stimulus-response (S-R) mapping was varied, Duncan (1977) obtained a pattern of errors suggesting that response selection was performed in two s teps: (1) selection of the hand and (2) selection of the finger. The presen t study is aimed at determining which of two hypotheses best accounts for D uncan's findings. According to the first hypothesis, the two-step selection is due to anatomical features of the to-be-executed response. In contrast, according to the second hypothesis, such a selection reflects a more gener al property of the response selection process. We used the same experimenta l protocol as Duncan (1977) but with unimanual instead of bimanual response s. The pattern of errors reported by Duncan was essentially replicated, tha t is, subjects often selected the correct side, but failed to correctly sel ect the finger involved in the required response. The effect of S-R compati bility on RT was similar for the two inner fingers (middle and ring) and th e two outer ones (fore- and little). However, this effect was larger for in ner than for outer fingers. These results suggest a two-step response selec tion process which allows one to discard the anatomical hypothesis.