Determinants of two-choice reaction-time patterns for same-hand and different-hand finger pairings.

Several studies of two-choice reaction times have compared situations in which the alternative responses are fingers from one hand (the same-hand pairing) or one finger from each hand (the different-hand pairing). Two patterns of results have been obtained: (a) equivalent reaction times for the same-hand and different-hand pairings and (b) faster reaction times for the different-hand pairing. Previously, these outcomes have been attributed to the adoption of different response-preparation strategies when response pairs are constant (low response-pair uncertainty) versus when they are varied from trial to trial (high response-pair uncertainty). However, response-pair uncertainty has been confounded with whether only the two relevant fingers were placed on response keys or whether more than two fingers were. Experiment 1 of the present study demonstrated that finger placement, rather than response-pair uncertainty, determines which reaction-time pattern is obtained. Experiments 2 and 3 investigated the nature of the finger-placement effect by placing the fingers that were irrelevant for the task on response keys or on immovable blocks. The experiments indicated that the crucial factor is the number of fingers on active response keys, with the type of preparation being different when only two fingers are on keys rather than when more than two fingers are.