| The uncertainty effect, commonly referred to as Hick's Law, is a general linear increase of response latency to one of N stimuli that indicates which of N associated responses, such as a button press, depends on the logarithm of the number of potential stimulus-response alternatives (Hick, 1952; Hyman, 1953). For over fifty years the accepted explanation of the uncertainty effect has implicated the stimulus-response (S-R) mapping process. This thesis comprises a series of six experiments which varies attributes of the stimuli which indicates a movement target and the responses made to the targets themselves. In Experiment 1, we report that visually guided, aimed hand movements are unaffected by both stimulus-response uncertainty and stimulus-response repetition. A second experiment demonstrated that this lack of an uncertainty effect persists for equiluminant stimuli. One way that these experiments differed from previous research studying stimulus-response uncertainty is that the stimulus was an abrupt change at the location which was the target of the to-be-made movement. Experiment 3 and 4 of this thesis investigates and rejects the hypothesis that this stimulus difference is what led to the elimination of the stimulus uncertainty effect in the research of Kveraga et al. and Wright et al. Experiment 3-5 compared, using as the target indicator, an abrupt change at the location of the target with an alternative. In Experiment 3, the alternative was an abrupt change at the location of all the non-targets; no stimulus uncertainty effect was found in either condition. In Experiment 4, the alternative was a line at the fixation mark that pointed toward the target; again no stimulus uncertainty effect was found in either condition. In Experiment 5, the alternative was a letter, whose association with the target was largely arbitrary; this condition showed that it is possible to obtain stimulus uncertainty effects for pointing movements under some conditions. Finally, the sixth and final Experiment of this thesis compared aimed, arm movements to targets made with either one or two effectors. These overall results are discussed in terms of a distinction between a response-selection process and an effector-selection process. |
