Van Maanen, L. 1 , Grasman, R. 1 , Forstmann, B. U. 1 , Keuken, M. 1, 2 , Brown, S. 3 & Wagenmakers, E. 1
1 Department of Psychology, University of Amsterdam, Amsterdam, The Netherlands
2 Max-Planck Institute for Human Cognitive and Brain Sciences, Leipzig, Germany
3 School of Psychology, University of Newcastle, Callaghan, Australia
The popular random-dot motion (RDM) task has recently been applied to multiple-choice perceptual decision-making. In this task, participants have to decide on a direction of motion of a cloud of moving dots. The response alternatives typically surround the stimulus in a circular way. However, this means that changes in the number of alternatives on an RDM display lead to changes in the angular distance between the response alternatives, complicating the study of multiple-choice effects with this paradigm. To disentangle the effects of angular distance and number of alternatives we analyzed behavior in the RDM task using a neurally-inspired optimal observer model. The model applies Bayesian principles to give an account of how changes in the stimulus influence the decision-making process. In addition to an analysis of optimal behavior, we applied a Linear Ballistic Accumulator (LBA, Brown & Heathcote, 2008) model to verify the predictions of the optimal model. The results show that (a) there is a natural interaction in the RDM task between angular distance and the number of alternatives, (b) the number of alternatives is encoded by the "response caution" parameter, and (c) behavior in the RDM task is near optimal when handling multiple choices.