The deciding hand: How an analysis of human reach movements reveals choice biases


Decision making, or the resolution of competition, is one of the most central components of human cognition: from low-level, brief, sensory events that compete for cortical activation and amplification, to high level symbols and complex objects that compete first for recognition then later for influence over decisions. Despite its centrality to understanding human thought, the science of decision making is usually restricted only to an analysis of what decisions people make. This approach overlooks the very important component of how people execute their decisions. Here, I will show results from a variety of studies demonstrating that an analysis of the physical reach movements people make to indicate a decision, and careful manipulation of the timing of decision stimuli, can be used to reveal subtle aspects of decision making and the precise timelines over which they operate. I will present evidence from studies where decisions are driven by low level visual properties (e.g. luminance), where decisions are influenced by arbitrary, more cognitively driven properties (e.g. reward associations) and finally, where decisions are made based completely on participant driven properties separate from any specific stimuli features (e.g. personal preference). The analysis of the resulting spatial reach trajectories as participants physically interact with the choice options reveals decision biases including: an initial bias toward high luminance that decays with time, a bias toward gain and a delayed bias away from loss, and reaches that reflect an individual's decision difficulty. Notably, these biases would have been invisible using conventional research methods.