"and it was all yellow": Response priming biases perceptual decision-making


Humans make perceptual decisions everyday that are enacted by the motor system. Recent evidence suggests that accumulating evidence for a perceptual decision and planning the action to enact the decision occur simultaneously, and that decision commitment occurs when the action reaches its motor threshold. This mechanism was tested by determining if increasing the response activation associated with a specific motor response for a decision influenced the amount of evidence needed to make that decision. Participants were presented with checkerboard stimuli that contained varying ratios of yellow-to-blue squares and were asked to determine if stimuli were yellower or bluer by pressing a left- or right-hand key, respectively. By varying the yellow-to-blue ratio, the number of yellow squares needed to make a yellow decision was obtained (i.e., the perceptual threshold). Varying response activations were induced by presenting stimuli on the left or right side of the screen (spatially compatible or incompatible with the decision response - leftward stimuli may activate left motor responses which were associated with 'yellow', and vice versa). Results indicated that when stimuli appeared on the left, the perceptual threshold for a 'yellow' decision was reduced - when the response associated with 'yellow' had increased activation via spatial compatibility mechanisms, participants were biased to report 'yellow'. A second experiment revealed that potential directional saccades occurring during this task do not account for the bias. Overall, the level of response activation influenced the evidence needed to make decisions providing support for the interconnected nature of the decision-making and action systems.

Acknowledgments: This research was supported by grants and scholarships from NSERC