(A) DDM model postulates that a decision is transformed into action when the evidence favouring one of the choices has been accumulated to a certain threshold level (decision bound) (left panel). The model makes a prediction about the pattern of choice probability and the decision time in respect to the strength of the motion signal (right panel). For the baseline, the starting point of the evidence accumulation is set to 0. (B) When there is more sensory evidence in favour of rightward motion (red line), the drift speed for the rightward decision will increase (left panel), and left would decrease. As a result, a rightward decision becomes more likely (shift of psychometric function) and the decision time pattern generally shifts to the left, showing a tendency to respond faster for the rightward motion (right panel). If the motor cost influences the sensory representation (Figure 2C), we would expect this pattern of results (sensory representation model). (C) A shift in the starting value of the accumulation process induces a prior bias towards a rightward decision, decreasing the required amount of evidence for rightward decision compared to the left (left panel). This will again bias the decision to favour the rightward decision. Instead of shifting the pattern of decision times to the left (as in B), the starting point model predicts an additional offset to the rightward and leftward decision time; shorter for the rightward decision and longer for the leftward decision. In this model, the bias therefore arises from a change in the decision layer transforming the sensory representation into the decision (Figure 2B), while the sensory evidence itself is not changing.