English: Illustration of a single correct response trial under the drift-diffusion model (DDM) of reaction time on a two-choice task. The distribution of reaction times across trials is determined by the rate at which evidence accumulates in neurons with an underlying "random walk". The drift rate (v) is the average rate (red line) at which this evidence accumulates in the presence of this random noise (blue line). The decision threshold (a), represents the width of the decision boundary, or the amount of evidence needed before a response is made. The trial terminate when the accumulating evidence hits the correct (a_1) or the incorrect (a_0) boundary.