A. Drift-diffusion with symmetric bounds. Noisy momentary evidence for/against hypotheses *h*_{1} and *h*_{2} is accumulated (irregular trace) until it reaches an upper or lower termination bound, leading to a choice in favor of *h*_{1} or *h*_{2}. In the motion task, *h*_{1} and *h*_{2} are opposite directions (e.g., right and left). The momentary evidence is the difference in firing rates between pools of direction-selective neurons that prefer the two directions. At each moment, this difference is a noisy draw from a Gaussian distribution (*inset*) with mean proportional to motion strength. The mean difference is the expected drift rate of the diffusion process. The process reconciles the speed and accuracy of choices with two parameters, bound height (±A) and mean of *e*. If the stimulus is extinguished before the accumulated evidence reaches a bound, then the decision is based on the sign of the accumulation.

B. Competing accumulators. The same mechanism is realized by two accumulators that race. If the evidence for *h*_{1} and *h*_{2} are opposite, then the race is mathematically identical to symmetric drift-diffusion. The race is a better approximation to the physiology, since there are neurons in LIP that represent accumulated evidence for each of the choices. This mechanism extends to account for choices and RT when there are more than two alternatives. If the stimulus is extinguished before one of the accumulations reaches a bound, then the decision is based on the accumulator with the larger value (as in ).

C. Certainty. The heat map displays the correspondence between the state of the accumulated evidence in panel A and the log of the odds that the decision it precipitates will be the correct one. The mapping depends on the possible difficulties that might be encountered. This corresponds to the possible motion strengths in the direction discrimination experiments. The mapping does not depend on presence or shape of the bound. Notice that the same amount of accumulated evidence supports less certainty as time passes. Cooler colors indicate low certainty (e.g., log odds equal to 0 implies that a correct choice and an error are equally likely). In the post decision wagering experiment, the monkey opts out of the discrimination and chooses the sure-but-small reward when the accumulated evidence is in the low certainty (cooler) region of the map.

Adapted from and .

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