Accuracy, Information, and Response Time in a Saccadic Decision Task

Reaction times generally follow a simple law economically described by the LATER model, in which a decision signal rises linearly in response to information about a target to a threshold at which a response is initiated, at a rate that varies from trial to trial with a Gaussian distribution. Functionally, LATER may be regarded as an ideal decision mechanism incorporating prior probability, information, and criterion level or urgency; this can be tested quantitatively by seeing whether LATER accurately predicts the effects on latency distributions of manipulating these variables: in this case, information and urgency. We presented subjects with random-dot kinematograms while fixating a central LED. The information content of the display was varied by altering the proportion of the dots moving coherently together either left or right rather than randomly. As soon as subjects detected the direction of coherent movement, they made a saccade in the same direction to one of a pair of LEDs on each side of the fixation target. Subjects responded either carefully, taking time to ensure an accurate judgement, or more hastily and with less regard for accuracy. The distributions of latencies under the different combinations of conditions were found to conform to LATER's predictions. Providing more information or increasing urgency both reduce latency; but they alter the observed distributions in different ways, equivalent to increasing the mean rate of rise on the one hand or reducing the criterion level on the other. Making only simple assumptions about the underlying mechanisms, the observed changes can be accounted for quantitatively.