Abstract
The accumulator model of two-choice discrimination conceives the decision process as a race between competing evidence totals in discrete time and continuous state space. General expressions for the terminating probabilities for the model are derived, and a tractable version considered in which the increment distributions are exponential. Expressions for response probabilities, first passage time distributions, and a “balance of evidence” theory of response confidence are presented, and the resulting model compared to that obtained under the more usual assumption of truncated normal increments. The exponential model is fitted to data from an experiment exhibiting both faster and slower mean error times across a range of discriminability levels, and is shown to satisfactorily account for these when augmented with the assumption of negatively correlated within-condition variation in discriminability and decision criterion. Cross-paradigmatic support for the fitted model is provided by an estimate of sampling time of the order of 100 msec, in agreement with estimates obtained from a technique employing brief stimulus exposures.
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