Figure 8. Shows RT differentials associated with the probability that a given target, drawn from a set of two, will appear. Ordinate values are RT (infrequent target)-RT (frequent target); larger differences are associated with speedier responses to the frequent target. When the target probabilities are equal (50%), the difference is negligible. The difference is sizeable when one of the targets appears on the majority of trials, and this effect is seen in sessions when no responses are reinforced (Ext).