Shifts in the psychometric function and their implications for models of timing

This study examined ho iv two models of timing, scalar expectancy theory (S ET) and learning to time (LeT), conceptualize the learning process in tempo ral tasks, and then reports two experiments to test these conceptualization s. Pigeons responded on a two-alternative free-operant psychophysical proce dure in which responses on the left key were reinforceable during the first two, but not the last two, quarters of a 60-s trial, and responses on the right key were reinforceable during the last two, but not the first two, qu arters of the trial. In Experiment 1 three groups of birds experienced a di fference in reinforcement rates between the two keys only at the end segmen ts of the trial (i.e., between the first and fourth quarters), only around the middle segments of the trial (i.e., between the second and third quarte rs), or in both end and middle segments. In Condition 1 the difference in r einforcement rate favored the left key; in Condition 2 it favored the right key. When the reinforcement rates differed in the end segments of the tria l, the psychometric function-the proportion of right responses across the t rial-did not shift across conditions; when it occurred around the middle of the trial or in both end and middle segments, the psychometric function sh ifted across conditions. Experiment 2 showed that the psychometric function shifts even when the overall reinforcement rate for the two keys is equal, provided the rates differ around the middle of the trial. This pattern of shifts of the psychometric function is inconsistent with SET. In contrast, LeT provided a good quantitative fit to the data.