Machado, A., & Guilhardi, P. (2000).
Shifts in the psychometric function and their implications for models of timing.
Journal of the Experimental Analysis of Behavior,
74, 25-54.
This study examined how two models of timing, scalar expectancy
theory (SET) and learning to time (LeT), conceptualize the
learning process in temporal tasks, and then reports two
experiments to test these conceptualizations. Pigeons responded
on a two-alternative free-operant psychophysical procedure 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, quarters of the trial. In Experiment
1 three groups of birds experienced a difference in reinforcement
rates between the two keys only at the end segments 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
quarters), or in both end and middle segments. In Condition 1 the
difference in reinforcement rate favored the left key; in
Condition 2 it favored the right key. When the reinforcement
rates differed in the end segments of the trial, the psychometric
function - the proportion of right responses across the trial -
did not shift across conditions; when it occurred around the
middle of the trial or in both end and middle segments, the
psychometric function shifted 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.
Key words: models of timing, learning to time, psychometric
function, temporal bisection, key peck, pigeon