Pigeons discriminated between the 1st and 2nd halves of a trial. Trial duration was varied both within and between sessions in a 2-alternative free-operant psychophysical choice procedure. Two keys were illuminated either both red or both green at the beginning of a trial. Half the trials were long (red keys), and half were short (green keys). Session duration was kept constant across conditions. Left-key responding was reinforced only during the 1st half of a trial, and right-key responding was reinforced only in the 2nd half of a trial. Contrary to predictions of the behavioral theory of timing (P. R. Killeen and J. G. Fetterman), estimates of pacemaker period increased with increases in the trial duration despite constancy of reinforcement rate. Weber fractions were relatively constant across timed intervals, consistent with Weber's law. The present experiment is discussed with regard to implications for current theories of timing.
Bizo, LA & White, KG 1997, 'Training with controlled reinforcer density: implications for models of timing', Journal of Experimental Psychology: Animal Behavior Processes, vol. 23, no. 1, pp. 44-55.