What is timed in a fixed-interval temporal bisection procedure?

Recent research on interval timing in the behavioral and neurological sciences has employed a concurrent fixed-interval (FI) procedure first reported by Platt and Davis (Journal of Experimental Psychology: Animal Behavior Processes, 9, 160-170, 1983). Studies employing the task typically assess just 1 dependent variable, the switch/bisection point; however, multiple measures of timing are available in the procedure and it is unclear (a) what is timed (i.e., learned) by subjects and (b) what other measures might tell us about timing in the task and generally. The main objective of the current experiment was to utilize multiple dependent measures of timing accuracy and precision derived from the task to assess whether the 2 FIs are timed independently or if timing 1 FI interferes with timing the other, and vice versa. Four pigeons were exposed to an FI temporal bisection procedure with parametric manipulations across two phases. In the constant phase, the short FI was always the same; the long FI was 2 to 16 times the short FI and changed across conditions. In the proportional phase, the long FI was always 4 times the duration of the short FI. Across both phases, pigeon mean bisection points were near the geometric mean of the 2 FIs. Coefficients of variance increased as the durations to be timed increased. Results suggested pigeons' timing of the short FI was affected by the presence of the long FI, and vice versa. The FI temporal bisection task offers multiple dependent variables for analysis and is well suited for studying temporal learning and decision making.