A bias-free test of human temporal bisection: Evidence against bisection at the arithmetic mean.

The temporal bisection procedure has been used to assess theories of time perception. A problem with the procedure for measuring the perceived midpoint of two durations is that the spacing of probe durations affects the length of the bisection point. Linear spacing results in longer bisection points closer to the arithmetic mean of the durations than logarithmic spacing. In three experiments, the influence of probe duration distribution was avoided by presenting a single probe duration of either the arithmetic or geometric mean of the trained durations. It was found that the number of participants that categorised the arithmetic mean as long was significantly larger than those that categorised it as short. The number of participants that categorised the geometric mean as either short or long did not significantly differ. This was true for trained durations of 0.4 s vs. 1.6 s (Experiments 1-3), 0.2 s vs. 3.2 s (Experiment 2) and 0.4 s vs. 6.4 s (Experiment 3). In Experiment 4, the probe trial distribution effect was replicated with logarithmic and linearly distributed probe durations, demonstrating that bisection occurs close to the arithmetic mean with linearly spaced probe durations. The results provide evidence against bisection at the arithmetic mean when probe spacing bias is avoided and, instead, the results are consistent with logarithmic encoding of time, or a comparison rule based on relative rather than absolute differences.