The location and interpretation of the bisection point

In a temporal bisection task with humans, the observer is required to decide whether a probe duration (t) is more similar to the short referent (S), an R-L response, or to the long referent (L), an R-L response. Temporal bisection yields a psychometric function relating the proportion of long responses, P(R-L), to probe duration t. The value of t at which R-S and R-L occur with equal frequency, P(RL) =.5, is referred to as the bisection point, T-1/2. Bisection models usually interpret T-1/2 as identifying the value of t that is equally confusable with S and L, but they differ in their predictions for the location of T-1/2. The present paper presents new data relevant to the location and interpretation of T-1/2. The data indicate that the empirical values usually are biased, the biases being influenced by duration range, L:S ratio, and probe spacing. Moreover, the biases often are not consistent across observers. It is concluded that empirical values of T-1/ 2 should not be interpreted as indicating the value of t that is equally confusable with S and L.