Relative importance of perceptual and mnemonic variance in human temporal bisection

We investigated the relative contributions of perception and reference memo ry to behavioural variability in a temporal discrimination with human subje cts. We used two temporal bisection tasks. In both tasks each trial consist ed of a sequential presentation of three intervals, two standards, and a pr obe, and subjects were asked to judge the similarity of each probe against the two standards. In a "single bisection", the standards' duration was con stant across trials. In a "roving bisection", the two standards were trial unique. We compared our results with the predictions from a model related t o Scalar Expectancy Theory, with the added assumption that the decision pro cess minimizes the expected number of errors given the information availabl e. The model shows that if errors in reference memory were dominant the psy chometric function should be identical for single and roving tasks, and if perceptual errors were dominant the psychometric function should be steeper for the single than for the roving bisection. As we found that psychometri c functions were steeper for the single than for the roving tasks, we concl uded that perceptual errors are dominant.