Scalar timing in animals and humans

Abstract John Gibbon's lifetime work provided a deep understanding of the mechanisms whereby the time sense indexes the passage of time (its accumulation) and records, that is, stores, relevant time intervals in memory, enabling behavior to occur at the right time. The Scalar Expectancy Theory (SET; Gibbon, 1977) remains the most prominent of the theoretical accounts of animal and human timing. SET deals with the three principle psychophysical properties of timing data: flexible accuracy, multiplicative variance, and ratio comparisons. It differs from many other timing theories in its emphasis on scalar variability, a term that refers to the linear increase in the standard deviation of timing errors as a task's criterion time increases. Recently, research based on the conceptual framework and analytic tools of SET in John Gibbon's lab was expanded from a decades-long focus on nonhuman species to an assessment of timing performance in “normal” and brain-diseased human subjects, aimed at understanding the functional and neural mechanisms underlying interval timing in humans. This review is aimed at showing that animal and human data obtained with a variety of timing paradigms are both amenable to analyses of accuracy and scalar variability under the SET framework. In the second part of this report we discuss advances made in our understanding of neurobiological mechanisms underlying interval timing by taking advantage of the SET framework. Issues awaiting new theoretical developments in modeling time production and perception, as revealed by psychophysical findings of recent clinical research that are still not well understood (i.e., sources of nonscalar variability), are raised at the end.