Scalar Timing in Animals and Humans

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.     

Effects of proactive interference on non-verbal working memory

Working memory (WM) is a cognitive system responsible for actively maintaining and processing relevant information and is central to successful cognition. A process critical to WM is the resolution of proactive interference (PI), which involves suppressing memory intrusions from prior memories that are no longer relevant. Most studies that have examined resistance to PI in a process-pure fashion used verbal material. By contrast, studies using non-verbal material are scarce, and it remains unclear whether the effect of PI is domain-general or whether it applies solely to the verbal domain. The aim of the present study was to examine the effect of PI in visual WM using both objects with high and low nameability. Using a Directed-Forgetting paradigm, we varied discriminability between WM items on two dimensions, one verbal (high-nameability vs. low-nameability objects) and one perceptual (colored vs. gray objects). As in previous studies using verbal material, effects of PI were found with object stimuli, even after controlling for verbal labels being used (i.e., low-nameability condition). We also found that the addition of distinctive features (color, verbal label) increased performance in rejecting intrusion probes, most likely through an increase in discriminability between content–context bindings in WM.     

Expectancy in humans in multisecond peak-interval timing with gaps

In two experiments, the peak-interval procedure was used with humans to test effects related to gaps in multisecond timing. In Experiment 1, peak times of response distributions were shorter when the gap occurred later during the encoding of the criterion time to be reproduced, suggesting that gap expectancy shortened perceived durations. Peak times were also positively related to objective target durations. Spreads of response distributions were generally related to estimated durations. In Experiment 2, peak times were shortest when gaps were expected but did not occur, confirming that the shortening effect of gap expectancy is independent of the gap occurrence. High positive start-stop correlations and moderate positive peak-time-spread correlations showed strong memory variability, whereas low and negative start-spread correlations suggest small response-threshold variability. Correlations seemed not to be influenced by expectancy. Overall, the peak-interval procedure with gaps provided relevant information on similarities and differences in timing in humans and other animals.     

The effects of aging on time reproduction in delayed free-recall

The experiments presented here demonstrate that normal aging amplifies differences in time production occurring in delayed free-recall testing. Experiment 1 compared the time production ability of two healthy aged groups as well as college-aged participants. During the test session, which followed a 24-h delay and omitted all feedback and examples of the two target intervals, the two groups of aged participants' over-produced a 6s interval. This effect is similar in form to errors shown by young participants, but twice the magnitude. Productions of a 17 s interval were generally accurate overall. However, further analysis indicated that the majority of aged participants over-produced the 17 s interval while a minority greatly under-produced the 17 s interval. Furthermore, aged participants showed violations of the scalar property of timing variability in the training session, while in the test session, only those who under-produced the 17 s interval showed this tendency. In contrast, training session performance was good for all participants. Experiments 2 and 3 investigated the ability of the participants in Experiment 1 to reproduce the length of a line from memory, under conditions analogous to those of the time production experiments. These experiments provided tests of the specificity of the errors observed in Experiment 1. Performance in the older participants was accurate, if more variable, compared to the young participants, in contrast to the time production results, indicating that production inaccuracy in free-recall is specific to interval timing in the current context.