About Skinner and time: behavior-analytic contributions to research on animal timing

The article discusses two important influences of B. F. Skinner, and later workers in the behavior-analytic tradition, on the study of animal timing. The first influence is methodological, and is traced from the invention of schedules imposing temporal constraints or periodicities on animals in The Behavior of Organisms, through the rate differentiation procedures of Schedules of Reinforcement, to modern temporal psychophysics in animals. The second influence has been the development of accounts of animal timing that have tried to avoid reference to internal processes of a cognitive sort, in particular internal clock mechanisms. Skinner's early discussion of temporal control is first reviewed, and then three recent theories-Killeen & Fetterman's (1988) Behavioral Theory of Timing; Machado's (1997) Learning to Time; and Dragoi, Staddon, Palmer, & Buhusi's (2003) Adaptive Timer Model-are discussed and evaluated.     

Traveling in time: a time-left analogue for humans

Two experiments studied normal humans in an analogue of the time-left procedure of J. Gibbon and R. M. Church (1981). In Experiment 1 the "standard" alternative (S) was always half the length of the "comparison" time-left link (C), and S ranged from 4 to 8 s. Humans showed an increasing preference for the time-left alternative with increasing elapsed time in the interval, and indifference points strongly supported the idea of a linear, rather than a logarithmic, time scale. Experiment 2 used some conditions in which S was greater or less than C/2, and preference for the time-left alternative varied systematically with the S/C ratio. Data from both experiments showed reasonable superposition, suggesting underlying scalar timing processes in time left in humans.     

Temporal bisection in children

Children aged 3, 5, and 8 years received training on a temporal bisection task, with standard short and long durations being presented as visual stimuli lasting 1 and 4 s or 2 and 8 s. Nonstandard comparison stimuli were spaced linearly between the standards. Psychophysical functions showed increasing proportions of "long" responses (responses appropriate to the long standard) with increasing stimulus duration, but were flatter in the younger children than in the 8-year-olds. Bisection points (the stimulus duration giving rise to 50% "long" responses) were close to the arithmetic mean of the short and long standards in most conditions. Statistical analyses and results from different theoretical models of the data all suggested that temporal sensitivity was higher in the 8-year-olds than in the younger groups, even when the possibility of random responding was controlled for.     

Scalar timing without reference memory? Episodic temporal generalization and bisection in humans

Three experiments tested whether the scalar property of timing could occur when humans timed short durations under conditions in which it was unlikely that they developed reference memories of temporal "standards". Experiment 1 used an episodic version of a temporal generalization task where judgements were made of the potential equality of two durations presented on each trial. Unknown to the subject, one of these was always 200, 400, 600, or 800 ms, and the other was of variable duration. Temporal generalization gradients showed the scalar property of superimposition at standard values greater than 200 ms. Experiment 2 used a variant of the "roving bisection" method invented by Rodriguez-Girones and Kacelnik (1998) modified so that the scalar property of timing could be observed empirically. Data from bisection with short/long standard pairs of 100/400, 200/800, and 300/1,200 ms showed nearly perfect scalar-type superimposition. Experiment 3 again used episodic temporal generalization, but durations were never repeated and came from three distinct time ranges. Superimposition was found across these ranges except for the shortest visual stimuli timed. The data suggested that scalar timing could occur in humans in conditions where the formation of reference memories of temporal standards was highly improbable.     

Subjective shortening with filled and unfilled auditory and visual intervals in humans?

Two experiments tested humans on a memory for duration task based on the method of Wearden and Ferrara (1993), which had previously provided evidence for subjective shortening in memory for stimulus duration. Auditory stimuli were tones (filled) or click-defined intervals (unfilled). Filled visual stimuli were either squares or lines, with the unfilled interval being the time between two line presentations. In Experiment 1, good evidence for subjective shortening was found when filled and unfilled visual stimuli, or filled auditory stimuli, were used, but evidence for subjective shortening with unfilled auditory stimuli was more ambiguous. Experiment 2 used a simplified variant of the Wearden and Ferrara task, and evidence for subjective shortening was obtained from all four stimulus types.     

Is the growth of subjective time in humans a linear or nonlinear function of real time?

The difficulties of deciding whether subjective time grows as a linear or nonlinear function of real time are discussed, and two experiments are presented to address this question. In Experiment 1, people received a 10-s standard duration and then had to judge what proportion other durations (ranging from 1 to 10 s) were of the standard. Counting was prevented by a concurrent task. The relation between judged and actual proportions was linear. In Experiment 2, people were required to average together three tone durations (mean duration 600 ms) and to judge whether subsequently presented comparisons were or were not the average. The spacing of the tone durations had no effect on judgements, suggesting a linear underlying time scale.     

Short-term memory for time in children and adults: A behavioral study and a model

This experiment investigated the effect of the short-term retention of duration on temporal discrimination in 5- and 8-year-olds, as well as in adults, by using an episodic temporal generalization task. In each age group, the participants' task was to compare two successive durations (a standard and a comparison duration) separated by a retention interval of 500ms, 5s, or 10s, with the order of presentation of these two durations being counterbalanced. The results revealed a shortening effect for the first presented stimulus in all of the age groups, although this was greater in the younger children, thereby indicating the presence of a negative time-order error. Furthermore, introducing a retention delay between the two durations did not produce a shortening effect but instead flattened the generalization gradient, especially in the younger children. However, this flattening of the generalization gradient with the retention delay was more marked between 500ms and 5s than between 5s and 10s. Thus, retaining the first duration in short-term memory during a task requiring the comparison of two successive durations reduced temporal discrimination accuracy and did so to a greater extent in the younger children.     

The differential contribution of executive functions to temporal generalisation,reproduction and verbal estimation

Evidence from dual-task studies suggests that executive resources are recruited during timing. However, there has been little exploration of whether executive recruitment is universal across temporal tasks, or whether different temporal tasks recruit different executive resources. The current study explored this further by examining how individual differences in updating, switching, inhibition and access affected performance on temporal generalisation, reproduction and verbal estimation tasks. It was found that temporal tasks differentially loaded onto different executive resources. Temporal generalisation performance was related to updating and access ability. Reproduction performance was related to updating, access and switching. Verbal estimation performance was only related to access. The results suggest that executive resources may be recruited when monitoring and maintaining multiple durations in memory at the same time, and when retrieving duration representations from long-term memory. The findings emphasise the need to consider timing behaviour as the product of a wide range of complex, integrated, cognitive systems, rather than as the output of a clock in isolation.     

Disparities in Current and Future Childhood and Newborn Carrier Identification

International carrier testing guidelines discourage testing in childhood to preserve autonomous decision making and prevent detrimental psychosocial consequences. Despite the discouragement of autosomal recessive carrier testing during childhood, some sickle cell disease (SCD) or cystic fibrosis (CF) carriers are incidentally identified through UK and international newborn screening (NBS). This creates a scenario where parents may have knowledge of their newborn’s, but not older child’s carrier status. In addition, there is wide variation in the identification of CF and SCD carriers due to the screening technologies implemented by different NBS programs. The current and future availability of childhood testing are determined to some extent by the impact of testing on children and parents (whether this is beneficial or detrimental to wellbeing). However empirical research informing carrier guidance and practice is conflicting. Echoing previous calls, this discussion highlights the need for further qualitative and longitudinal research with children to consider the psychosocial impact of carrier testing on children and role of disclosure from parents on adaptation to results. It is recommended that professionals aim to minimize harms resulting from carrier identification by providing support for parents and children following NBS. Support for non-genetics specialists from genetic counselors to enable discussion of carrier results with children is suggested.     

Interference between auditory and visual duration judgements suggests a common code for time

Auditory stimuli usually have longer subjective durations than visual ones for the same real duration, although performance on many timing tasks is similar in form with different modalities. One suggestion is that auditory and visual stimuli are initially timed by different mechanisms, but later converted into some common duration code which is amodal. The present study investigated this using a temporal generalization interference paradigm. In test blocks, people decided whether comparison durations were or were not a 400-ms standard on average. Test blocks alternated with interference blocks where durations were systematically shorter or longer than in test blocks, and interference was found, in the direction of the durations in the interference blocks, even when the interfering blocks used stimuli in a different modality from the test block. This provides what may be the first direct experimental evidence for a “common code” for durations initially presented in different modalities at some level of the human timing system.