Summary People plan to act in the future when an appropriate event occurs, a capacity known as event-based prospective memory [1]. Prospective memory involves forming a representation of a planned future action, subsequently inactivating the representation, and ultimately reactivating it at an appropriate point in the future. Recent studies suggest that monkeys, chimpanzees, and rats display elements of prospective memory [2–5], but it is uncertain if the full sequence (activation-inactivation-reactivation) that occurs in humans also occurs in nonhumans [6–8]. Here we asked if rats exhibit event-based prospective memory. Rats completed an ongoing temporal-discrimination task while waiting for a large meal. To promote the use of event-based prospective memory, an event (tone-pulses) provided information that the meal could be obtained soon. Event-based prospective memory was suggested by the dramatic decline in ongoing-task performance after the event, with excellent performance at other times. To document that the event initiated memory activation, the event occurred at novel times. Finally, multiple, repeated presentations of the event on the same day demonstrate that rats inactivate and reactivate the memory representation in an on-demand, event-based fashion. Development of an animal model of prospective memory may be valuable to probe the biological underpinnings of memory disorders [7, 9].

A fundamental question about time perception concerns the form in which time is represented. Psychophysical approaches to answering this and related questions have focused on the various scales of representation that might be available to an animal. For example, Stevens (1951) described a hierarchy of measurement scales ranging from nominal to ordinal, interval, and ratio. In this hierarchy, higher order scales contain the properties of lower order scales plus the addition of an extra property. For example, with an ordinal scale, it is possible to represent the order of values on the scale. An interval scale has added to it information about the spacing of successive values, permitting the addition and subtraction of values on the scale. Finally, a ratio scale has added to it information about the relative position of values on a scale with an absolute zero point, permitting the multiplication and division of values on the scale. For example, finalists in a race are ranked first, second, and third. This ordinal ranking conveys no information about how close the finish times were among the finalists. Therefore, there is no guarantee of a meaningful outcome if higher order operations are performed on these ranks. However, in some cases, a meaningful outcome might occur (e.g., when the finish times for the finalists are about equally spaced).Identifying the representation of time is an important issue for theories of timing. For example, in the scalar timing theory (Gibbon, 1991), it is proposed that an accumulator integrates the number of pulses from a pacemaker. According to the behavioral theory of timing (Killeen & Fetterman, 1988), an animal proceeds through a sequence of behavioral states with transitions produced by a pacemaker. Staddon and Higa (1999) proposed that timing abilities are based on the magnitude of a memory trace since the last reinforcement. Theories of timing have generally not explicitly stated what type of measurement scale should be used. In some cases, however, a theory can be more compatible with one measurement scale than with others. For example, an important feature of scalar timing theory is the use of ratio decision rules, which are naturally compatible with a ratio scale. However, in other cases, the compatibility is less clear. For example, although the behavioral theory of timing is based on an ordinal sequence of behaviors, the theory has been used successfully in a variety of tasks that imply higher order scales (e.g., it can predict bisection at the geometric mean, which implies a ratio scale).One approach to identifying the representation of time involves a psychophysical method. For example, in Gibbon and Church's (1981) time-left procedure, animals were required to choose between a standard interval (e.g., a fixed 30 sec) and a comparison interval (e.g., 60 2 t, where t is the elapsed time into the trial when the comparison lever is inserted). For example, in one of their experiments, a trial began with the comparison lever entering the chamber followed by the standard leve...

The differential reinforcement of low rate (DRL) schedule is commonly used to assess impulsivity, hyperactivity, and the cognitive effects of pharmacological treatments on performance. A DRL schedule requires subjects to wait a certain minimum amount of time between successive responses to receive reinforcement. The DRL criterion value, which specifies the minimum wait time between responses, is often shifted towards increasingly longer values over the course of training. However, the process invoked by shifting DRL values is poorly understood. Experiment 1 compared performance on a DRL 30-s schedule versus a DRL 15-s schedule that was later shifted to a DRL 30-s schedule. Dependent measures assessing interresponse time (IRT) production and reward-earning efficiency showed significant detrimental effects following a DRL schedule transition in comparison with the performance on a maintained DRL 30-s schedule. Experiments 2a and 2b assessed the effects of small incremental changes vs. a sudden large shift in the DRL criterion on performance. The incremental changes produced little to no disruption in performance compared to a sudden large shift. The results indicate that the common practice of incrementing the DRL criterion over sessions may be an inefficient means of training stable DRL performance.

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The current understanding of the activity of mammalian pheromones is that endocrine and behavioural effects are limited to the exposed individuals. Here, we demonstrate that the nasal exposure of female mice to a male murine pheromone stimulates expansion of mammary glands, leading to prolonged nursing of pups. Subsequent behavioural testing of the pups from pheromone-exposed dams exhibited enhanced learning. Sialic acid components in the milk are known to be involved in brain development. We hypothesized that the offspring might have received more of this key nutrient that promotes brain development. The mRNA for polysialyltransferase, which produces polysialylated neural cell adhesion molecules related to brain development, was increased in the brain of offspring of pheromone-exposed dams at post-natal day 10, while it was not different at embryonic stages, indicating possible differential brain development during early post-natal life.