Effects of the number and spacing of conditioning sessions on spontaneous recovery from extinction

Although it has been more than 80 years since Pavlov first observed spontaneous recovery from extinction, the mechanisms underlying this phenomenon remain a mystery. The majority of the proposed models attribute spontaneous recovery to a time-induced change in extinction-related processes. Recent findings, however, point also to the importance of elapsed time since acquisition, not just since extinction. These findings suggest that various temporal parameters of the learning episode may be explicitly represented in order to guide future choice on whether an animal should invest to a signal that has produced more than it has failed. In the present experiments, we used a conditioned magazine approach in the mouse to investigate whether and how various temporal parameters of acquisition affected spontaneous recovery. We found that prolonging the duration of acquisition, either by distributing the same number of acquisition trials across more sessions, or by spacing the same conditioning sessions more widely, augmented spontaneous recovery. Further investigation of the former effect revealed that the session is an important unit of learning experience, while the number of trials within a session is not a primary determinant for spontaneous recovery. Finally, we quantitatively characterized extinction at the level of the individual subject and found it to be abrupt. It took a few trials to appear but became complete almost immediately.Ph.D.Includes bibliographical references (p. 82-87)

Female rats learn trace memories better than male rats and consequently retain a greater proportion of new neurons in their hippocampi

Learning increases the survival of new cells that are generated in the hippocampal formation before the training experience, especially if the animal learns to associate stimuli across time [Gould E, Beylin A, Tanapat P, Reeves A, Shors TJ (1999) Nat Neurosci 2:260–265]. All relevant studies have been conducted on male rats, despite evidence for sex differences in this type of learning. In the present study, we asked whether sex differences in learning influence the survival of neurons generated in the adult hippocampus. Male and female adult rats were injected with one dose of bromodeoxyuridine (BrdU; 200 mg/kg), to label one population of dividing cells. One week later, half of the animals were trained with a temporal learning task of trace eyeblink conditioning, while the other half were not trained. Animals were killed 1 day after training (12 days after the BrdU injection). Hippocampal tissue was stained for BrdU and a marker of immature neurons, doublecortin. Both sexes learned to emit the conditioned eyeblink response during the trace interval. As a consequence, more new neurons remained in their hippocampi than in sex-matched controls. In individual animals, the number of surviving cells correlated positively with asymptotic performance; those that expressed more learned responses retained more new neurons. However, animals that learned very well retained even more new cells if they required many trials to do so. Because females emitted more learned responses than males did, they retained nearly twice as many new cells per unit volume of tissue. This effect was most evident in the ventral region of the hippocampal formation. Thus, sex differences in learning alter the anatomical structure of the hippocampus. As a result, male and female brains continue to differentiate in adulthood

Autoshaped Head Poking in the Mouse: A Quantitative Analysis of the Learning Curve

In autoshaping experiments, we quantified the acquisition of anticipatory head poking in individual mice, using an algorithm that finds changes in the slope of a cumulative record. In most mice, upward changes in the amount of anticipatory poking per trial were abrupt, and tended to occur at session boundaries, suggesting that the session is as significant a unit of experience as the trial. There were large individual differences in the latency to the onset of vigorous responding. “Asymptotic” performance was unstable; large, bidirectional, and relatively enduring changes were common. Given the characteristics of the individual learning curves, it is unlikely that physiologically meaningful estimates of rate of learning can be extracted from group-average learning curves