DRL performance in the weanling rat: a comparison with adult subjects

Temporal regulation of children with autism spectrum disorder exposed to a differential-reinforcement-of low-rates schedule

This study investigated temporal adjustment of children with autism spectrum disorder under a differential-reinforcement-of-low-rates (DRL) schedule. Sixteen participants, aged 3.2 to 7 years, were exposed to two conditions, DRL 5 s and DRL 20 s. Children participated in 7 sessions in each condition, except for 1 participant who attained the adjustment criteria in the DRL 5-s schedule. Temporal adjustment was measured with the proportion of reinforced interresponse times (IRTs) and the mean IRT. The operant response was a press on a touch screen and the reinforcers were cartoons. IQ and receptive language were measured prior to the DRL sessions. Results showed that the mean proportion of reinforced IRTs was slightly higher in the DRL 5-s schedule. The mean IRT was above the IRT requirement in both conditions. However, substantial individual variability was observed. Children with higher IQ and receptive language scores presented a greater proportion of reinforced IRTs in both conditions. Moreover, participants who adjusted their responses to the DRL 5-s schedule were more likely to adjust responding to the DRL 20-s schedule. This suggests that some children might be more sensitive to reinforcement contingencies than others. This study points at future research in the field of timing in children.

Developmental emergence of fear/threat learning: neurobiology, associations and timing

Pavlovian fear or threat conditioning, where a neutral stimulus takes on aversive properties through pairing with an aversive stimulus, has been an important tool for exploring the neurobiology of learning. In the past decades, this neurobehavioral approach has been expanded to include the developing infant. Indeed, protracted postnatal brain development permits the exploration of how incorporating the amygdala, prefrontal cortex and hippocampus into this learning system impacts the acquisition and expression of aversive conditioning. Here, we review the developmental trajectory of these key brain areas involved in aversive conditioning and relate it to pups' transition to independence through weaning. Overall, the data suggests that adult-like features of threat learning emerge as the relevant brain areas become incorporated into this learning. Specifically, the developmental emergence of the amygdala permits cue learning and the emergence of the hippocampus permits context learning. We also describe unique features of learning in early life that block threat learning and enhance interaction with the mother or exploration of the environment. Finally, we describe the development of a sense of time within this learning and its involvement in creating associations. Together these data suggest that the development of threat learning is a useful tool for dissecting adult-like functioning of brain circuits, as well as providing unique insights into ecologically relevant developmental changes.

Reinforced variability in rats: effects of gender, age and contingency

Operant variability was compared in four groups of Long-Evans rats (young males, young females, mature males and mature females) under two different conditions. Under VAR contingencies, where response variability was required for reinforcement, a sequence of four responses on left (L) and right (R) levers had to differ from each of the preceding four sequences. If LLLL, LRLL, RRRR, and RRLL had just occurred, then a RLRL sequence, for example, would be reinforced in the next trial, but LRLL would not. Sequence variability was compared to that under YOKE contingencies, where reinforcement was provided whether or not the rats varied their responses. We found that young rats behaved more variably than mature rats, this effect most pronounced under the YOKE contingencies, where variability was not required. On the other hand, variability was not related to gender under either VAR or YOKE conditions. Thirdly, all groups were sensitive to the schedule contingencies, behaving more variably under VAR than under YOKE. Thus age and schedule requirements influenced operant variability, but gender did not.

Long-term memory for DRL: a comparison between weanling, adult and senescent rats

Weanling (24-day-old), adult (3- and 7-month-old) and senescent rats (24-month-old) were submitted for 10 days to a DRL 20 seconds schedule, at a pace of 4 sessions a day. After a 3 month interval, the subjects were tested for retention in an identical experimental setting. In training, the median Interresponse-Time (IRT) is directly related to the age of the subjects, whereas response rates are inversely related to it. Age-related differences subside at the end of training. In testing, adult rats improve their efficiency, whereas senescent and particularly former weanling rats do not catch up with training scores. These data are discussed in relationship with developmental variables in the temporal regulation of behavior, collateral activity, contextual and methodological variables in the study of memory.