Developmental changes in the duration of attention to unfamiliar stimuli in the rat.

The purpose of the present study was to examine developmental changes in the duration of attention to novel auditory stimuli in rats from infancy to adulthood. In all experiments, the cardiac component of the orienting response was used as the index of attention. In the first experiment 16-, 23-, 30-, and 75-day-old rats were exposed to a novel auditory stimulus for 60 s. Animals of all four ages exhibited a marked decrease in heart rate in response to this stimulus. Although there were no significant effects of age on the magnitude of the cardiac response, there were dramatic developmental differences in the persistence of the response. The 16-day-olds maintained a decrease in heart rate for the entire stimulus period, whereas heart rate returned to baseline in the 75-day-olds after only 40 s. The duration of the cardiac response of the 23- and 30-day-olds was between these extremes. In Experiments 2A and 2B the effect of stimulus complexity on duration of the cardiac response was examined. Increasing stimulus complexity prolonged the deceleratory response in the 30-day-olds (Experiment 2A), and decreasing stimulus complexity decreased the duration of the cardiac response in the 16-day-olds (Experiment 2B). The results of these experiments were discussed in the context of Cohen's and Porges's two-phase theories of attention. The findings demonstrate that the initial attention-getting or reactive phase of attention did not change with age, but that the duration of the attention-holding or sustained phase decreased substantially during development.