Habituation of the forelimb-withdrawal response in neonatal rats

The purpose of the present research was to examine the ontogeny of habituation in the neonatal rat, using the forelimb-withdrawal response. Thresholds and latencies of the response, changing patterns of responding to shock stimuli, and habituation of the response were studied in rats 3 to 15 days of age. It was found that although response thresholds do not change during this period of development, response latencies decrease and amplitudes increase. Three- and 6-day old pups remain active much longer following shock than do 10- and 15day-olds. When compared for habituation to different frequencies of stimulation, 3-day-old rat pups were found to be much more susceptible to habituation at low frequencies than 15-day-olds. Insertion of a simple intense shock in the habituation series produced marked dishabituation in the 3-day-olds but little or none in the 15-day-olds. This pattern of results was obtained regardless of the locus of the dishabituating shock. Analysis of response latencies showed that although 15-day-olds responded 2 to 3 times more quickly than 3-day-olds, both age groups responded more rapidly to a strong shock than to a weak one. During habituation, response amplitudes of both 3- and 15-day-old pups declined with no change in latency. Age-related differences in habituation were shown to be independent of differences in reactivity to shock and more likely due to the emergence of response sensitization.