Rapid plasticity attenuation soon after birth revealed by habituation in newborn chicks.

Habituation reflects a form of experience-dependent plasticity whereby the organism progressively learns to ignore the irrelevant information repeatedly encountered. Here, we measured the freezing response to a repeated loud noise in three groups of newborn chicks (Gallus gallus) of different ages (1-2 Day old, 2-3 Day old, and 3-4 Day old) to investigate the ontogeny of habituation in this avian species. Habituation was already present 1 Day after hatching, revealing that the neural mechanisms underlying this form of plasticity are immediately active. Unexpectedly, however, we also found that in the second Day of stimulation the amount of learning was significantly attenuated in chicks of 3-4 days of age as compared to the younger animals, thus showing that 24-48 hr of maturation were sufficient to reduce plasticity. While previous findings in other animal species have proved the existence of a precocious critical period of plasticity in early cortical areas by means of sensory deprivation, our results demonstrate that during the initial development of an intact avian brain also the degree of plasticity underlying a learning process like habituation is maximal soon after birth, and then is subject to a rapid attenuation.