Psychologists at the University of Massachusetts at Amherst have found that infants fine-tune their visual and auditory systems to stimuli during the first year of life, essentially "weeding out" unnecessary discriminatory abilities.
Lisa Scott and her colleagues examined several studies suggesting that infants begin to hone their perceptual discrimination to environmentally relevant distinctions as they become nine to 12 months of age.
In one study, six-month-old infants were able to differentiate between two human faces as easily as between two monkey faces, whereas nine-month-olds could only differentiate between two human faces.
The study also showed that infants maintained the ability to tell the difference between two monkey faces if they were familiarized with monkey faces when they were six to nine months old.
This phenomenon, called "perceptual narrowing" also occurs in other perceptual systems, say the researchers.
In another study examining speech, six-month-old infants could discriminate one sound from another from virtually every language, but the ability declined by the ninth month if they did not receive any experience with such sounds.
"What is most intriguing about these findings is that they collectively suggest that typical perceptual specialization and development is characterized by the gradual decline of abilities, not just gaining new ones," said Scott.
Coincident with this decline, the brain is experiencing an exuberance of synaptic connections, followed by the pruning of these connections to adult levels, say the authors.
"It is important to note that this does not suggest a developmental regression, but progression towards greater efficiency at perceiving and processing salient rather than less-salient environmental input," write the authors.
The study, published in Current Directions in Psychological Science, suggests that environmental input modifies and shapes of these neural connections throughout development, allowing infants to differentiate between non-native sounds, faces, and even musical rhythms.