[6] The prevailing theory is that human infants are born with the ability to sense a wide variety of stimuli, and as they age, they begin to selectively narrow these perceptions by categorizing them in a more socio-culturally relevant way.
Most of the research done to date in the area of perceptual narrowing involves facial processing studies conducted with infants.
[9] Cross species studies have been conducted where human infants at 6 months of age were familiarized with individual monkeys.
When the monkey faces were associated with unique proper name labels, the infants maintained their ability to discriminate between them when retested at nine months of age.
If the exposure was just to monkey faces in general, without name labels, the infants were unable to discriminate between them when retested at the nine months mark.
While plasticity is evident throughout the human lifespan, it occurs most often at younger ages, during sensitive periods of development.
There is evidence that a failure in the perceptual narrowing process can, in rare cases, lead to adult synaethesia.