Newborn infants begin to perceive language inputs from their parents or caregivers within the first few weeks of life. This time period is thus important for subsequent linguistic development and it can be influenced by several factors. Previous research has shown that the infants of mothers suffering from postpartum depression may have reduced language comprehension and production within the first years of life.
While around 15 percent of new moms may be clinically depressed after the birth of the new baby, a much larger percentage experience the “baby blues,” characterized by a depressed mood but not by full-blown clinical depression. It is not known to what extent mild depressed mood may affect an infant’s early linguistic development, despite the fact that this condition is far more prevalent than clinical postpartum depression.
Scientists from the Max Planck Institute for Human Cognitive and Brain Sciences in Leipzig, Germany, have now investigated whether subclinical depressed mood in mothers also has a negative effect on how well young babies can distinguish speech sounds from one another. This ability is considered an important prerequisite for the further steps towards a well-developed linguistic ability. If sounds can be distinguished from one another, individual words can also be distinguished from one another, and this forms the basis for recognizing both familiar and novel words in the child’s future.
The study involved 46 mothers who completed surveys about their different mood experiences and levels of stress after giving birth. Their moods were measured using a standardized questionnaire typically used to diagnose postnatal upset. The researchers also used electroencephalography (EEG) to measure how well babies were able distinguish speech sounds from other, deviant sounds. The so-called mismatch response is used for this purpose, in which a specific EEG signal shows how well the brain processes and distinguishes between different speech sounds.
The newborns were presented with speech stimuli consisting of repeated “ba” sounds, with irregular and unexpected deviant sounds, namely “ga” and “bu,” introduced at random intervals. The mismatch response was measured at ages of two and six months in each infant. This test of neurolinguistic perception does not require the infants to pay any conscious attention to the sounds and can be conducted perfectly comfortably while infants lie down (2 months) or sit in the laps of their parents (6 months). It is considered an accurate measurement of the infant’s ability to distinguish between sounds.
The results, published in the Journal of the American Medical Association Network Open, revealed that if mothers experience a more negative mood two months after giving birth, their children are less able to process speech sounds when they are tested at six months of age. This applied particularly when the deviant sounds involved differences in pitch, rather than content, with infants from depressed mood mothers being unable to distinguish variations in pitch. These infants showed a more immature mismatch response indicating that their ability to distinguish sounds with different pitches was not as developed as it was in infants whose mothers reported positive moods.
When people speak directly to infants they often use variations in pitch and emphasis, and speak more slowly. The researchers hypothesized that mothers with subclinical depressed mood engage in less infant-directed speech and that when they do engage, their speech may be more monotonous in nature.
“We suspect that the affected mothers use less infant-directed-speech,” explains study first author Professor Gesa Schaadt. “They probably use less pitch variation when directing speech to their infants.” This also leads to a more limited perception of different pitches in the children, she said. This perception, in turn, is considered a prerequisite for further language development, and infants who have delayed development of a mismatch response are considered to be at increased risk of suffering from a speech disorder later in life.
The results show how important it is that parents use infant-directed speech for the further language development of their children. Infant-directed speech that varies greatly in pitch and emphasizes certain parts of words more clearly – and thus focuses the little ones’ attention on what is being said – is considered appropriate for infants and children, and is critical for encouraging early language development.
The findings of the research suggest that the postpartum period is a critical time for infant speech perception and that early support for mothers with postpartum depressed mood may also be beneficial for their infants’ speech perception and language development. Such support could come from fathers and other caregivers who are able to engage in infant-directed-speech appropriately.
“To ensure the proper development of young children, appropriate support is also needed for mothers who suffer from mild upsets that often do not yet require treatment,” says Professor Schaadt. That doesn’t necessarily have to be organized intervention measures. “Sometimes it just takes the fathers to be more involved.”
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By Alison Bosman, Earth.com Staff Writer