A human baby, while a foetus in the mother’s womb, cannot see nor speak or understand language. Yet, it is able discriminate between the voices of the mother and other women! Experiments using the heart beat rate of the foetus reveal a higher signal when the mother speaks. Early exposure to the mother’s voice facilities recognition of the sound sources and establishes it as a preferred stimulus. How is this? As the mother-to-be speaks, the accompanying movements and vibrations are transmitted through her body to the baby in the womb, thus fixing her voice as the most perceived. Such perception is not necessarily of speech or language, but of the voice. It is “paralingual,” meaning not the language but the tone, pitch volume, rate, fluency and accent. All these are felt and registered by the foetus.
Such hearing and discrimination of speech sources seems to start in the last trimester of pregnancy. At this stage, the foetuses are able to discriminate their mother’s voice from a female stranger’s, suggesting recognition and learning of some features of her voice. The voice is thus referred to as the ‘auditory face’. Researchers at the Queen’s University in Canada studied 143 foetuses and recorded their continuous heart rates as they listened to recorded sounds (two-minute reading of the story of the young Bambi).The foetuses could tell apart their mother’s voices from those of the other women, even in these recordings. The heart beats, the signal strengths and related features were distinctly different. This suggests that not only had each of these fetuses recognised its mother’s voice but had learnt and stored this ‘auditory face’ of the mother in its brain.
Of course, the foetuses did not know who Bambi was, nor understand the language or the story. It was not speech perception or understanding, but voice perception. What they reacted to was the tone, the timbre and other features that characterised the mother’s voice, which were different from those of the ‘strangers’. The neural network in the brain had already started forming and getting established.
Lest we get too human-centric or bombastic, it is good to be told that other mammals too have such ability for voice recognition and discrimination. Macaque monkeys do so. Their babies tell apart their mothers from others. Dr. Insley from UC Davis in the US and Dr I. Charrier from Saint-Etienne in France have studied babies (pups) of seals from the ocean. “Not only do the pups and their mothers have the ability to recognize each other’s vocalizations during the breeding season, despite the large population of the colony, but they are able to retain these memories for at least four years; this learning appears to occur as rapidly as the first 2-5 days of life” quotes a review of these papers.
The recent paper titled “Neural circuits underlying mother’s voice perception predict social communication abilities in children”, by Dr. DA Abrams and others from Stanford, in the May 31, 2016 issue of PNAS (US), points out that mother’s voice is a constant and familiar presence in a child’s environment, beginning at a time when the vocal sounds and vibrations are conducted through the uterus to the foetus’ developing auditory pathways. This stimulus is seen to guide the emotional and social functions during development- a ‘fingerprint’, as it were.
Just as the French studies showed how mother’s voice holds on for 4 years in seal pups, the Stanford group studied children as old as 7-10 years of age. They listened to their mothers’ voices (mothers were asked to simply repeat four-syllable nonsense words, thus no cue or hint, just babbles), and as they did, the researchers used functional MRI to measure the brain activity of the child. The results revealed that hearing a mother’s voice, a critical source of emotional comfort and social learning, is reflected in a wide range of brain parts that include auditory, speech, reward, and affective processing. The child’s social abilities are tightly linked to the function of this brain network. Surprisingly, this lasts as long as 10 years of childhood.
Hearing and registering mother’s voice thus generates a neural fingerprint of the child’s social communication abilities. The study also suggests that in cases of children with autism or attention deficit, what the disturbance in this fingerprint might be, and how one may approach to correct it.
Finally, the big question that fathers ask: what about us and our voices? Well, we do not know yet, but it certainly is not the same as the mother’s. While some early studies do suggest that voice recognition and fixation in the child does happen, what changes, if any, occur in the child brain are yet to be studied using methods such as fMRI. Till that is done, the jury is still out.
D. Balasubramanian
dbala@lvpei.org
