Research by Indian scientists has suggested that foetal brain cells detect select sounds for long-term retention much earlier than hitherto assumed and could explain maternal voice recognition by foetuses in the third trimester and infancy.

The scientists at IIT Kharagpur, have discovered through studies on newborn mice that the earliest sound-driven brain activity starts much before ear canal opening, a critical developmental point in the brain’s auditory maturation.

They have found that six-day to 10-day old mice pups exposed to repetitive sound sequences with embedded so-called salient or deviant sounds, can selectively retain memories corresponding to the deviant sounds into adulthood.

Sensory development in mice continues for a few days after birth and the auditory system in mice six to 10 days old is at a stage it would be observed in 22-week to 26-week human foetuses.

Multiple earlier studies have indicated that human foetuses can detect sounds between 18 and 24 weeks and discern the maternal voice in the third trimester — 27 to 40 weeks. But the mechanisms underlying the brain’s capacity to process sounds in earlier periods have remained unclear.

The Kharagpur study led by assistant professor Sharba Bandyopadhyay has found that in its earliest stages of development, the auditory system retains only deviant — or relatively rare sounds — and not the most frequent sound.

“It is surprising and counterintuitive,” said Adarsh Mukesh, a research scholar and co-author of the study.

“All earlier studies have shown that retention comes from repeated exposure to something. Here, we find only the memory corresponding to rare sounds gets imprinted for retention.”

The study, published in the Journal of Neuroscience, has also shown that the capacity for detecting and retaining deviant or rare sounds is strongest at early ages of mice and reduces overtime to values observed in adults.

The study’s other co-author is Muneshwar Mehta.

The scientists believe they have observed a phenomenon unique to the early stage of the auditory system’s development where certain cells detect sounds but the brain isn’t fully developed yet to determine which sounds are important.

“The available neural circuits discriminate between sounds — but against a background of routine sound or noise, it retains only rare sounds,” Mukesh said. “But as the brain continues to develop, it develops a mechanism to what we’re all familiar with — experience-based retention.”

Their findings may explain the foetal recognition of maternal voice in the third trimester and infancy.

The foetus is exposed to a constant background of sounds of the mother’s heartbeat, breathing and blood flow against which, Mukesh said, her voice could serve as the salient or rare sound that gets imprinted for retention.

Sourav Banerjee, a scientist at the National Brain Research Centre, Manesar (Haryana), said the work by the Kharagpur scientists has pushed back the critical time window in which the brain’s circuitry acquires the capacity to receive and process auditory signals.

“They’ve shown that auditory signals are encoded much earlier than established until now,” said Banerjee, who was not associated with the study.

“They’ve done this through elegant experiments that provide multiple snapshots of the auditory system under development.”