The study shows that inheritance can go far beyond the classic genetic theories, researchers report in The Journal of Neuroscience.

They found that young mice raised in an enriched environment – with toys and other stimulation – passed along the learning benefits to pups they had after they grew up.

The stimulated mothers did not simply have better parenting skills, because the researchers showed pups swapped at birth still learned better if their biological mothers – but not their foster parents – had been raised with the extra toys.

"You inherit to some degree some aspects of your parent's experience," Larry Feig, a professor of biochemistry at Tufts University School of Medicine in Boston, said in a telephone interview.

"This is a protective mechanism a mother passes on to her offspring," Feig, who helped lead the study, added. "The mother is changed for months. Her brain is changed so that when she is old enough to get pregnant, the effect is still there."

Feig and colleagues raised mice, some in plain cases with wood chips and others in "enriched" cages with boxes, a running wheel, toys, and constant rearrangements of nesting material.

They tested learning with an unpleasant "shock chamber" to condition the mice to be afraid.

Mice born to mothers raised in the "enriched" cages learned much more quickly that the shock chamber was a scary place, Feig's team found. This was true even when the mothers did not become pregnant until weeks after they lived in the special cages.

BRAINS OF MICE

When the researchers looked at the brains of the mice, they found clear changes in what is called long-term potentiation – a measure of how well nerve cells communicate with one another. These changes were inherited by the pups, even if the pups themselves never saw a toy or running wheel.

Feig said what is being changed is a mystery.

"It is probably some hormonal effect," he said.

Learning and stimulation may raise levels of hormones – he does not know which ones – and these levels may stay high for part of an animal's lifetime and affect the developing foetus.

"There is a lot of evidence that during embryonic development the offspring are greatly sensitive to things the mother is exposed to," Feig said – including food, chemicals and perhaps hormones.

Dean Hartley, a neurosciences researcher at Rush University Medical Centre in Chicago who worked on the study, said this could work in two ways. "Does that mean an unenriched environment could have a detrimental effect?" he asked.

"Because the environment can affect us in both ways – good and bad – we need to be cautious about the environmental exposures pre-pregnancy."