Memory is one of our most fundamental and important cognitive skills, but how it forms is not entirely clear.
Now a new study has filled some gaps, highlighting the complex, bi-directional interaction between the brain and behaviour. Early changes in our ability to remember past events impact the brain, it shows, and the brain in turn influences memory development.
“Most adults cannot recall events from the first few years of their lives,” says Tracy Riggins from the University of Maryland, US, senior author of a paper in the Journal of Neuroscience.
“What happens during our childhood years that helps to stabilise memories so they are better able to stand the test of time? Does our brain mature and then our ability to remember improves? Or does our memory improve which then changes our brain?”
Most research on the typical development of the hippocampus – a key player in the brain’s memory processing – has come from cross-sectional studies and post-mortems from non-human primates.
Riggins and colleagues imaged the brains of 200 healthy children aged four to eight (long-term memories start appearing from around four) and traced the development of four- and six-year-olds over three years to untangle this.
In parallel, they tested what’s known as source memory, an aspect of episodic memory that shows considerable development during childhood and is related to hippocampal growth.
“Episodic memories have details, such as when and where events occurred,” explains Riggins, and “is known to show prolonged development across childhood. This may relate to why, as adults, we are unable to recall events from our early childhoods.”
Source memories refer specifically to the ability to remember contextual details. To test this, children were taught 12 facts they didn’t know, via video, by a female adult called Abby or a male puppet called Henry.
They were given six facts from each source – such as “A group of rhinos is called a crash”, “A group of kangaroos is called a mob” or “A group of goats is called a tribe” – then asked to remember the facts as they would be tested on them a week later. But they weren’t told they would be tested on the information’s source.
On their second visit the children were given 22 trivia questions to answer – some commonly known, such as “What colour is the sky”, and others more obscure, such as “What is the coloured part of your eye called?”
If correct, they were asked whether they had learned the answers from Abby, Henry or someone else. Their brain development was tracked at each visit using functional magnetic resonance imaging (fMRI). Analyses controlled for intelligence scores.
As well as highlighting the two-way interaction between neural development and memory changes, results showed that timing and brain location matter.
Early changes in memory ability had more impact on the brain than later changes, and not all brain regions showed the same effects. Also, some regions were more influenced by early changes in memory, whereas other regions were more predictive of later memory changes, Riggins explains.
“Together these results highlight the intricate and reciprocal relationships between brain and behaviour across development.”
There were also substantial differences between recall skills and brain development in children of the same age.
“For example, some four-year-old children have very good memories and strong brain connections,” says Riggins, “whereas other four-year-olds have poorer memory abilities and weaker brain connections.”
The team is currently exploring possible reasons for these differences, such as sleep quality.