The finding could lead to better cognitive training for those at risk of dementia, researchers said.

Episodic memories are those associated with autobiographical events, such as a past birthday party or first trip to an amusement park. This type of memory is crucial to our ability to accurately retell stories, researchers said.

Chandramallika Basak and Margaret O'Connell from the University of Texas in US tested episodic memory in 46 adults between the ages of 60 and 86 at three different stages - before memory training, immediately after training and one and a half months after completing the training.

Participants were separated into two groups – predictable training or unpredictable training - and did not differ in terms of education or cognitive abilities.

For both groups, sequences of digits in different colours were presented. Participants were asked to indicate when the colour of the current digit matched an earlier one of the same colour.

In training that involved a predictable element, the changing colours occurred in a fixed order, whereas the colour switching was random in the training that involved unpredictability.

The two groups of participants demonstrated equivalent story recall before training, but the group given training with the unpredictable element was able to narrate a previously heard story more accurately than the other group.

Working memory involves the ability to keep information in the focus of attention and to manipulate or reorder it despite distractions - the sort of mental juggling required in everyday life.

The findings were published in the journal Frontiers in Psychology.

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