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.
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.
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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.
"Completing the task when the colour changes occur unpredictably requires more cognitive resources, or control," said Basak.
She compared the effect to what happens when you take a new, previously uncharted way home from work. The cognitive demands required to navigate new landmarks that are part of the journey increase with the new route's unpredictability.
That benefit, however, appeared to fade when the same group was tested a month and a half later, researchers said.
"Training-related improvements from our novel approach dissipated when performance was tested awhile after completion of the training," said Basak.
"Studies such as this one shed light on the role of cognitive control in memory training," she said.
"They also highlight the differences in training-related performance gains between people, and could help researchers and clinicians develop better cognitive training strategies for older adults who are at risk for dementia," Basak added.
"When you have multiple items to remember, you need to focus your attention on what is most relevant and up-to-date, setting aside what may be distracting," Basak added.
The findings were published in the journal Frontiers in Psychology.