We frequently reflect on events that have happened in the past and plan for events that we know will happen in the future.
Professor of Psychology Harlene Hayne, vice-chancellor of Otago University, describes this phenomenon as a kind of mental time travel using a special kind of memory that is commonly referred to as episodic memory.
It allows us to take a trip down memory lane, revisit the past in our mind's eye and allow us to consider future possibilities that have yet to occur.
Most modern theorists of memory agree our ability to remember the past does not rely on a single memory system, but rather a series of different memory systems comprised of different neural substrates and which operate according to different principles.
The term episodic memory is used to refer to the recollection of personal, past experiences.
More recently, the term has also been used to refer to the ability to use past experience to make plans for the future; this latter ability has been referred to as episodic foresight.
According to some memory experts, episodic memory is a uniquely human ability - no other animal has the ability to scan the past or plan for the future.
Professor Hayne has been studying memory development in infants and children for 30 years and her interest in episodic memory began a decade ago, motivated by the phenomenon of childhood amnesia, or our inability to recall events that took place during our infancy and early childhood.
"This phenomenon is particularly puzzling because infants and young children have outstanding memory ability," Professor Hayne said.
"The key question is what happens to those memories and why."
Some researchers argue that a new kind of memory - episodic memory - emerges between the ages of 3 and 4 years old.
"Although there is little debate among memory researchers about the importance of episodic memory, there is considerable debate about when episodic memory might emerge during the course of human development."
Professor Hayne's research seeks to answer some of the area's burning questions. At what age do children first show signs of "mental time travel" - and how do these skills over life as a function of age and experience? When do children begin to use what they have learned in the past to make predictions about similar events in the future?
Solving these riddles will have important theoretical implications for views about the normal course of memory development.
"It also has practical implications in settings in which children must rely on their memories, including clinical, legal, and educational contexts.
"Furthermore, given the importance of episodic foresight for coping and planning, we also suspect that individual differences in this component of episodic memory plays an important role in a wide range of psychological processes that may either enhance or impede mental health, well-being and success."
Memory, she said, was a fundamental aspect of human cognition that emerged early in development - even newborns exhibit memory for their mother's face and voice.
Depending on their age, human infants exhibit memory when tested after hours, days, weeks, or even months.
"Our research and that of others has shown that the basic building blocks of memory emerge early, but change rapidly during infancy and early childhood."
She and fellow researchers developed two new tasks to explore the episodic memory's development in children.
In the "timeline task", children aged 3 to 5 were provided a personal timeline consisting of a large piece of paper displaying the child at different points in their life.
This reinforced the linear notion of time, while also allowing the researchers to ask questions about different events without relying exclusively on terms like later, earlier, before and after, that young children struggle to understand.
The researchers then compared the ability of the children to describe events that happened to them in the recent past or that would happen in the near future.
They found all the children could provide some information about these different kinds of events.
"On every measure, even 3-year-olds exhibited evidence of both episodic memory and episodic foresight," Professor Hayne said.
The second task, modelled on a test developed by renowned memory researcher Dr Endel Tulving, also showed 3-year-olds were capable of forming an episodic memory of the task.
"One potential explanation for childhood amnesia has been that prior to 3 years of age, children do not have the capacity to form episodic memories. Our data render this particular explanation unlikely. Instead, our findings clearly show that children as young as 3 years old form episodic memories - the difference appears to be in how long they can remember them."
Ultimately, she believed the key to unlocking childhood amnesia was children's ability to use language in the service of memory.
"Although their memory skills, including their episodic memory skills, emerge very early in development, their ability to use their language and the language of others to retrieve and express those memories allow them to be maintained over significant periods of development."
Treasure hunt helps uncover secret to memory
Professor Harlene Hayne and Otago University researchers adopted the famous "spoon test" by Dr Endel Tulving, a world authority on human memory function, to study "mental time travel" in a group of Kiwi children.
The researchers tested 3- and 4-year-old children in a large outdoor sandbox, over two phases separated by delays ranging from 15 minutes to a week.
In the first phase, each child was taken to the sandbox and told: "I saw a pirate around here earlier today and I think that he might have hidden his treasure in our sandbox. Can you help me find it?"
The experimenter and the child then dug for the gold-painted treasure chest.
Once it was uncovered, they found it was locked and the experimenter asked the child if they had a key to open the padlock.
"When the child responds no, the experimenter looks in her pockets and says, 'I don't have a key to unlock it either, oh well, we're just going to go back inside now and do something else'," Professor Hayne said.
After a delay, the child took part in the second phase, where their language skills were assessed.
When these had been completed, the experimenter told the child they were going to go back outside to the sandbox, but first there were some things to show them, and they could pick one to take home.
The experimenter then displayed three objects in front of the child: a small coloured ball, a small coloured wind-up toy, and a key. The child was then encouraged to select one of the three objects and the child was taken back out to the sandbox.
"Using our version of the spoon test, we have shown that 3-year-olds clearly form an episodic memory of the task, correctly selecting the key when they are tested after one 15-minute delay," Professor Hayne said.
"Unlike their older counterparts, however, they do not retain this memory over delays longer than 15 minutes."
In contrast, 4-year-olds exhibited robust evidence of episodic memory, even when the delay was as long as a week.
The series
• Part 1: Tackling the obesity epidemic
• Part 2: Solving the human jigsaw puzzle
• Part 3: The Kiwi-made biotech wonder
• Part 4 (Today): Learning mental time travel
• Part 5: The birth of the artificial muscle
• Part 6: The age of wearable computing
