Previously it was believed that general semantic, personal semantic and episodic memory relied on different areas of the brain. Instead researchers, including UEA’s Dr Louis Renoult, suggest that different long-term memory types could be viewed as a spectrum, where they rely on activating the same areas of the brain at differing magnitudes.
The group’s study was published today in eLife.
Dr Renoult, Associate Professor in Psychology in UEA’s School of Psychology, said: "Taken together, our data suggests that there is a clearer distinction in the subjective experience elicited by the retrieval of semantic (general knowledge expressed in the present) and episodic memory (recollection of personal past episodes) than in the brain regions they rely on."
Long-term memory can be classified into different categories, including semantic memory, which refers to an individual’s non-personal, general knowledge of the world, and episodic memory, which concerns the recollection of contextually specific events from their personal past.
For instance, the thought ‘I know that a sauna is very hot, steamy and relaxing’ is an example of a semantic memory, while ‘I remember winning a dare of who could stay longest in the sauna. It was a Friday night and we celebrated with a large glass of cucumber water’, is an example of episodic memory.
This distinction has been called into question in recent years. Previous research has discovered an overlap between the neural correlates of semantic and episodic memory.
Neural correlates refer to the specific patterns of brain activity that are associated with the creation, storage and/or retrieval of memories and are used to help researchers understand how memories are formed and accessed in the brain.
Furthermore, several types of long-term memory exist that do not neatly fit into the semantic-episodic distinction. For example, personal semantics includes knowledge of personal facts (for example, ‘I have a sauna membership’) and summaries of repeated events (for example, ‘I always start my gym routine with stretches and good music’).
As such, personal semantics exists conceptually between the boundaries of semantic and episodic memory; it is personal, like episodic memories, but detached from the context of how they were learned, like in semantic memories.
Some forms of personal semantics, such as autobiographical facts, appear to have neural correlates similar to semantic memory, while others, such as memories of repeated events, have neural correlates more similar to those of episodic memory.
“Personal semantics, despite its importance, remains an understudied area of long-term memory,” said lead author Annick Tanguay, who conducted the study at the University of Ottawa, Canada, and UEA, and is currently a Postdoctoral Research Fellow at Vanderbilt University Medical Center in Nashville, United States.
“Few studies have directly compared personal semantics to semantic or episodic memory, and even fewer have compared different types of personal semantics to one another or to both semantic and episodic memory. With our study, we were able to do so.” said co-author Patrick Davidson, Associate Professor at the School of Psychology, University of Ottawa.
Tanguay and colleagues used a neuroimaging technique called functional magnetic resonance imaging (fMRI) to record the brain activity of 48 participants whilst they answered yes/no questions designed to involve either memory for general knowledge (semantic memory), unique events (episodic memory), autobiographical knowledge or memory for repeated events (two types of personal semantic memory). The team then used multivariate analysis methods to compare the participants’ brain activity across the four memory conditions.
Their results indicated that personal semantic, general semantic and episodic memory had both shared and unique neural correlates. When they contrasted four memory conditions – autobiographical facts, repeated events, general semantics, and episodic memory – with a control condition, they identified the shared neural correlates, finding that all four types of memory involved activity within several regions of the brain’s ‘core memory network’.
On the other hand, when they examined the four memory conditions individually, the team found that they engaged the network to a different degree of magnitude.
In the large regions of the brain’s medial frontal cortex, posterior parietal cortex, and medial temporal lobes, there was a relatively small increase in activity when remembering autobiographical facts compared to general knowledge facts.
There was a further, larger increase in activity when remembering repeated events and another small increase when remembering unique events.
In other areas of the brain, such as the right frontal inferior gyrus and the superior parietal lobule, the team also observed a progressive decrease in activity in the same order of memory types.
Another 106 participants made ratings on the same statements to gain additional insights into what might explain similarities and differences between memory types.
In sum, the neural activity in many regions of the ‘core memory network’ seemed to coincide with an increase in personal relevance, an increase in the visual details people perceived in their mind’s eye, and whether the memory involved a scene.
The authors note, however, that caution must be exercised when drawing parallels based on data from distinct groups of participants.
“Rather than looking at a memory type overall, more work needs to be done to understand links between individual pieces of information across the spectrum of memory,” said co-author Daniela Palombo, Associate Professor at the Department of Psychology, University of British Columbia, Canada.
Dr Renoult said: “For future studies, it would be interesting to better understand the nature of the elementary components that underlie the similarities and differences between these memory types, such as the perceptual and spatial aspects of these memories and how much self-reflection they involve.”
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