Have you ever thought about a really good memory – the day you graduated school, or maybe when you won a prize – or a very sad memory, like the death of a family member, and then wondered what specific part of your brain was activated for that certain memory? A team of neuroscientists from UC Davis is using virtual reality in a really neat experiment to identify and learn how different areas of the brain assemble memories in context.

We all know that one memory can trigger others which are related to the first one – if you remember getting your diploma at graduation, you might next remember going out to dinner with friends and family after the ceremony, which could lead to a memory about another time you went to the same restaurant. Specific events are remembered with specific context, such as who was there, what happened, and where it took place. But multiple memories can also have context, along with the information that links the two together: events which happen at the same place, for instance.

Graduate student Halle Dimsdale-Zucker and Professor Charan Ranganath at the university’s Center for Neuroscience and Department of Psychology are studying how a person’s brain puts together all of the pieces in memories, by using functional magnetic resonance imaging, or fMRI, to look at which areas are activated as a person recalls a certain memory. They also used virtual reality, which Dimsdale-Zucker said makes it possible to carry out controlled laboratory experiments regarding a certain type of memory.

Dimsdale-Zucker’s work was supported by a graduate research fellowship from the National Science Foundation (NSF), and her team recently published a study in the journal Nature Communications, titled “CA1 and CA3 differentially support spontaneous retrieval of episodic contexts within human hippocampal subfields.” It details how they are using a virtual reality environment to train subjects, in order to determine which areas of the hippocampus are activated for which type of memory; co-authors include Dimsdale-Zucker, Maureen Ritchey from Boston College, Arne D. Ekstrom and Andrew P. Yonelinas with UC Davis, and Professor Ranganath.

Experimental approach. Participants encoded objects uniquely located within one of two spatial locations (spatial contexts) across a series of 20 videos (episodic contexts). Next, they were scanned while performing an object recognition test which required differentiating old and new objects presented without any contextual information. Representational similarity analyses (RSA) were used to examine the similarity of voxel patterns elicited by each recollected object relative to other recollected objects that were studied in the same (or different) spatial and episodic context.

The abstract reads, “The hippocampus plays a critical role in spatial and episodic memory. Mechanistic models predict that hippocampal subfields have computational specializations that differentially support memory. However, there is little empirical evidence suggesting differences between the subfields, particularly in humans. To clarify how hippocampal subfields support human spatial and episodic memory, we developed a virtual reality paradigm where participants passively navigated through houses (spatial contexts) across a series of videos (episodic contexts). We then used multivariate analyses of high-resolution fMRI data to identify neural representations of contextual information during recollection. Multi-voxel pattern similarity analyses revealed that CA1 represented objects that shared an episodic context as more similar than those from different episodic contexts. CA23DG showed the opposite pattern, differentiating between objects encountered in the same episodic context. The complementary characteristics of these subfields explain how we can parse our experiences into cohesive episodes while retaining the specific details that support vivid recollection.”

Dimsdale-Zucker used SketchUp to create two homes in a 3D virtual environment. Each house contained ten pieces of furniture in differing colors and a total of 300 neutral objects, like a teddy bear and a vase, were scattered throughout both.

The study subjects watched a series of 20 videos, in which they went into one of the houses, and then the other. Different objects were positioned in different places within the houses in the videos, and the subjects were tasked with memorizing those objects in two specific contexts: spatial memory (which house was the object in) and episodic memory (which video was the object in). Then, in the second phase, the subjects had to try and remember the objects while they were being scanned by fMRI.

The team’s hypothesis was proved when the subjects, once asked about the objects, spontaneously reactivated contextual information, and different parts of the hippocampus were activated for different types of this information. The researchers associated the CA1 area in particular with representing memories that share information about contexts, like objects appearing in the same video.

“What’s exciting is that it is intuitive that you can remember a unique experience, but the hippocampus is also involved in linking similar experiences. You need both to be able to remember,” Dimsdale-Zucker explained.

Pattern similarity in CA1 and CA23DG is sensitive to episodic context. Pattern similarity was higher in left CA1 for items studied in the same video than for items in different videos. Left CA23DG showed a reversal of this pattern such that pattern similarity was higher for items studied between videos vs. within the same video. Neither CA1 nor CA23DG patterns were sensitive to spatial context similarity alone.

The study also revealed that the hippocampus was involved in episodic memories that linked space and time, which contradicts conventional thinking that it’s mostly involved in spatial memories.

Continued research into how memories are made, stored, and recalled could be helpful in terms of better diagnosis and treatment of memory problems for people with aging or degenerative disorders, like Alzheimer’s disease.

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[Source: UC Davis]

 

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