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Developmental biology - Hippocampus

Our Early Development of Memory

Remembering past events develops rapidly in first years of life...

The hippocampus is a small, horseshoe shaped structure deep in the brain. Its two horns are located one behind each eye, and each is capped by a brain structure called the amygdala. The amygdala is synonymous with "fear". Although it is the brain region inciting all human emotions, fear is the first and most durable one in our memory perhaps as it keeps us out of danger.


To observe how signals travel between areas of our brain, researchers use fMRI (functional Magnetic Resonance Imaging) to measure brain regions for signal output. But, it is a difficult system to use on very young children as it requires lying still while a noisy magnet circulates around your head.

In order to study brain activity in the 22 toddlers involved in the study, a team of scientists led by Simona Ghetti PhD, Professor at the University of California at Davis Center for Mind and Brain, worked out a plan with postdoctoral researcher Janani Prabhakar to measure children's brain activity during natural day activities, and during nighttime sleep by having children sleep in the fMRI scanner.

Their results are published in the June 4th Proceedings of the National Academy of Sciences [PNAS].

Brain signals followed by the research team were produced by the children as they listened to a familiar song while playing with a stuffed toy. Then at bedtime, as each child slept inside the fMRI scanner, she/he heard a familiar tune that had played whil the child played with a stuffed toy. Also, a new, unfamiliar song was heard - but only while a child was sleeping. The fMRI scans registered stronger signals from the hippocampus of each child who heard the familiar song. Additionally, children who had previously shown good memory skills also showed stronger hippocampal activity while sleeping in the fMRI machinery.

Toddlers were also tested when awake, to see if they could remember in which room they heard the familiar song, or if they could associate the song with a toy they were playing with when they heard the song for the first time. To be able to remember things in context where it happened and what you were doing at the time it happened is known as "episodic memory."

Hippocampal activity generated by a familiar tune was higher while children were sleeping, even when that same song was played in reverse. The highest hippocampal response was observed in toddlers who correctly remembered (1) where they learned the song and (2) a toy associated with the song. This tells us how important the hippocampus is for development of episodic memory.

These results establish that hippocampal activity in toddlers reflects memory of past experience. Also, that memory can persist despite slight alteration of the stimulus (such as hearing a familiar song played backwards). Memories associated with place and emotional attachment, such as the stuffed toy represented to the children - appears to produce the strongest memories.
Knowing this about our hippocampal memory, may offer an approach to re-stimulate the neural substrate of early memory following periods of loss of speech due to trauma or stroke.

One of the most fascinating questions in psychology and neuroscience pertains to how young children gain the capacity to remember their past. Early hippocampal processes have been implicated in this ability, but a lack of viable methods has hindered assessments of their contribution in early human development. We employed a functional magnetic resonance imaging paradigm that captures memory-related hippocampal function during natural nocturnal sleep in toddlers. Our results provide direct evidence of a connection between hippocampal function and early memory ability. This experimental approach overcomes previous challenges and promises to pave the way to investigations linking changes in brain function to early development of learning mechanisms, including applications to typical and atypical development.

Nonhuman research has implicated developmental processes within the hippocampus in the emergence and early development of episodic memory, but methodological challenges have hindered assessments of this possibility in humans. Here, we delivered a previously learned song and a novel song to 2-year-old toddlers during natural nocturnal sleep and, using functional magnetic resonance imaging, found that hippocampal activation was stronger for the learned song compared with the novel song. This was true regardless of whether the song was presented intact or backwards. Toddlers who remembered where and in the presence of which toy character they heard the song exhibited stronger hippocampal activation for the song. The results establish that hippocampal activation in toddlers reflects past experiences, persists despite some alteration of the stimulus, and is associated with behavior. This research sheds light on early hippocampal and memory functioning and offers an approach to interrogate the neural substrates of early memory.

Authors: Janani Prabhakar, Elliott G. Johnson, Christine Wu Nordahl, and Simona Ghetti. Additional authors are graduate student Elliott Johnson and Christine Nordahl, associate professor at the UC Davis MIND Institute and Department of Psychiatry and Behavioral Sciences.

The authors declare no conflict of interest.


Funding: The work was funded by the National Institutes of Health.

This article is a PNAS Direct Submission.

Data deposition: The data reported in this paper have been deposited in Open Science Framework and can be accessed using the following website: https://osf.io/6nu9s/. Data is provided in wide form and has identifying and demographic information (subject ID number, age in months, sex), as well as outcome level data for each subject (mean parameter estimates for target > novel and reversed > novel contrasts in the left and right hippocampus, source memory accuracy, and composite memory score).

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Jun 25, 2018   Fetal Timeline   Maternal Timeline   News   News Archive

MRI study of 22 toddlers found that hippocampal activity was higher when sleeping children heard a song, previously learned, than when hearing a new song. This was regardless of whether the learned song was played as recorded or played in reverse. The highest response was observed in toddlers who remembered where they learned the song and a toy associated with the song. Image credit: pixabay.com

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