amnesia in children -- 10/20/21
Today's selection -- from Wayfinding by M. R. O'Connor. Why can’t we remember things from very early childhood?:
"It was a neuroscientist who first told me about the universal phenomenon of amnesia in children. Kate Jeffery is an English neuroscientist whose laboratory at University College London studies the behavior of hippocampal cells in rats. At the core of her interest is the mystery of why the human brain seems to use the same neural circuit for navigating space and episodic memory; she has called it one of the most outstanding questions about the brain. 'Why would nature have used the same structures for both space and memory, which seem so very different?' she wrote in Current Biology. 'An intriguing possibility is that the cognitive map provides, in a manner of speaking, the stage upon which the drama of recollected life events is played out. By this account, it serves as the "mind's eye" not only for remembering spaces, but also the events that happened there and even -- according to recent human neuroimaging evidence -- imagination.'
"I met Jeffery at a conference in London and asked her about my own experiences with childhood memory. Was there an age at which our cognitive powers for spatial mapping are 'turned on,' so to speak? How the spatial system develops in infancy, Jeffery told me, is still very much an open-ended question; we don't know how much of our brains are hardwired or how much spatial experience is necessary in order to condition the functions of the brain. Some studies have shown that animals raised in featureless or small confinements struggle with simple spatial tasks, but how this translates to humans is unclear. 'I think the field is still grappling with these issues. We're not exactly sure. But there is a phenomenon, this period of time in infancy and beyond during which we don't have lasting episodic memories, we don't seem to be laying down those memories,' she said. And, Jeffery pointed out, young infants don't form cognitive maps in the way adults do. 'Their spatial organization of information is a lot less rich,' said Jeffery. 'It's possible that memories you form as an infant, because the hippocampus is still developing, may get overwritten or disturbed by the new circuitry that is still developing. And as an adult, you can't retrieve those early life memories the way you can later ones.'
"The hippocampus of rats is anatomically similar to humans, and for Jeffery, looking into their brains and listening to the activity of their neurons firing as they move provides tantalizing glimpses into the physiology of spatial mapping and memory. As we sat together talking about these questions, I asked Jeffery if she could explain the process by which neuroscientists think the hippocampus perceives and creates representations of space. Jeffery graciously took a piece of paper and pencil and began sketching a series of boxes and arrows, building a classic circuit diagram to illustrate the neural components of the hippocampus. She started with a box representing the entorhinal cortex, labeled it 'EC,' and split it into five layers representing various cell types. The entorhinal cortex, she told me, is the main interface between the neocortex, the part of the human brain associated with higher intelligence, and the hippocampus. All of the primary sensory areas -- vision, olfaction, audition, touch, what Jeffery described as 'a little bit of this, a little bit of that' -- feed into the entorhinal cortex. From that box she began drawing arrows to other boxes labeled 'DG,' 'CA3,' 'CA2,' 'CAI,' and 'SUB.' These were the main components of the hippocampal circuit, each one fed by the various layers of the entorhinal cortex. 'By the time you get to the hippocampus, quite a lot of stuff has happened, these senses are very highly processed,' she explained. 'But it turns out that layer two goes to CA3, layer three goes to CAI and the subiculum, but there is an output from CAI that goes back into layer five of the entorhinal cortex.' She paused, looked at my furrowed brow, and chuckled. 'So it's sort of like that but there's lots of backwards and forwards.'
"In recent years some of the most stunning images of the hippocampus have emerged from Harvard University's Center for Brain Science. There the neuroscientist Jeff Lichtman has pioneered a way of using microscopes to map neural connections in the brains of mice. By fiddling with genes, Lichtman causes mice to express different fluorescent proteins in individual neurons, which appear under magnification in bursts of beautiful pinks, blues, and greens. These 'brainbow' photographs show how cells in the hippocampus are condensed into single orderly layers. Whereas neurons of the cortex look like a galaxy of randomly strewn stars, those in the hippocampus are aligned in elegant curving arcs. [Click here to look at images].
"These are the cells, called pyramidal neurons, that Jeffery and so many other neuroscientists are fascinated by. And they are a key to understanding the phenomenon of amnesia in our early lives.
"Sigmund Freud coined the term 'infant amnesia' and explained it in terms of repression; the brain was hiding the desires and emotions of infancy from the adult psyche, and these could be accessed through psychotherapy. 'Hitherto it has not occurred to us to feel any astonishment at the fact of this amnesia, though we might have had good grounds for doing so,' Freud wrote in 1910. 'For we learn from other people that during these years, of which at a later date we retain nothing but a few unintelligible and fragmentary recollections, we reacted in a lively manner to impressions, that we were capable of expressing pain and joy in human fashion, that we gave evidence of love, jealousy, and other passionate feelings by which we were strongly moved at the time, and even that we gave utterance to remarks which were regarded by adults as good evidence of our possessing insight and the beginnings of a capacity for judgment. And of all this we, when we are grown up, have no knowledge on our own! Why should our memory lag so far behind the other activities of our minds?' Freud thought of memory as a permanent storage system that enacts a lasting influence over our behavior into adulthood, even if our conscious minds can't unlock it. What he didn't know is that this period of infant amnesia until the age of two-followed by childhood amnesia until the age of six or so -- is not only universal among humans but some mammals as well. All altricial species who raise their young, including rats and monkeys, experience a period of amnesia, hinting at a potential evolutionarily conserved necessity for this developmental period.
"From the 1970s to the 1990s, another explanation for infant amnesia was a child's lack of language: early memories become inaccessible once babies transition from nonverbal to verbal communication. It's precisely around the age of eighteen months that there is an explosion of language in infants, and shortly thereafter infant amnesia dissipates. As Nora Newcombe, founder of the Spatial Intelligence and Learning Center at Temple University, explained to me, '[They believed] that the advent of memories has to do with both language acquisition and is then tied up with cultural norms about the importance of remembering unique events. These are obviously not unimportant; we speak, we live in social groups. But that idea wasn't going to be enough. It wasn't going to be the only explanation.' Further complicating the language hypothesis was the fact that so many animal species that never develop language nevertheless seem to remember events in their lives."