Memory development
Functional Neuroanatomy of Memory Systems
The Medial Temporal Lobe and Hippocampus
Structural Overview: Humans possess two hippocampi (left and right). This region is localized within the Medial Temporal Lobe (MTL) and is the primary hub for processing declarative (explicit) memory. Hippocampal volume increases dramatically in the first two years of life, aligning with the offset of infantile amnesia.
Vulnerability: The hippocampus is exceptionally sensitive to hypoxic conditions (lack of oxygen), which can lead to significant memory impairment.
Neurogenesis: It is one of the few areas in the adult brain capable of neurogenesis (generating new neurons postnatally), especially in dentate gyrus. These new neurons could destabilize memory traces, making it impossible to form long lasting memories. Factors such as learning and cognitive demand can influence hippocampal volume, as seen in studies of London taxi drivers who showed structural changes correlating with spatial memory requirements.
The Cerebral Cortex and Prefrontal Cortex (PFC)
Evolutionary Comparison: The human cerebral cortex is significantly larger and more complex () compared to other mammals like cats ().
Role of the PFC: The development of the prefrontal cortex is closely linked to higher-level cognitive abilities and the organization of memory. While lower-order animals exhibit associative memory, humans demonstrate advanced cognitive control and source monitoring due to PFC maturation.
Neuronal Development
Gestation and Early Growth: Brain structure begins to emerge at weeks gestation. By weeks, the brain transitions from a smooth surface to one with convolutions (folds), allowing it to pack approximately neurons (or the commonly cited ) into the skull.
Synaptogenesis and Pruning: In the first years of life, the brain forms roughly million synapses per second. This is followed by neuronal pruning, which streamlines circuit efficiency but reduces the brain's plasticity and its ability to recover from specific injuries.
Myelination: Sensory pathways develop first, with myelination continuing through the first two years of life, providing the foundation for later cognitive and memory functions.
Memory Development Trajectories
Explicit Memory (Declarative)
Autobiographical Memory: Typically absent before age ; significant development occurs between ages .
Episodic memory: the ability to remember past events along with elements of the spatial and temporal context in which they occurred.
Semantic memory: memory for facts that are not associated with a particular context.
Children as young as 3 years of age produce verifiable narratives of both
naturally occurring events and staged events, and start to deploy basic
memory strategies.
▪ However, evidence has emerged that preschool children are more
suggestible than older children or adults (Bruck & Ceci, 1999), and are
relatively poor at source and reality monitoring (Drummey and Newcombe,
2002).
▪ Dramatic growth of explicit memory across preschool and into the early
childhood years.
During the first two years of life, children learn many words (Frank et al., 2017).
They also become familiar with event scripts such as what happens at bedtime and acquire gender labels and stereotypes (Weinraub et al., 1984).
Place learning is the ability to locate a position using information about its distance and direction from distal landmarks.
In children, place learning is first seen around 21 months of age. Place learning continues to develop well into the school years, with children being able to take advantage of more distal landmarks in more complex settings with increasing age
Skills Growth: Improvements in episodic and semantic recall are observed through adolescence as the MTL circuits and frontal lobe mature.
Types:
Prospective memory (PM) enables the execution of an intended action at an appropriate moment e.g. remembering to buy cat food before you return home.
Event-based prospective memory refers to instances in which the action needs to be performed in response to a cue (e.g. buying cat food when you walk past the pet food store).
Time-based prospective memory refers to when the intended action has to be carried out at a certain time or after a specific delay.
Implicit Memory (Non-Declarative)
Early Maturation: Unlike explicit memory, implicit memory (skills, habits, and conditioning) develops very early. Most investigators agree that implicit memory is in place within the first months of postnatal life, and that there is little if any further development in performance beyond the age of 3 years.
Trajectory: There is no significant growth trajectory for implicit memory beyond early childhood; once established, its performance remains relatively stable compared to the evolving explicit system.
Procedural memory: the acquisition and retention of particular motor or cognitive skills or habits and is implicated in the skills and habits that underlie language learning and social interaction.
Assessment Paradigms for Memory Systems
Explicit Memory Assessment
Imitation: This paradigm is used to measure explicit memory in non-verbal infants. A researcher performs an action, and the child's ability to replicate that action immediately (elicited imitation) or after a delay (deferred imitation) is assessed.
12-, 18-, and 24-month-old infants exhibit clear evidence of imitation following a 24-hour delay (deferred imitation).
18- and 24-month-old infants reproduce more target actions during the test than 12-month-olds.
6-month-olds perform as well as 12-month-olds when they are tested in the absence of a delay (elicited imitation).
With additional exposure to the target actions, even 6-month-old infants exhibited deferred imitation following a 24-hour delay.
Source Memory Tasks: Researchers evaluate whether a child can identify the source or context of a learned fact. Proficiency in source memory typically develops between ages and.
Tested children 4, 6 and 8 years on a source paradigm adapted from adults. Children were presented with ten facts by either an experimenter or a puppet. After a delay of one week the children were asked questions on the facts (“item memory”) and also asked to identify the source (experimenter, puppet, teacher, parent, “source memory”).
Children showed a steady improvement with age in their ability to remember the facts, but showed an abrupt improvement between 4 and 6 year in their ability to monitor the source.
Visual preference technique involves presenting two different stimuli at once to determine if the baby displays a preference by looking at one longer than the other. If so the baby must be able to tell the stimuli apart. Introducing a delay between stimulus presentation and test is used to test memory retention of presented stimuli.
Habituation: using this technique an investigator presents an infant with a novel stimulus that captures the infant’s attention, and then continues presenting it until he gets bored and stops paying attention to it. The next step is to change some aspect of the stimulus, such as the pitch of a musical tone. If the infant shows renewed interest after the change in the stimulus, the infant is said to exhibit dishabituation, and the investigator can conclude that the infant perceived the change.
Familiarisation and Recognition: images are presented on a background of one colour during familiarization and on a different coloured background during the recognition test. Results showed that recognition memory is impaired by a change in context at 6 and 12 months of age but is unaffected at 18 and 24 months of age.
Increase in strategy use and metamemory skills during adolescence, whereby participants increasingly organize the information to be remembered to facilitate recall and increasingly reflect on the contents and accuracy of their memories.
Categorisation task: Participants are shown a set of stimuli that can be categorized semantically (e.g. animals, food, furniture) and are told that they have to remember as many stimuli as possible and to do whatever may facilitate learning and remembering the items.
Strategic behavior during encoding is reflected in sorting and during recall by clustering.
Studies show that recall, and sorting and clustering increase between childhood and adolescence and that recall performance correlates with the use of strategy.
Implicit Memory Assessment
Procedural Tasks: Measuring the acquisition of motor skills or habits over repeated trials where the subject may not consciously recall the learning process.
Associative Learning Paradigms - serial reaction tasks: Utilizing classical and operant conditioning to observe behavioral changes based on reinforcement or stimulus association. participants must learn the association between a set of response keys and a corresponding set of locations on a display screen, so that they press the correct button when a location is cued.
Priming and recognition paradigm: 3 year olds’ priming and recognition memory for pictures of animals from a children’s book they had read 3 months earlier. While the children showed no evidence of explicit recognition of the pictures, they did show evidence of implicit priming by identifying blurred images of the animals more quickly than control children.
A second experiment showed that explicit memory improved between 3 and 5 years whereas priming did not.