Infant memories: Lost but not gone?

Infantile amnesia is a phenomenon that causes people to forget their early childhood experiences. Sigmund Freud suggested that these early memories are emotionally charged and thus repressed. However, modern psychology attributes infantile amnesia to the immaturity of the hippocampus, which is vital for forming episodic memories in older children and adults. Infants' hippocampal circuits are not developed enough to form enduring memories of life events.

The theory argues that infantile amnesia does not indicate a failure to encode memories, but rather a difficulty in retrieving them. This is not unique to humans; most mammals exhibit accelerated forgetting during infancy. Research on mice has shown that memories formed in infancy persist but cannot be naturally accessed. Optogenetic stimulation of hippocampal neurons, active during learning, can reactivate these memories.

This suggests that early-life memories are encoded and persist despite being inaccessible through natural sensory retrieval cues, pointing to a failure in retrieval rather than encoding.

The debate isn't limited to humans and mice. Most mammals exhibit similar patterns of forgetting early-life experiences. However, highly precocial animals like guinea pigs and degus, whose brains are more mature at birth, do not demonstrate this phenomenon. Recent advancements in human infant memory research have shown that memory systems are active from birth.

While hippocampal-based memory in infants can be measured through brain activity, it is challenging due to technical limitations. However, functional MRI in awake infants can now provide insights into hippocampal activity and memory formation. The inability to naturally retrieve early-life memories might be due to the brain's changing interpretation of the world as it develops.

Language acquisition, object recognition, and understanding of concepts contribute to this shift, making it difficult for infants to access memories formed during their earliest years.