How Does the Hippocampus Work?

The process of storing our thoughts and experiences occurs during sleep.

Emotionally laden experiences are transferred from the hippocampus to the brain’s hard drive or neocortex. This transfer occurs during the first deep sleep phases. This process must be repeated several times since the neocortex learns more slowly than the hippocampus. Perhaps this is the reason sleep cycles are repeated during sleep.

New memories are connected to existing memory content for contextual remembering. This occurs in the later stages of sleep when we dream or during rapid eye movement sleep (REM). This is why people often like to “sleep on it” when making important decisions. This process of connecting new to old memories often gives us valuable new insights.

The hippocampus stores the access information to all neocortical memory fragments like an index in so called index neurons. These index neurons remain in the hippocampus’ dentate gyrus throughout life anchoring all our memory content. Without these neurons it would be impossible to reconstruct our memories. The dentate gyrus has the ability to produce thousands of new neurons every day, even in adults. If this regeneration didn’t happen, we would have to overwrite the memories in our index neurons to make space for new memories. This neurogenesis takes place mainly during REM sleep. Approximately 1.5% of all hippocampal neurons are formed each year.

Contrary to many commonly held beliefs, healthy seniors have the potential to remain cognitively and emotionally intact until the end of life because of the persistence of neurogenesis. In a healthy state, new index neurons are constantly produced, however they can only survive if they encode new experiences or if their synapses (the points of contact with other neurons) become linked to the hippocampal network.

Meaning in life is associated with manageable tasks and challenges, or in other words, healthy levels of stress. These new tasks and challenges stimulate adult hippocampal neurogenesis. Studies show schematically the facets of our lifestyle that directly affect the growth and maintenance of our memory.

Cognitive decline, or death of hippocampal neurons has become common. Most of us know at least one person who suffers from dementia. Noted physician and molecular geneticist, Michael Nehls, MD, PhD states that our modern lifestyle is resulting in an average loss of about 1.4% of brain volume per year!

There is no decline in brain volume even at age 100 years. In contrast, the dotted line at the lower end of the graph represents hippocampal shrinkage as a result of our modern lifestyle due to poor sleep, lack of exercise, insufficient nutrients due to excessive consumption of processed foods, social isolation and loss of purpose in life.