Possible link between post-memory neurogenesis and memory retention
A study published in the journal Science on May 9, 2014 has suggested that memory loss in mice is linked to the formation of new neurons in the dentate gyrus region of the brain. The study, whose findings claim that neurogenesis in white mice leads to reduced memory retention, was published by researchers Katherine G. Akers et al. at the University of California, San Francisco.
“Increasing neurogenesis after the formation of a memory was sufficient to induce forgetting in adult mice,” explains the study’s abstract. “By contrast, during infancy, when hippocampal neurogenesis levels are high and freshly generated memories tend to be rapidly forgotten, decreasing neurogenesis after memory formation mitigated forgetting.”
To test the ability of white mice to retain memory, an electric shock conditioning task was utilized. Mice were placed in a box and given electric shocks to determine how long the mice showed fear behaviour. Adult mice were capable of remembering fear for over a month, whereas young mice were unable to remember fear after a day of conditioning.
Adult mice were given medication that boosted neuron proliferation like Prozac to investigate whether neurogenesis is responsible for forgetfulness in young mice. The Akers team learned that an increase in neurogenesis was responsible for an increase in forgetful behaviour during the shock test.
In mice, much like in humans, neurons are continuously replaced from birth in a process known as neurogenesis. Neurogenesis occurs in a part of the brain specialized for memory, known as the dentate gyrus of the hippocampus. Much like all cell growth, neurogenesis occurs at a greater rate earlier on in life.
Interesting is that young mice in whom neurogenesis was reduced showed longer lasting fear behaviour during the shock tests. These young mice were able to remember the shocks for over a week, rather than forgetting their fear within a day.
The mammalian brain is comprised of two kinds of cells: neurons and glial cells. Despite having an entire scientific field (neurology) as their namesake, the brain actually contains fewer neurons compared to glial cells.
Neurons form an expansive set of electrical and chemical connections throughout the human body allowing signals and responses to be sent from every part of the body to the brain and back. Glial cells aid in the maintenance and propagation of neural connections.
The relationship between neurogenesis and memory formation remains a controversial subject. The issue is that concurrent research argues that increasing neurogenesis in white mice has shown to both improve and diminish memory retention.
Previous research demonstrated that suppressing neurogenesis in mice can impair pattern separation learning, which allows organisms to distinguish between similar but slightly different circumstances. However, computational models created by the Akers team now reveal that increasing neurogenesis can lead to the degradation of existing memories.
“More neurons increase the capacity to learn new memories in the future,” explained Sheena Josselyn, one of the lead researchers of the study. “But memory is based on a circuit, so if you add to this circuit, it makes sense that [more neurons] would disrupt [memory formation].”
For scientists interested in applying the Akers team’s research to humans, the study’s results pose an explanation for infantile amnesia – the inability for humans to recall memories between the ages of two to four years of age.
The data found by the Akers team marks a continuing search for a greater understanding of the nervous system and the brain.
Sources: IFLS article on study, Hippocampal Neurogenesis Regulates Forgetting During Adulthood and Infancy study, How the Brain Deletes Old Memories (Science Magazine article)