21 Neurogenesis and Hippocampal Memory System

When discussing a brain function such as memory, one should relate it to brain plasticity. One definition of plasticity is an alternative way of performing the same function. Anecdotal evidence suggests that the human brain can perform amazing memory feats in unexpected, alternative ways. For example, the established ability of savants (individuals with partial brain damage) to memorize events, sequences of numbers, letters, or musical notes, and to perform arithmetical calculations, suggests that compensatory rewiring of brain circuits after injury can affect learning. Which particular form of brain plasticity could be responsible for such astounding learning abilities as those seen in Kim Peek (“Rain Man”) and Daniel Tammet (“Brainman”), two individuals diagnosed as autistic savants (www.savantsyndrome.com)? In this chapter, we describe a radical form of plasticity, adult neurogenesis, in hippocampal formation (HF). The discovery of adult neurogenesis (production of new neurons in adult brain) has radically changed our ideas of how the brain can adapt to physiological and environmental challenges. The process of neuronal production is highly regulated and is involved in hippocampal functions under physiological conditions. In some cases, neurogenesis can respond to hippocampus-related pathologies such as epilepsy, ischemia, mood disorders, and addiction. Understanding neurogenesis, along with other forms of brain plasticity, may help us to understand normal memory and perhaps the enhanced memory such as that seen in individuals with the Savant Syndrome (Treffert and Christensen 2005). LESIONS OF THE NEUROGENIC REGION The HF is part of an integrated network involved in learning and memory (Eichenbaum 2000, 2001;...