The decay theory indicates that forgetting is caused by the passage of time. It states that the memory is physically no longer present. (Benjamin, Hopkins, & Nation, 1994. p.278)

An engram , or a pattern of neural activity, can be thought of as though they it were a set of paths through the forest. If no one treads through the paths, vegetation will begin to grow over them over time, making it difficult to walk through. Likewise, as time goes by with a memory path of neurons not being used, it becomes more difficult for that path to be reactivated (remembered). The path has eroded, or faded away. (Benjamin, Hopkins, & Nation, 1994. p.277)

Most scientists believe that neurons , die off gradually as we age. We start out with over 100 billion neurons. A person at the age of one hundred will have lost a mere 5 - 10 percent of that. (Kurland and Lupoff, 1999, p. xix) This used to be the reasoning for the decline in mental abilities as people age. However, recent experiments show that new brain cells do grow in some parts of the brain to replace the dying ones. (Remembering Well, p. 53)