Forgetting is the process by which information stored in the brain appears to vanish, degrade, or otherwise become inaccessible. It is a common process, appearing from youth to old age in a seemingly random pattern. Forgetting cannot be predicted except in the case of trauma to the brain or mind, such as Alzheimer's Disease, concussion, or emotional trauma that affects the memory. It is a normal part of life, and many scientists believe that the ability to forget is crucial to mental and emotional health. For example, Solomon Shereshevsky, a Russian journalist, had a photographic memory so powerful that it interfered with his daily life; he couldn't hear or see anything without triggering an enormous flow of previously-retained information in his brain. Forgetting can also be a function of improper memory storage in the brain; if information is not linked to other information, it might not persist because it has no anchor.
There are several major theories of forgetting. One is cue-dependent theory, which states that memories can fade because of their missing stimulus cues. If a memory is strongly linked with specific stimulus, it will slowly fade if those external stimuli are not regularly re-experienced. In this theory, memories may never be fully forgotten, but simply are hidden unless the stimulus is repeated.
Another major theory is decay theory, which states that the storage of memories creates a physical neurochemical which allows the mind to replicate the mental conditions of the memory more easily. As the brain ages, the memory slowly fades if it is not revisited. This theory depends on time and states that memories are not permanent.