According to the Stanford Research Institute, a person has between 90,000 and 150,000 thoughts per day. But a study at Stanford also indicates that we have some control over which thoughts we have and which we suppress.
The question of repressed memories has caused controversy, especially with reports of psychotherapists encouraging patients to "remember" events that, in reality, never happened. But research shows that the brain does have a biological mechanism for blocking out unwanted memories and that if these memories are blocked over and over for a long enough period of time, they cannot be recalled, not even at will.
A study using fMRI and memory exercises on words showed that when people were asked to purposely try not to remember one word of a pair of words they were shown, they did remember them less well than ones they were trying to remember, even when exposed to those words for a longer period of time.
The brain imaging scans showed that controlling unwanted memories was associated with increased activation of the left and right frontal cortex (the part of the brain used to repress memory), which in turn led to reduced activation of the hippocampus (the part of the brain used to remember experiences).
If we are actively choosing to forget certain memories, are they still lurking and ready to be called up under hypnosis or does the brain dispose of them as unwanted and unnecessary?
It does appear that repressed memories may be recalled years later when triggered by some event or sensory experience and events like this have been recorded. There have been many cases where those memories called up under hypnosis have not withstood the test of evidence.
The existence of repressed memories is still not fully accepted by mainstream psychology. There are experts in the field who believe that no evidence exists for the theory while others speculate that repression may be a method used by individuals to survive traumatic experiences. As research continues, perhaps science will find the key to all memory.