Much of this knowledge derives from study of profoundly amnesic patients, who retain certain components of memory at normal levels. Memories spared in the amnesic syndrome Implicit memory Warrington and Weiskrantz had amnesic patients and normal controls look at a list of words. They were not told that some of these words were on the list they had seen earlier in the day. On the other hand, if asked explicitly to tell what words had been presented on the list, amnesics performed much worse than controls. There are many kinds of implicit memory, including priming memory, skill memory, and conditioning.
Every stimulus leaves a memory trace in the regions of the brain that it activates.
The neurons activated by the stimulus are primed to respond to a similar stimulus. It can also be assessed by accuracy measures, as in the cueing experiment described in the previous paragraph. This has been called priming memory, and it is a form of implicit memory, since it does not depend upon instructions to remember, and it remains intact in patients with severe amnesia. It has been shown that amnesic patients can learn new skills normally, even when they can't recall the circumstances of learning. HM, the well-studied patient who was amnesic following bilateral medial temporal ablations, showed improvement over many days in his ability to keep a stylus within a small area of a rotating disk, even though he had no explicit memory of the task or training sessions.