Our understanding of learning and memory has been informed by studying individuals who exhibit cognitive impairments after sustaining some form of trauma to the brain. For instance, the study of individuals with amnestic disorders suggests that different mechanisms support procedural and declarative memory: Despite anterograde amnesia, H.M. showed evidence of learning, E.P. could learn new sentences, and Clive Wearing retained his musical ability. Though research on amnesia is insufficient and sometimes controversial (and wildly misrepresented in media), combining this evidence with results obtained with other approaches (e.g., studying healthy individuals, studying expert performance) has resulted in a clearer understanding of how human memory does not work.
Medved, M.I., Hirst, W. (2006). Islands of memory: Autobiographical remembering in amnestics. Memory, 14(3), 276-288. doi: 10.1080/09658210500233524
for fun, see also: https://soundcloud.com/radiolab/memory-and-forgetting