In humans, the phenomenon of temporally graded retrograde amnesia has been
described in the clinic and the laboratory for more than 100 years. In the
1990s, retrograde amnesia began to be studied prospectively in experimental
animals. We identified 13 published studies in which animals were given eq
uivalent training at two or more separate times before damage to the fornix
or hippocampal formation. Eleven of these studies found temporally graded
retrograde amnesia, with the extent of amnesia ranging from several days to
a month or two. We consider these studies and also suggest why temporally
graded retrograde amnesia has sometimes not been observed. Although the evi
dence in favor of temporally graded retrograde amnesia is substantial, the
inference from this work, that memory is reorganized as time passes, is rat
her vague and depends on mechanisms yet to be identified. It is therefore e
ncouraging that many opportunities exist for moving beyond purely descripti
ve studies to studies that involve treatments or manipulations directed tow
ard yielding information about mechanisms. Published 2001 Wiley-Liss, Inc.d
agger