In that brief interval, the members of the group carrying the e4 gene who did not exercise had undergone significant atrophy of their hippocampus. It had shrunk by about 3 percent, on average.Those volunteers who carried the e4 gene but who regularly exercised, however, showed almost no shrinkage of their hippocampus. Likewise, both groups of volunteers who did not carry the e4 gene showed little change to their hippocampus.