it basically comes from it's neurogenesis attribute.
cannabinoids induce neurogenesis in areas of the brain in which cannabinoid receptors are present (which is almost all areas . . ., especially the hippocampus and frontal cortex)
those two areas of brain are the ones deteriorated by alzheimers, cannabis works against this by being neuroprotective and having neurogenesis.
http://www.jci.org/articles/view/25509/version/1
that should be all the evidence you need. good luck.