Table 3.

Stimulus Pairs Used

Odor pairs	Medium pairs
Jasmine vs. vanilla	Foam rubber vs. plastic beads
Mulberry vs. patchouli	Gravel vs. BBs
Cinnamon vs. gardenia	Pine shavings vs. shredded  manila folders

The exemplars within a dimension were always used in pairs, for example, whenever jasmine appeared as one odor within a discrimination, the other odor was vanilla. No two rats within the same group received the same combinations, but the aged and control groups were matched. The order of presentation of exemplars and the combination of exemplars into positive (+) and negative (−) stimuli was determined by a pseudorandom series generated before testing.