Sleep learning experiments are especially difficult conduct,
researchers have to make sure the participants are actually
asleep. The team at the Weizmann Institute of Science exposed
sleeping subjects to a tone followed by a smell, neither of
which woke them up. The odor was either pleasant of unpleasant –
generating a deep or a shallow breath – and each was associated
with a particular tone. When the corresponding tones were
replayed, the participants either elicited a deep or shallow
breath, without the presence of the odor. When exposed to tones
that had been paired with pleasant odors, they sniffed deeply,
while the second tones – those associated with bad smells –
provoked short, shallow sniffs.