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Smell is hard to forget, according to a new rat study. Scientists found that rat brains "remember" odors they were exposed to while deeply anesthetized.

It was previously believed that sensory information received by the brain under general anesthesia couldn't be perceived. However, the latest study suggests that under anesthesia, the brain can receive sensory information and register that information at the cellular level without behavioral reporting of the same information after waking up from anesthesia.

In the study, researchers exposed rats to specific odors while under general anesthesia. While rates behaved as if they have never encountered the odor before when they recovered from anesthesia, scientists found evidence of cellular imprinting after examining rats' brain tissue.

"It raises the question of whether our brains are being imprinted during anesthesia in ways we don't recognize because we simply don't remember," lead researcher Yan Xu, Ph.D., vice chairman for basic sciences in the Department of Anesthesiology at the University of Pittsburgh School of Medicine, said in a news release. "The fact that an anesthetized brain can receive sensory information - and distinguish whether that information is novel or familiar during and after anesthesia, even if one does not remember receiving it - suggests a need to re-evaluate how the depth of anesthesia should be measured clinically."

The study involved 107 rats that were assigned to 12 different anesthesia and odor exposure paradigms: some were exposed to the same odor during and after anesthesia, some to air before and an odor after, some to familiar odors, others to novel odors, and controls who were not exposed to odors at all.

"This study reveals important new information about how anesthesia affects our brains," said Xu. "The results highlight a need for additional research into the effects of general anesthesia on learning and memory."

The findings are published in the journal Anesthesiology.