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To arrive at their findings, the team measures the sniff response repeatedly over time in patients with severe brain injuries. To do this, they used jars with different smells for 43 severely brain-injured patients. The experimenter left the patient to smell a jar containing a shampoo, an unpleasant smell of rotten fish, or no smell at all. Each jar was given ten times in random order. The volume of air sniffed by the patient was measured. Study findings
The researchers found that patients who are minimally conscious inhaled less in response to smells but did not discriminate between pleasant and unpleasant odors. The patients also modified the air into their nose in response to the jar with no smell, showing awareness of the jar or learned anticipation of smell.
In vegetative patients, the results varied, with some not changing their breathing in response to both odors, while others did. They also found that if an unresponsive patient had a sniff response, it is tied to being able to regain consciousness in the future.
"We found that if patients in a vegetative state had a sniff response, they later transitioned to at least a minimally conscious state. In some cases, this was the only sign that their brain was going to recover -- and we saw it days, weeks and even months before any other signs," Anat Arzi, a researcher in the University of Cambridge's Department of Psychology and the Weizmann Institute of Science Israel, said.
Patients who are in a vegetative state may open their eyes, wake up and fall asleep regularly, and have basic reflexes. However, they also do not show meaningful signs of awareness. On the other hand, a minimally conscious patient may experience periods where they can show signs of awareness.
A follow-up research about three and a half years later revealed that over 91 percent of the patients who exhibited a sniff response was still alive, but 63 percent of those who showed no response at all, had died.
"In addition, olfactory sniff responses were associated with long-term survival rates. These results highlight the importance of olfaction in human brain function and provide an accessible tool that signals consciousness and recovery in patients with brain injuries," the team concluded in the study. Journal reference: Arzi, A., Rozenkrantz, L., Gorodisky, L. et al. (2020). Olfactory sniffing signals consciousness in unresponsive patients with brain injuries. Nature. https://www.nature.com/articles/s41586-020-2245-5
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