Performance usually improves with practice, but not if training is a rotten time. A new study shows that people’s ability to identify noises declines when the sounds are paired with putrid smells—a phenomenon that may allow our brain to detect danger more quickly.

In a study published in May in Nature Neuroscience, neurobiologist Rony Paz of the Weizmann Institute of Science in Rehovot, Israel, and his colleagues exposed volunteers to auditory tones presented with no other stimuli or immediately followed by a rancid or fragrant odor delivered through a nose mask.

After this training session, the subjects were played a series of tone pairs—notes of very similar or identical frequencies—and asked whether the tones in each pair were the same or different. The subjects became better at distinguishing tones similar to those that had been presented alone or with a pleasant scent. But their ability to discriminate tones resembling those linked to a foul stench worsened—an effect that persisted one day later.

Such sensory confusion could be an adaptation that allows our defenses to rapidly mobilize. “This likely made sense in our evolutionary past,” Paz says. “If you’ve previously heard the sound of a lion attacking, your survival might depend on a similar noise sounding the same to you.”