Quoted from New Scientist articlehere

“You might think that being able to distinguish between a noise associated with danger and a similar but innocuous one would be a useful skill. Yet people find it hard to tell similar sounds apart if one is linked to a bad experience. The finding could help explain how people with post-traumatic stress disorder (PTSD) may become hypersensitive to certain types of sound.

Rony Paz and colleagues from the Weizmann Institute of Science in Rehovot, Israel, repeatedly played one of two tones to volunteers. One group heard a tone followed by an unpleasant smell, the other a tone followed by a pleasant, melon-like odour. The team then tested how well each person could distinguish between the tones they had heard and similar sounds.

On average, those who had heard the sound that was followed by an unpleasant smell performed worse at this task. The effect persisted 24 hours later.
Evolutionary sense

This makes sense from an evolutionary point of view, says Paz. “If you hear a lion and you see a zebra get eaten, that should be enough for you to know that a lion is bad and to avoid it.” If you subsequently hear a different lion, you want your system to respond quickly to the threat rather than try to distinguish between the two lions.

Paz thinks this conditioning may involve rewiring of the amygdala, the part of the brain which controls the fear response. Understanding this mechanism could lead to better treatments for PTSD, he says.”