We all know that certain songs and musical phrases can have a tremendous impact on the way we feel, but did you know that the way we feel has a tremendous impact on the way we perceive sounds?

Sound does not only affect our emotions — our emotions also affect the way recognize and process sound. It’s the neurological phenomenon by which we shrug on hearing signals similar to our alarm clock, and by which combat veterans suffering from post-traumatic stress disorder (PTSD) can have horrifying memories stirred up by the sound of thunder.

For a long time, the neurological mechanism behind these subjective, often troubling associations have remained a mystery. Now, in a study published in Nature Neuroscience, researchers from the Perelman School of Medicine at the University of Pennsylvania may have taken the first step towards a comprehensive model of the associative process underpinning the phenomenon.

"Emotions are closely linked to perception and very often our emotional response really helps us deal with reality," senior study author Dr. Maria N. Geffen, an assistant professor of head and neck surgery, told reporters. "For example, a fear response helps you escape potentially dangerous situations and react quickly. But there are also situations where things can go wrong in the way the fear response develops. That's what happens in anxiety and also in PTSD — the emotional response to the events is generalized to the point where the fear response starts getting developed to a very broad range of stimuli."

In other words, our “auditory plasticity” — or, the process whereby our perception of sound adapts according to the aural patterns we experience — can be influenced by strong emotions to a point where similar sounds are mistakenly yoked together and interpreted as connoting one and the same thing. In PTSD cases where thunder and other harsh aural patterns trigger harrowing memories, the soldier’s fear has effectively altered the way he or she understands such sounds — and instead of discriminating between “gunshot” and “door slamming,” the brain recognizes the enter spectrum as a unit corresponding to anxiety and danger.

In the study’s experiment, lab mice were taught how to distinguish between safe and potentially dangerous sound frequencies — a process referred to in the study as “emotional discrimination learning.” By varying the difficulty of the task, researchers were able to develop different levels of sound comprehension among the mice, with certain subjects exhibiting much higher hearing acuity that others. The subjects exhibiting low acuity were intended to represent PTSD cases, where sound comprehension has been altered by a traumatic event.

"The animals presented with sounds that were very far apart generalize the fear that they developed to the danger tone over a whole range of frequencies, whereas the animals presented with the two sounds that were very similar exhibited specialization of their emotional response. Following the fine conditioning task, they figured out that it's a very narrow range of pitches that are potentially dangerous," Geffen explained.

By identifying the neurological mechanisms associated with PTSD, the study may anticipate key features of future treatment programs for the debilitating condition. In addition, it may potentially help researchers determine why seemingly traumatic events affect different people in vastly different ways.