Identical sounds can be perceived entirely differently, depending on context.
During a football match, if we hear someone shouting at the top of their lungs, we might not be particularly perturbed.
However, if we heard the exact same shouting while reading in a library, our reaction is likely to be significantly more intense.
Scientists have long known that we respond to auditory stimuli in a context-specific way.
Our reaction to the sound of a car horn, for instance, elicits a different response if we hear it as we cross a busy street, compared with hearing it from the comfort of our sofa at home.
Although we understand why context matters, the neural mechanisms behind it have proven more difficult to pick apart.
Researchers from NYU Langone Medical Center's Skirball Institute of Biomolecular Medicine in New York designed a study to investigate the neurological changes regarding context-specific sound perception.
Senior investigator Robert Froemke, postdoctoral fellow Kishore Kuchibhotla, and their team set out to map how the same sensory inputs could be perceived and coded differently in the brain.
Understanding the sound in context
Although the project has been underway for around 5 years, the most recent findings still came as a surprise to the team.
Froemke has been interested in the plasticity of the brain and its ability to assign meaning to sounds for many years. Medical News Today recently took the opportunity to ask why this field of study was so interesting to him. He said:
"I think one of the most amazing things about the brain is that it changes, and it can learn all throughout life. This makes us all individual and different from one another, allowing us to learn from our mistakes and try to be better tomorrow than we were today."
He continued: "Words and music, the names of our romantic partners, these can be fairly simple sounds that have such powerful meaning to us, and in really interesting individual ways that drive strong emotional reactions and profoundly affect our behavior. The sound of a crying baby, for example, evokes a very different reaction if it's your baby versus if it's three rows behind you on a plane."
The way in which the human mind can change its response to certain stimuli is nothing short of amazing. As Froemke says, "I couldn't not study it."
Hung against this backdrop of deep fascination, Froemke and Kuchibhotla set out to understand these interactions by measuring nerve circuit activity in mice.
The battle between inhibition and excitation
In the brain, roughly speaking, nerve cells can be divided into excitatory and inhibitory. Excitatory nerves produce chemicals that encourage the next nerve cell to pass on and consequently amplify the message. Conversely, inhibitory nerve cells prevent the message from being further transmitted.
The balance between these two subsets is vital to ensure that information is received and understood, and neither gets ignored unnecessarily or amplified disproportionately.
Inhibition and excitation must be finely balanced.
To process incoming sensory information, such as sounds, signaling levels are adjusted through the interplay between these two cell types.
It is thought that the brain attaches more or less importance to a particular signal by dialing up or down the excitatory or inhibitory nerve signals.
In the experiments, mice were split into groups; some were trained to expect a reward when they heard a specific musical note, while others were not trained to expect anything at the sound of the same note.
Froemke and his team examined how sets of neurons responded to sounds which they either did or did not expect to signal a reward.
They found, to their surprise, that most of the excitatory nerve cells in the auditory cortex fired less when the mice expected a reward and received one.
Conversely, a second set of excitatory neurons in the same situation, increased their activity when they expected a reward.
Froemke admits that the results were "very surprising." He recently explained the findings to MNT:
"We [imaged] the same population of cortical neurons over days as we trained animals. But, rather than a general increase or decrease in neural activity (which is what we expected), some neurons radically changed how they responded to sounds. Basically, we played the sound A (like a key on a piano) or sound B, and some neurons respond to one or the other sound, or neither sound, just as we expected."
"We also expected that if we rewarded sound A, the cells originally responding to A would respond even more so," Froemke continued. "But instead, some brain cells that originally responded to B or did not respond to any sound became very responsive the moment the task began, and resumed their original lack of response when the task was over. [...] Even more surprising, most of the cells originally responding to A stopped responding."
Explaining the inhibition
On further investigation, the team found that these unexpected changes were being controlled by the activation of populations of inhibitory neurons; specifically, parvalbumin, somatostatin, and vasointestinal peptide neurons. All of these subtypes were working in concert to switch the cortical network from the "passive" state to the behaviorally "active" state.
A brain region important in focusing attention - the nucleus basalis - releases acetylcholine, which, in the auditory cortex, influences the inhibitory neurons and changes the way a sound is perceived.
To pin down acetylcholine's role, the researchers inhibited its release in the trained mice's brains. When this was done, the mice only responded to the reward signal half as often.
Froemke hopes that, in the future, these findings will be used to help improve learning. He told MNT that "we've shown how behavioral context can activate the system important for attention, which is also critical for learning (you usually don't learn if you don't pay attention). We're very interested in when this system is engaged, and when it fails to be engaged, and how we can improve training procedures to more effectively control acetylcholine release to promote and enhance learning (eventually in people)."
The team plans to continue their research in this regard and investigate the roles of other important neuroactive chemicals. Froemke is particularly interested in "noradrenaline (the brain's version of adrenaline [norepinephrine], for increasing arousal and quickly paying attention to surprising or potentially dangerous things) and oxytocin (a hormone important for social interactions and maternal care), which help us notice other things in the environment and in our social lives."