The impact of loud noise on the brain: Part II
When preparing for a solo music recital, one might spend a significant amount of time repeatedly playing the same piece. This repetition continues until the performance has no mistakes, perfect timing, and ultimately, is as consistent as possible. The brain performs a similar function. In response to a repeated sound, distinct brain regions respond consistently each time the sound is played. Researchers have found a way to quantify the brain’s consistency in responding and have termed it coherence.
In a music recital, one might also take part in a duet performance. In this case, two individuals would not only practice being consistent within themselves, but also playing in sync with their partner. Similarly, when the brain is processing repetitive sounds, different brain regions talk to each other in a coordinated manner. The consistency of the responses of two brain regions, like two people playing a duet, is termed phase-locking value.
A group of researchers at Western University set out to explore if brain signalling changes in response to repetitive sound exist following noise-induced hearing loss (see Part I of article summary here). In this study, Dr. Wieczerzak and colleagues investigated brain signalling in response to a sound before and after rodents had acquired hearing loss. They recorded signals from two brain regions: 1) the auditory cortex, where sound information is ultimately processed, and 2) the prefrontal cortex, an important area for cognitive function.
When investigating coherence and phase-locking value, a higher value corresponds to increased consistency within and between brain regions, respectively. The researchers found that there was decreased coherence in the prefrontal cortex following hearing loss, despite no change in auditory cortex coherence. Furthermore, the phase-locking value between the two brain regions was also decreased following hearing loss. Ultimately, these findings point to changes in brain signalling in the prefrontal cortex and how it interacts with the auditory cortex.
Aberrant signalling within and between brain regions in response to repetitive sounds has been observed in a variety of neurological disorders such as schizophrenia and dementia. So far, researchers are still trying to uncover what cellular and molecular changes occur alongside these changes in signalling. As mentioned in Part I of this article, the researchers found no changes in cognitive function dependent upon the prefrontal cortex in these same animals. These novel findings in the field of hearing loss will prompt researchers to further investigate the interplay between aberrant signalling, protein changes, and cognitive function in specific brain regions.