This past Friday (June 21st) is the annual Paris Fete de la Musique, or the Paris music festival. European countries such as France have a long history of successful musicians creating magnificent masterpieces. To this day, we often have the opportunity to listen to street musicians perform in the metro station, on the RER, or just by the side of the streets. Because of the easily available music in pretty much every single metro station of Paris, you might think that the Fete de la Musique is nothing special. However, it is completely different.
The party started around 4pm as musicians began to set up their own speakers and instruments on almost every single street and bar of Paris. We went to Notre Dame as our first stop and we were already welcomed with all different genres of music from classical to folk to choral to bass-thumping club music. As we wandered around on the side streets near Notre Dame along with hundreds of other people, stopping at different concerts and listening to different street musicians playing, I noticed my ability to focus on the music I want to enjoy despite all these background noises. Somehow, my brain was able to do a descent job ignoring noises consisting off motorcycles, tourists asking for directions, nearby musicians, and drunk people singing out of tune. However, the occasional car honking sounds can still distract me from the beautiful music of the Fete de la Musique.
A little bit a research shows that there is biological basis behind our ability to ignore background noise and focus on the wonderful melodies of music. A study done by Perez-Gonzalez et al. (2005) found a type of neuron that will respond to novel sounds but not a repetition of sounds; it is located in the inferior colliculus (IC) of rats, part of the midbrain nuclei that receives input from the auditory cortex and peripheral auditory pathway. Specifically, this type of neuron is named “detector neuron” and shows stimulus-specific adaptation (SSA) in which these neurons are able to detect all frequencies of sound within the rats’ hearing frequency range, but will stop firing if the same pitch of sound is repeated at 0.5 hertz or higher. However, the firing of these neurons can be brought back when a sound of a different pitch is introduced (Perez-Gonzalez et al., 2005). Although discovered in a rat model, it is possible that humans also have the same kind of neurons. This would explain why I was able to filter out constant repeated background traffic noise when listening to the changing notes of a classical masterpiece that a violinist was playing. However, when a car honked, I would get distracted because this type of detector neuron will fire to the sudden change in pitch of the traffic noise caused by the honking sound.
More recently, research on shifting attention between different sound sources further differentiated between a top-down (voluntary) and bottom-up (involuntary) shift of auditory attention. For example, I made the conscious choice of focusing on the violinist playing music; this is a top-down shift of attention. However, when a car honking noise surprised me and caught my attention, it is a bottom-up involuntary shift of attention. To test the difference in brain activation, Huang et al. (2012) used fMRI while testing 19 healthy subjects on hearing tests. Specifically, in a 10-second trial, the healthy subjects were informed to wait for a “cue” (sound) at the ear where a subsequent “target” sound is likely to appear. After the cue sound, the subjects were instructed to pay attention to the ear that received the cue and press a button as fast as possible right after hearing the target sound. However, in 20% of the trials the “target” sound is replaced by a novel sound opposite to the ear that received the cue in order to trigger involuntary attention shift (Huang et al., 2012). The fMRI results showed different activation pattern of the brain between voluntary cued attention shift and the involuntary novel sound attention shift. For voluntary attention shift, superior / posterior intra-parietal sulcus (IPS), located on the surface of the parietal lobe and precentral areas such as Pontine micturition center (PMC), part of the brainstem and frontal eye fields (FEF), a region of the prefrontal cortex, are more activated. For involuntary attention shift, inferior IPS, posterior superior temporal sulcus (STS), and temporal parietal junction (TPJ) are more activated.
While knowing all these different brain regions can be confusing and might not be necessary, it is more important to recognize the idea that this study demonstrated two types of auditory attention shifts supported by the evidence of different brain activations using fMRI. One potential flaw of this study is the inherent difficulty to distinguish whether the brain areas activated are due to attention shifting or other pathways that happens to be activated by the auditory stimuli. In addition, a slightly bigger sample size would increase the credibility of this study. Despite these flaws, combining the more macro view of brain area activations of voluntary and involuntary attention shift to the micro view of specific neurons that fire when a novel auditory stimulus is introduced, researchers have gotten closer to understanding the complex auditory system that enables us to filter out sound waves that are not important and only focuses on the sounds that are more important such as the wonderful music at the Fete de la Musique.
Huang S, Belliveau JW, Tengshe C, Ahveninen J (2012) Brain networks of novelty-driven involuntary and cued voluntary auditory attention shifting. PLoS One 7:e44062.
Perez-Gonzalez D, Malmierca MS, Covey E (2005) Novelty detector neurons in the mammalian auditory midbrain. The European journal of neuroscience 22:2879-2885.
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