Tagged: arcuate fasiculus
It was an average Wednesday night at 8, and I was channel surfing. As I changed the channels I heard singing; I knew instantly that the show was American Idol. Most of you watch or have watched the show in the past and time and time again it befuddles me to think how these individuals think that they can sing. Most of the singers not only have piercing voices, but they are off key and sound terrible. After most auditions, the contestants – although I know it was horrible – still believe their rendition of a Whitney Houston song was outstanding. If you are like me then you want to know why.
Tone-deaf individuals do not have brain damage or trouble hearing, and they are definitely not suffering from a lack of exposure to music. So what actually makes people unable to understand their inability to sing? Researchers conducted an experiment that tested the connectivity of the arcuate fasciculus (AF), which connects the temporoparietal junction (the place where the temporal and parietal lobes meet), with the frontal cortex in the brain. This junction allows neural substrates of sound perception and production to be connected. The researchers hypothesized that there are structural and functional abnormalities that contribute to tone deafness.
To test their hypothesis, structural MRIs with diffusion tensor imaging (DTI) were performed on the patients. DTI is a type of MRI that allows researchers to map internal structures with the diffusion of water. After processing the information, the maps identified that the right superior AF was diminished compared to control, signifying that the AF is disrupted in tone-deaf individuals. Also, resultant fibers in tone-deaf individuals projected dorsally toward the parietal lobe and/or translocally to the left hemisphere and not toward the ipsilateral inferior frontal gyrus where normal individuals have projections.The imaging and testing of the AF led researchers to conclude that the superior branch is responsible for fine-grained discrimination, and the inferior branch is responsible for automatic matching of sound output to its target. They also tested the volume of the fibers connecting each part of the brain and discovered that tone-deaf individuals have a lower volume of fibers than the control, which is important for conscious pitch determination and the degree of action-perception mismatch. According to the experiment, both the superior and inferior branches of the AF are needed for accurate perception and production.
Figure 1: A comparison between the regions of interest of the posterior superior temporal gyrus (pSTG) and the posterior inferior frontal gyrus (pIFG) of the right side of the brain
Tone-deafness is a new disconnection syndrome that deals with impaired pitch perception and vocal sound projection. There are no known genes that are associated with this condition that affects the AF fibers and their connection between the superior and inferior areas of the brain. So for all of you non-tone-deaf American Idol viewers, you will just have to sit through the next episode and know that most of singers cannot help but obliviously sing off-key.
Other Reading of Interest:
Tone Deafness – Scientific American
The amusic brain – BRAIN: A Journal of Neurology