When we listen to music, our brains naturally integrate the lyrics and the melody into one cohesive song. But to patients missing a crucial connecting structure in the brain, a song is made up of two components – music and lyrics – which remain distinct in their minds.
These people lack something called the corpus callosum, which is a structure that connects the brain’s two hemispheres. This connector is crucial because the right and left hemispheres often have different specializations. For example, music processing occurs more in the right hemisphere, while language processing is more prominent in the left hemisphere. Therefore, split-brain patients who lack the corpus callosum, due either to its failure to develop or to surgery for severe epilepsy, perceive the world in a completely different way than we do.
When split-brain patients are asked to listen to two different songs, one in each ear, and then repeat what they hear, something interesting happens: they repeat the lyrics of the song they heard in their right ear, and repeat the melody of the song that played in their left ear. Imagine you are wearing a weird set of earphones that plays two different songs, one in each earbud. In the right one, “Masterpiece” by Jessie J is playing. In the left one, it’s St. Lucia’s “Elevate”. If your corpus callosum is intact, you’ll hear a confusing jumble of Jessie J’s powerful lyrics and St. Lucia’s catchy tune. But if your brain’s hemispheres are not connected, you may hear the lyrics of “Masterpiece” layered on top of just the tune of “Elevate”. The melody of “Masterpiece” and St. Lucia’s lyrics will magically disappear.
Since the left ear is connected to the right hemisphere and the right ear is connected to the left hemisphere, the likely explanation for this is that when the song presented in the left ear projects to the right hemisphere, the brain mainly perceives the melody. In the same way, the left hemisphere focuses on the lyrics of the song in the right ear, due to its language specialization. Therefore, split-brain patients may feel as if they are hearing one song composed of the lyrics heard in the right ear and the melody heard in the left ear, instead of a confusing jumble of two songs that people with normal corpus callosums would hear. This curious experience that split-brain patients have shows us the corpus callosum’s crucial role in integrating the tune and lyrics of songs by connecting the two hemispheres.
Further research on the behavior of split-brain patients has revealed that the left hemisphere has dominant control over speech. This is a problem in split-brain patients because what they say may be uninformed by the information processed in the right hemisphere. Imagine that the word “beach” is presented to the right hemisphere of a split-brain patient. When asked to choose a picture that best represents summer from among several choices, the patient is likely to choose a picture of a beach. So far, this makes sense. But when the subject is asked why they chose the image of a beach over other images of things such as sunglasses, a swimming pool, or ice-cream, they will make up an explanation, like the fact that they saw a poster of a beach earlier that day. They are not intentionally lying. They do this because the left hemisphere, which controls the content of speech, is uninformed by the fact that the right hemisphere saw the word “beach” before choosing the image. This tells us that the left hemisphere is not only in charge of our mouth but also that it attempts to make a plausible narrative that explains our actions.
Observing the behavior of split-brain patients has offered a deeper understanding of the corpus callosum’s crucial role of integrating the information in the left and right hemispheres to create a coherent understanding of our surroundings. The left hemisphere’s dominant control over speech leaves us with many future topics of research, including the topic of consciousness. Given that a lot of our consciousness is made up of interpreting our actions and our surroundings, it seems plausible that the narrative-making role that the left hemisphere plays in split-brain patients is associated with consciousness. It remains to be seen whether there are specific areas in the brain’s left hemisphere that are responsible for making narratives. If so, studying patients with lesions in that area of the brain has the potential to shed light on what life may look like with diminished awareness, and thereby solidify our understanding of the nature of consciousness.
Featured image created by the author, Yuki Hayashi.
Yuki Hayashi is a Sophomore in Ezra Stiles College. Contact her at firstname.lastname@example.org.