Close your eyes… what do you see? What passes before your shut eyes in all likelihood depends on the sounds occurring around you, whether that be a lawn mower or a passing bus. Now, Dr. Lars Muckli of the Institute of Neuroscience and Psychology at the University of Glasgow and his co-researchers have found your brain’s visual cortex uses information gleaned from your ears as well as your eyes in order to see the world. “This research enhances our basic understanding of how interconnected different regions of the brain are,” Muckli said. “The early visual cortex hasn’t previously been known to process auditory information, and while there is some anatomical evidence of interconnectedness in monkeys, our study is the first to clearly show a relationship in humans.”
Your visual cortex is located in the occipital lobe, at the back of the brain. It is part of the cerebral cortex, which is the folded and ridged outermost layer of your brain. To study what occurs in the visual cortex, the researchers enlisted the help of 10 healthy volunteers. In five separate experiments, the participants wore a blindfold and were instructed to keep their eyes closed at all times while the researchers examined activity in the visual cortex using functional Magnetic Resonance Imaging (fMRI) and in one experiment performed retinotopic mapping. Retinotopy, as it is sometimes calls, traces the visual input path from the eye’s retina to neurons in the brain; fMRI measures brain activity by detecting associated changes in blood flow.
In one experiment, the research team asked the blindfolded volunteers to listen to three different sounds: birdsong, traffic noise, and a talking crowd. The researchers were able to identify unique patterns in brain activity and discriminate between the different sounds being processed in the visual cortex activity. In a second experiment, the participants simply imagined images, in the absence of both sight and sound, yet this too spurred activity in the visual cortex.
“Sounds create visual imagery, mental images, and automatic projections,” Muckli said. “So, for example, if you are in a street and you hear the sound of an approaching motorbike, you expect to see a motorbike coming around the corner.” He theorized that the visual cortex uses information gleaned from the ears to better predict what might be seen. In turn, the visual cortex can better see and focus on surprising events, which confers a survival advantage.
In the future, Muckli and his team hope to further test how auditory information supports visual processing. “This might provide insights into mental health conditions such as schizophrenia or autism and help us understand how sensory perceptions differ in these individuals,” he said in a press release.
Source: Vetter P, Smith FM, Muckli L. Decoding Sound and Imagery Content in Early Visual Cortex. Current Biology. 2014.