A German research team is helping to overturn traditional theories by mapping the response to visual stimuli of distinct areas of the auditory (hearing) cortex in the brains of macaque monkeys. The scientists, who report their findings in this week's issue of The Journal of Neuroscience, reveal that pockets in the rear of the region used for processing sounds actually show a heightened response to watching a video, even if the sound is muted. This phenomenon could possibly be part of an innate mechanism to localize the source of a sound and may be useful to scientists trying to combat conditions characterized by audition deficits, such as dyslexia.
Christoph Kayser, a research scientist at the Max Planck Institute for Biological Cybernetics in Tübingen, Germany, says that his team mapped the auditory cortex in monkeys, segmenting it into 11 fields based on the frequency of sound each section processes. "This allows us to search for what parts of the brain are generally activated by auditory stimuli," he says, "and to separate an auditory field—for example [the] primary auditory cortex from higher auditory fields."
Using fMRI, the team monitored anesthetized (as well as alert) macaques as they watched videos (with and without the volume turned up), and while they listened when just the audio track of the videos were played. In the anesthetized monkeys, the researchers observed activation in the caudal belt of the auditory cortex (two fields at the back of the entire bean-shaped region) when the animals were exposed to the visual stimulus as well as to the sound accompanied by video. More fields responded in the fully alert animals, but they were all in the hindmost regions of the auditory cortex. The increased activation, Kayser says, "could be a reason of attention and more conscious processing of the stimuli, which in a way is uncontrolled in the awake animal, because we don't really know what they perceive or what they think about it."
The current finding confirms previous studies that suggested caudal sections of the auditory cortex in humans are involved in interpreting visual speech, but could not use fMRI to properly localize this correlation. Kayser and his colleagues are now working to build a better understanding of the mapping of auditory fields in the human cortex. Josef Rauschecker, a physiologist and biophysicist at the Georgetown University Medical Center in Washington, D.C., says that several studies indicate that the organization of the human brain is similar to that of lower primates, especially in the visual system. "That is likely to be true for audition as well, even though the exact homologies will need to be worked out," he says. "Functional MRI is a great technique that can be applied in both species and can therefore serve an important function in comparing them."
Insight into the way the human auditory cortex manages multisensory effects could lead to better understanding of conditions that involve a deficit in audition, such as dyslexia. "I wouldn't say that we have any applications right now," Kayser says. "For many of these [deficits], we don't really know where they happen in the auditory system, and that's something that needs to be carefully localized."
Daniel Tranel, a neurologist at the University of Iowa, is more optimistic, saying that knowledge of sensory integration at early stages "could help scientists pinpoint sources of extraordinary sensory processing, such as creativity and genius, as well as abnormal sensory processing, as seen in schizophrenia."