Virtual ears help scientists get to grips with the cocktail party effect
14 November 2008
The study, published in the ‘Journal of Neuroscience’, has implications for the emergence of hearing abilities in children and for restoring hearing after fitting hearing aids and cochlear implants.
Humans begin to develop their hearing at a very early stage. Even a 28-week-old fetus will respond to sound, and newborn infants can distinguish different types of speech sound. Our hearing continues to develop throughout childhood, including the ability to distinguish between sounds coming from different directions and to understand speech in difficult acoustic environments, such as a busy room with many echoes.
Nerve cells in the superior colliculus - one of the brain regions responsible for processing sound - mature during infancy, gradually developing a preference for specific sound directions. Which of these nerve cells are active therefore signals where sounds are located, essentially forming an auditory map of the environment.
Now, researchers led by Professor Andrew King, a Wellcome Trust Principal Research Fellow at the University of Oxford, have developed a method for enabling them to separate out the changes in the selectivity of the nerve cells for different directions of space from the development of the auditory map. The technique, used on ferrets, involves using 'virtual ears', which can enable an infant ferret to hear sounds as if it were an adult.
The researchers placed tiny microphones inside the opening of the ears of adult ferrets to capture the sound as modulated by an adult head. This sound, when played back over headphones to an infant ferret, appears to be coming from outside the ferret's head, but mimicking what an adult would hear. By measuring the responses of nerve cells in the brain, the researchers were able to see how the infant brain differs from the adult and to show that the development of the selectivity of the nerve cells for sound location and their assembly into the auditory map are influenced by independent factors.
"Our research showed that the region of space to which the nerve cells respond is determined by the shape of the ferret's ears and their distance apart, both of which change with age," says Professor King. "On the other hand, the gradual development of the auditory map is influenced by the experience of the sounds that are heard."
Professor King believes that this has implications for a child’s ability to learn how to hear after a hearing aid or cochlear implant has been fitted. Other work from this group has shown that even the adult brain is remarkably plastic.
"We have shown that the neural circuits of our hearing apparatus can adjust to a loss of hearing in one ear," says Professor King. "Clearly, the adult brain is still plastic and able to adapt, so fitting hearing aids and cochlear implants in adults is worthwhile."
The research was funded by the Wellcome Trust, the Lister Institute for Preventive Medicine and the Biotechnology and Biological Sciences Research Council.
Image: Oliver Burston, Wellcome Images