Young audiologist finds the right note
31 January 2008
Deafness Research UK recently awarded a scholarship to Salma Mahmood, an audiology student at Aston University, to study the way in which the brain processes echoes. The work, carried out with Dr Bernhard Seeber at Nottingham's MRC Institute of Hearing Research, is important for understanding how we localize sounds indoors.
The aim of Salma’s investigation was to find out which factors influence a perceptual phenomenon known as the precedence effect. In listeners with normal hearing, reflected sounds – echoes – are suppressed by the auditory system so that they don’t interfere with localizing the direct sound-source. This effect can be demonstrated in the laboratory using an array of loudspeakers in different positions around the room. When a sound is played from one direction and followed very quickly – within a few milliseconds – by a sound from another speaker, just one sound-source is heard as coming from the first direction. As the delay between the leading and lagging sounds is increased, the effect breaks down and instead two sounds are heard.
Previous work has shown that this ability to ignore echoes is disrupted in people who wear cochlear implants on both ears. One reason for this could be because current cochlear implants make it very difficult to judge differences in pitch, for example it may be hard to separate the voices of two people talking even though they will each have a different tone. In normal hearing a difference in pitch should ultimately lead to hearing two separate sounds, but impaired pitch processing could cause problems with integrating and segregating direct sounds and their echoes.
Initial results from the study showed that the length of delay needed before sounds are perceived separately – the echo threshold – is reduced as soon as lead and lag do not share the same pitch. For example, for a lead sound at a frequency of 200 Hz and a lag sound at 201 hertz (Hz), echo thresholds were 6 milliseconds (ms). Echo thresholds reduced to 3 ms at 203 Hz and to just 1 ms at 205 Hz, while for larger separations lead and lag were always perceived separately. These findings suggest that the ear is extremely sensitive to differences in pitch between source and echo and the work has lead to a number of follow-up studies currently taking place in Dr Seeber’s laboratory.
Commenting on the progress made during Salma’s eight weeks in his lab, Dr Seeber says she “gained the skills to structure and cope with large amounts of information and to work efficiently” and he added that “throughout this learning process, in which she continuously advanced her abilities, she grew a more in-depth interest in science such that she presently continues the project within her BSc in Audiology.”
