Often times, the importance of hearing is overlooked in the midst of psychobiological research . Along with taste, touch, sight, and smell, the ability to hear is one of the most fundamental senses we have that helps us distinguish and encode information every day. Although auditory research communities continue to make tremendous strides in research, many processes continue to awe scientists in many ways, such as the precedence effect.
The Precedence effect is an acoustic phenomenon that describes one’s ability to distinguish the spatial location of a primary sound in the presence of another non-dominant sound . Dependent on the differences of Interaural time and Interaural level differences determines how well a person can accurately locate the direction of sound from the primary sound via Binaural hearing. Therefore, the onset of the precedence effect means two independent sounds are perceived as one. Binaural hearing’s mechanisms are particularly interesting in sound localization due to the physiology of the human head. Each ear is located at bilateraly end and at equal distance of the skull; dependent on the initial location of the sound means either ear will experience a natural delay. The amount of delay time experienced from this feature is accounted for as the interaural difference of time, or IDT. On the other hand, interaural differences of time (ITD) are dependent on the frequency of sound and how the human head creates acoustic shadows for the indirect ear.
In an effort to shed more light on the precedence effect, Dr. Dye gave a lecture regarding Dr. Brown and Dr. Stecker’s work at the University of Washington. Dr. Brown and Dr. Stecker coded virtual listening tasks in order to quantify the precedence effect in a variety of difference frequencies and time (echo) delays. The frequencies chosen were at 1500,3000, or 5000 Hertz to resemble acceptable noise ranges while time delays were studied in fluctuating increments between 5–175 milliseconds with headphones. As a result, the two scientists found that, in higher presence of echo delay (ITD), or secondary noise, attention towards the primary target sound becomes more difficult to locate. On the other hand, the precedence effect was found to be insignificant when comparing it to ILD’s.
According to the National Institute on Deafness and Other Communication Disorders, over 37.5 million people have been diagnosed or have reported hearing issues. Because our auditory system essentially never stops receiving information from birth, hearing loss is degenerative and increases with age. In conjunction with Stecker and Brown’s research, the MRC Institute of Hearing Research have also found significant data on the precedence effect over issues with hearing loss. “The effect of hearing impairment on localization dominance for single-word stimuli” by Dr. Akeroyd and Dr. Guy resembles to Stecker and Brown’s methods; however, a a ring of speakers were used to test ILD and ITD’s instead of headphones. Regardless, a strong correlation between the precedence effect and hearing loss was found: the more progressed hearing loss was, the lower amount of primary sound localization was recorded. Both papers also highlight the importance of considering individual differences between subjects which could account for skewed data.
Although the precedence effect is seen as an acoustic principle, it could be used to explain why individual’s with progressive hearing loss find themselves having a harder time localizing where certain sounds are coming from. In addition, because the data also took into account the bilateral nature of the ears and skull, perhaps inner and outer hearing aids for patients who lack hearing one ear could be modified in order to account for the precedence effect. By analyzing both studies, scientists are able to ascertain more information about the precedence effect and use the data to find innovative methods to improve the quality of hearing loss for millions worldwide.
Citations:
Akeroyd MA, Guy FH. The effect of hearing impairment on localization dominance for single-word stimuli. J Acoust Soc Am. 2015;130(1):312-23.
Stecker GC, Brown AD. Temporal weighting of binaural cues revealed by detection of dynamic interaural differences in high-rate Gabor click trains. J Acoust Soc Am. 2010;127(5):3092-103.
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