Ears and sound localization
Rik J. Otte, Martijn J. H. Agterberg, Marc M. Van Wanrooij, Ad F. M. Snik, A. John Van Opstal Age-related Hearing Loss and Ear Morphology Affect Vertical but not Horizontal Sound-Localization Performance JARO, 14(2): 261-273, 2013. doi: 10.1007/s10162-012-0367-7.
Sound localization involves the use of several differnt kinds of cues that are present in the sound arriving at the eardrum. The classical cues are the interaural time differences that occur because of different travel times for a sound from it’s source to the two ears, and interaural level differences that arise from the acoustic shadow of the head. Another rich set of cues arises from the structure of the external ear, or pinnae. These cues are largely found at high frequencies, and arise from reflections of the sound within the pinnae, which in turn shape the spectral structure of the sound reaching the ears. Pinnae cues are especially important for localizing sounds in elevation (as opposed to azimuth).
The pinna grows and changes shape as we age, and so the cues that are available are not constant over life. In a paper current in JARO On-line first, Ostal et al. tested the sound localization abilities of individuals at different ages using brief broadband sounds, for which spectral cues are important. Ear size was found to influence how well individuals at the different ages could localize sounds, and the larger ears of older adults enabled the use of lower frequency cues. However, the shift in the ability to use cues in different spectral ranges was not sufficient to offset the effects of age-related hearing loss, as older individuals were also poorer at the elevation localization task.
The paper raises an interesting set of questions regarding central processing. If the pinna is growing, then the relationship between the spectral structure and sound location in azimuth is always changing. The central auditory system clearly has to adapt to this shift, and I leave it as an excercise for the reader to provide hypotheses and mechanisms regarding where and how such adaptations occur. Access the full article at Springer.com.