Page 52 - 2016Summer

TECHNICAL COMMITTEE REPORT
Psychological and Physiological Acoustics
ogy, and genetics continue to solve puzzles and to raise new questions and controversies. One seemingly simple question is whether the sharpness of frequency tuning within the co- chlea is similar across different species of mammals. Early work suggested that it was, and so researchers have gener- ally been comfortable with extrapolating the results from in- vasive studies of cochlear mechanics in laboratory animals such as guinea pigs and chinchillas to explain human hear- ing. Over the past 15 years or so, suggestions that human cochlear tuning is considerably sharper than that in other mammals (Shera et al., 2002) has led to renewed interest and controversy in the topic of human cochlear mechanics (Rug- gero and Temchin, 2005; Shera et al., 2010), a topic that was pioneered by P&P’s own Georg von Békésy, who won the Nobel Prize for his work in the area in 1961.
The inner and outer hair cells, which line the cochlea and sense its vibrations, are an astounding feat of biology and continue to fascinate and confound researchers. While the inner hair cells transduce vibrations into a neural code that is sent along the auditory nerve, the outer hair cells form part of a complex process that amplifies the vibrations, sharpens tuning, and produces “otoacoustic emissions,” sounds that are generated in the ear. Since their discovery, published in a landmark JASA article by David Kemp (1978), otoacoustic emissions have been used to provide us with a window into the functioning of the human ear that is now employed as part of the health screening of every newborn infant in the United States.
Hearing loss affects a large number of people around the world and is particularly common among older individuals. Many forms of hearing loss involve damaged or dysfunc- tional inner or outer hair cells. However, a new form of hear- ing disorder was recently discovered in animals when it was found that a loud noise that produced only a temporary shift in thresholds resulted in a loss of up to 50% of the synapses that connect the inner hair cells to the auditory nerve (Ku- jawa and Liberman, 2009). A current hot topic of research is to discover the prevalence and perceptual consequences in humans of this “hidden hearing loss,” which remains un- detected by traditional clinical screening tools (Schaette and McAlpine, 2011; Plack et al., 2014).
One of the great triumphs of auditory research has been the cochlear implant. This device is surgically implanted, with an electrode array inserted into the spiral turns of the cochlea to directly stimulate the auditory nerve with elec- trical pulses. The cochlear implant can restore some func-
tional hearing in people who were previously deaf to the ex- tent that many cochlear-implant recipients can understand speech, even in the absence of lip-reading cues. Well over 300,000 devices have been implanted worldwide, and it is now common to provide deaf infants as young as 12 months with a cochlear implant. Despite its tremendous success, us- ers of the cochlear implant still face numerous challenges, including understanding speech in noisy environments and perceiving pitch in music. Because of these remaining chal- lenges, the push to better understand perception via a co- chlear implant and to improve its performance continues; in 2015, a total of 15 articles on cochlear implants appeared in JASA alone. Exciting new work is being done in the area of alternative auditory implants, in the brainstem and even in the midbrain, for patients for whom a traditional cochlear implant is not an option, perhaps because of a tumor or the lack of an auditory nerve.
At a less invasive level, hearing aids still remain the best op- tion for most people with a hearing loss that ranges from mild to severe. Although the technology itself goes back a long way, cutting-edge new signal-processing algorithms are constantly being updated in these devices to take advantage of the more rapid and powerful digital signal processing that can now be fitted within hearing aids. Here, too, researchers and companies are experimenting not only with the type of processing but also with the type of stimulation, be it via bone conduction or direct mechanical stimulation of the eardrum.
The auditory brain still remains something of a mystery for researchers despite the enormous strides that have been made over the past 50 years in understanding how signals are passed from the cochlea to the brainstem and midbrain structures and then on to the auditory cortex. Although perceptual attributes and features, such as pitch, loudness, brightness, and perceived location, have been identified and studied psychophysically, it is often challenging to find clear neural correlates of these features. The percept of pitch is one where neural correlates have been identified (e.g., Ben- dor and Wang, 2005), although considerable uncertainty regarding the location and underlying mechanisms remain. Neuroimaging techniques, such as EEG (electroencephalo- gram), MEG (magnetoencephalography), and fMRI (func- tional magnetic resonance imaging), are being recruited to solve some of these mysteries in the human brain. In addi- tion, cutting-edge technologies, such as two-photon imag- ing and optogenetics, are being employed in other species to decipher how the brain processes sound and to discover
50 | Acoustics Today | Summer 2016