Page 33 - Spring 2015

status of CI Research and the
field of Hearing Perception
Almost any issue of the Journal of the Acoustical Society of America (JASA) has a number of papers concerned with CIs or simulations of CI processing (i.e., vocoders; see be- low). Two things began to occur in the 1990s to produce this trend. First, CIs started providing high levels of speech understanding in a majority of the users. This increased the prevalence of CIs and the significance of understanding how they worked as well as how the brain was processing elec- trical stimulation. Second, there has been a slow shift away from understanding basic auditory phenomena to how these phenomena occur or do not occur in hearing-impaired indi- viduals, either hearing aid or CI users. Although still in the realm of basic science, the translational aspects of hearing and how we can help people hear better seems to be the new direction of the field.
However, although we know a lot about hearing now, trans- lating our findings to help people should be the end goal, CIs have opened up a Pandora’s box of what we did not know about hearing. Take the field’s obsession with envelope ver- sus fine-structure encoding. Surely, CIs, the devices that primarily encode envelope information, provided a major impetus for all the attention paid to envelope versus fine- structure encoding. Every day, it seems that I stumble over questions that have not been asked for the typical acoustic hearing system alone but seem incredibly important when considering both typical hearing and CIs together.
The following is my opinion of how we are doing with un- derstanding CIs and electrical stimulation of the auditory. Essentially these are some highlights from the field.
The Good
For the high-performing CI users (those who have typically had a short duration of profound hearing loss or deafness), speech-processing strategies do a wonderful job of provid- ing access to hearing, particularly in quiet situations (see Figure 4). Although most of the temporal fine structure is removed from the signals, a small amount of temporal pitch information is conveyed by the temporal envelope, which can help CI users discriminate different gender speakers (Fu et al., 2004). As mentioned above, many people get bilater- al CIs, which help improve the abilities to locate sounds in space and speech understanding in background noise. Most of the latter occurs through a monaural mechanism called the “better ear” effect, which is simply that one ear might have a better signal-to-noise ratio (Loizou et al., 2009).
Probably the only reason a CI works as well as it does is that we have
an input signal that is so robust to degradation.
CIs started as a treatment for adult postlingually deafened individuals. However, children who get implants often de- velop excellent language, speech production, and speech un- derstanding (Svirsky et al., 2000). At the other end of the age spectrum, there was a worry that older people would not be good candidates for CIs because of possible medical com- plications or a lack of plasticity in the brain that would al- low for speech understanding. What has become clear is that there appears to be no upper limit on age of implantation; CIs seem to be safe and effective and can improve the quality of life across the life span (Lin et al., 2012).
The bad
Although we generally laud the successes of the high-per- forming CI users, there exists a wide variability in perfor- mance (see Figure 4). Even though many CI users achieve near-perfect speech understanding in quiet, many do not. The reasons are wide and varied, but factors such as etiology, duration of deafness, and age of implantation affect speech understanding with a CI. Sometimes it is easy to forget about the relatively poor-performing CI users, which may be a re- sult of how researchers like me choose their participants. The adult CI literature is composed mostly of middle-aged high performers with short durations of deafness. But there are populations such as prelingually deafened individuals who have gone for decades of their life without auditory stimula- tion that fall into a population that would be typically poor CI users. In general, the field could pay more attention to the poor performers.
Coming back to the two big problems, pitch and spatial hearing, several investigators have attempted to develop novel stimulation strategies to reintroduce temporal fine structure by lowering the rate of the carrier on a subset of electrodes. In general, it is unclear if those strategies provide a benefit for pitch or spatial hearing, which is a major disap- pointment. However, the field seems to be moving in a good direction. Recently, Churchill et al. (2014) found a method to present both temporal fine structure and envelope infor- mation using a mixed rate stimulation strategy (where low rates are used to present envelope information and coherent temporal fine structure across several apical electrodes and high rates are used to present just envelope information at basal electrodes), which improves sound localization with- out degrading speech understanding in quiet.
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