Hello Sampo,
Many thanks for your thorough and interesting reply to my post. Even if
binaural listening with head tracking could be perfected, headphones are still
out of the question when it comes to presenting stimuli to hearing aid and
cochlear implant users. A need for presenting sound in three dimensions (or, at
the very least, surround sound on the ear-level plane) was the whole reason I
became seriously interested in Ambisonics.
It is interesting that you mentioned ultrasonic frequencies. I recall, from
years back, one manufacturer's attempt to market an ultrasonic device that
would compete with cochlear implants. (If I remember correctly, it was a
German-based company called Hearing Innovations.) The ultrasonic signal was a
60-kHz carrier and presented to the user via bone conduction. One might
ascertain that inertia alone would mechanically filter out the 60-kHz signal,
thus leaving only the signal used to modulate the carrier to stimulate the
inner ear. But, according to the manufacturer, individuals with profound
sensorineural hearing loss could "hear" with the device, thus precluding the
idea that it was merely a fancy bone-conduction hearing aid. I don't know what
physiological processes would have been involved or if further research is
being done.
Although it's pretty well established that most neurons can't go through the
whole action potential / depolarization cycle within a 1 ms time-frame, the
motile, outer hair cells can vibrate at frequencies on the order of 8 kHz (or
higher?) when electrically stimulated (at least in vitro). Although OHCs don't
innervate afferent fibers, they certainly affect hearing, and add to the
complexity (and mystery) of the peripheral auditory system. I worked on a
project with Lin Bian, M.D. and Ph.D. to show how the cochlear partition moves,
cycle-by-cycle, and it definitely isn't linear. This has a bit to do with the
"rectification" process. That particular study didn't make it into the pages of
JASA, but Lin's subsequent studies did.
Your paragraph "Now that I've read some basics of cochlear implant tech, I
don't see how such considerations are taken into account. Thus, Eric, since you
seem to be worried about the effects of real life background noise on CIs,
maybe you could go double the mile by trying out a CI analysis algorithm which
hybridizes your typical Shannon-esque noise-band vocoder with a selective
application of pure, rectified, time-domain information, straight from the
sampler" was well-taken in that I'm not trying to copy what other researchers
and CI manufacturers are doing. The following is taken directly from my website:
"The overall design results in a virtual 'electrical transmission line' along
the implanted membrane. The active array doesn't suffer from current smearing
when a large number of electrodes are simultaneously energized... The high
channel count isn't merely an attempt to achieve better frequency
discrimination based on the place theory of frequency coding; instead, our
implant design and multiplexing strategy follows many established principles of
psychoacoustics -- some of which are ignored in other CI designs. The
high-density electrode will improve dynamic range and speech quality (as well
as frequency discrimination) when one CI is used, and nearly-normal interaural
time difference (ITD) coding when two implants are used. Additionally, there
are theoretical benefits to our high channel density design when severe nerve
ganglion damage is present... Because our design works on different principles
than other implants, it is important to note that our
acoustic simulations are considerably different from noise-band vocoder
demonstrations used to simulate CI listening." (This paragraph, by the way, is
about my CI design.)
In brief, a lot of things haven't been taken into account when it comes to
auditory neural prostheses, and I'm not sure why they haven't. What I do know
is that I have a heck of a lot to learn, and I keep an open mind. What most
people may not realize is that hearing science is more of a hobby than vocation
for me. I'm excited about Ambisonics because it's new information for me
(although I realize it's not exactly new technology). I'm enjoying the plethora
of recordings that are available, and my interest in Ambisonics extends well
beyond hearing research. In fact, I'm giving a demo to an Audio Production and
Technology class this Thursday. Should be fun!
Kind regards,
Eric
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