Greetings everyone,
I enjoyed the recent posts regarding binaural demos and headphones versus
loudspeaker arrays.
I am writing this post off the cuff, so I can’t site peer-reviewed articles
immediately. But I believe what follows is accurate.
Audiograms for individuals with profound hearing loss oftentimes suggest
measurable, low-frequency hearing. Naturally, there is reason to question
whether the sensation is, in fact, hearing or a vibrotactile response. It is
easy to demonstrate our sensitivity to low-frequency vibration by lightly
touching the cone of a loudspeaker. Not too many years back, the standard
transducer for measuring hearing was the Telefonix earphone. Some profoundly
deaf persons would indicate a ‘tickling’ sensation at the low (below 500 Hz)
frequencies. Probably not a surprise when the presentation level was 90 dB HL
(which equates to an even greater SPL at the low and high frequencies).
Of greater concern to audiologist is that of unilateral or asymmetric hearing
losses. Above a certain presentation level, bone conduction makes the sound
clearly audible in the ‘better’ ear, which may not be the ear being tested.
This ‘crosstalk’ is greatly minimized by use of insert phones (predominately
EAR phones), but narrowband masking noise is used to ‘mask’ the test signal
from the non-test ear. Of course, the masking noise itself could be audible in
the opposite ear if the masking noise level (in HL or SPL) is greater than the
inter-aural attenuation (dB) of bone and tissue.
Aside from masking and audiometric protocols, the idea of sound transmitted via
bone conduction is important when it comes to normal-hearing listeners. We
probably don’t give it a lot of consideration to it because the predominant
‘sensation’ is that of hearing, so we assume that sound only reaches the inner
ear via the outer ear. What we ‘feel’ at loud concerts is identifiable as
low-frequency vibration, but this certainly adds to the ‘sound’ (I wonder
whether we could induce motion sickness if the vibration wasn’t in time with
the airborne sound).
I don’t have knowledge of persons who can discern pitch through touch (skin),
but any type of vibrotactile (or even visual) response that is rhythmic could
be discerned as a pattern that can be associated with music or speech. For
persons who were deaf at birth (congenital deafness; not necessarily genetic
causes), there is certainly reason to believe they have developed heightened
sensitivity to such vibration, and, in extraordinary persons, pitch could be
also be discernible. By the way, percussionists definitely tune their
instruments to match other instruments in the orchestra/group.
RE profound hearing loss at birth: Whether the brain creates synapses to
accommodate hearing via tactile input at a young age (i.e. when the brain is
plastic enough to do amazing things) could be studied using EEG mapping or fMR.
The sense of hearing is generally associated with the temporal lobe (and
further reduced to specific areas, or gyri and sulci). I do not know if these
areas would ‘light up’ in response to other sensory input (such as applying
vibration to the feet) for those persons. Please be aware that the temporal
lobe represents higher level cortical function (pattern recognition and
language), not necessarily sound-source direction. The latter may occur at the
mid-brain (inferior or superior colliculus).
Scientists have a good grasp on how we hear (meaning the physiological aspects
of the peripheral auditory system), but I fear we know little on how we
‘listen’. Isolating simple sounds allows us to study and understand important
aspects of hearing, but humans (and other critters) as well as our environments
are complex. I’ll leave discussions of man-environment interactions to
Ecological Psychologists or their opponents.
Regarding dentally-implanted hearing aids, my scant knowledge of this is that
they are bone-conduction devices. Some children (and adults) may have stenosis
or atresia that precludes them from hearing despite a normal inner ear system.
This type of hearing loss is a conductive hearing loss. By providing sound
vibration to boney surfaces (such as the mandibular process), the inner ear
receives stimulation as though the footplate of the stapes were acting on the
oval window (middle and inner ear components). Sound travels faster in a solid
medium than it does for air, so the usual directional cues are probably lost or
would take time to ‘learn’ (the other aspect of localization).
You can take a tuning fork and, while it’s vibrating, touch it to your
forehead, teeth, or prominent (hard) area just behind your ears. It is evident
from a simple demonstration that the increase in level is not merely a result
of sound reaching the outer ear. There’s a limit to high frequency response via
bone conduction because of inertia. It takes more energy to accelerate a large
mass. The light weight of the eardrum and bones of the middle ear (ossicles)
makes the middle ear an efficient moving system over a broad range of
frequencies. The limits are set by the sensory cells (inner and outer hair
cells) of the inner ear, not the mass or compliance of the middle ear.
Real-world hearing is multi-dimensional. I would be curious to know whether our
sense of distance is at all affected by the feeling sensation that reaches us
(via ground vibration) before airborne waves reach us. We are conscious of the
obvious, dominant cues that affect our perception of the world. In the absence
of the dominant cues, nature finds ways to fill gaps and make sense of our
environment. Pitch and melody may not be easily recognizable when distorted,
but rhythm is often identifiable. Speech, like music, has rhythm and intonation
(prosodic features). So ‘hearing’ with our skin seems entirely plausible,
particularly if we define hearing as an active process of making sense of
vibration. Reptiles are sensitive to infrasonic frequencies: Maybe their
hearing mechanism is akin to the utricle and saccules that are responsible for
our sense of balance. In fact, the organs of balance reside in the inner ear,
so why shouldn’t they also
contribute to our perception of sound (in addition to sound-source direction).
Just some thoughts, and me rambling on another Super Bowl Sunday.
Best to everyone,
Eric
Still trying to figure things out.
And a surround array of loudspeakers still rules.
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