Naked Science Forum
Life Sciences => Physiology & Medicine => Topic started by: thedoc on 22/09/2016 16:23:02
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Ted asked the Naked Scientists:
Dear Dr Smith
Can you please answer the following questions
Are the sounds 'heard' by sufferers generated in the inner ear if so how?
If a person with normal hearing listens to an audible signal say a 1000 cycles per second can this signal be detected within the brain? If so is it a digital signal?
I ask these questions because I wondered whether it is possible to introduce an anti phase signal to cancel or minimise the unwanted signal
Thank you
Sincerely
E Banstead
What do you think?
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Hearing occurs due to the excitement resonant structures in the inner ear that do a frequency analysis of the incoming sound and pass it onto the brain.
To narrow the band width of these resonators there is a degree of positive feedback in each and it is believed that tinnitus occurs when the degree of feedback gets out of control and oscillation occurs.
If a microphone is placed in the ear of a tinnitus sufferer the sound generated by the oscillating resonators can be heard so it has a real existence and it is not generated by the brain.
the only remedy that I know is to inject white noise into the ear
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Since tinnitus is produced in the nerves of the ear, rather than from sound in the ear, anti-phase noise would not help much.
It is possible to mask the sound of tinnitus with other sounds.
The best cure is prevention - wear earplugs to rock concerts (you still feel the sound in your chest)!
https://en.wikipedia.org/wiki/Tinnitus#Prevention
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To narrow the band width of these resonators there is a degree of positive feedback in each and it is believed that tinnitus occurs when the degree of feedback gets out of control and oscillation occurs.
I'm sceptical. To sustain oscillation at a constant amplitude, a loop gain of exactly unity is required. Exactly as in ±nothing. If the loop gain were even slightly below unity the amplitude of oscillation would decay exponentially, if in excess of unity the amplitude increases until limiting takes place. Not all tinnitus sufferers hear a noise which is maximum amplitude.
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If a microphone is placed in the ear of a tinnitus sufferer the sound generated by the oscillating resonators can be heard
The ear is not just a passive receiver - there are muscles that can activate the tiny hairs in the ear, thus producing sound.
It is thought that these may enhance the sensitivity of hearing.
In most people, these "otoacoustic emissions" are inaudible to the individual producing them, while for others they are audible as an annoying noise, and classified as tinnitus. However, this accounts for a minority of tinnitus sufferers (1-9%, according to Wikipedia).
See: https://en.wikipedia.org/wiki/Otoacoustic_emission#Clinical_importance
I'm sceptical. To sustain oscillation at a constant amplitude, a loop gain of exactly unity is required. Exactly as in ±nothing.
This is true if you wish to produce a pure sinusoid with a linear circuit.
However, the ear is rather non-linear, and the response of nerves is rather non-linear.
In the case of spontaneous otoacoustic emission, the frequency appears fairly stable, but the measured amplitude varies randomly over a wide range (a factor of 10,000:1)- it seems to be a chaotic oscillator rather than a linear oscillator.
But the majority of tinnitus appears to be neurological, generated in the nerves and/or brain.
Neurones that are not stimulated for some time (eg due to frequency-selective hearing loss) could increase their sensitivity in a futile attempt to extract some signal, and could produce spurious sensations of sound. Or they could make cross-connections with other nerves that seem to be doing more interesting things.
The formation and maintenance of synapses to produce learning and useful information processing is a bit of a mystery in brain studies. As are the processes when this goes awry.
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I'm sceptical. To sustain oscillation at a constant amplitude, a loop gain of exactly unity is required. Exactly as in ±nothing.
This is true if you wish to produce a pure sinusoid with a linear circuit.
However, the ear is rather non-linear, and the response of nerves is rather non-linear.
In the real world there is no such thing as a linear oscillator. Any practical oscillator is a combination of positive feedback to sustain oscillation, and negative feedback to stabilise the amplitude. The negative feedback is often simply gain compression as the signal limits against the supply for RF or non sinusoidal oscillators, but for low distortion oscillators a thermistor or an AGC are used.
If you put feedback on a logarithmic amplifier there still has to be stabilisation at a single amplitude that provides unity loop gain.
In the case of spontaneous otoacoustic emission, the frequency appears fairly stable, but the measured amplitude varies randomly over a wide range (a factor of 10,000:1)- it seems to be a chaotic oscillator rather than a linear oscillator.
But the majority of tinnitus appears to be neurological, generated in the nerves and/or brain.
Neurones that are not stimulated for some time (eg due to frequency-selective hearing loss) could increase their sensitivity in a futile attempt to extract some signal, and could produce spurious sensations of sound. Or they could make cross-connections with other nerves that seem to be doing more interesting things.
The formation and maintenance of synapses to produce learning and useful information processing is a bit of a mystery in brain studies. As are the processes when this goes awry.
But is any of this positive feedback?