Binaural Beats Mystery

  • #1
NatanijelVasic
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Hello everyone,

I have been thinking about binaural beats, and after learning about the inner workings of the ear, there is something that I cannot explain. A binaural beat is the pulsating effect perceived by the brain when slightly offset frequencies are played in each ear.

In my example, the left ear receives a sine wave of 100 Hz, and the right ear receives a sine of 106 Hz. The person is wearing headphones, so the audio waves do not mix in the air, and therefore there is no interference in the air.
( ) ,
Inside each ear, the mechanical vibrations cause a certain region of hair cells to vibrate and each region corresponds to a certain resonant frequency. This means that the information sent by neurones from the ear to the brain is a Fourier representation of the original sound (neurones cannot handle fast switching, which is why audio frequencies cannot be transported in the original form through the nerve).
From the example above, this means that the brain is receiving a CONSTANT signal input from the left ear, and a constant signal input from the right - the only difference being that the left and right neurones correspond to different frequency ranges.
The mystery is this: where does the signal interference occur in order to cause the beating effect? An inverse Fourier cannot occur in the brain as the neurones "don't support" frequencies as high as audio. The brain might well add the Fourier representation of the signals from each ear, but that won't explain where the interference comes from. What is even more interesting, is that the beating effect from the interference is exactly the correct frequency i.e. the same frequency as would occur in the medium of sound. This suggests that the perceived interference is not than just an illusion, but "mathematical" signal progressing within the brain.

I would be extremely grateful for any responses :)

Nat

An example of binaural beats:

(try taking out the Left earbud only, then the right only, then listen to both at the same time)
 
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Answers and Replies

  • #2
BillTre
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At least some of the neurons innervating the ear sensory cells in the cochlea receive input only from hair cells responsive to a single frequency.
In addition, at lower frequencies, some of these neurons can fire 1:1 with the vibrations.
 
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  • #3
NatanijelVasic
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At least some of the neurons innervating the ear sensory cells in the cochlea receive input only from hair cells responsive to a single frequency.
In addition, at lower frequencies, some of these neurons can fire 1:1 with the vibrations.

Thank you very much for your response. Does that mean neurones in the ear can fire at 1000Hz and perhaps even slightly above?
 
  • #4
BillTre
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There is a cut-off where the neurons can't keep up with faster rates. I forget what that cut-off frequency is. Above that, the response is not 1:1.
It would be either an increase in a base rate of neuron firing or it might fire only when the hair cells fire (another thing I forgot).
 
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  • #5
Fervent Freyja
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No one is arguing with you. This 'mathematical' signaling process likely occurs.

More recent work on the topic: https://www.researchgate.net/publication/264887690_Mismatch_negativity_to_acoustical_illusion_of_beat_How_and_where_the_change_detection_takes_place [Broken] https://www.researchgate.net/publication/264887690_Mismatch_negativity_to_acoustical_illusion_of_beat_How_and_where_the_change_detection_takes_place [Broken]
"These findings suggest that the auditory periphery has an important contribution to
novelty detection process already at sub-cortical level. Overall, a signif-
icant outcome of this study is that the detection of acoustic novelty in
the human brain is represented via a multiple-level hierarchy extending
beyond the auditory cortex. "
 
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  • #6
NatanijelVasic
19
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No one is arguing with you. This 'mathematical' signaling process likely occurs.

More recent work on the topic: https://www.researchgate.net/publication/264887690_Mismatch_negativity_to_acoustical_illusion_of_beat_How_and_where_the_change_detection_takes_place [Broken] https://www.researchgate.net/publication/264887690_Mismatch_negativity_to_acoustical_illusion_of_beat_How_and_where_the_change_detection_takes_place [Broken]
"These findings suggest that the auditory periphery has an important contribution to
novelty detection process already at sub-cortical level. Overall, a signif-
icant outcome of this study is that the detection of acoustic novelty in
the human brain is represented via a multiple-level hierarchy extending
beyond the auditory cortex. "

Ah I think that would explain it then. I did a test myself and the beating effect does seem to disappear at higher frequencies (i tried at around 5 KHz).
 
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