• How would such neurons be able to signal higher frequencies based on a timing code
(because, as we know, people can hear frequencies of up to 20,000 Hz)?
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Volley Principle
• A simple version of the timing code can't work, but if neurons work together according to the volley principle , it's possible to produce a timing code even for higher frequencies.
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Volley Principle
• Wever suggested that while one neuron alone could not carry the temporal code for a 20,000 Hz tone, 20 neurons, with staggered firing rates, could.
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Volley Principle
• Each neuron would respond on average to every
20th cycle of the pure tone, and the pooled neural responses would jointly contain the information that a 20,000 Hz tone was being presented.
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• Auditory nerve fibers sensitive to a particular frequency range fire at the same part (phase) of every cycle of a sound in that range.
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• No individual neuron could fire at each peak, but a bunch of phase-locked neurons working together can produce a burst of activity at each peak, and so the firing frequency of a collection of neurons can indeed mimic the frequency of the stimulus.
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• The effect of the missing fundamental
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• Remember…
– suppose we have a 400 Hz fundamental plus its harmonics (800, 1,200, 1,600, 2,000). This should sound like a pitch of 400 Hz with a rich timbre.
– What happens when we remove the fundamental frequency (the 400)?
• The perceived pitch of the tone doesn't change!
• This is called periodicity pitch or “the effect of the missing fundamental.”
• Why is this a problem for place-coding for pitch?
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• It is now widely accepted that the brain processes the information present in the harmonics to calculate the fundamental frequency.
• The precise way in which it does so is still a matter of debate, but the processing seems to be based on an autocorrelation involving the timing of neural impulses in the auditory nerve.
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• Music is recognizable even on crappy stereos that can't reproduce the fundamental frequencies
• Voices are recognizable on the phone, even though phones can't reproduce the fundamental frequency of the human voice
• Also useful for the construction of pipe organs
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• As you know, loudness is related to amplitude, but that’s not the whole story. Loudness, like pitch, is a perceptual (not a physical) quality.
• Two sounds that have the same physical sound pressure levels (but different frequencies) are often perceived to have different loudnesses.
• How is loudness encoded by our auditory system?
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Firing Rate Hypothesis
• Big idea: neurons fire more to a louder sound
– For a weak tone, the basilar membrane is displaced little, hair cells are not pushed very far, and few spikes in the auditory nerve fibers.
– Rising (high pressure) phase of each cycle of the sound signal evokes bursts of spikes in a collection of auditory nerve fibers.
– The amplitude of the sound determines the number of spikes per burst. Low amplitude signals evoke few spikes while high amplitude signals evoke more spikes.
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Firing Rate Hypothesis
• Big idea: neurons fire more to a louder sound
– Were this hypothesis the only correct one, we could control the perceived loudness simply by injecting patterns of current into the neuron thereby causing it to respond at a more rapid rate.
• Big problem: loudness increases over a range of
120 dB, while neurons can only increase their firing rate over a range of 40 dB
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Number of Neurons Hypothesis
– Big idea: more neurons fire to a louder sound
– As a traveling wave passes down the membrane, each point of the membrane oscillates at the frequency of the tone.
– When the sound is weak, displacements are generally quite small and only a small region of the basilar membrane moves sufficiently to evoke any responses.
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Number of Neurons Hypothesis
– With increases in sound intensity, the membrane is displaced by a larger amount at each point.
– This is particularly true for the part of the basilar membrane that is most sensitive to the tone we are playing (peak of the envelope), but there is also a region nearby that did not respond to the weak stimulus but does if the intensity is increased.
– In the auditory nerve, this means that additional nearby auditory nerve fibers will be recruited when the sound intensity is increased.
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– We are in the same position with respect to loudness perception as we were with respect to the perception of pitch!
– Both firing rate hypothesis and number of neurons hypothesis could plausibly serve as the mechanism for our perception of loudness.
– In fact, we now know that both mechanisms play a role in loudness perception.
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– Even though we have an understanding of the physics of sound, the response
(motions) of the basilar membrane, and the response (firing rates) of the auditory nerve fibers, this information alone can not yet answer for us what the mechanism of loudness perception is.
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• Pitch
– depends on place code & timing code.
• Loudness
– depends on firing rates & number of neurons.
• How do these 4 neural codes co-exist?
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• (1) low freq, low intensity
• (2) low freq, high intensity
• (3) high freq, low intensity
• (4) high freq, high intensity
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The Central Auditory System
• Each auditory nerve sends information to the cochlear nucleus.
• From there, projections diverge to many different pathways.
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The Central
Auditory System
• There are many parallel pathways in the auditory brainstem.
• The binaural system receives input from both ears.
• The monaural system receives input from one ear only.
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• Interaural intensity differences:
– If a sound is played at a position off to the right side, sound intensities will be slightly different in the two ears
• 1) paths are of different length because sound has to travel past the head to get to the left ear and sound intensity decreases with distance (1 over the square of the distance)
• 2) head interferes with the sound-wave, casting the auditory equivalent of a shadow on the far ear
• Interaural time differences:
– Sound from the right arrives at right ear first because its closer
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• Lloyd Jeffries, a psychophysicist, proposed a theory in the early 1950s that medial superior olivary (MSO) neurons act as coincidence detectors .
– Coincident spikes arriving from the two ears evoke a response in the MSO neuron.
– Inputs from the two ears are delayed by various amounts relative to one another by the relative length of the axons.
– For this to work, the timing of individual spikes must be very precise, and it is (at least for low frequency tones) because of phase locking.
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Each MSO neuron is tuned to a specific ITD
• This neuron responds best at an ITD of zero and less well at progressively greater ITDs.
– It responds again when the two sounds are an entire cycle out of phase.
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Each set of auditory pathways has a specialized function
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