5 ms·

This is probably because the neurons involved can't switch fast enough. Our neural hardware is simply too slow to work in the time domain directly.

monocasa 5y ago
They do just fine with acoustic frequency ranges.
- philipswood 5y ago
  Firing rates for neurons are in the order of 10hz - under the minimum acoustic frequencies of about 20hz. Peak neuron firing rates basically only touch the lower bounds of frequencies that need to be processed. Without the "hardware acceleration" of the cochlea that preprocesses the time domain signals to frequency domain first, basically our whole audio sensorium is out of processing range. Normal nervous signal transmission speeds are in the order of the speed of sound, switching rates are in Hz range. Voltages are in the 10s of millivolt range.
  - monocasa 5y ago
    Neuron spiking isn't rate encoded, it's temporal encoded. Ie. the distance from the next expected regular interval is the signal, not the spiking interval itself. Additionally there's large variations in neuron spiking rates.
    - philipswood 5y ago
      Agreed. It's encoded in an elegant way. It has to be, due to the signalling constraints. It's not like a microphone-based electronic system that's fast enough to directly transmit an analog sample of air pressure changes.
  - nomel 5y ago
    Are you suggesting that anything over 10Hz is perceived with the representation of a rate, rather than direct stimulus?
    - philipswood 5y ago
      It has to be encoded somehow. Maybe not a rate. It can't be "amplitude modulated" because in general signals are pulses. So it's kind-of digital. It sometimes can't be a pulse rate encoded signal due to frequency cutoffs. As far as I understand they've identified several different encoding schemes and will probably find several more.
      - candiodari 5y ago
        Yes it is encoded. Essentially the rate of firing for a neuron encodes an exponential value to be represented. This is called "spike trains". You can see this clearly if you do an extreme slowdown of a human movement. Then, suddenly, what looks like a smooth movement, like raising an arm (and because of inertia it is smoothed of course), isn't really smooth. A pulse arrives in the muscle, and there is 20ms where the muscle is tensioned, and then it's back to neutral for 100ms. Then another spike arrives, another 20ms where a lot of tension is put on the muscle, the movement accelerates, and the muscle goes back to neutral. It's not a continuous movement at all. https://en.wikipedia.org/wiki/Biological_neuron_model https://en.wikipedia.org/wiki/Biological_neuron_model But odds are good that it's not just the value that's encoded. Many experiments have shown that it matters a lot if the signals are in phase (ie. they encode the same or some multiple of a value, but that the signals started at the exact same time matters, maybe more than the value itself. Or in the encoding: while for the value only the distance between 2 spikes matters, if 2 spikes on 2 different neurons occur at the exact same time, this will be interpreted as very relevant, even those both spikes may have very different firing rates)
saltcured 5y ago
I think I understand your sentiment, but consider our tactile sense of vibrations. We are able to distinguish different frequencies with the nerves in the skin, without cochlea doing the frequency domain transform. I think there are very different spectral limits and aliasing artifacts, but it's not like we can only sense a 5 Hz vibration either...
- philipswood 5y ago
  Again, I think that the sensor itself is detecting the frequency. The actual signal isn't "direct". IIRC from reading up on haptics - different tactile sensors are sensitive to different ranges of stimulus. Different cell bodies sense different effects and frequency ranges and send an encoded signal. We tend to forget that the underlying technology is very different and needs different encoding, signalling and computation approaches. The main sentiment is that our nervous system is quite slow compared to our electronics and needs to use different design approaches and novel encodings, to get things done. In much the same way our electronics can't engage with visible optic phenomena "directly". I mean visible light frequencies are too high to directly sample and transmit electronically the sensor used has to deal with that.
  - saltcured 5y ago
    Right, I know it isn't direct. The ability to introduce aliasing and fool this perception system is the basis for haptic feedback systems. I was responding more narrowly to the topic of a frequency domain transform as you get from the cochlea. I didn't mean to suggest that there are not other "hardware" signal transforms or feature extraction happening prior to the nervous system signal processing. As I understand it, the touch sensing structures are more like a zero, first, and second derivatives (deformation/tension, velocity, acceleration) and these coincidentally have different frequency sensitivity curves. My reading is that in some ways this might be more similar to how our visual system integrates different photon detection signals to interpret color. Rather than the clear frequency domain transform of the cochlea, there are broadly overlapping sensitivity curves with only 2-3 peaks.

monocasa 5y ago

They do just fine with acoustic frequency ranges.

philipswood 5y ago
Firing rates for neurons are in the order of 10hz - under the minimum acoustic frequencies of about 20hz. Peak neuron firing rates basically only touch the lower bounds of frequencies that need to be processed. Without the "hardware acceleration" of the cochlea that preprocesses the time domain signals to frequency domain first, basically our whole audio sensorium is out of processing range. Normal nervous signal transmission speeds are in the order of the speed of sound, switching rates are in Hz range. Voltages are in the 10s of millivolt range.
- monocasa 5y ago
  Neuron spiking isn't rate encoded, it's temporal encoded. Ie. the distance from the next expected regular interval is the signal, not the spiking interval itself. Additionally there's large variations in neuron spiking rates.
  - philipswood 5y ago
    Agreed. It's encoded in an elegant way. It has to be, due to the signalling constraints. It's not like a microphone-based electronic system that's fast enough to directly transmit an analog sample of air pressure changes.
- nomel 5y ago
  Are you suggesting that anything over 10Hz is perceived with the representation of a rate, rather than direct stimulus?
  - philipswood 5y ago
    It has to be encoded somehow. Maybe not a rate. It can't be "amplitude modulated" because in general signals are pulses. So it's kind-of digital. It sometimes can't be a pulse rate encoded signal due to frequency cutoffs. As far as I understand they've identified several different encoding schemes and will probably find several more.
    - candiodari 5y ago
      Yes it is encoded. Essentially the rate of firing for a neuron encodes an exponential value to be represented. This is called "spike trains". You can see this clearly if you do an extreme slowdown of a human movement. Then, suddenly, what looks like a smooth movement, like raising an arm (and because of inertia it is smoothed of course), isn't really smooth. A pulse arrives in the muscle, and there is 20ms where the muscle is tensioned, and then it's back to neutral for 100ms. Then another spike arrives, another 20ms where a lot of tension is put on the muscle, the movement accelerates, and the muscle goes back to neutral. It's not a continuous movement at all. https://en.wikipedia.org/wiki/Biological_neuron_model https://en.wikipedia.org/wiki/Biological_neuron_model But odds are good that it's not just the value that's encoded. Many experiments have shown that it matters a lot if the signals are in phase (ie. they encode the same or some multiple of a value, but that the signals started at the exact same time matters, maybe more than the value itself. Or in the encoding: while for the value only the distance between 2 spikes matters, if 2 spikes on 2 different neurons occur at the exact same time, this will be interpreted as very relevant, even those both spikes may have very different firing rates)

saltcured 5y ago

I think I understand your sentiment, but consider our tactile sense of vibrations. We are able to distinguish different frequencies with the nerves in the skin, without cochlea doing the frequency domain transform. I think there are very different spectral limits and aliasing artifacts, but it's not like we can only sense a 5 Hz vibration either...

philipswood 5y ago
Again, I think that the sensor itself is detecting the frequency. The actual signal isn't "direct". IIRC from reading up on haptics - different tactile sensors are sensitive to different ranges of stimulus. Different cell bodies sense different effects and frequency ranges and send an encoded signal. We tend to forget that the underlying technology is very different and needs different encoding, signalling and computation approaches. The main sentiment is that our nervous system is quite slow compared to our electronics and needs to use different design approaches and novel encodings, to get things done. In much the same way our electronics can't engage with visible optic phenomena "directly". I mean visible light frequencies are too high to directly sample and transmit electronically the sensor used has to deal with that.
- saltcured 5y ago
  Right, I know it isn't direct. The ability to introduce aliasing and fool this perception system is the basis for haptic feedback systems. I was responding more narrowly to the topic of a frequency domain transform as you get from the cochlea. I didn't mean to suggest that there are not other "hardware" signal transforms or feature extraction happening prior to the nervous system signal processing. As I understand it, the touch sensing structures are more like a zero, first, and second derivatives (deformation/tension, velocity, acceleration) and these coincidentally have different frequency sensitivity curves. My reading is that in some ways this might be more similar to how our visual system integrates different photon detection signals to interpret color. Rather than the clear frequency domain transform of the cochlea, there are broadly overlapping sensitivity curves with only 2-3 peaks.