I guess not many neuroscientists read/. (or at least not "ask/."). There are some elements of truth in what has been said so far, but also some key misunderstandings.
First: what is a nerve? When people talk about nerves they usually mean the very fine greyish fibres that run between the brain (or spinal cord) and various organs. In general, one organ, one nerve: thus "optic" nerves, "auditory" nerves, "cranial" nerves, "sciatic" nerves etc. These fibres are actually bundles of finer fibres, called axons, each of which is a projection emerging from a single nerve cell (or neuron).
The bandwidth of the nerve is thus something like the sum of the bandwidths of each of the axons (think of a T3 line for a technological analogy).
What is the bandwidth of an axon? There are two parts to this question:
1. The axon transmits information in short pulses called "action potentials" (or, more familiarly, "spikes"). Each pulse is the same shape as all the others, so no information is carried in the shape: only in the fact that the spike happened. The action potential is an electrochemical phenomenon: it requires both conformal changes to proteins in the cell membrane as well as electrical field effects in order to be initiated and to propogate. The electrical part of this is, of course, fast. The chemical part is slow (actually, slow to recover the base state), and, as a result, two pulses cannot be any closer than a few milliseconds (peak to peak). This is called the refractory period. I have seen cells fire an action potential every 1.2 milliseconds, but this is rare. More commonly, they will max out at around 100 to 200 spikes/second. NOTE: All of these numbers are approximate, not because we don't know them, or because I don't remember them, but because the properties of different nerve cells are different. I could tell you in great detail about the neurons I study, but these aren't the ones that contribute to the peripheral nerves. Indeed, not all nerves are the same, anyway. Axons in the auditory nerve (bringing information from your ear to your brain) will happily fire upto 1000 spikes/sec. Not so axons in other nerves. In fact, any number between 10 and 1000 would be defensible. So we'll split the difference (logarithmically) and say 100 spikes/sec.
2. OK. So 100 spikes/sec. How many bits is that? Frankly, we don't really know. You might think: well a spike is either there or not, there are at most 100 spikes/sec, so this is a binary code at 100 baud. The problem with this reasoning is that there is no clock. In a binary stream (like a modem signal) you know when the next bit will be. So you simply ask, was there a one or a zero at that time? For the axon the information is in *when* the spike happened relative to the previous one. In principle, this interval could be chosen arbitrarily precisely, thus conveying infinite information with just two spikes. Of course, this doesn't happen, but the question of how many bits is really an experimental one. One famous study on the H1 cell of the blowfly (Bialek et al., SCIENCE 252: (5014) 1854-1857) JUN 28 1991) put the number on order of 3 bits/spike (I don't have the paper in front of me to check the exact value). Other studies have suggested similar numbers in other preps. These studies have all been carried out on invertebrates, not on human nerve fibres.
So much for the axon. What about the nerve? Recall that the nerve is a bundle of many many axons. How many? Depends on the nerve. But a hundred thousand is a reasonable figure.
So the answer is something like 100 spike/sec/axon * 3 bit/spike * 1e5 axon/nerve = 3e7 bit/sec. But that number could be off by as many as two orders of magnitude depending on which nerve you look at and whether we really know how many bits are conveyed per spike.
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I guess not many neuroscientists read /. (or at least not "ask /."). There are some elements of truth in what has been said so far, but also some key misunderstandings.
First: what is a nerve? When people talk about nerves they usually mean the very fine greyish fibres that run between the brain (or spinal cord) and various organs. In general, one organ, one nerve: thus "optic" nerves, "auditory" nerves, "cranial" nerves, "sciatic" nerves etc. These fibres are actually bundles of finer fibres, called axons, each of which is a projection emerging from a single nerve cell (or neuron).
The bandwidth of the nerve is thus something like the sum of the bandwidths of each of the axons (think of a T3 line for a technological analogy).
What is the bandwidth of an axon? There are two parts to this question:
1. The axon transmits information in short pulses called "action potentials" (or, more familiarly, "spikes"). Each pulse is the same shape as all the others, so no information is carried in the shape: only in the fact that the spike happened. The action potential is an electrochemical phenomenon: it requires both conformal changes to proteins in the cell membrane as well as electrical field effects in order to be initiated and to propogate. The electrical part of this is, of course, fast. The chemical part is slow (actually, slow to recover the base state), and, as a result, two pulses cannot be any closer than a few milliseconds (peak to peak). This is called the refractory period. I have seen cells fire an action potential every 1.2 milliseconds, but this is rare. More commonly, they will max out at around 100 to 200 spikes/second. NOTE: All of these numbers are approximate, not because we don't know them, or because I don't remember them, but because the properties of different nerve cells are different. I could tell you in great detail about the neurons I study, but these aren't the ones that contribute to the peripheral nerves. Indeed, not all nerves are the same, anyway. Axons in the auditory nerve (bringing information from your ear to your brain) will happily fire upto 1000 spikes/sec. Not so axons in other nerves. In fact, any number between 10 and 1000 would be defensible. So we'll split the difference (logarithmically) and say 100 spikes/sec.
2. OK. So 100 spikes/sec. How many bits is that? Frankly, we don't really know. You might think: well a spike is either there or not, there are at most 100 spikes/sec, so this is a binary code at 100 baud. The problem with this reasoning is that there is no clock. In a binary stream (like a modem signal) you know when the next bit will be. So you simply ask, was there a one or a zero at that time? For the axon the information is in *when* the spike happened relative to the previous one. In principle, this interval could be chosen arbitrarily precisely, thus conveying infinite information with just two spikes. Of course, this doesn't happen, but the question of how many bits is really an experimental one. One famous study on the H1 cell of the blowfly (Bialek et al., SCIENCE 252: (5014) 1854-1857) JUN 28 1991) put the number on order of 3 bits/spike (I don't have the paper in front of me to check the exact value). Other studies have suggested similar numbers in other preps. These studies have all been carried out on invertebrates, not on human nerve fibres.
So much for the axon. What about the nerve? Recall that the nerve is a bundle of many many axons. How many? Depends on the nerve. But a hundred thousand is a reasonable figure.
So the answer is something like 100 spike/sec/axon * 3 bit/spike * 1e5 axon/nerve = 3e7 bit/sec. But that number could be off by as many as two orders of magnitude depending on which nerve you look at and whether we really know how many bits are conveyed per spike.
5.0 works for me. Redhat 5.2. Kernel 2.0.36. /usr/lib/Real.
I downloaded an rpm from real.com. Had to change
LD_LIBRARY_PATH to include
Now, if I could get more to listen to more than a
minute w/o a "server disconnected"...