On 6/9/06, Eugen Leitl <[EMAIL PROTECTED]> wrote:
Most of information is visual, and retina purportedly compresses 1:126
(obviously, some of it lossy).
http://www.4colorvision.com/dynamics/mechanism.htm
claims 23000 receptor cells on the foveola, so I would just
do a rough calculation of some 50 fps (you don't see this, but
the cells do), and 16 bit/cell (it's probably 12 bit, but it's
a rough estimate, anyway). Estimate gives some 20 MBit/s, which
I think is way too low.
That is the kind of info that I'm looking for, but I already have it
for the retina.
Each human eye has about 6 million cones and 125 million rods
(Wikipedia). A cone has a max frequency of perhaps 60Hz, since movies
are 30 frames/sec, while a rod has a refractory period of about 50 ms
(Varsányi et al. 2005). Assuming a signal/noise ratio of 3 (typical
of some other neurons), you can calculate that the information
provided by human photoreceptors is about 2.7 billion bits per second:
Photoreceptors: 125 million rods with frequencies up to 20Hz:
125Mx20log10(3?)=1.2Gbps
6 million cones wi freqs up to 60?Hz: 6Mx60log10(3?)=172Mbps
Total for 2 eyes = 343.57Mbps + 2385Mbps = 2729Mbps
Much of this, however, is highly correlated, because the info coming
to the left eye is so strongly correlated with that in the right eye,
so total info is probably around 1.5Gbit/sec.
This info goes from the photoreceptors, to bipolar cells, to ganglion
cells, all within the retina. The information coming from the retinal
ganglion cells to the lateral geniculate nucleus is a lot less. A
ganglion cell has a refractory period, on average, of 3.75
milliseconds (Berry & Marker 1998), which means it can fire at rates
up to 267Hz, while its lowest firing rate is about 10Hz (Berry &
Marker 1998). This gives it a bandwidth of 257Hz. A retinal ganglion
cell has a signal-to-noise ratio of about 3 at most contrast levels
(Dhingra et al. 2005). Hence, we can calculate the channel capacity
of a retinal ganglion cell using the Shannon-Hartley channel-capacity
formula as 257log10(3) = 123 bits per second. There are about 1.35
million ganglion cells in a human retina (Wikipedia, "Ganglion cell").
(There are 5 different types of ganglion cell, but I don't have data
on all the different types.) With two eyes, this gives us 331 million
bits per second coming into the human visual system, again wi
redundancy between the two eyes, total info probably less than
200Mbits/sec.
For the human auditory modality, we know the frequency sensitivity of
the entire auditory system combined, and hence need not deal with
individual neurons. If we believe that humans can hear sounds from
30Hz to 20,000Hz, and that the background-noise level of the human ear
is 0dB, while a typical environment might have 40dB of sound, we then
have that the auditory bandwidth is 19,070Hz, with a signal-to-noise
ratio of 4000. The channel capacity of each human ear is then
19070log_10(4000) = 69,000 bits/second.
Note a possibly general rule: Primary sensors, like photoreceptors,
have high refractory periods, meaning few bits per second, but there
are a lot of them. Summarizing neurons, like ganglions, have much
higher frequencies, so they transmit more bits/sec, but there are
fewer of them.
If the information preservation factor of .1 found in the
photoreceptor-ganglion passage is typical, one could calculate the
number of steps between retina and frontal lobe (along various paths)
and estimate info left after all those steps. Probably that
assumption would be unjustified.
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