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the eye and the brain: biological image acquisition and processing 181
Ref. 135
will indeed uncover several of them. But let us now turn to see what the eyecan do.
The human eye
The human eye is a so-calledcameraeye.Likeaphotographiccamera,andincontrastto
insecteyesandothercompoundeyes,the vertebrate camera eye works by producing an
image of the outer world on a surface consisting of light sensors, the retina. The retina
covers more than half of the inside of the eye ball, whose typical diameter in an adult is
about16.7 mm. The pupil has a diameter between2 mm – below which one gets problems
with diffraction – and7 mm – for which lens aberrations are just acceptable. The
imageontheretinahaslowimagedistortion,lowchromaticaberrations(about1dioptre
betweenredandblue)andlowcoma;theeyeachieves this performance by using andeformableasphericgradient-indexlensandacorneawhoseshapeisalwaysneartheideal
shapewithin30µm–anextremelygood value for a deformable body. The eye, together
with the brain, also has a powerful autofocus – still not fully understood – and an excellent
motion compensation and image stabilization system built in. A section of this
amazingdevice is shown in Figure 133.
The retina is an outgrowth of the brain. It contains 120 million rods, or black and
white pixels, and 6 million cones, or colour pixels. Each pixel can detect around 300 to
500 intensity levels (9 bit). The eye works over an intensity range of 8 to 10 orders of
magnitude; the involved mechanism is incredibly complex, takes place already inside the
receptors, involves calcium ions, and is fully known only since a few years.The region of
highest resolution, the fovea, has an angular size of about1°. The resolution of the eye
is about1 . The integration time of the retina is about100 ms – despite this, nothing is
seen during the saccades.The retina itself is200 µm thick and is transparent: this means
that all cables leading to the receptors are transparent as well.
The retina has very low energy consumption and uses a different type of neurons that
usual nerves: instead of using spikes, the neurons in it use electrotonic potentials, not
the action potentials or spikes used in most other nerves, which would generate interferences
that would make seeing impossible. In the fovea, every pixel has a connection
to the brain. At the borders of the retina, around 10 000 pixels are combined to onesignal
channel. (If all pixels wereconnected1to 1 to thebrain, thebrain would need to be
as large as a typical classroom.)As a result, the signals of the fovea, whose area is only
about 0.3 % of the retina, use about 50 % of the processing in the brain’s cortex. To avoid
chromatic aberrations, the fovea has no blue receptors.The retina is also a graphic preprocessor:
it contains three neuronal layers that end up as 1.3 million channels to the
cortex,wheretheyfeed5million axonsthatin turnconnectto500million neurons.
Thecompressionmethodsbetweenthe125millionpixelintheretinaandthe1.3million
channels to the cortex is still subject of research. It is known that the signals do
nottransportpixeldata,butdatastreamsprocessedinaboutadozendifferentways.The
streamsdonotcarrybrightness values, but only contrasts, and they do not transmit RGB
values, but colour differences.The streams carry motion signals in a compressed way and
the spatial frequency data is simplified. Explorations have shown how the ganglions in
the retina provide a navigational horizon, how they detect objects moving against the
background of the visual field, and how they subtract the motion of the head.The comingyearsanddecadeswillprovide
many additional results; several data channels between
Motion Mountain – The Adventure of Physics copyright © Christoph Schiller June 1990–November 2015 free pdf file available at www.motionmountain.net