The human eye is a so-called camera eye. Like a photographic camera, and in contrast to insect eyes and other compound eyes, 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 about 16.7mm. The pupil has a diameter between 2mm – below which one gets problems with diffraction – and 7mm – for which lens aberrations are just acceptable. The image on the retina has low image distortion, low chromatic aberrations (about 1 dioptre between red and blue) and low coma; the eye achieves this performance by using an deformable aspheric gradient-index lens and a cornea whose shape is always near the ideal shape within 30 μm – an extremely good 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 amazing device is shown in Figure.
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 about 1°. The resolution of the eye is about 1'.The integration time of the retina is about 100ms – despite this, nothing is seen during the saccades.The retina itself is 200μm 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 one signal channel. (If all pixels were connected 1 to 1 to the brain, the brain 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, where they feed 5 million axons that in turn connect to 500 million neurons.
The compression methods between the 125 million pixel in the retina and the 1.3 million channels to the cortex is still subject of research. It is known that the signals do not transport pixel data, but data streams processed in about a dozen different ways. The streams do not carry brightness 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 coming years and decades will provide many additional results; several data channels between the eye and the brain are still unknown.
Apart from rods and cones, human eyes also contain a third type of receptor.This receptor type, the photosensitive ganglion cell or intrinsically photosensitive retinal ganglion cell, has only been discovered in the early 1990s, sparking Ref. 136 a whole new research field.
Photosensitive ganglion cells are sensitive mainly to blue light, use melanopsin as photopigment and are extremely slow.They are connected to the suprachiasmatic nucleus in the brain, a small structure of the size of a grain of rice that controls our circadian hormone cycle. For this reason you should walk a lot outside, where a lot of blue light is available, in order to reset the body’s clock and get rid of jet-lag. Photosensitive ganglion cells also produce the signals that control the diameter of the pupil.
