The basis of visual information is light reflected from objects. These signals enter the eye, are turned around (upside down) and then fall on the back of the eye. This area is called the retina, and contains receptors that are sensitive to light. There are about 260 million photoreceptors on the retina. These photoreceptors contain photopigments that break apart when light falls on them. This creates an electrically charged current, and signal transmission between neurons can occur. In the center of the retina is the fovea, this is where the signals coming from the center of your visual field fall. On the fovea there are a lot of photoreceptors, so the center of your visual field is also where you can see the most details.

There are two types of photoreceptors, rods and cones:

• Rods are sensitive to subtle stimulation and thus become active when there is little light. Consequently, these receptors are mainly used at night.
• Cones are sensitive to strong stimulation, and thus mainly active during the day. This is because cones have photopigments that can be reproduced very quickly. As a result, they recover quickly from light, and can start decaying again. In addition, cones are also essential for seeing colors.

Behind the photoreceptors lie ganglion cells, which ensure that signals are projected to the brain. We distinguish two different types of ganglion cells:

• Magno cells or ‘M-cells’, these are large cells with large receptive fields

• Parvo cells or ‘P-cells’, these are smaller cells

All cells of the visual system have a receptive field. This is a concept that is difficult for some people to understand, because it is not part of the cell. We describe the receptive field of a cell as the place in the visual space where light must be to make a particular cell active. You can also represent that in another way, as a place on the retina. Then the receptive field is the part of the retina where light must fall in order to activate the cell further down the brain.

The axons of ganglion cells send signals to the brain. These axons pass through three structures before they reach the brain and can form synapses:

• Optic Nerves - a bundle of axons that leaves the eye, and enters the brain through cavities in the skull. A lesion in an optic nerve causes blindness in one eye.
• Optic Chiasm - the optic nerves of both eyes meet here, after which partial decussation occurs. Decussation is the crossing of a bundle of fibers from one side of the brain to the other. In the optic chiasm this happens only partially, because half of the fibers stay on the same side, and the other half cross over. All axons that contain signals from the left visual field go to the right hemisphere, and vice versa.
• Optic Tracts - the axons from the various optic nerves form an optic tract on both sides of the brain. A lesion here causes blindness to the entire left or right part of the visual field.

More than 90% of the neurons from the optic nerves project to the LGN in the thalamus. The other 10% of neurons go to sites responsible, for example, for visual attention (these are the superior colliculus and the pulvinar nucleus). From the LGN, a bundle of axons goes to the primary visual cortex (V1).

From the bottom to the top, the LGN has six different layers, with the bottom layer being designated by the number 1. The different layers receive input from different types of cells from the eye, as well as from the different eyes.

The right LGN processes information from the left visual field. The axons from the ipsilateral eye (same side, in this case right) project to layers 2, 3 and 5. The axons from the contralateral eye (opposite side, in this case left) project to layers 1, 4 and 6. The bottom two layers of the LGN are also called magnocellular LGN layers, because larger neurons can be found here. These neurons also receive input from the M-ganglion cells from the eye. Layers 3 through 6 receive input from the P-ganglion cells and are called the parvocellular LGN layers. Between the large six layers lies a small number of neurons. These neurons form the konio-cellular layers, and receive input from non-P-non-M cells from the eye.

The different types of cells in the LGN are also involved in different aspects of vision. For example, the connections from the magnocellular layers of the LGN are important for detecting movement in the visual field. In contrast, the parvocellular layers are essential for seeing and recognizing shapes. The koniocellular layers also have their own function, which is to detect colors.

As mentioned above, neurons from the LGN project primarily to the primary visual cortex. However, there are also a lot of areas outside of V1 that are involved in processing visual stimuli. These areas together are called the extrastriate cortex. There are two pathways through which visual information can be further processed after V1:

• Dorsal pathway: this pathway is important for the analysis of movements and the control of executive actions. This path begins with the magnocellular layers of the LGN, and from here goes to V1, V2, V3 and V5

• Ventral Pathway: this pathway is important for object recognition and receives most input from parvocellular and koniocellular layers of the LGN. From here the signals go to V1, V2, V3 and V4.

Author: Myrthe Princen (translated by Melanie Smekal)