What’s in a rat’s eye

The photos show various parts of the rat's eye. There are many similarities between the eyes of the rat and those of humans, but also some striking differences. A few of these are mentioned here.

The eyeball of the rat’s eye can be rotated to change the viewing direction without having to turn the head. What is special is that rats can move both eyes in opposite directions. This both horizontally and vertically. Researchers at the Max Planck Institute found this out in 2013 and were able to record this using high-speed cameras. They are able to keep an eye on the sky while also looking forward. They can interpret a double field of view. Probably this has evolved in the wild to deal with great threat to raptors.

Both eyes are on the side of the head. This gives a very wide field of view. The images from both eyes go to the brain separately. However, to have three-dimensional vision, it is required that the field of view be overlapped by both eyes. The eyes therefore change direction for only a fraction of a moment. But because the field of view is so wide, the rat has a different technique for seeing depth better. This by moving the head back and forth and changing its position. In this way they can better interpret depth.

Human lenses allow only visible light and almost no ultraviolet light to pass through. Rat lenses, on the other hand, allow all visible light plus almost 50% of ultraviolet A light to pass through.

The human lens is flexible: the ciliary muscles pull on the lens and thus change its shape. This change in shape causes the light passing through the lens to bend in different ways, which allows the lens to focus light on the retina. Rats appear unable to change their lenses' shape.

Humans have two types of photoreceptors: one type that senses light and dark, called rods, and one that senses colours, called cones. Humans have three types of cones: green, blue, and red. Rats have rods and cones as well, but only two types of cones: green and blue. Therefore, rats are unable to see reds. In addition, the rat's blue cones are sensitive to shorter wavelengths than our blue cones, which means that rats can see into the ultraviolet.

The rat retina has a very ‘coarse’ neural grain. Each neural cell in the rat retina is responsive to a much larger number of photoreceptors than those of the human retina, which increases sensitivity at the expense of acuity.

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