The difference is not so much in the brain as it is in the eye. The cells in the retina (inner cladding of the eyeball) are sensitive to a certain range of wavelengths of the electromagnetic spectrum. This wavelength is a property of the light which is related to the color of the light, and to whether the light is visible, infrared or ultraviolet. The process of light detection occurs as follows: when light arrives to the eye, it's absorbed by some molecules that are present in these cells in your retina. These molecules then undergo some changes, and the result is an excitation of the optical nerves, that connect the eye to a part of the brain which is on the back of your head, where it is processed. The portion of the electromagnetic spectrum that we can see depends not on what the brain can process, but to which wavelengths (colors) of light the cells in your retina are sensitive to, and this in turn depends on which light-absorbing molecules are present in these cells.

Two more interesting pieces of information about vision are the following:

+Not all animals can see "in color". In fact, in the retina or the human eye there are two types of cells; one detects the intensity of light, allowing us to see "in black and white", and the other one is responsible for making us distinguish between different colors. The animal species that don't have the second type of cells are therefore color-blind. As an interesting anecdote, bulls are color-blind, so the fact that, in a bull-fight, the bull is attracted by the red cape, or in general, that bulls are attracted by red-colored objects, is not true. What they are attracted by is movement, and it's the movement that the bullfighter gives to the cape what makes the bull go for it, not its color.
+The property of vision that does depend on the brain process is, however, the threedimensionality. Most animals see only in two dimensions, but humans see in 3-D. This is possible because of the slightly different angle with which both your eyes see objects; the brain then processes these differences allowing us to perceive sensations such as depth, distance, volume, and so on. This property is used on the 3-D books, in which apparently meaningless spots on a page take volume and "grow" in front of your eyes to give you a full sensation of three dimensions. The spots are distributed around the page in such a way that, when looked at from the right distance, the brain produces this sensation of volume and depth.

Hi inquirers.Your questions show that you know some important things about the system. For one thing, you know that in order for us to "see" something, our eyes have to pick up the information and send it to our brain. Then the brain itself has to make sense of the message. In this case, the reason we can only see part of the spectrum is because we don't have all possible sensors in our eyes.

We don't "see" infrared, but we feel it as heat. Some snakes, like pythons, have special organs to sense heat. (Why do you think they have them? Does the type of prey they eat matter in whether they can use them?)

We also do not have ultraviolet receptors. Bees have them, so flowers that use bees as pollinators often have markings that bees can see and we can't. (Why should flowers "advertise" to bees?)

So why don't we have all of the possible sensors? For one thing, there are many tradeoffs in building something if your resources are limited. If you go to your favorite restaurant and only have a little money, you have to order only the most important food and skip the less important things. This is an example of making a tradeoff. Night vision (which requires receptors called rods) is important to cats, so they give up color vision (which uses receptors called cones). Having no color receptors allows them to have more night vision receptors.

Animals that had every possible sensor would be very expensive for their parents to produce. Since energy and nutrients are almost always in short supply, they might not be able to make any offspring at all. They certainly couldn't make as many as a parent that only gave each offspring the essentials. Over time, then, the offspring with all the extras would disappear, and the ones with the essentials would be more common. Of course, parents don't really "choose". The map for their offspring is encoded in their genes.

Why do we have the receptors we do have instead of having great night vision, visual UV receptors, and infrared receptors?

Basically vision (or more generally stated light perception) in any organism is accomplished via one or more compounds that have evolved to detect light. The visual compound in human eyes is called opsin (sometimes also called rhodopsin for rods). These compounds, also generally called pigments work such that when light strikes opsin it causes a physical change in the shape of the compound which works to activate opsin. Activated opsin causes a whole sequence of events to occur known as second messenger events. The eventual result is that there is a change in the flow of ions across the photoreceptor cell membranes and this signals the cell that light has been perceived. Opsins in humans are specifically designed to detect light of specific wavelenghts. Rhodopsin (the opsin responsible for dim light vision) has a maximum sensitivity at 510nm which is blue-green light. Humans also have cone vision or color vision. We have 3 different opsins to see red, blue and green light. The "blue" opsi n
is very specifically designed to have a max sensitivity to light of 455nm, the "green" opsin is very specifically designed to have a max sensitivity to light of 530nm, and the "red" opsin is very specifically designed to have a max sensitivity to light of 625nm. The max sensitivity means that only light of that wavelength or close to it has the energy necessary to cause that opsin to change its physical structure and thus induce the cell that houses the opsin to "detect the light". So it's all in the compound that initially absorbs the light energy. It doesn't actually have anything to do with differences in the brains of different organisms. Some deepsea fish can see far red/infrared light. This is because they have a compound like our opsins that physically change their structure when light of that long wavelength strikes it.[There is a good website about this see: http://lifesci.ucsb.edu/~biolum/organism/dragon.html] The difference does not lie in their visual processing centers in their brains. There are
certain shrimp which are sensitive to UV radiation, and again it is due to the presence of a certain compound in the shrimps eyes (specifically in the retina) that allows them to be sensitive to this part of the electromagnetic spectrum. If a scientist wants to find out what part of the electromagnetic spectrum that a particular organisms is sensitive to, they would take the retina from that organisms eye and run a pigment analysis. Pigment analysis is done by shining light of different wavelengths onto the retina sample and looking for wavelengths that are absorbed by the retina versus wavelengths that pass through without being absorbed. The wavelengths that are absorbed will tell the scientist which wavelengths the organism sees. What wavelengths do plants "see"? What compounds do they use to do this?