Very fundamental questions about brain function can be investigated using evolutionarily primitive species. The investigation of complex cognitive functions, however, is only possible in species whose behavior is similar to that of humans: the non-human primates.Primates are more similar to humans than flies, mice, rats or cats. Over the course of evolution, similar structures and functional principles have developed in the brains of primates that are not shared by any other group of mammals. For example, both humans and non-human primates perceive the world primarily via their eyes. The important role of the sense of vision is reflected in the organization of the brain: Not only the visual system but the executive system as well, that system responsible for the control of behavior, is organized in a similar fashion in all primates.  

As a matter of principle, primates are only used in cognitive neuroscience if more distant species are unable to provide the answers to the questions being pursued.  

Very simple cognitive processes can be studied in experiments on the living rat brain. The limitations of this model become obvious as soon as we remember that rodents rely primarily on their nose and whiskers for orientation, so their senses of hearing and vision are much weaker than in primates. In order to understand higher cognitive functions of our brains, we depend on experimental animals that are more similar to humans. And this is where the non-human primates play a role, especially rhesus monkeys.  

On the other hand, with today’s methods direct invasive studies of brain activity on healthy humans are simply not justified. In rare cases invasive diagnostic or therapeutic procedures are necessary with which it is possible to gather data about neural information processing. Such studies are interesting, but we must never forget that these patients are gravely ill, for example with severe epilepsy. The results of such studies must always be treated with caution, because functions studied in the diseased brain may be affected by the disease, and the results cannot be validated by a comparison with the function of healthy brains.  

Apes have even more similarity to humans than rhesus monkeys, but scientists have agreed not to use these animals for research. Since 1991 no more animal experiments have been carried out with apes (Source: Animal Welfare Report of the German Federal Governmemt 2007).