ABSTRACT
An understanding of how the somatosensory system processes and uses sensory information depends in large part on knowing how the system is organized. One might think that our understanding of the system would have been complete sometime ago, but actually our knowledge is far from complete, and considerable research is yet needed. There are several reasons for our incomplete understanding. Most importantly, evolution has created considerable variability in the organization of the mammalian somatosensory system. Part of the variability stems from adaptations that lead to the use of different parts of the body to gather important sensory information. Many mammals use the whiskers on the face, nose, and mouth for exploring the environment, but this is a bit risky for primates, which have emphasized binocular vision with forward directed eyes and a short snout. Explorations with the face risk damage to the eyes, and reaching and feeling with the hand became a suitable substitute. Some New World monkeys also use their highly sensitive prehensile tail. But primates have adapted in other ways as well, and have evolved into one of the most varied orders in terms of brain size and organization. Prosimian primates occupy a range of environmental niches, but have only moderate brain expansion. New World monkeys vary from small marmosets which have brains that are yet large relative to body size, to spider and capuchin monkeys with large brains that at least superficially resemble those of Old World macaque monkeys. Old World 2monkeys are less varied, and they have impressively large, complex brains. Chimpanzees have even larger brains, similar in size to those of our early hominoid ancestors, while our greatly expanded brains have allowed us to populate and dominate the planet. While basic aspects of somatosensory processing are likely to be similar, it seems unlikely that these greatly varied primate brains would all process somatosensory information in exactly the same way. Thus, we should try to determine how somatosensory systems of primates differ and what the common features are. In particular, do primates with larger, more complex brains have more extensive and more complex processing? Also, since the extant somatosensory systems evolved through gradual modifications of an ancestral system, what features have been commonly retained as parts of the basic framework? Here we begin to address these questions. We start by considering the basic features of the mammalian somatosensory system. To do this, we need to determine what features are commonly found in the somatosensory systems of non-primate mammals.
