We have learned a great deal about the neural structures involved in representing and processing language since Broca's first scientific paper on the subject in 1861. In Part IV we reviewed some of the advances in our knowledge of this subject, and began to explore some of the newer techniques for investigating the neural basis of linguistic knowledge. We may divide the neurolinguistic topics that we have discussed into three groups: the study of general areas of the brain in which language processing takes place; the development of theories of the functional neuroanatomy within a specified area of the brain; and the exploration of the cellular and the neurophysiological basis for language functions. We know progressively less about each of these three topics. This is not surprising. Neuroscience has always progressed “from the outside in”, beginning by identifying the general area of the brain where a function is carried out, then developing a model of the internal workings of that area in general terms, and finally dealing with the cellular and sub-cellular events actually responsible for the function's operation. Neurolinguistics is making slow but steady progress along these lines.

In Chapters 18 and 19 we reviewed studies from the time of World War II onwards which gave us a much more detailed and clearer understanding of the general areas of the brain that are involved in language functions. These studies show that, although Broca was right that the left hemisphere is specialized and dominant for language in the vast majority of right-handed individuals, the situation is much more complicated in left-handers, and in right-handed individuals who have left-handed family members or who are not strongly right-handed.