Although by 370 b.c. Hippocrates had concluded that the brain played a role in the generation of behavior (Carlson, 2003), it was not until seminal studies performed in the 1860s that specific human cognitive abilities began to be mapped onto regions of the brain with some accuracy. Postmortem studies by Paul Broca and Karl Wernicke allowed individuals' cognitive deficits to be linked tentatively to lesions discovered in specific brain regions. It was not until almost a century later that the advent of noninvasive neuroimaging techniques allowed the functional neuroanatomy of the brain of a living individual to be visualized for the first time. The striking contemporary images produced by functional magnetic resonance imaging (fMRI) have allowed researchers to examine in detail the neural correlates of a wide range of cognitive processes such as visual perception, speech perception, and short-term memory.

Contemporary neuroimaging studies of speech production are typically designed to isolate regions of the brain involved in one of the “core processes” (Indefrey & Levelt, 2000) of speech, such as lexical retrieval (selecting an appropriate word) or phonological code retrieval (retrieving information about the sound of the word). Such studies have generated extensive documentation of the neural correlates of overt and inner speech, as well as the neural activation that differentiates them. The latter is of particular relevance for private speech researchers because it may help shed light on what happens when private speech “goes underground” (Vygotsky, 1934/1986, p. 33) to form inner speech.