Evaluating evidence for the cortical localization for language: Systematic reviews in the 1860s and 1870s

The specialization of cortical function for language was proposed by Paul Broca (1824-1880) in 1861 and further elaborated to include the principle of hemispheric lateralization in 1865. Broca and other French colleagues argued for and against these hypotheses, employing clinical and pathological observations of individuals with acquired language disorders as evidence. These ideas became a topic of widespread interest after the debates at the Paris Academy of Medicine in 1865 were reported internationally. During this period until the end of the decade, hundreds of publications appeared on the localization and laterality of findings in aphasic individuals and case series. Several large-scale systematic reviews of historic (pre-1861) and contemporary (post-1861) clinical findings were published only a few years after the syndrome had been proposed. These aimed to determine the strength and quality of evidence regarding the specialization and lateralization of brain areas for language. However, their authors held distinct theoretical assumptions and ideological concerns and were motivated by varied research questions. These comprehensive efforts using systematic review methodology to assess the evidence for and against hypotheses about the organization of language in the brain are examined to expose the issues of live debate in early neuroscience.