Slicing the cortex to study mental illness: Alois Alzheimer's pictures of equivalence.

Alois Alzheimer (1864-1915) was a German physician who specialized in psychiatry, and who is today known for the first description of a-in his own words-peculiar ailment (eigenartige Erkrankung), which was named after him. In his time, however, he was foremost recognized for his work in refining histopathological techniques and thereby contributing to the methodological arsenal for differential diagnosis in clinical psychiatry. In his laboratory that was based at the renowned Munich Psychiatric University Clinic led by Emil Kraepelin (1856-1926), Alzheimer, his assistants, and students conserved, prepared, and studied slices of deceased patients' brains under the microscope. How could histological postmortem research better clinical diagnoses? Against what norm should the pathologies be compared? What was the normal brain in a context of highly invasive preparation techniques and the artifacts that they produced? In an unpublished lecture series, Alzheimer explicitly addressed these questions and framed them in terms of practical problems and possible solutions: where to get normal brains from; how animal studies could help to enlighten the normal brain and infectious mental disorders; how the study of hereditary idiocy might yield knowledge about normal brain development and general brain pathology. This chapter offers a close reading of parts of Alzheimer's lectures, his habilitation thesis, and his programmatic opening paper of a journal that he cofounded. These sources provide us with an introduction into the making of the normal and the pathological brain in histopathology that focuses more on problems and controversies than providing an undisputable, easy-to-use framework. I examine the premises of Alzheimer's conceptualization of "pictures of equivalence" (Aequivalentbilder) to elucidate how the epistemological gap between postmortem research and clinical psychiatry was managed in this particular context. The excavation of this historical epistemology not only fills a gap in the rich history of brain research that has mostly focused on brain localization theory, of which Alzheimer and Kraepelin were skeptical. It also provides a case study for how "the normal" and "the pathological" were put to work, and were, literally, pictured. In conjunction with the other chapters of this volume, this contribution thereby raises the historiographical and philosophical question of what to include into an assessment of the making and use of models of the brain.