Charles Darwin's emotional expression "experiment" and his contribution to modern neuropharmacology

In the late 1860s and early 1870s, Darwin had corresponded with the French physician and physiologist, G. B. A. Duchenne, regarding Duchenne's experimental manipulation of human facial expression of emotion, by applying Galvanic electrical stimulation directly to facial muscles. Duchenne had produced a set of over 60 photographic plates to illustrate his view that there are different muscles in the human face that are separately responsible for each individual emotion. Darwin studied this material very carefully and he received permission from Duchenne in 1871 to reproduce several of these images in The Expression of the Emotions in Man and Animals (1872). Darwin had doubted Duchenne's view that there were individual muscle groups that mediate the expression of dozens of separable emotions, and he wondered whether there might instead be a fewer set of core emotions that are expressed with great stability worldwide and across cultures. Prompted by his doubts regarding the veracity of Duchenne's model, Darwin conducted what may have been the first-ever single-blind study of the recognition of human facial expression of emotion. This single experiment was a little-known forerunner for an entire modern field of study with contemporary clinical relevance. Moreover, his specific question about cross-cultural recognition of the cardinal emotions in faces is a topic that is being actively studied (in the twenty-first century) with the hope of developing novel biomarkers to aid the discovery of new therapies for the treatment of schizophrenia, autism, and other neuropsychiatric diseases.