…… The portraiture that flourished in Vienna at the turn of the 20th century is a good place to start. Not only does this modernist school hold a prominent place in the history of art, it consists of just three major artists — Gustav Klimt, Oskar Kokoschka and Egon Schiele — which makes it easier to study in depth.
As a group, these artists sought to depict the unconscious, instinctual strivings of the people in their portraits, but each painter developed a distinctive way of using facial expressions and hand and body gestures to communicate those mental processes.
Their efforts to get at the truth beneath the appearance of an individual both paralleled and were influenced by similar efforts at the time in the fields of biology and psychoanalysis. Thus the portraits of the modernists in the period known as “Vienna 1900” offer a great example of how artistic, psychological and scientific insights can enrich one another.
The idea that truth lies beneath the surface derives from Carl von Rokitansky, a gifted pathologist who was dean of the Vienna School of Medicine in the middle of the 19th century. Baron von Rokitansky compared what his clinician colleague Josef Skoda heard and saw at the bedsides of his patients with autopsy findings after their deaths. This systematic correlation of clinical and pathological findings taught them that only by going deep below the skin could they understand the nature of illness. 

I’ve read many a book and chatted with art therapists about the psychological process involved in art and art making and this article comes from a different perspective; brain science.