What type of operation does Charlie undergo?

Charlie undergoes neurosurgery.

Strauss dealt largely with the theory and techniques of neurosurgery, describing in detail how pioneer studies on the mapping of hormone control centers enabled him to isolate and stimulate these centers while at the same time removing the hormone-inhibitor producing portion of the cortex. He explained the enzyme-block theory and went on to describe my physical condition before and after surgery. Photographs (I didn't know they had been taken) were passed around and commented on, and I could see by the nods and smiles that most people there agreed with him that the "dull, vacuous facial expression" had been transformed into an "alert, intelligent appearance." He also discussed in detail the pertinent aspects of our therapy sessions-especially my changing attitudes toward free association the couch.