All neurologists and neurosurgeons are also neurophysiologists; it's part of their training and they use the principles every day.
Usually, the person doing neurophysiology research is a fellow in neurology. A fellow is a physician who has completed his/her residency in neurology, but completes research to specialize further in the field. There are also neurophysiologists who are Ph.D. trained and have master's and Ph.D. candidates as lab assistants.
Of course, there are grunts with a BS degree in neuroscience who do a lot of the day-to-day basic research under the guidance of another researcher with a higher academic degree.