It depends what you want aspect of the interface you want to work on. Typically labs that work on brain-machine interfaces have people with a variety of backgrounds. AKA PhD students and postdocs from computational neuroscience, systems neuroscience, electrical engineering, mechanical engineering, physics, computer science, statistics, etc.
With highly interdisciplinary areas like this (or most everything in neuroscience) the actual field listed on your degrees is of little importance. What matters most is being motivated and competent to work in the area.