I'm not sure that's the best way to look at it.
Graduate studies require a solid foundation in the subject matter. Thus, if you want to enter a psychology graduate program you need a psycholgy undergraduate degree. That would be the general rule, to which there may be exceptions. In some cases one can cross over from related programs. For example, one may be able to enter a physics graduate program with an undergraduate degree in engineering or mathematics, provided one has an acceptable (as determined by the graduate admissions process) number of courses in physics.
Graduate programs in subjects like neuroscience are interdisciplinary and may take students from an array of undergraduate majors: physics, biology, computer science, psychology, mathematics, etc. - often depending on the particular focus of the program.
The take home message is this: don't assume that you can cross over from one subject to another. Do your homework and as soon as you may be interested in a given program for graduate study, figure out what the entrance requirements are for it.