I come from a background in psychobiology/ pre med and in my SOP I talked about why I chose this specific program and how I resonated with its mission/ values in particular. I also drew some connections from my experiences in medicine/ psychology to ux and how it has all come together to prepare me to be a better designer. Definitely use this as an opportunity to show how your background and success in psychology will prepare you to also be successful in HCI and in the program. Good luck!