I am not sure if you received an answer to your question or if you have already sent off your application to your preferred schools or not, but I have some advice that may be helpful.
I am applying to Clinical Psyc Master's programs, and I was curious about the same thing. Of course, PhD programs are more rigorous than Master's so you may want to look into more information on the program's site.
Before applying, I was not sure what I wanted my research to be on so I came to a conclusion that I enjoyed cognitive-behavioral therapy psychological approach. I included this in my SOP. Then with more research, I looked into more interest that were covered while using CBT. I became fascinated when I discovered that so many topics were being covered in CBT. Now, I do not think that PhD programs require you to know exactly what your dissertation will be on just like Master's programs don't require me to know exactly what I want my thesis to cover. I believe that the advice from your mentor/advisor will help you with this when you get into the program.
So maybe if you include your psychological approaches that interests you as well as you research that you may have done while in your undergrad, you should be fine on this section of your SOP.
Hope this helped!!! Good luck!