SoP for applying CS PhD program with research experience that is not my interest
In my own SoPs I wrote about my past experience and how it informed what I wanted to do in the future. For me in particular, I had done some computational research and some bench work unrelated to neuroscience, but I wanted to do some more wet biology in neuroscience. I wrote about how those relatively unrelated pieces of experience would apply to what I wanted to do next. I think it makes a lot more sense in a statement of purpose to talk about what you want to to study and learn rather than only what you've already done. If I said "I've been growing walnuts my whole life and I want to do this PhD to grow more walnuts" - that might be great in a job application to be a walnut-farmer, but if I read that in someone else's SoP I'd wonder why they were after a PhD and if they had any idea what a PhD actually was.

Presumably in your work with machine learning in computational neuroscience, you've learned about some of the benefits and limits of machine learning in that domain. You now know a bit what it's like to work with those tools, and you'd like to apply them to a new problem. Presumably, in a CS PhD, you'd be expected to do more work developing the tools rather than just applying them. That seems like a pretty straightforward SoP to me.