In my humble opinion, I'd say you can discuss them VERY briefly in your SOP. Don't put a ton of attention on them at all. Maybe squeeze all of them into one paragraph, essentially outlining all of the other awesome cool research you've done in your spare time. But you wanna concentrate on REALLY highlighting your applicable research interests, and as such any other information added to the paper will detract from that.
If any of the aformentioned research was done with one of your letter writers, it may be best to have them mention it in your letter instead of in your SOP. It'll sound more like icing on the cake that way, rather than something you're trying to highlight yourself.