Good idea to write the direct boss. You might also want to copy it to the boss's boss who gave you the feedback. Make a point of thanking the b's b for the feedback -- even though it was hard to hear. When you thank them, it creates the possibility of more valuable openness. If you react badly to feedback, people might stop telling you stuff. Ask her if she has any specific suggestions of how you might seem more engaged around those specific people who complained. The complaints don't mean you are that way. It just means that those people were uncomfortable. The boss or boss's boss might have hired those people and know specific things about what they need or want. Good luck!
follow up time! I talked to my boss and she said she totally disagreed with her boss's comments. she had to be as political as possible about it because she still works there, but she thinks people were unfairly comparing me to the favorite intern (who is admittedly amazing but the position was directly related to his skillset, interests, and course of study) and that she isn't worried so much about the people who purportedly made these complaints because they didn't see that I produced twice as much work as was asked of me originally. I also think there's an element of east/west coast culture clash. My primarily serious jobs have been in DC where people are more to the point and less worried about feelings and charm than in CA.
the take away is that I will try and be more inquisitive in my future endeavors so as to project that sense of engagedness that seemed to be lacking. it will be tough because I like to ruminate on things for a while before asking questions which usually doesn't fly as well in a meeting scenario. i also found this today which I wanted to share because hellooooo relevant to my life http://www.fastcompany.com/3034895/strong-female-lead/the-one-word-men-never-see-in-their-performance-reviews
Interesting article. If a direct boss "totally disagreed" with her boss, you may face more contradictions in the future. Try and be warm, competent, and consistent towards everyone and let your record of achievements be your witness. It is better to ruminate, if that is your tendency, than rush into statements. But questions can always give you more information and asking them might show that you're engaged - good luck!