This isn't scientific or even academic but I'd like to throw my two cents in on this problem from a broader conceptual level.
I think asking how does porn effect the brain is similar to asking how do action movies effect the brain? How does modern art effect the brain? How does junk food effect the brain? How does rock and roll music effect the brain? Not necessarily hopeless questions, but perhaps ill defined starts.
Porn is such an enormous genre (I've been told ). You might have something like fisting scatological bloodletting rape fantasy on one end (actually god knows what at one end!). And on the other we might put erotica which is at least an attempt to create real art via explicit sexual imagery.
In the erotica direction I've seen some rather genuine, even poignant sexual moments captured between two people. This leads me to have trouble seeing how explicit imagery itself is categorically good or bad to the brain.
So the first monkey wrench in a study is going to be trying to define the "vibrational quality" of porn we are doing studies with. Now a modern neuroscientist is not going to listen to the concept of "vibrational quality" (even though we all know that is a valid statement that makes perfect experiential sense). Anyway, setting that aside I think the vibrational qualities of a particular porn, like the vibrational qualities of any meme, boil down to the intention that went into creating it; the quality of consciousness possessed by the creator and all participants.
Second monkey wrench is the developmental lines of the viewer.
The general status of aesthetic development in our culture is uniquely bad in my opinion. There is an atypically massive gap between the completely naive and the highly refined, whereas the stratification between levels of, say, reading comprehension skills, is much more gradient. I think the lack of aesthetic development is confounded by an almost complete lack of study done on how to teach it (compared with say, mathematics) and by a near universal (and flawed) insistence that art appreciation is 100% subjective. Personally I believe it's a a faculty that can be developed like any skill. (See A Summary Of The Housen Aesthetic Response Theory for example).
Then aesthetics are just one line of development in the viewer. There is also moral development of the person. Have they found their own internal moral compass, or are they still subjugating to an internal parental voice of authority? Were they raised to think porn is sinful and internalized guilt and shame around viewing it? I believe one of those researches suggests that guilt/shame + sexual arousal is a recipe for addiction, no?
Does the person meditate? Have they ever moved sexual energy beyond their genitalia? On and on.
My point, tongue in cheek, is that it's likely that the average random person whose response to porn is being counted in a study is the same average random person that goes to church out of a guilty sense of duty every Sunday, thinks Adam Sandler movies are films, ejaculates every time they are aroused and has never meditated.
How would someone at Stage I, in Housen's stages of aesthetic development, view a hardcore porn clip vs. someone at Stage III or IV or V? How about someone who meditates four hours a day vs. someone who has never meditated? A fundamentalist Christian vs. a tantric Yogi? On and on.
Frankly, I have never really understood the compulsion for porn or the idea that it's addictive. Never purchased a porn in my life that I can recall. Working in web design, I do take on clients in the adult industry at times. So I have some firsthand experience with the people and they run the whole range as might be expected.
One thing I've been curious about for a long time, I have to admit --- what would postconventional, heart-centered, high art, enlightened porn look like? Is that even possible? What would a Stanley Kubrick do if he ended up in the porn genre? I don't think we'll know the answers for awhile. Our culture is uncomfortable with open discussion of sexuality and carte blanche ostracizes almost anything that meets this criteria, high vibration or not. So if you are an actress with talent or a gifted filmmaker and on the fence about participating in a film that could be considered erotica/porn and potentially ruin your career, it's likely you would decide against it. Unless you already felt socially ostracized, lacking in talent, victimized by the community, "I could never become a real director anyway", etc. This and the fact that the porn industry is so well funded -- well there you have a recipe for a vicious circle of attracting least common denominator participants.
Best,
Sean
