But what I would like to focus is the pleasure system of the brain, which makes my experiment looks really similar to a psychology EE.
My experiment is about seeing whether positive or negative reinforcement have any impact in the emotion which in turns affect the brain.
How could I shift the focus onto biology only?
First and foremost, have you done your research on the pleasure system on the brain in terms of biology and physiology? Your EE will require a sophisticated understanding on how that works. The way to shift your focus from psychology to biology would be to focus on biological reactions that occur and physiological effects on the body. From my very limited understanding of neurobiology, that would mean you're talking about the nervous system, stimuli, neurones, neurotransmitters, activation of certain centres in the brain, release of dopamine, serotonin, stress chemicals, or the lack thereof etc. You can also talk about the effects on physical health and possible cause of other diseases. So your RQ may have to shift from "stimuli->effects on emotion -> effects on brain" to "stimuli->effects on brain -> effects on emotion and health" You can still talk about emotions but you have to discuss the biological and chemical pathways that produce them, if you understand what i mean. That gives it a stronger biological focus. The basis of your research will also be on neurobiology.
And what's more important, is whether it is doable, which you should ask your EE supervisor. From my knowledge, IB doesn't allow animal/human experimentation for EEs. It may be hard for you to obtain results/test for reactions in the brain at pre-college level laboratories. If that is the case, your EE will be mostly if not fully based on researching articles and you will not be doing much experimentation yourself. This HAS been done in EEs in the past (or least I've heard of), to both good and disastrous effects. It may not be what you want to do (not to mention, may be hard to do). So it is doable, but I wouldn't recommend it unless you're incredibly good at peer-reviewing. And maybe I wouldn't recommend it even if you were incredibly good. Find a way to obtain results. Check with your teachers on how you will do the experiments. You can ask your teachers to attach at a bigger facility at a university to use their resources if really need be but I'm not sure if that's allowed. If you have already have a way to experiment, then you're good to go. My advice would be to check with the Bio EE supervisors and ask for their advice on whether you can do it.
If you really can't and don't want to do your Bio EE based on articles, you may want to put in a special request for psychology with your school. I'm not familiar with how a psychology EE is done so I shall not comment on how to do it. You'll have to do some work on your part to figure that out as well having no supervisor. But I think the demands are definitely different from biology in terms of experimentation. Please check with your teachers.
Lastly, I assume you have a more specific RQ in mind, because there are many stimuli you can provide to the brain and many studies you can do. Make sure you are able to cover whatever you want within your EE. Try and make it innovative as well, because a part of the EE rubric also asks for how is your study significant, or different from studies done in the past, and there have been studies done on neuro before.
Nevertheless, your EE sounds like it'll be something fun to do. Theoretically, it can be researched and experimented on. Just make sure you know how you're going to plan and write your EE.
I wish you all the best.
Feel free to ask me any questions