Cognitive sciences are an interdisciplinary field between:
- medical sciences
- palaeontology / anthropology
- information sciences
- I believe I forgot something
The core is probably somewhere around psychology, neurobiology (including endocrinology) and artificial intelligence, though others would lay the focus elsewhere.
It does not matter much which of these fields you study, if you aim at an academic career studying cognition, because you will have to make yourself familiar with, or even an expert, in most of the other fields, if you want to go beyond a master's degree. Where you put your focus, is a matter of personal preference, and no matter where that is, you will be working in a team of experts from different fields. I'm studying psychology, and at our institute (of psychology) we have researchers who studied medicine, physics, information science, and pedagogy. In the neuroscience institute next door, there are psychologists, biologists, medical scientists (including psychiatrists) and probably more that I don't know. Despite their different backgrounds, they now all work on the same research topics.
So while it does not matter (much) which first topic you choose, you need to be open and curious of mind, to be successful in science. If, as you say, you are interested only in a very narrow field, interdisciplinary work will be impossible for you. You won't be able to follow your topic to where it leads you. Sometimes, when you research something, you realize that the answer to a relevant question, that you need to answer before you can go on, lies in another field. Then you must study that field. If you don't, that will hamper your understanding and research.
The best scientists, in my opinion, are those that are basically curious and want to understand and learn. They all have their individual talents, and certainly no one can know and understand everything, but the most successful researchers today are experts in several fields and interested in several others.
On the other hand, there are those hard working lab rats that get things done, and they are invaluable, too. The question is, can you see yourself as doing the grunt work, or do you want to really understand it all?
But then maybe that is all something you cannot know in advance. You need to experience things to know if they are for you or not. When I was fresh from school, I had very limited interests and a wide array of clearly defined dislikes (everything I had to learn in school). But in the years that followed, I understood that I am basically interested in everything, even the subjects that I hated in school. And what I found the most interesting at first, I lost interest in after studying it for a few years. Now I study something else.
What I would recommend is:
- read the internet sites of the university institutes and some introductory textbooks in the three or four fields that you are most interested in
- choose the one whose methodology you like best: do you want to dissect brains, do statistical data analysis, program software, work on people or animals, theorize or spend nights in the lab, ... ?
- study that
- if after a year or two you realize you want something else, switch (but understand that every field includes stuff you find boring)
- finish your master's
- reflect on what you learned and understand what interests you for the next few years
- find a research group and do your Ph.D.
- again reflect and decide on the next steps
- don't worry, there are many successful non-linear careers in science