Can I become a research psychologist by studying maths and computer science? I am currently studying maths and computer science in University of Barcelona, Spain, and I love research psychology. The question is: "is there a way to end up as a research psychologist following maths and CS studies? If not, what are the paths I could take without going far from maths?"
Thanks for your attention and apologies for my English!
Sure you can! There are some things you'll need to keep in mind, though, to make sure your current plans for undergraduate fit your longer-term desire to be involved with psychology research.

First, psychology research take many, many forms. There are many inter-disciplinary fields of psychology research that actually prefers at least some background in computing and strong quantitative skills, especially in areas like human factors (also called engineering psychology or ergonomics in Europe - in the US ergonomics usually means something a little different), Human-Computer Interaction, and some programs of "applied psychology" and industrial-organizational psychology. Cognitive psychology also has quite a lot of use of computing backgrounds, while some fields of psychology lean heavily upon advanced statistics, such as some experimental social psychology. On the other hand, some areas of research psychology - such as clinical/behavioral/counselling - typically do not make much use of computing or advanced math at all. If you wanted to go into clinical research I'd strongly question your plan to study math/computing instead of a more related field like psychology, sociology, etc.

The second issue is being perceived as needing to make a switch, and so having to try to convince people that you have sufficient background in psychology to be accepted into a psychology program. Most programs in the US I've seen request someone have the equivalent of about 5 classes in psychology if they don't have a degree in a psychology-related field. Will you be able to get any psychology classes with your current plant? If not, then wanting to go into any research field without actually studying that field first is a questionable plan, at best. It's a different matter if you want to switch after completing your studies - but if you are still studying, why not switch now?

The third issue, noted partially in the paragraph above, is your preparation to do original research and so be able to make the most of your time in a graduate program. If you are studying math, this may or not be actually helpful in research. Psychology research most commonly uses quantitative skills in the areas of data analysis and advanced statistics. While a math background can help prepare you to learn the stats, it may not actually teach you the stats at all, and it most likely has no course in research or experimental methods. Similarly, computer science only sometimes includes any courses in stats, and rarely stats you'll actually need to use (ANOVA, linear regressions of all varieties, non-parametric stats, factor analysis, etc.), and it very likely contains no course in research methods or experimental design. 

My personal path was actually to start studying in computing and I considered a minor in applied math, and I became interested in the intersections of computing and psychology after I discovered fields like human factors and HCI existed. I used the research-based focus of the psychology field to give me the stats and experimental design training I needed, and used the computing background to enable my particular research interests in applied technology. This combination has worked very nicely for me so far, but it is particularly challenging at times and requires you to be very self-driven and willing to teach yourself things that aren't the focus of any of your classes and may be outside the area of expertise of any of your professors.

The fields of computing, math, and psychology are very large and varied, and so while there may be an area that might be just right for you that allows you to combine all your interests, you'll need to work hard to 1) begin to locate just what that area might be, 2) ensure you are getting the preparation you need to thrive in that environment, and 3) be willing to get creative, because most established pathways, advertisements, and listed opportunities are often tailored for more "traditional" students wanting to go from Undergraduate Program in A to Graduate Program in A. 

Finally, you should be aware that inherently multi-disciplinary fields can be a bit..."different". The same basic area of research might be called a half-dozen different things, and the home of a department might vary by University. An example of this is human factors-related programs are sometimes hosted by a psychology department, sometimes by an engineering department, sometimes by a computer science department, and sometimes even by a business department, each with their own unique flavor and research focus - and you might be a great fit for one but a terrible fit for another. Meanwhile one program may be called one thing but seems to really do research in another. My first research project was considered primarily psychology research at my home institution, yet is being published in the top conference in the area of HCI; the topic of the project was actually the topic studied as "human factors" 10+ years ago, but is now considered primarily the domain of HCI. My next research project is in HCI, but a lot of related previous research was in the field of aerospace engineering, geospatial information systems, and remote sensing. If being in such weird situations terribly bothers you, you might want to consider a more "traditional path". I thrive on the complexity, but it really isn't for everyone!