I'm a computer scientist and would like to get involved in biology-related research such as optogenetics, stem cells or synthetic biology. What field could I most smoothly transition to and how?

First off, as cliche as it sounds, I want to tell you that nothing is impossible and you should follow where your heart is. I was trained in biology as an undergrad who only had experience in cell biology but was very interested in synthetic biology. I am now lucky enough to be going to one of the best PhD programs in bioengineering where optogenetics was first developed. I first would like to point out that your interests in "synthetic biology, stem cells research, optogenetics and everything neuro-cognitive related" contain very different classes of research topics. Bioengineering (including synthetic biology and optogenetics) follows a clearly distinct and opposite paradigm than does biology and other basic sciences (including neurosciences, stem cells research). Bioengineering is driven by real problems that need to be addressed with economic and effective measures; the research topics in bioengineering tend to be much more concerned with design, prediction, and optimization of a solution. The outcome of bioengineering is usually not some novel insights about how our world works, but rather more useful tools. Sometimes, these tools may make our discovery process more efficient. On the other hand, basic sciences are driven by phenomena; basic scientists are trained to measure and explain phenomena (whether physical, chemical or biological) with minimal assumption. The outcome of which is a working model of how a particular phenomena is related to other phenomena, which does not necessitate the production of more useful tools (there often can be, but not usually). There can be significant overlap between the two fields, as there was in the development of optogenetics, but you should be aware of the distinction in the ultimate goals the two fields aim to achieve. Since you mentioned that you are more interested in the production of knowledge, which informs me that you are more interested in making discoveries as a basic scientist. My following answer responds to that particular inquiry. Just to get this out of the way: Jobs in basic sciences are more scarce and don't pay as well as those in  engineering, there's no question about it. If you have a family, that's  obviously a big deciding factor for you and your loved ones. Many basic scientists go the academic route PhD -> Post-doc -?> Faculty. Others do end up working in big pharma, biotech, journal reviews after PhD. No one can (and should) tell you what research topics you can focus on. You need to figure it out yourself. Your next best course of action, assuming you have competency in the fundamentals of biology which you can learn about from textbooks, is to read recent peer-reviewed articles written by neuroscientists out there. You can search through faculty webpages in some of the good institutes. Find out specifically what research topics interest you; they need to be more specific than what you listed. For example, are you interested in the application of optogenetics in exploring voltage-gated channel activity of the pain sensory neurons in the brain etc. Find specific faculty members that you can see yourself working with. Then you should prepare a C.V. of your past academic / research work. Whether that was in biology or computer science, it doesn't matter. Emphasize your research skills (occupational skills don't count). What programming languages you know, what programs you have written etc. Write a compelling email to the faculty you are interested in that entails the reason why you want to learn about their research topics, what you can bring to his/her research, and ask for an opportunity to work in their lab. If it goes well, the faculty may offer you a chance to meet or interview with them; then you need to convince them that you know what you are talking about and what you would like to learn in their lab. You may be able to land yourself a research assistant position. Do it for a few years before you consider PhD in the basic sciences. From my personal experience, graduate candidates with computational skills are in high demands.  Particularly in biology, where the field is moving fast towards the end of descriptive phase within the foreseeable future. We are now  acquiring more biological data than we know how to interpret or analyze  them. Your computational skills may come very handy in helping yourself  and others to write programs that can efficiently organize and analyze  high-throughput data. It is very common for outsiders like computer  scientists, physicists, or mathematicians to move into biology; so do not feel like you are at a disadvantage. Be encouraged that many great biologists likely went through the same thinking process that you did. Learn what you can about each field and determine for yourself if this path is truly your calling.