It's definitely more computing than biology, but it does depend slightly on the type of signal acquisition device. You have to know more about the underlying biological processes of the brain if you are recording from an implanted microelectrode array rather an an EEG cap.
Reading doctoral theses on this subject is also a great way to learn more. There are at least a few online that do a great job of explaining BCIs from the ground up.
Here is a decent resource, but specific to one BCI research platform: http://www.amazon.com/Practical-Guide-Brain-Computer-Interfa...
Reading doctoral theses on this subject is also a great way to learn more. There are at least a few online that do a great job of explaining BCIs from the ground up.