One reason the brain is difficult to study is that many individual neurons or brain areas are active at once; conventional techniques allow one to monitor only one or a few neurons or locations at a time. Lawrence B. Cohen primary research involves the development of fluorescent protein sensors of membrane potential. At present the voltage signals are just large enough to be useful in monitoring activity in invertebrate and mammalian nervous systems.
A second area is understanding the role of the mammalian olfactory bulb in olfactory processing by comparing the input (from the nose) and the output (carried by mitral/tufted cells) to determine the olfactory transformation carried out by the bulb.

Image courtesy of interviewee