It depends on what you mean by "measuring brain activity". fMRI has a spatial resolution on the order of the summed activity of tens of thousands of neurons an

It depends on what you mean by "measuring brain activity". fMRI has a spatial resolution on the order of the summed activity of tens of thousands of neurons and a temporal resolution of 2-4 seconds, so it's often very imprecise if you want to know what one neuron in particular is up to. I've seen single-unit recordings done with electrodes and simultaneous Ca2+ imaging with a 2-photon microscope. You trade real-time speed and single unit specificity for a loss of knowledge about the population. MEG can give you real-time summed behavior, but the inverse problem is indeterminate so you're less sure of _where_ that activity is coming from. "what's the good tool to measure brain activity" is very often a function of "what do i want to find out about the brain?"