From Neural Wiki
In order to study how an intact brain functions, it is preferable that one does not disrupt the natural flow of the brain with electrodes.
Inserting multiple electrodes into the brain is like planting telephone poles all over your house, they are going to get in the way -paraphrased from G.W. Gross
- However, it should be noted that current non-invasive techniques are not detailed enough to see the activity of an individual neuron, but are extremely useful for studying which areas of the brain respond to certain stimuli.
 Methods of Non-invasive neuronal monitoring
Magnetic resonance imaging MRI
Functional magnetic resonance imaging FMRI
positron emission tomography PET