In this diagram, a conductance change occurs at point A causing inward current flow Im.
By observing the direction of current flow and noting that potential drop across a resistor is in this direction of current flow, you can determine the polarity of extracellular field potentials.
Questions
To see if you have mastered the concepts presented, try to answer the following questions:
1) In the diagram above, what does the uppermost point represent? Does it represent ground or your recording electrode?
2) Assume that you are recording extracellularly in neocortex. If your electrode is in the cell body layer, would you measure a source or a sink during the arrival of an antidromically activated action potential? Would it be active or passive? What would be the potential with respect to a distant ground electrode be when the action potential arrives?
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