tube Tetrode), though more electrodes may be used. Recording electrodes can be metal wires or fine print on a PCB with gold or platinum plated at their exposed tips to ensure good contact and prevent change in resistance while the experiment is going on. Principal Component weights of spikes from two different neurons Spike shapes colored according to their assignment to different neurons. The blue trace could not be assigned unequivocally. See also Spike sorting article in Scholarpedia Electrophysiology Neurophysiology Articles containing video clips