Biophysics and variations of extracellular spike waveforms. A, The biophysical phases of the extracellular waveform (blue) are approximately associated with its intracellular waveform (red) and closely associated with the inverted derivative of the intracellular waveform (dotted-red), at least for the negative phase. The phases of the intracellular waveform are described as (1) resting membrane potential, (2) depolarization, (3) repolarization, and (4) hyperpolarization. These illustrations are inspired from Henze et al. (2000). B, Schematic illustration of three example neurons: (i) cortical pyramidal, (ii) inhibitory, and (iii) thalamic pyramidal. The schematic illustrates varying waveform shapes from different spatial locations on a multielectrode array recording (gray). Note that the spike shape varies between neuron types and timing depends on the distance from the cell body. C, Several examples of extracellular waveforms that have not been well documented.