A central concern in LFP analysis is that with use of distant, extracranial reference electrodes, there is uncertainty as to the precise neural generator of the LFP, which is in part why the first and second spatial derivatives of the LFP were explored as additional measures (Mitzdorf, 1985). The second derivative of the LFP, known as current source density (CSD) also estimates the net local pattern of neuronal transmembrane current flows that generate an LFP distribution in the extracellular medium (Nicholson, 1973;Nicholson and Freeman, 1975), and is a centerpiece of our analysis. We directly compared the vertical and lateral spread of the LFP recorded with a distant reference, with that of the derived CSD signal and that of the concomitant multiunit activity (MUA) signal. LFP and MUA signals were sampled with linear array multielectrodes (100 or 200 μm spacing) placed in and near A1 in awake monkeys.
