Abstract The slow voltage variation coincident with the spreading depression of activity, in the rabbit's cerebral cortex, was measured with electrodes inserted into the cortex and the underlying white substance, led against a distant, reference one. The voltage curves obtained were compared with that from an electrode applied to the pial surface, such as has been described previously. Inside the cortical gray substance a negative elevation was recorded, but, in any given region reached by the response, it appeared later than at the pial surface, and was preceded by an initial positive phase. As at the pial surface, the negative elevation was followed by a final, long-lasting, slowly subsiding positivity. The more deeply the electrode was inserted into the cortex, the later appeared the negative elevation, and also, the lower was its amplitude (fig. 1). Inside the subcortical white substance this negative phase did not occur — the curves presented only the initial and final positive elevations, separated by a recess of the voltage (fig. 2). The early stages of the voltage variation of spreading depression are remarkably similar to those of the variation induced by prolonged arterial occlusion (fig. 3). It is concluded, therefore, that they are referable to the same events. It is assumed that the voltages recorded during these early stages may correspond to an electric field of currents established, in the conducting medium, by differences of potential which appear between distinct parts of the surface of neurones.
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