[Origin of the slow negative potential of direct cortical responses in the cat]

Neuronal responses and changes in an electrocorticographic slow negative potential (SNP) over cortical depth were studied in anesthetized cats in order to elucidate the mechanisms of formation of a slow negative potential (SNP) elicited by direct cortical stimulation. Neuronal membrane hyperpolarization accompanied by cessation of spike generation was observed during SNP. At the same time neuronal excitability and membrane resistance decreased. Potential distribution over cortical depth allowed no decision about the origin of SNP while the vertical current gradient during SNP revealed that the current was directed both upward and downward from a maximum source, as might occur with IPSP's generated in the somata of cells located there. The results indicate that SNP is a field effect produced by IPSP.