Dynamic interactions of evoked potentials in the association cortex of the cat

Interactions of acoustic (Ac) and somatosensory (SS) evoked potentials were studied in the suprasylvian association cortex of cats anaesthetized with chloralose and pentobarbital. Ac and SS stimuli were synchronously applied so as to their phases of identical polarity overlapped each other completely. The amplitude of the summated potential was compared with the algebraic sum of the separately recorded Ac and SS potentials. Three kinds of phenomena were observed in these conditions: (a) algebraic summation; (b) occlusion and (c) facilitatory interaction of the two sorts of evoked potentials. Under chloralose anaesthesia only algebraic summation occurred, while use of barbiturate favoured the appearance of occlusion and facilitatory interactions. Synchronous activation of the Ac and SS inputs with 2 cps frequency for 2 min enhanced the occlusion for several minutes. Samples taken during synchronized cortical activity also showed a marked tendency for occlusive interactions. Direct stimulation of the visual cortex or optic flashes at a frequency of 10--20/sec and stimulation of the thalamic association nuclei enhanced the occlusion of the Ac and SS evoked potentials as well. The observed phenomena are explained with heterosynaptic facilitation taking place on the cortical pyramidal cells and interneurons.