The contribution of background electrical activity to the form of averaged evoked potentials in chloralose anesthetized cats

An examination of potentials recorded from the hypothalamus of the chloralose-anesthetized cat following cutaneous or visual stimuli revealed the existence of complex interactions between evoked potentials and background electrical activity. These interactions led to alterations in the form of evoked potentials or to their nonappearance. The evoked potentials could be stabilized to a large extent by timing stimuli in relation to background activity so that stimuli were delivered in periods of little or no activity. Further, it was found that the effects of one evoked potential on subsequent evoked potentials lasted for periods of up to 6 sec. When averages of evoked potentials were computed it was found that the form of the average was markedly affected by background activity, which exaggerated stable wave-form components and distorted others. Averages computed from stimuli delivered in periods of little or no background activity showed clear differences from averages computed from stimuli delivered at the same time intervals, but without regard to background electrical activity.