The purpose of this paper is to describe the interaction between two spontaneous biological rhythms and the visual evoked response. However, a study of the evoked response in humans is already complicated enough, and complicating it further by mixing in spontaneous biological rhythms may require some justification. Just suppose human beings came in genetically matched bunches like pedigreed litter-mate pups, and could be brought to the laboratory with approximately identical past experiences. It would simplify some problems, certainly. A first approach to this is actually possible. Spontaneous rhythms afford a kind of biological lazy Susan. Each cycle presents the same selection of differing physiological states in the same subject. Suppose, for example, we select two phases of such a cycle. We may alternately take a measure at first one and then the other of these phases, but average together the measures at each of the phases separately. The results are data on two differing physiological states collected for all practical purposes simultaneously on the same subject. In averaging an evoked response we may present stimuli at two or more selected phases, but average the stimuli according to the phase of the cyclic process at which the stimulus is presented. This paper will deal with only two particular spontaneous physiological cycles: the EEG alpha cycle; and the cardiac cycle. First let us consider the EEG alpha cycle. Something that looks like alpha rhythm can be produced by photic stimulation. The idea of presenting photic stimuli at various phases of the spontaneous alpha rhythm to alter degrees of photic driving has occurred to many investigators. An adequate device to achieve such a result was described as far back as 1951 (Hewlett, 1951), yet references to practical applications of such a device are scarce. Nevertheless, renewed interest in the possibility that photic stimuli might have quite different effects depending upon alpha phase at stimulation can be justified on two grounds: first, reports of the influence of alpha phase on behavior; and second, reports of the influence of alpha phase on the visual evoked response. Both simple visual reaction time and brightness judgment depend upon alpha phase at stimulation. Unfortunately, there is no apparent consistency from person to person; and although a given individual will show * This work was supported by Contract NONR 2931( 00), of the U. S. Office of Naval Research; and by the California Department of Mental Hygiene Grant NO. 61-1-24. Reproduction in whole or in part is permitted for purposes of the United States Government.
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