TONIC INTEROCULAR SUPPRESSION, BINOCULAR SUMMATION, AND THE VISUAL-EVOKED POTENTIAL

Purpose. Psychophysical studies have shown that a dark-adapted eye exe rts a tonic interocular suppression (TIS) upon spatial vision mediated by the contralateral eye. The present study was designed to demonstra te TIS by means of visual evoked potential (VEP) procedures. Methods. Evoked cortical potentials were obtained in response to reversing chec kerboard patterns with fundamental Fourier frequencies between 3 and 1 2 cycles per degree. Responses were obtained under monocular viewing c onditions when the contralateral ''adapting'' eye was dark adapted, un der monocular viewing conditions when the adapted state of the adaptin g eye was experimentally manipulated, or under binocular viewing condi tions. Data were collected from three healthy young men, two naive reg arding purpose of experimentation. Results. Regardless of spatial freq uency, monocular responses evoked by stimulating a ''test eye'' were a lways smaller in amplitude when the contralateral adapting eye was dar k adapted than when adapted to a dim, homogeneous field. The monocular evoked response obtained in the presence of an interocular adapting f ield was similar in amplitude to the binocular evoked response. During dark adaptation of the contralateral adapting eye, the amplitude of t he monocular evoked response decreased: the time course of this declin e follows that of psychophysically measured rod thresholds in the dire ctly adapted eye. Conclusions. TIS is easily demonstrated by means of VEP as well as psychophysical procedures. The well-known increase in V EP amplitude resulting from binocular viewing may be attributable to t he removal of TIS rather than to ''physiologic, binocular summation.''