ON THE OSCILLATORY POTENTIALS OF THE HUMAN ELECTRORETINOGRAM IN LIGHT AND DARK ADAPTATION

Abstract

The thresholds and the relation to stimulus intensity of the oscillatory potentials and the slow potentials (a‐ and b‐wave) of the human ERG were studied on adaptation to a steady background of different intensities. The total energy and dominant frequency of the oscillatory potentials were calculated by a combined impulse response and Fourier analysis when different intensity of stimulus light was used on adaptation to a maximal background illumination (about 1.5 × 103 photopic cd/m2).The curve of incremental thresholds of the oscillatory potentials shows a flat range on exposure to background light below 1 cd/m2 (the level of adaptation, at which the Purkinje shift of the b‐wave occurs) before rising along a linear section. The sensitivity of the oscillatory potentials declines on adaptation to bright background illumination when the sensitivity of the cones is higher than that of the rods.The linear section of log δI (incremental threshold) vs log I (background light) does not obey the Weber‐Fechner law [log δI = K log (I + ID)] but will approach Barlow's square root formula [log δI = K log (I + ID) 1/2] and the line has a slope of 0.33. There is, as expected, no sign of saturation, i. e. rapid decrease in sensitivity of the oscillatory potentials on adaptation to the brightest background illumination used.Oscillatory potentials recorded just at threshold have a high frequency (about 160 Hz), but the frequency decreases (to around 120 Hz) with stronger stimuli on adaptation to maximal background illumination. The stimulus‐response curve appears linear in weak background illumination, but changes to a plateau at the background intensity at which the Purkinje shift of the b‐wave occurs (about 1 cd/m2). Thus prominent cone activity as such does not seem to be able to grade the oscillatory potentials.The incremental threshold curve of the oscillatory potentials was similar, to that of the a‐wave but differed from that of the b‐wave.The slow potentials (a‐ and b‐wave) act independently of the oscillatory potentials in response to stimulus of different intensities, supporting the view that the origin of the oscillatory potentials is quite separate from that of the a‐ and b‐wave.