The electrophysiological investigation of chromatic responses is not a routine part of clinical ophthalmology, for better diagnostic methods exist. The interest lies in the information about intraretinal processing that is revealed by such recordings, mostly studied in congenital red-green color deficiency, as reviewed by Armington.¹ Most of the previous publications along this line dealt with the b-wave, flicker electroretinogram (ERG), or early receptor potential (ERP).^{6–8,11,13,14,18–20,22,23} The spectral sensitivity of the b-wave and the flicker ERG were reported to be reduced at long wavelength in protans.^6,7 Yokoyama and his coworkers^20,22,23 demonstrated in protans and deutans abnormal spectral response curves of the b-wave and abnormal ERG responses to a mixture of red and green stimuli sinusoidally flickering in counterphase. The b-wave or flicker ERG is not solely indicative of the receptor activity.

Two kinds of electrical responses have been reported as being generated in photoreceptor cells: the early and late receptor potentials.^2,3 The major difference in waveform of the late receptor potential between the cones and rods lies in the off-response (response to a cessation of stimulus light); the off-response is rapid in the cones and slow in the rods.^2,3 (The off-response of blue cones is slow,^20,22,25 but blue cones are not concerned, insofar as we know, with red-green color deficiency.)

In the human ERG the off-response begins with a rapid positive-going deflection (the rapid off-response) at a stimulus intensity above about 6 lux at the retina.²⁴ The rapid off-response in humans follows flickering stimuli of high frequency (not less than 34Hz)²⁴ and is resistant to light adaptation,²⁴ and the relative spectral sensitivity function curve approximates the psychophysical photopic curve.¹⁰ This ERG rapid off-response is unchanged in congenital stationary night blindness,⁹ but no rapid off-responses can be obtained in rod monochromatism.⁹ The rapid off-response is preserved in vitro after treatment of the retina with aspartate or glutamate,²⁴ which is known to abolish the post-synaptic responses of the retina without abolishing the receptor potential. Thus, the rapid off-response is photopic in nature and is useful for an objective examination of the photopic function at the photoreceptor level.