To assess the influence of selective adaptation of long (L) and middle (M) wavelength sensitive cones with electroretinography (ERG) and psychophysics, a novel adaptation procedure was developed, which comprises a selective and quantifiable change in the state of adaptation in the different cone types. One adaptation condition was used as a reference. In four additional conditions, the M-cones or the L-cones were selectively adapted, so that they absorbed either more or less photons. At each of these five states of adaptation, the ERG response amplitudes to 30Hz L- and to M-cone selective stimuli were measured. Furthermore, the psychophysical sensitivities to L- and M-cone selective stimuli were measured at different temporal frequencies. In subjects with normal color vision, adaptation can have a strong influence on the L- and M-cone driven response amplitudes in the ERG and on both the L- and the M-cone sensitivities in the psychophysical luminance channel. As a result, the L- to M-cone ERG and psychophysical ratios can change dramatically at the different states of adaptation. The cone sensitivity thresholds and the L- to M-cone sensitivity ratio in the psychophysical chromatic channel are about unity at all states of adaptation, suggesting the presence of a compensatory mechanism. In dichromats, the responses and sensitivities to stimulation of the absent cone type were generally small at all states of adaptation. But, with reddish backgrounds residual ERG responses and residual psychophysical sensitivities were observed, indicating the presence of either a robust rod driven signal or an additional adaptation mechanisms that are not cone driven and that have not been described before.

It is implicit in many cone-specific ERG studies that the amplitude is proportional to the numbers of cones stimulated. The objective of these experiments was to test this idea by comparing ERGs obtained from different areas of the retina with histological data on cone-density distributions. The histology (Curcio et al., 1990) shows that the cumulative number of cones in the human retina increases exponentially with stimulus diameter between 0- and 40-deg eccentricity. L-, M-, and (L+M) cone-driven 30-Hz ERGs were obtained from a series of stimuli with one of the following configurations: (1) Circular stimuli of different angular subtense up to 70-deg diameter. (2) Annuli with 70-deg outer diameter but variable inner diameter. (3) Annuli of constant area but increasing eccentricity. Cone contrasts were equalized for each stimulus condition. The modulated and nonmodulated regions of the screen had the same mean hue and luminance. The data suggest that the L+M cone ERG amplitude increases with stimulus diameter in direct proportion to the estimated number of cones stimulated. Furthermore, the total L+M responses appear to be predicted from individual L and M responses by simple linear summation for both the disc and annular stimuli.