Spectral sensitivity of the visual system as revealed by evoked potentials in normal and anomalous trichomats.

The spectral sensitivity of the visual system upon adaptation by white light at intensity of 10,500 td was studied in Maxwellian view. The monochromatic stimuli had a size of 8 degrees and duration of 700 ms. They were superimposed on an adaptation field of 14 degrees. The spectral sensitivity was determined by the off-VEP amplitude measured from the negative peak with a latency of 75-80 ms to the positive peak with a latency of 100-110 ms. The EEG was led off monopolarly from the occipital scalp area. Eight normal trichomats, 2 protanomalous and 2 deuteranomalous subjects were investigated. Some specificity of the spectral sensitivity curve, determined by the VEP amplitude, were studied. Special attention was placed on the sensitivity decrease between lambda = 550 nm and lambda = 590 nm, which was much pronounced in the normal trichomats. Maximum sensitivity was observed at lambda = 527 nm. The sensitivity at lambda = 578 nm was lower than the maximum one by 0.8 logarithmic units or more. It is assumed that this sensitivity decrease is a manifestation of opponency between the middle wavelength and long wavelength mechanisms. In the anomalous trichomats the sensitivity decrease between lambda = 550 nm and lambda = 590 nm was much less pronounced. Maximum sensitivity was obtained at lambda = 551 nm. In the long wavelength range of the spectrum the deuteranomalous subjects showed a higher sensitivity as compared to the protanomalous ones. The obtained results suggest disturbances of color opponency in the anomalous trichomats.