New observations on the violet mechanism in man as revealed by the VECP

The violet mechanism of the human visual system was studied in 14 trichromats by recording visually evoked cortical potential (VECP). Stimulus was a 10 degrees test light of 451 or 489 nm and of 700 ms duration, superimposed on a 15 degrees steady adaptation field of 574 nm (20,000 td). The violet mechanism was identified by measuring the relative radiation intensity necessary for a constant response to 489 and 451 nm. Both for the sensory threshold and for a constant VECP the radiation intensity was found to be 0.4 log unit higher for 489 nm than for 451 nm. The VECP components elicited by violet light (451 nm) exhibited individual and intraindividual variations more than those to a red (630 nm) test light. A full sequence of VECP components (P1, N1, P2, N2, P3) to violet light was seen occasionally. Regularly recorded were N2 and P3. The component P2 was missing as soon as the radiation intensity exceeded the sensory threshold by more than 1 log unit. Similarly, saturation of the N2--P3 amplitude occurred at test light intensities 1.3 log unit above the sensory threshold. Increasing the size of test field from 8 degrees to 14 degrees resulted in an increase of the P2 amplitude. The VECP to violet as compared to red light exhibited smaller amplitudes and a slower time course of all potential components as well as 20 to 55 ms longer latencies. In contrast to the red and the green mechanism where VECPs of larger amplitude were seen both at onset and offset of the stimulus, off-components were recorded to violet light only occasionally.(ABSTRACT TRUNCATED AT 250 WORDS)