Purpose. Although colour information is carried only by a subdivision of the parvocellular stream, chromatic isoluminant stimuli may activate the achromatic system. The magnocellular system responds to isoluminant red/green patterns, whereas blue/yellow stimuli, seemingly isoluminant, may produce luminance-contrast intrusions due to variability of macular pigmentation and chromatic aberration. We attempt to validate chromatic visual evoked potentials and isolate achromatic intrusions using criteria established in human psychophysical and macaque single/multi-unit experiments. Methods. Occipital scalp VEPs were compared following stimulation with red/green or blue/yellow gratings generated on (1) an apparatus which minimises chromatic aberration or (2) on a tv monitor (adjustable hues). Stimulus presentation was either phase reversal or on-off (no change in mean luminance or mean hue). Results. The chromatic VEPs show low-pass temporal frequency tuning similar to psychophysically defined colour mechanisms and macaque visual cortical cells of high chromatic sensitivity. VEP dependence on spatial frequency is more variable, probably related to the number and distribution of activated units close to the skull, which contribute to slow potentials. Achromatic intrusions are different for red/green and blue/yellow gratings, probably because different populations of cortical units are activated. Adherence to the tritanopic confusion line is less critical than expected for S-cone generated VEPs. Even gratings containing dominant wavelengths of 480/570 nm elicit VEPs which resemble tritan rather than red/green waveforms. Individuals show a surprising range of responses, especially to tritan versus achromatic stimulation. Conclusions. Colour selectivity of VEPs depends not only on the chromatic purity of the stimulus, but also on the individual sensitivity of the subjects to chromatic/achromatic stimulation.
|Journal||Investigative Ophthalmology and Visual Science|
|Publication status||Published - 15 Feb 1996|