Chromatic VEP points to two systems for processing colour

摘要

Recent optical imaging experiments have demonstrated a complicated arrangement of colour and luminance processing that is carried out in Area V2 (Wang et al. 2006). While imaging can give some idea of function, electrical activation measured with the cortical visual evoked potential (VEP) may add to this picture. We aimed to study surface versus contour based colour processing through diffuse heterochromatic exchange compared with that generated by the appearance/disappearance of a heterochromatic pattern. Six participants with normal colour vision volunteered for the study. The VERIS mfVEP system was used. Diffuse stimulation corresponded to the hexagons (subtence 4 deg) alternating between the colour and background grey either at the same luminance (36 cd/m2) or with a constant luminance contrast of 30%. Patterned coloured stimuli comprised line patterns presented in appearance-disappearance mode with the same colour and luminance contrasts as for the diffuse exchange. For red stimulation, a positivity (140 ms, recorded Oz to Fz with ear ground) demonstrated an amplitude which increased with saturation of red. Blue diffuse stimulation resulted in a negativity (130 ms) in the first order response. Patterned stimulation though producing a larger signal than diffuse stimulation was almost independent of the level of saturation. Spectral dependance of the diffuse chromatic VEP showed a null point at yellow (570 nm). However, the response to yellow pattern appearance was still considerable, with a manifestly different waveform from the diffuse chromatic VEP. Thus, there are separate neural processes for serving surface and contour colour signalling. The diffuse response shows a separation between luminance and colour while the pattern response shows no such separation. The contribution of the blobs of (V1) and thin stripes (V2) to these processes are yet to be established.