A rod monochromat's pattern VECP at high and low background levels: spatial tuning of colour mechanisms and rods...

The visually evoked cortical potential spatial-tuning function, i.e. checksize vs amplitude of response, was determined for a rod monochromat at a low background level of 0.2 scot td or a high background level of 200 scot td. The results showed evidence of a peak at 36' for the low-background spatial-tuning curve, but almost no peak was evident when the high background level was employed. This latter finding was interpreted as consistent with a rod saturation effect. Additionally, the shape of the spatial-tuning function found with the low background level was compared with that obtained from the blue or red (R) and green (G) cone mechanisms of normals. It was concluded that the blue cone mechanism may not use, as has been hypothesized, the same neural transmission pathway as rods because its spatial-tuning curve differed from both the rod and R and G systems.