Colour and brightness components of foveal visual evoked potentials in man

The different effects of colour and brightness on the transient visual evoked potentials to a foveal stimulus have been investigated in a psycho-physically controlled stimulus set-up, in which equally bright red, orange and green stimuli were substituted for a standard yellow stimulus. These colour-evoked responses were compared with the responses to additional brightness increments and decrements of each of the colour stimuli. An initial component of small amplitude, P55, was followed by a colour-dominated component, N87, and a luminance-dominated component, P120, with a maximum at the occipital electrode. Both N87 and P120 showed a decline in amplitude at the parietal electrode and P120 had a reversed polarity at FZ. These results indicate that the responses to equally bright green and red stimuli have closely similar wave forms, but that this changes rapidly with additional brightness differences. Comparison with the reports of subdural recording of colour-evoked potentials in the macaque striate cortex suggests that P55 corresponds with the primary excitation via geniculo-cortical fibres and that N87 and P120 represent later stages of cortical processing.