We examined in which form an additional structural stimulus in the white checks influenced the visual evoked potential of a checkerboard reversal induced response. A pattern of vertical bars was introduced with a short delay to the checkerboard reversal. The purpose of these experiments was to find out about the discrimination threshold of the occipital cortex for timely spaced stimuli. The double stimulus was separated from the next change minimally for 800 ms. With this arrangement we avoided "steady states", which are shown by repetitive interstimulus intervals of less than 200 ms. Nine normal volunteers between 22 to 28 years of age were measured. We found a typical waveform after the P 100 potential consisting of a positive peak followed by a negative long-lasting baseline elevation. Only separations of more than 60 ms yielded this configuration. Interstimulus times of less than 60 ms resulted in refractory effects on the visual evoked response. All the subjects reported colour or brightness changes of the additional bars. These pseudo colours were very probably due to the Prevost-Fechner-Benham-Effect which is common with repetitive white and black pattern changes. Our pseudo colours showed remarkable differences to the classical designs which has led us to the conclusion that colour perception on TV monitors is influenced so far by unpredictable flicker factors.