Contrast colour and contrast colour from flicker

Classical phenomena of contrast colours are obtained by placing a light achromatic circle surrounded by a coloured annular region (say, red): the circle no longer appears achromatic, but is tinged with the opponent hue. Here, we present a new phenomenon: when the red annulus is temporally modulated with an annulus also achromatic as the inner circle, observers perceive a greenish annulus surrounding a circle which now appears reddish. We call this phenomenon contrast colour from flicker (CCFF). We determined the temporal characteristics of the CCFF, associated with inversion of the perceived colour of the circle, along different colours of the annulus and different spatial contexts. We found that both phenomena persist when changing the annulus colour to blue, green, or yellow, and are strongly affected by background context. However, they differ in the temporal dynamics: both phenomena occur over the whole scale from several milliseconds up to two minutes, but CCFF depends on an instantaneous adaptation mechanism for colour appearance. This suggests that these phenomena may depend on different adaptation mechanisms.