Chromostereopsis - Reversal Effect

Reversal Effect

Evidence for the stereoptic effect is often quite easy to see. For instance, when red and blue are viewed side by side on a dark surrounding, most people will view the red as "floating" in front of the blue. However, this is not true for everyone, as some people see the opposite and others no effect at all. This is the same effect that both Goethe and De Wilde had indicated in their observations. While a majority of people will view red as “floating” in front of blue, others experience a reversal of the effect in which they see blue floating in front of the red, or no depth effect at all. While this reversal may appear to discredit chromostereopsis, it does not and instead, as originally proposed by Einthoven, can be explained by an increase in the effect and subsequent reversal via blocking of the eccentric position of the pupil with respect to the optical axis. The diverse nature of the chromostereoptic effect is due to the fact that the color depth effect is closely intertwined with both perceptual and optical factors. In other words, neither the optical nor the perceptual factors can be taken in insolation to explain chromostereopsis. This multifactorial component of chromostereopsis offers one explanation of the reversal of the effect in different people given the same visual cues.

Another interesting reversal effect was observed in 1928 by Verhoeff in which the red bars were perceived as farther away and the blue bars as protruding when the bars are paired on a white background instead of a black background. Verhoeff proposed that this paradoxical reversal can be understood in terms of the pupil’s luminance contours (see: Illusory Contours). The pupil has lines of constant luminance efficiency, with each subsequent line marking a 25% decrease in efficiency. Around 1998, Winn and co-workers confirmed Verhoeff’s interpretation of this reversal using experiments on different colored backgrounds. Other research has also suggested that border contrast changes could lead to color depth reversal with the switch from black to white backgrounds.

In 1933, Stiles and Crawford discovered that the light sensitivity of the fovea differs significantly for rays entering the eye through the center of the pupil versus rays entering from its peripheral regions. They observed that the usual "intensity multiplied by aperture" rule did not apply in foveal vision and that rays entering the eye via peripheral regions of the pupil were less efficient by roughly a factor of five. This effect is now known as the Stiles-Crawford effect and also has implications for the reverse chromostereoptic effect.