The modulation of simultaneous chromatic contrast induction

PhD ThesisA coloured background may induce a contrasting colour in a target figure set against it. This effect, known as 'simultaneous chromatic contrast induction', can strongly influence the way in which we perceive colours and may therefore play an important role in our colour perception. Yet in order to support putative roles in phenomena such as colour-constancy, simultaneous contrast effects must be in some way 'regulated', otherwise they would cause objects to change colour when moved from one setting to another. Here we investigate a number of candidate cues that might be expected to influence the strength of simultaneous chromatic contrast. In experiment 1, we investigate the influence of global illumination cues that can indicate whether a colour change is consistent with a change in the illumination over the entire scene, or with a localised change in the background colour. Global illumination changes have already been shown to facilitate simultaneous contrast induction, and we further find that this effect is diminished by increasing the luminance contrast between the elements that make up the scene. Contrary to our expectations, increasing the number of distinct chromaticities in the surround does not have any effect. Finally, we attempt to discover whether this effect occurs at a monocular or a binocular site, by presenting different elements of the stimuli either to the same eye as the target, or to different eyes. Here, our results suggest that the effect is probably binocular. In experiment 2, we investigate how the strength of simultaneous contrast induction changes when textured backgrounds and targets are substituted for the uniformly shaded backgrounds and targets that have been studied in most recent investigations of simultaneous contrast induction. Texture in the background reduces the strength of simultaneous contrast induction for uniform targets. However, texture in targets set against a uniform background strongly inhibits simultaneous contrast induction. When both background and target are textured, simultaneous contrast induction is partially restored suggesting that texture may exert its influence due to segmentation effects. However, another result shows that simultaneous contrast is primarily determined at a local level, arguing against this interpretation of the effect. We do show that the effects of texture are tightly tuned to the cardinal axes, implicating early visual mechanisms in the functioning of simultaneous chromatic contrast effects. In experiment 3, we investigate two other strong segmentation cues: binocular disparity and differential motion between a target and its immediate surround. Our results show that differential motion may facilitate simultaneous contrast induction, strengthening its effects. However the effect is small and is often inconsistent between observers.The Physiological Society, The Barbour Foundation, The Ridley Family Fun