Colour assimilation in Munker - White patterns: Asymmetric shifts in hue and saturation for blue and yellow flanking conditions

B Lingelbach, B Neustadt, O Schmidt, W H Ehrenstein

ECVP 2003 abstract

When short coloured target bars are placed within more extended differently coloured flanking bars, the perceived colour of the target bars shifts dramatically towards the colour of the flanking bars (Munker - White effect). We measured this effect comparatively for blue and yellow flanking conditions. A calibrated colour CRT display (Eizo monitor with standard RGB) served for computer-assisted stimulus presentation and adjustment. Stimuli were 36 thin (11.2 min) horizontal bars (8.2 deg) alternating in colour (blue/yellow); part (2.15 deg) of the yellow or blue bars was replaced by eight red bars so that two rectangular areas were formed, to the left and right, separated by 1.8 deg. Six emmetropic, colour-normal observers adjusted the colour of the target in one rectangle to match it with that in the other. Settings (RGB data) were transformed into CIE Lab (uniform colour space) values. The results indicate symmetric shifts in brightness (reduction by blue, enhancement by yellow flankers), but asymmetric shifts in hue and saturation. Saturation was enhanced in both conditions, but approximately twice as much with yellow as with blue flankers. Hue shifts were more pronounced (by a factor of 1.4) for blue flankers (towards purple) than for yellow flankers (towards orange). These asymmetries are consistent with the notion that the yellow - blue colour opponent mechanism is split into two subsystems (eg Sankeralli and Mullen, 2001 Visual Neuroscience 18 127 - 135) with different spatial frequency characteristics (Hutchinson and Logvinenko, 2002 Perception 31 Supplement, 66).