Abstract
We investigated color tuning mechanisms in perceptual grouping by pitching two global patterns with different chromatic properties competing each other for a final stable percept. We use a tripole Glass patterns (tGPs) paradigm. A tGP is composed of randomly distributed sets of three dots, or tripole. Each tripole contained one anchor dot and two context dots. An observer would perceive a clockwise (CW) or a counter-clockwise (CCW) spiral by grouping the anchor dot with either one of the context dots respectively. The hue of the anchor dot was fixed at an intermediate color direction half way between +L-M and S color directions (45° azimuth on equiluminance plane). One of the context dot has the same hue as the anchor dot, whereas the other dot varied in color directions within the range of ±135° relative to +L-M. The contrast of the anchor dot was fixed at twice the detection threshold, while the contrast of both context dots varied from near threshold to four times of the threshold. Participants were asked to answer whether they perceived a CW or CCW spiral. When the CW dot had the same hue as the anchor dot, the probability of perceiving a CW spiral, or P(CW), first increased and then decreased when the CW contrast increased, resulting in an inverted-U curve. The peak of P(CW) curve decreased and shifted rightward with CCW contrast when CCW dot had a color direction at around the +L-M axis. However, when the color direction of the CCW dot was near the S axis, P(CW) curves saturated and was not affected by a change in CCW contrast. The result suggests that, instead of narrowly tuned mechanisms centered at intermediate color directions (Mandelli et al., 2005), broadly tuned mechanisms, centered at around S and +L-M axes respectively, dominate perceptual grouping.