Role of chromaticity, contrast, and local orientation cues in the perception of density

We compared the role of the red-green, blue-yellow, and luminance post-rece ptoral mechanisms in the perception of density. The task requires the compa rison of densities between two stimuli composed of oriented bandpass elemen ts, pseudo-randomly scattered across an area of constant size. The percepti on of density differences was measured by a temporal 2AFC procedure for all pairs of mechanisms and for four possible densities. We found that stimuli of identical physical densities are not perceived equally: there is a cons istent bias in favour of blue-yellow stimuli which are perceived as signifi cantly more dense than red-green and achromatic stimuli. We considered thre e factors that could have differentially affected the density perception of blue-yellow stimuli: an increase in the perceived size of the individual b lue-yellow elements, a perceived contrast difference, and the presence of l ocal orientation cues. We found that the increased perceived density of the blue-yellow stimuli occurred despite the fact that there was no increase i n perceived size of the individual elements, and remained despite correctio ns for the two other factors. We conclude that the significant increase in perceived density for the blue-yellow mechanism is a global effect, associa ted with a perceived colour 'melting' of the elements in the array. Our dat a were fitted with the occupancy model of Allik and Tuulmets (1991, Percept ion & Psychophysics 49 303-314) and we found that blue-yellow stimuli have a greater 'occupancy' than red-green or achromatic stimuli.